Earth History and Mineralisation

Session 1 Tectonic evolution and metallogenic potential throughoutearth history

Although it is possible to identify various tectonicprocesses conducive to mineralisation, the problemremains to identify the critical factors that control theformation of a large ore deposit at a particular time andlocation within an evolving orogenic system. To do this,it is best to examine mineralisation in modern, activesubduction complexes where it is possible to determinethe physical properties and dynamics of thelithosphereasthenosphere system, image plate archi-tecture and measure the rates at which tectonic andmineralising processes occur. Tectonic processes thatgenerate magmatism and deformation at convergentmargins influence the timing and location of ore depositsby inducing melting, by providing pathways for magmasand by varying the rate of supply of volatile components.Magmatism provides heat to generate hydrothermalsystems and also releases fluids that redistribute metalswithin the crust of the overlying lithosphere. Asorogenesis progresses from subduction of oceaniclithosphere and generation of oceanic island arcs, toarccontinent, then continent continent collisionthrough to post-collisional extension and orogeniccollapse, so various transient effects due to changes inplate configurations and subduction architecture cancreate various conditions conducive to magmatism andmineralisation. King et al.6 demonstrated that plate re-organisations driven by instabilities in mantle conventioncan occur on time scales of < 4 Ma. However, transientchanges in plate configurations are insufficient alone togenerate giant ore deposits. The same types of porphyrycopper and epithermal gold deposits occur in differenttectonic scenarios, so that although magmatism may begenerated by various subduction and mantle wedgeprocesses, the concentration of mineralisation in largeore deposits has more to do with the structural evolutionand the state of stress in the upper plate.

Subduction of oceanic lithosphereIn the SE AsiaSW Pacific region, for example, seismictomography not only images the present configuration ofsubduction slab anomalies in the upper mantle but canbe interpreted in terms of the past history of subductionto test tectonic plate reconstructions based on surfacegeology and palaeomagnetic data.4,5 Cenozoic magmaticarcs are richly endowed with magmatic hydrothermalmineral deposits such as high- and low-sulphidationepithermal Au, porphyry CuAu and skarn CuAudeposits.7 Because the magmas are associated with

CuAu mineralisation in both oceanic and continentalarcs, it seems that the metals were derived from themantle source of the magmas, rather than a local crustalsource. Most of the magmatism and ore deposits formedduring three short intervals of plate re-organisation ratherthan during periods of steady-state subduction. Theseplate re-organisations were initiated by the collision of theAustralian continent with the Philippine Sea plate at 25Ma, rotation or extrusion of Indochina and the cessationof spreading of the South China Sea at 17 Ma and animportant period of tectonic reorganisation at 5 Ma.1,7

Arc-related magmatism in transient tectonic settingsproduced the most abundant and largest ore deposits,most of which have formed since 5 Ma. For the periodsince 25 Ma, most subduction zones in the SE AsiaSWPacific region experienced significant arc volcanismduring hinge retreat (rollback), which in many cases wasaccompanied by marginal basin formation.7 Mostvolcanic episodes were completed within 35 Ma. Incontrast, reduction or cessation of volcanic activityoccurred during hinge advance. Hinge movement appearsto be a first-order factor in the control of both volcanismand ore formation. Increased volcanic activity duringperiods of hinge retreat can be explained by corner flow inthe mantle wedge that is induced to conserve massbeneath the arc as the slab descends. Magmatism mayhave resulted from slab melting, melting in the wedgeabove the subduction zone caused by slab dehydration,or melting in sub-arc lithosphere caused by inflow of hotmantle during slab rollback. Mantle melting is mostlikely around 80 km depth, with 50 km lateral transportof water from the slab.3

Changes in slab architectureSubduction of lithosphere can jump from one locationto another, usually because there is some impedimentto subduction, such as the presence of continental crustand/or thickened lithosphere entering the subductionzone, for example the collision of Australian con-tinental shelf with the Banda Arc, where subductionsouth of Timor has recently jumped north across theBanda Arc to Wetar.9 Subduction can reverse polarityas a consequence of changes in plate configurations.Northward subduction south of Timor has switchedto southward subduction of the Banda Sea plate atWetar. Stalling (and subsequent melting) of subductedlithosphere, slab steepening and subduction polarityreversal all appear to have been common regimes for the

Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003 Vol. 112 B107

Tectonic processes conducive to magmatic-hydrothermal mineralisation

Derek Blundell

Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK ([email protected])

DOI 10.1179/037174503225001604

formation of magmatic-hydrothermal deposits. The ratesof subduction and rollback may control the volume ofmelt and the amount of magmatic activity.

Slab detachmentThere is good evidence from earthquake hypocentrelocations and from seismic tomographic images thatsinking segments of subducted lithosphere becomedetached, thus relieving the remaining lithosphere of aload and allowing asthenosphere to flow through the gap.The resulting changes in temperature and pressure areconducive to melting of both asthenosphere andlithosphere. Wortel and Spakman13 have modelled amechanism of slab detachment through lateral propag-ation of a tear in the lithosphere. Where torn, thelithosphere is relieved of the load from the deeper part ofthe slab and rises, whilst the intact part of the lithospherecarries the full load and is depressed. Propagation of thetear can be followed by tracking the null point betweenuplift and downwarping as it progresses around the arc.For example, migration of the null point started in thewestern Carpathians at 16 Ma and migrated clockwisearound the arc to the Vrancea region at its south-easternend, where the slab is still attached. The tear is initiatedwhere the weight of the slab exceeds its internal strength,most probably when weak continental crust in thelithosphere arrives at the trench and starts to be sub-ducted. Temperatures in the continental lithosphereappear to be the main factor in determining the depth atwhich the tear initiates, which can be as little as 35 km. Asthe tear develops, hot asthenosphere can well up throughthe widening gap, producing a transient heat pulsethrough the lithosphere of the overriding plate. Modellingthe thermal consequences, van de Zedde and Wortel12

demonstrate the possibilities, under various transientconditions, of partial melting of asthenosphere associatedwith shallow slab detachment, partial melting of themantle lithosphere of the subducting plate or that of theover-riding plate, and anatexis of the over-riding crust.Slab detachment is the natural last stage of the subductionprocess of continentcontinent collision, when relativeplate motion ceases. It can thus be the underlying cause ofpost-collisional extension and orogenic collapse.

The magmatism and mineralisation of the Carpathiansbetween 15 Ma and 10 Ma2,8 is a well-documentedexample of slab tear and back-arc extension. Slab tearexplains a diachronous trend in magmatism clockwisearound the arc. Mineralisation is synchronous but owesmuch to complexity in the upper plate.

Localised extension within a transpressional regimeLocalised extensional settings in the over-riding platewithin an overall transpressional tectonic regime appearto be favourable for the location of igneous intrusionsand ore deposits. Tosdal and Richards11 explained thatthe locations of porphyry copper systems are oftenclosely related to major strike-slip fault systems, invol-ving both transpression and transtension, which aresteeply dipping or near-vertical, offering good pathwaysfor upward movement of magmas. They argued that

periods of normal compression or large-scale extensionare not conducive to the formation of porphyry copperdeposits but, instead, many porphyry copper depositsdeveloped under short-lived, transient conditions duringperiods of plate re-organisation.

Discussing the origins of Variscan mineralisation,Tornos et al.10 presented an account of oblique northwardsubduction of oceanic lithosphere beneath SW Iberia andsubsequent collision-induced thrusting and left-lateraltranscurrent motion of crustal blocks. Most of themineralisation in the area was associated withmagmatic/hydrothermal activity controlled by strike-slipfaulting related to large-scale transpressional deformationbut located within areas of localised extension. Variousstyles of ore deposits were formed, involving abnormallyhigh heat flow and large volume fluid flow along openfractures driven by the interplay of fault and magmaticactivity to shallow depths. In the Iberian Pyrite Belt, themineralisation is restricted to (sub-) surficial levels.Oblique collision and associated lateral escape promotedthe opening of crustal-scale fractures with crustal meltingat relatively low pressures, which required extensionaltectonics and high heat flow conditions, possibly in a fore-arc extensional setting. Second- and third-order pull-apartsedimentary basins were also formed, the locations of thelargest massive sulphide deposits. This tectonic setting isdistinct from the subduction-related arc and back-arcgeodynamic environments postulated for most otherequivalent metallogenic provinces, which may be the keyto understanding what made this part of the world sofavourable for hosting about 5 Ma of intensely productivehydrothermal activity around the end of Devonian times.One possibility is that the mineralisation is associated withslab break-off and the consequent major heat pulse,providing conditions for magma generation and theappropriate deformation in the over-riding plate.

References1. M. E. BARLEY et al.: Geol. Soc. Spec. Publ., 2002, 204,

3947.2. H. DE BOORDER et al.: Earth Planet. Sci. Lett., 1998, 164,

569575.3. J. W. DAVIES and D. J. STEVENSON: J. Geophys. Res.,

1992, 97B, 20372070.4. R. HALL: J. Asian Earth Sci., 2002, 20, 353434.5. R. HALL and W. SPAKMAN: Earth Planet. Sci. Lett.,

2002, 201, 321336.6. S. D. KING et al.: Earth Planet. Sci. Lett., 2002, 203,

8391.7. C. G. MACPHERSON and R. HALL: Geol. Soc. Spec. Publ.,

2002, 204, 4967.8. F. NEUBAUER: Geol. Soc. Spec. Publ., 2002, 204, 81102.9. A. N. RICHARDSON and D. J. BLUNDELL: Geol. Soc.

Spec. Publ., 1996, 106, 4760.10. F. TORNOS et al.: Geol. Soc. Spec. Publ., 2002, 204,

179198.11. R. M. TOSDAL and J. P. RICHARDS: Rev. Econ. Geol.,

2001, 14, 157181.12. D. M. A. VAN DE ZEDDE and M. J. R. WORTEL: Tectonics,

2001, 20, 868882.13. M. J. R. WORTEL and W. SPAKMAN: Science, 2001, 290,

19101917.

B108 Applied Earth Science (Trans. Inst. Min. Metall. B) August 2003 Vol. 112

Session 1 Tectonic evolution

This presentation focuses on improving the under-standing of processes controlling the tectonic genesis ofore systems by analysing the tectonic mechanisms andassociated stress regimes. The timing and unique tectoniccharacter coincides with the mineralising stages. Apply-ing tectogenetic analysis, tectonic mechanisms and theirnature has successfully identified and explained anumber of ore systems of various commodities, includ-ing Au, Cu, Ni, ZnPb and diamond.2,3

Tectonic mechanismsTectonic mechanisms and the corresponding tectonicand mineralising processes developing in orogenic andcratonic regimes have been investigated. The TelferAuCu (Proterozoic Paterson Orogen) and Golden MileAu (Archaean Yilgarn Craton) world-class deposits ofWestern Australia are used as examples to explain thetectonic mechanism also commonly observed elsewherein various modifications.

Tectonic mechanisms at Telfer (orogenic environment)The Telfer ore system developed within a domal structure(Fig. 1A,B). Most explanations employ regional com-pression related tectonic processes, particularly strike-slip

shearing, folding and flexural-slip folding, and thrust typedeformation mechanisms to explain the formation of thedeposit domal shape and controls on mineralisation.The results of a study on the tectonic mechanisms atTelfer indicate that its domal geometry is not a regionalcompression related feature, but rather a result of localshear-extensional processes propagated from thebasement.3 This gives rise to an asymmetric extensionalflexural bend of the sequence with normal dip-slipkinematics along the bedding surfaces during themineralising processes (Fig. 1A). This flexural-bend isa deposit-scale tectonic feature closely linked with theactivity of the NWSE oriented and basement-rootedmajor shear/fault structural system, or lineament thatparallels the Paterson Orogen.

The forces controlling the Telfer-scale extensionaldeformation siting of mineralisation propagate fromthe basement upwards, and are mostly related to steepSSWNNE oriented structures mostly with reverse-slip kinematics. In a number of cases, these structuresdisplay a specific convex geometry and a tendency tofade out upwards. In this model, Main Dome is a firstorder (NWSE elongation), while West Dome is asecond order (NNESSW elongation) mineralisedfeature. This interpretation explains the lack ofcorrelation between the reefs forming these twotectonic domains.4 Although formed during thedevelopment of the extensional flexural bend, the twodomes evolve via differing tectonic forming processes,tectonic genesis and structural geometry: as a result, theycannot be considered as en echelon structures in theclassical sense.

Tectonic mechanisms at Golden Mile (cratonicenvironment)Most considerations on the Golden Mile (Kalgoorliecamp) tectonic deformation are concentrated on variousaspects of tectonic evolution. These include a recent sug-gestion that Golden Mile is an early orogenic-stagerelating ore system with significant rotation of thesequence and mineralisation in the later tectonic events.1

The most prominent fault of the Kalgoorlie region,the Boulder-Lefroy Fault Zone, is a near-surfaceexpression, generally with oblique-slip tectonic move-ment, of a major basement fracture striking NNWSSE.The active role of this basement-rooted feature duringmineralisation was to propagate the extensional struc-tural environment upwards, favouring prominent multi-lode Au mineralised system at Golden Mile and othermajor deposits in the region.

The moderate-to-steep westward dip of the GoldenMile Dolerite follows the orientation of the contactzone between the Dolerite and the underlying ParingaBasalt. This contact, together with steep to vertical



Tectonic mechanisms and their role in forming the major ore systems of Western Australia

W. V. Bogacz

Archon Resource Technologies Pty Ltd and BFP Consultants Pty Ltd, Level 2 Eastpoint Plaza, 233 Adelaide

Terrace, Perth, WA 6000, Australia ([email protected])

1 (A,B) Tectonic mechanism forming the Telfer oresystem

A

B

DOI 10.1179/037174503225001613

lithologies for the Western Lodes, determines aspecific triangular-like tectonic feature (Fig. 2A,B).

The formation of steep reverse-slip faults withconvex-type geometry and consistent E up/W downmovement, is believed to be the principal drivingmechanism for propagation of basement forces upwardsand the creation of the Golden Mile extensional regimeand accompanying mineralisation. This mechanismpermits the explanation of the tectonic genesis of theGolden Mile Dolerite hosted mineralisation (Fig. 2B).This also explains the rich Oroya shoot (23 Moz), whichis developed within Paringa Basalt, as a local lower orderfeature controlled by the formation of basement-propagated reverse-slip Kalgurli and Kalgurli Northfaults (Fig. 3).

ConclusionsThe understanding of the tectonic mechanismscontrolling extensional deformation and, therefore,the space preparation processes and correspondingmineralisation pattern, the nature of structures active

during the mineralising processes, and the overalltectonic deformation settings in which mineralisationis confined, are critical for the explanation of thetectonic genesis of ore systems and their geometry. Iftectonic mechanisms are understood, prediction ofmineralised zones becomes possible.

Comparing tectonic deformation and mechanismsof mineralisation emplacement, many similaritiesexist between Proterozoic (orogenic) and Archaean(cratonic) deposits. Processes related to the formationof ore systems are strongly influenced by thebasement activity. Basement-propagated structuralfeatures are dominated by steep reverse-slip andnormal dip-slip structures that predominantly controla deposit and local scale extensional deformation,associated mineralising processes, and determinegeometry and the internal pattern for many deposits.

More brittle type geomechanical environments,competency and other geomechanical contrast andboundary zones such as faults and flexures are moreprone to the propagation of the extensional deformationand mineralisation in their surroundings. As a rule,these are basement-propagated zones, which developextensional openings (controlling mineralisation) alongpre-existing foliation and other tectonic anisotropysurfaces within the host rock, thus making them lowerorder secondary structures. Later stage processes (morebrittle) propagating tectonic deformation along pre-existing structures are unique when compared to thehost rock (more ductile) earlier stage tectonic evolution.

References1. BATEMAN et al.: Appl. Struct. Geol. for Miner. Expl.

Min., AIG Internal Symp., 2325 September 2002,Kalgoorlie, WA, 2002, 68.

2. BOGACZ: Mineral deposits at the beginning of the 21stcentury, Balkema, 2001, 713.

3. BOGACZ: Appl. Struct. Geol. for Miner. Expl. Min.,AIG Internal Symp., 2325 September 2002,Kalgoorlie, WA, 2002, 2225.

4. ROWINS et al.: (1997) Econ. Geol., 1997, 92, 133160.5. TRAVIS et al.: Spec. Publ., Geol. Soc. Aust., 1971, 3.



B

A

2 (A,B) Tectonic mechanism forming the Golden Mileore system

3 Tectonic interpretation of the Oroya shoot

Most passive margins in earth history have experiencedthe same chain of events: (i) they form by rifting; (ii)subside as the adjacent ocean basin opens, then closes;and (iii) finally end up colliding with an arc. Thelifespan of passive margins is of interest as an indicatorof the earths tectonic regime, and as a framework forunderstanding their contained ore deposits. Heatproduction on earth was about 4 times the presentvalue at the end of the Archaean. Two of the commonlyinvoked mechanisms for loss of this extra heat aregreater ridge length (implying more, smaller plates) andfaster plate motions. Either mechanism, or both incombination, would have resulted in comparativelyshort-lived ocean basins and, by implication, short-lived passive margins. As a geologic test of thishypothesis, we examine the lifespan of the passivemargins within orogenic belts. The lifespan of a passivemargin is the time from the rift-drift transition to thedrowning of the platform at the onset of collision, i.e.when the passive margin evolves into a foredeep.

Ancient passive marginsData from a preliminary compilation of Proterozoic andPhanerozoic passive margins are plotted in Figs. 1 and 2.Age control is adequate for all the Phanerozoic marginsand for a few of the Proterozoic ones, but is poor forArchaean margins. The margins in our dataset had amean duration of 145 million years (Fig. 1). Surprisingly,the longest-lasting passive margins yet to be documentedare among the oldest (i.e. the Palaeoproterozoic southernand eastern margins of the Superior Craton, at ~320 and~285 million years). Presuming that the geochronology isreliable and the tectonic interpretations are sound, this

result invites an explanation, though the only ones thatoccur to us are ad hoc. The lack of a secular trend (Fig.2) is unexpected; the available data certainly do notrequire some combination of greater ridge length andfaster plates in the Early Proterozoic.

Modern passive marginsThe worlds extant passive margins range in age fromabout 5 million years (Red Sea) to about 180 millionyears (Central Atlantic). These margins are only partway through their life cycles and thus have a youngerage distribution than the ancient margins. Theirregularity of the age distribution (Fig. 2) is mainly aresult of the staggered breakup of Pangea. Large partsof these margins are likely to endure for many millionyears longer, before finally colliding with something.The worlds population of extant passive margins willthereby increase in both mean age and maximum agefor the foreseeable future. If one of the Central Atlanticmargins can survive unscathed for another 140 millionyears, it will have matched the lifespan of the southernmargin of the Superior Craton.

Ore deposits of passive marginsMineral resources associated with the various phases ofpassive margin evolution include Mississippi Valley-type (MVT) PbZn, banded iron formation (BIF), andsedimentary barite. Many of the worlds largest MVT



Lifespan of passive margins through earth history

Dwight Bradley1 and David B. Rowley2

1US Geological Survey, 4200 University Dr., Anchorage, AK 99508, USA ([email protected])2Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave, Chicago, IL 60637, USA([email protected])

1 Duration of ancient passive margins through earthhistory

2 Histograms of the lifespans of extant and ancientpassive margins. The extant margins have yet to liveout their lives and hence are younger on average. Forextant margins, each age sector along a diachronousmargin is counted as one, and conjugate pairs countas two; other protocols yield similar histograms

DOI 10.1179/037174503225001622

Mesothermal or, as more recently termed, orogenic lodegold deposits are the predominant gold deposit type inArchaean greenstone belts.8 Their defining characteristicsand spatial and temporal distributions are well-documented.7,9 However, a discrete sub-set of gold depositswith atypical metal associations have been identified as acontentious group. They are most abundant in Late-Archaean terrains, and include several world-classexamples. The Hemlo Deposit, Canada, is characterised byan anomalous enrichment in Ba, Mo, and Hg, amongstother elements, and appears unique. The remainingatypical deposits fall into two groups: those enriched in Cu Mo (e.g. McIntyre-Timmins, Canada; Boddington,Australia), and those enriched in CuZn Pb Ag and/orabundant pyrite (e.g. Bousquet, Canada; Mount Gibson,Australia). Owing to the abundance of volcanic-hostedpyrite and ore-stage Cu-enrichment, Bulyanhulu has beenassociated with the latter category of atypical deposits.8,9

Models for these deposits invoke a consistent spatial ordirect genetic association with magmatic intrusions, arelationship that has been long debated in the literature.

Geological setting of BulyanhuluBulyanhulu is situated within the Sukumalandgreenstone belt, one of a series of Nyanzian (Late-Archaean, 2825 Ga) terrains developed in theTanzania Craton.3,4 The terrain, regionally metamor-phosed to greenschist facies, consists of two oval, sub-concentric belts. They comprise an inner (LowerNyanzian) belt characterised by basaltic and andesiticlavas and tuffs, and an outer (Upper Nyanzian) arcconsisting of banded iron formation with volcanic-lastics. The terrain has been considered to youngoutwards; however, recent geochronology contradictsthis, and suggests significant structural complexity.2,3,10

Bulyanhulu is situated within the inner arc. Syn- andpost-orogenic granitoids, as well as several generationsof dykes of lamprophyric and basaltic composition,have been identified. In the Bulyanhulu area, a series ofNWSE trending shear zones occur and the main zoneof mineralisation, Reef 1 (105 Moz Au resource),1 ishosted in one such shear structure, with furthermineralisation in parallel structures termed Reef 2.

deposits are hosted by carbonate sequences that formedalong passive margins and were later mineralised in

collisional forelands;1 MVT deposits are known as farback as 2300 Ma but peaked around 300 Ma (Fig. 3).Many Proterozoic Superior-type BIFs formed aspassive margins evolved into collisional foredeep outerramps of foredeeps.2 Some Phanerozoic bedded bariteslikewise formed during onset of collision, along theforedeep axis.4 Secular trends in the existence andabundance of these deposit types (Fig. 3) are mostlikely due to plate interactions, superimposed onchanges in climate, ocean chemistry and atmosphericcomposition.

References1. D. C. BRADLEY and D. L. LEACH: Mineral. Deposita,

2003, 38, In press.2. P. F. HOFFMAN: Am. Geophys. Union Geodynamics

Ser., 1987, 17, 8598.3. P. W. JEWELL: SEPM Spec. Publ., 2000, 66, 147161.4. J. B. MAYNARD and P. M. OKITA: Econ. Geol., 1991, 86,

364376.5. C. W. MEYER: Annu. Rev. Earth Planet. Sci., 1988, 16,

147171.



3 Histograms showing the age distribution of threeore-deposit types sometimes associated with passivemargins

The Bulyanhulu enigma: an atypical Archaean lode gold deposit evidence for a pre-oremagmatic input?

C. M. Chamberlain1, J. J. Wilkinson1, R. J. Herrington2 and A. J. Boyce3

1Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK2Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK3Scottish Universities Research and Reactor Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, UK

DOI 10.1179/037174503225001631

Characteristics of mineralisationObservations of the Bulyanhulu host rocks and min-eralisation show that its evolution includes thefollowing.

Syn-volcanic activity and carbonaceous clasticsedimentationTholeiitic volcanic rocks dominate the stratigraphy, andare intercalated with calc-alkaline rhyodacites andsubordinate interflow sedimentary units, including theKisii Shale Unit, which largely hosts the Reef 1 gold-orebody at Bulyanhulu. The felsic rocks host pyriteclasts similar to those present in volcanic-hostedmassive sulphide (VHMS) systems. These volcanicsinclude hyaloclastite flows and polymict sediment-bearing lava breccias, and show evidence of extensivespilitisation.

Intrusions and related vein fluidsShallow-level porphyries, geochemically associatedwith calc-alkaline extrusive facies, intrude thevolcanics and sediments. These show variable degreesof alteration and mineralisation. The earliest stage ofquartz veining recognised (stage I) is characterised byfluid inclusions that contain 1248 wt%equiv. NaCl,with additional CaCl2, and have a maximum homo-genisation temperature of 420C. The most likelysource for such a saline fluid is magmatic. This issupported by the occurrence of directly measuredfluid dD and inferred d18O compositions that lie in thefield of magmatic water. Taken together, the fluidcharacteristics are inconsistent with VHMS-related orepithermal origins, or basinal brines.

Regional metamorphismPeak regional Kahaman metamorphism at around27002650 Ma was associated with an approximatelynorthsouth oriented, compressive event. Regionalmetamorphism to greenschist facies is characterised bythe pervasive development of chlorite-calcite alteration,with a generally weak fabric development. Barren shearstructures are enriched in LOI, CaO and Fe2O3 con-sistent with syn-metamorphic carbonatisation. Thesezones are distinct from mineralised shears in that theylack quartz-carbonate veining and gold-bearingsulphides.

Shearing related to shorteningThe principal deformation event observed atBulyanhulu is characterised by reverse high-angleshearing and associated quartz vein emplacement.The strong rheological contrast that the Kisii ShaleUnit provided resulted in intense focusing of strainallowing the Bulyanhulu Shear Zone, hosting Reef 1,to develop.

Structurally controlled mineralised veinsSecond-stage quartz veins (stage II quartz) cut acrossthe shear foliation which, in turn, overprints theregional metamorphic fabric indicating that veinformation postdated peak metamorphism. The main

stage of quartzcoppergold-sulphide veining atBulyanhulu is characterised by a series of steeplydipping lenses, varying in thickness and continuityalong strike, which overprint the pre-existing stage Iquartz. Other vein sets cut the shear zone foliationdisplaying variable degree of progressive deformationduring shearing events.

Pyrite, the dominant sulphide in the Reef 1 oreassemblage, occurs at the margins of stage II quartzveins. This pyrite is accompanied by chalcopyrite,pyrrhotite, microscopic gold and accessory monazite, intextural equilibrium with arsenopyrite. Parageneticallylater quartz, associated with boudinage textures, occurswith coarser gold and chalcopyrite, and accessorysphalerite and bismuthotellurides. It is not clear whethergold and chalcopyrite were introduced at this time, or ifthey were simply remobilised. Sulphur isotopecompositions of stage II sulphides show a relativelynarrow range, with the dominant population occurringbetween +20 and +45 supporting a homogeneoussulphur reservoir.

Aqueo-carbonic fluid inclusions enriched involatiles in addition to CO2 characterise the stage IIquartz veins, with homogenisation temperatures of~300450C, and show evidence of phase separation.Directly determined dD and inferred d18O values liewithin the overlapping fields of magmatic andmetamorphic waters.

The results of d13C analysis of vein carbonates arecomparable to those documented in Archaean golddeposits in the Yilgarn and Superior Cratons. Modellingof the carbonate alteration halo demonstrates a two-stage influx of hydrothermal fluids causing the resultingd13C profile.5

Statistical analysis (specifically principal componentanalysis) of lithogeochemical data is consistent with twogold associations AuCu and AuAg.6

DiscussionBulyanhulu has many characteristics consistent withthe classic model of mesothermal lode gold depositsbut significantly has a number of atypical features.5

Typical features(i) Like other gold deposits in the Sukumaland

Greenstone Belt (e.g. Geita, Buck Reef), mineral-isation at Bulyanhulu is shear-hosted.

(ii) Sub-parallel structures in contrasting lithologicalunits host gold mineralisation.

(iii) Fluid inclusions indicate low-to-moderate salinityCO2CH4-bearing fluids homogenising at 300450Cwith evidence of phase separation during veinformation.

(iv) The narrow zone of largely symmetrical alterationaround the vein system is consistent with fluidintroduction into wallrocks from the main shearzones.

(v) Gold is distributed over an extensive strike and diprange, similar to other major gold camps (e.g.Kolar, Timmins).



The duty of the geologist and the prospector is in fact todeliver the goods

Sir Lewis L. Fermor, 1951, 6th geological President of theInstitution of Mining and Metallurgy

Archaean cratons are underlain by relatively thin crust(~3040 km) and thick mantle lithosphere (up to ~250km) that is invariably enriched in diamonds ranging inage from Phanerozoic to Mesoarchaean.1,6,8,9 The distri-bution of mineral deposits across at least seven globallydistributed Archaean cratons also indicates that many ofthese fragments of early continents each have uniquepolymetallic fingerprints of their own.2 These differ-ences appear to reflect regional geochemical hetero-geneities of early earth. Some of these cratons have Au,Cu, Pb and Zn signatures that fit with simple collisionalmodels involving accretion of island arcs with contin-ental and oceanic fragments (e.g. Superior Province,Yilgarn and Zimbabwe Cratons). Others, however, areremarkably enriched in siderophile elements such as Ni,Cr, PGE, both in their crustal and mantle sections (e.g.Pilbara and Kaapvaal Cratons), whilst others still arerelatively enriched in Sn, W and U/Th (e.g. Amazonian,Leo-Man, Ntem and South China Cratons). How theseold continental fragments inherited their metallogeniccharacteristics is unresolved. In the case of the oldestArchaean cratons, their dominant metallogenic

fingerprints were formed near the time of their separationfrom the mantle; thereafter, their inherited metals weremostly remobilised and redistributed during subsequenttectono-metamorphic and erosion-sedimentary processes(e.g. Sn in South America; PGE in Southern Africa).Because different cratons are only small remnants of oncemuch larger and varied continents, their initial metalinventories were also in parts recycled. In this contri-bution, we compare quantitatively (using our extensive in-house GIS database of mineral deposits2,10) the mineralinventories of 12 Archaean cratons against mineral distri-bution across larger Phanerozoic continents (Gondwana,600200 Ma), Africa and South America (2000 Ma) tocompare and contrast the changing metallogenic finger-prints of earths continental lithosphere. This workattempts inter alia to chart evolving partition coefficientsof metalliferous elements between mantle and lithosphereduring growth and recycling of the continents over 35billion years of earths history.

MethodsA simple linear relationship exists between litho-diversity and mineral-resource diversity.5,7 This can beused to predict mineral potential of a specified region,although it ignores factors such as the infrastructureand exploration history of the study region. We use

Atypical features(i) Early syn-genetic sulphides in felsic volcanic shale

package, providing a focus for paragenetically laterhigh-grade gold mineralisation.

(ii) Evidence for an early saline fluid generation inthe vein system with a likely magmatic source.

(iii) Statistical lithogeochemical evidence for a two-stage introduction of metal into the Bulyanhulusystem.6

(iv) Mineralised and altered porphyry stocks inter-sected in drilling marginal to the ore zone.

(v) High tenor of copper in the gold assemblagealong with highly elevated Bi and Te.

SummaryWhilst it is clear that structure is the key to generation ofthe Bulyanhulu deposit, a number of the atypicalfeatures may be crucial components of the genetichistory of the deposit. Early syn-genetic pyrite associatedwith black shales was essentially barren of economicallyimportant elements. However, it may have acted as ageochemical trap for the precipitation of gold mineral-isation later on. Early saline fluids are recorded in quartz

in the gold-bearing vein mineralisation. They suggest anearly magmatic event which, based on lithogeochemicaldata, probably involved pre-ore metal enrichment. TheBulyanhulu deposit is, therefore, interpreted as a two-stagehydrothermal process with the main lode gold eventoverprinting an earlier CuAu enriched magmatic system.

References1. BARRICK: www.barrick.com (accessed 2002).2. G. BORG and T. KROGH: J. Afr. Earth Sci., 1999, 29,

301312.3. G. BORG and R. M. SHACKLETON: Oxford Monogr.

Geol. Geophys., 1997, 35, 608619.4. G. BORG et al.: Geol. Rundschau, 1990, 79, 355371.5. C. M. CHAMBERLAIN: (2003) Unpublished PhD thesis,

Imperial College, 2003, 1401.6. C. M. CHAMBERLAIN et al.: Appl. Earth Sci.(Trans.

Inst. Min. Metall. B), 2002, 111, 137138.7. D. I. GROVES et al.: Ore Geol. Rev., 1998, 13, 727.8. D. I. GROVES et al.: Econ. Geol., 2003, 98, 129.9. S. G. HAGEMANN and P. E. BROWN: Rev. Econ. Geol.,

2000, 13, 1559.10. S. MANYA and M. A. H. MABOKO: Precambrian Res.,

2003, 121, 3545.



Metallogenic scents of Archaean cratons: changing patterns of mineralisation duringearth evolution

M. J. de Wit and C. Thiart

CIGCES and AEON, Departments of Geological and Statistical Sciences, University of Cape Town, Rondebosch7701, South Africa ([email protected] and [email protected])

DOI 10.1179/037174503225001640

these methods to construct normalised fingerprintsof Archaean cratons (Fig. 1) to facilitate visualisationof relationships between cratons and specific mineralgroups (see for example 2-D contingency table andbar-chart of mineral groups and cratons of Africa;

Table 1, Fig. 2). Resulting profiles of cratons can beeasily compared against the total enrichment ofminerals across all cratons. Data in Table 1 are used asan input for correspondence analysis,4 a technique toconvert rows and columns of contingency tables into



1 Selected cratons used in this study (numbers 17 refer to craton names listed in Table 1; other cratons for whichanalyses is still to be completed are: 8, Superior; 9, Amazonian; 10, Sao Fransisco; 11, Pilbara; and 12, Yilgarn

Table 1 Cross-tabulated counts of mineral groups by cratons (6 cratons only)

Map Mineral groups/number of deposits Craton

Id Name Au CrNiPGETi CuZnPbBa MoSnSb W UThREE Other total

1 Kaapvaal 894 241 152 96 5 77 356 18212 Limpopo 0 40 33 0 8 9 15 1053 Zimbabwe 625 114 106 81 305 15 31 12774 Congo 53 2 24 1 0 17 63 1605 Tanzania 7 0 3 1 0 5 4 206 Leo-Man 47 22 11 10 2 9 53 1547 Requibath 0 5 21 0 3 4 10 43

Total deposits 1626 424 350 189 323 136 532 3580

2 Bar-chart of row-profiles (% across rows) for selected cratons. Total = all 7 cratons listed in Table 1

Mineral deposits have a heterogeneous temporal distri-bution, with characteristic peaks in the abundance ofparticular mineralisation styles at specific times in earthhistory. This uneven distribution can be explained by: (i)temporal changes in the processes that produce mineraldeposits; and (ii) preservational potential of the environ-ments in which the deposits form. Temporal changes inmineral-deposit forming processes can, in turn, beascribed to: (i) the evolution of atmosphere hydrospherebiosphere systems; (ii) a secular decrease in global heatflow; and (iii) long-term changes in tectonic processes (e.g.Barley and Groves1). As shown below, (iii) may be a directconsequence of (ii), and these factors also affect the long-term preservational potential of terrains. Althoughindividual lines of evidence for change in the oxidationstate of the atmospherehydrosphere system are hotlydebated, the nature of metal deposits for which transportand deposition are highly affected by redox state (e.g. Fe,Mn, U), and which formed in sedimentary environments,show marked evolutionary changes over time. A directcontrol on mineral deposit distribution by the seculardecrease in global heat flow is the restriction of NiCudeposits in high-Mg komatiite volcanic rocks to the LateArchaean and Palaeoproterozoic. In contrast, Palaeo-proterozoic to Tertiary NiCuPGE deposits areassociated with large intrusions and giant layered com-plexes that are less magnesian.

Gold deposits as potential tracers of tectonic trendsGold deposits are particularly useful for testing of secularchanges in tectonic processes because most such deposits

formed below the influence of surficial processes. There-fore, most are potentially unaffected by redox changes inan evolving atmospherehydrosphere biosphere system.Porphyry-and epithermal-type deposits form at highcrustal levels (< 3 km to surface) in arc and back-arcenvironments in convergent margins with high upliftrates and hence they are rarely older than Mesozoic andTertiary, respectively. Orogenic gold deposits, in contrast,form over a wide range of crustal environments (320km depth) in the same convergent margin settings, butduring the main stage of compressional to trans-pressional deformation that stabilised their host orogens.These deposits formed over 3 billion years and hence arepotentially sensitive tracers of temporal changes intectonic processes.

Orogenic gold deposits through time a preservational patternMesozoic to Tertiary gold deposits coincide withexternal ocean margins where accretion of juvenilecrust took place in environments in which largethermal anomalies were related to crustal thickeningor upwelling of asthenosphere due to ridge sub-duction, subduction rollback or lithospheric delamin-ation. Older deposits appear to have formed in similartectonic settings in which there were anomalousinputs of thermal energy during gold mineralisation.Orogenic gold deposits of all ages thus record orogen-wide fluxes of deeply-sourced auriferous fluid as apart of the orogenic process in convergent marginsettings. The formation ages of the orogenic gold

multi-D plots. This exploratory data analysis techniquemakes no distributional assumptions and is merely auseful preliminary step towards more structured andtraditional multivariate modelling of categorical data. Thecorrespondence maps show how the cratons and theirmineral groups cluster in n-dimensional space. Anormalised density value (index) for each craton andeach mineral group can then be calculated. A majoradditional challenge is to normalise the data further sothat each craton index is weighted per unit area of craton,as well as for its infrastructure (roads, mining activity). Forexample, cratons in well-developed mining countries(Kaapvaal) have been more thoroughly explored thanothers (Congo). We incorporate this in our finalmetallogenic fingerprints of cratons, before we comparethem to larger and younger continental fragments.

References1. M. J. DE WIT: Precambrian Res., 1998, 91, 181226.2. M. J. DE WIT et al.: J. Afr. Earth Sci., 1999, 28, 3551.3. L. L. FERMOR: Trans. Inst. Min. Met., 1951, 60, 421465.4. M. J. GREENACRE: Correspondence analysis in practice,

1984.5. J. C. GRIFFITHS and C. M. SMITH: Comput. Geosci., 1992,

18, 447486.6. D. E. JAMES et al.: Geophys. Res. Lett., 2001, 28,

24852488.7. M. J. MIHALASKY and G. F. BONHAM-CARTER: Nat.

Resources Res., 2001, 10, 209226.8. S. B. SHIREZ et al.: Science, 2002, 297, 16831686.9. J. STANKIEWICZ et al.: Phys. Earth .Planet. Interiors,

2002, 130, 235251.10. C. THIART and M. J. DE WIT: S. Afr. J. Geol., 2000, 103,

215-230.



Gold deposits as sensitive indicators of tectonic environments and their preservationpotential throughout geological history

D. I. Groves1, R. J. Goldfarb2 and R. M. Vielreicher1

1Centre for Global Metallogeny, School of Earth and Geographical Sciences, University of Western Australia,Crawley, Western Australia, 6009, Australia ([email protected] and [email protected])2US Geological Survey, Box 25046, M.S. 964, Denver Federal Center, Denver, CO 80225, USA([email protected])

DOI 10.1179/037174503225001659

deposits define two major Precambrian peaks at27502550 Ma and 21001750 Ma, a marked lack ofdeposits at 1750600 Ma, and a more-or-less continuous,but cyclic, formation from ~60050 Ma (Fig. 1a). Thepost-600 Ma deposits clearly equate to major orogenicevents, and are defined by goldfields distributed alongelongate fold belts normally along the margins ofPrecambrian cratons or older Phanerozoic fold belts (e.g.Goldfarb et al.3). The pre-1750 Ma deposits also definegoldfields along elongate belts, commonly greenstonebelts, but also fold belts of probable accretionary wedges.These belts are distributed within roughly equidimen-sional cratons, which contrast in location with theyounger elongate orogenic belts, which are commonlydeveloped along the craton margins. Importantly, thepeaks in Precambrian orogenic gold-deposit formationcorrespond to two major periods (super-events) ofgrowth of continental crust (Fig. 1b). These periods arecommonly interpreted to be due to overturn of a layeredmantle followed by transient whole-mantle convection,with resulting mantle plumes generating huge volumes ofnew crust by decompression melting of the lithosphere.These super-events led to the generation of distinctivebuoyant Archaean sub-continental mantle lithosphere(hereafter termed lithosphere) and buoyant to neutralPalaeoproterozoic lithosphere, which contrast with thenegatively buoyant Phanerozoic lithosphere (e.g. Griffinet al.4).

Archaean and Palaeoproterozoic orogenic golddeposits were generated in settings similar to those ofmodern orogens, but were incorporated into distinctive,relatively equidimensional, buoyant continental cratonsdue to penecontemporaneous plume activity. Thiseffectively preserved them from subsequent orogenesisand erosion. Thus, despite their antiquity, suchArchaean and Proterozoic gold provinces are still

equivalent in size to much younger gold provinces. Incontrast to the relatively equidimensional Archaean andPalaeoproterozoic cratons, juvenile crust created fromabout 1000 Ma to recent is preserved in microcontinentsor in accretionary collages. These elongate orogenic beltsare interpreted to have evolved subsequent to a shiftfrom a strongly episodic, plume-influenced plate tectonicstyle to a style of less episodic plate-tectonics in a coolingearth. Orogenic gold deposits formed in juvenile crust insuch belts would have been susceptible to erosion duringuplift on the reworked margins of the cratons. The lackof orogenic gold deposits between ~1750 Ma and ~600Ma could then be explained by progressive erosion downto the gold-poor high metamorphic-grade roots of thesethin orogens. Post-600 Ma, segments of the belts havepresumably not yet been so deeply eroded with,therefore, an abundance of large orogenic gold provinces,particularly post-~450 Ma.

Age of giant palaeoplacer gold deposits confirmatory evidenceMost modern placer gold deposits are related to theerosion of younger Phanerozoic orogenic gold concen-trations, mostly in MesozoicRecent convergent marginssurrounding the Pacific Rim (e.g. New Zealand, EasternRussia, California, Alaska), although some formed due todeep tropical weathering of older orogenic gold deposits(e.g. Ashanti Belt, Ghana; Tapajtrations, mostly inMesozoicRecent convergent margins surrounding thePacific Rim (e.g. New Zealand, Eastern Russia,California, Alaska), although some formed due to deeptropical weathering of older orogenic gold deposits (e.g.Ashanti Belt, Ghana; Tapajs, Amazon). Palaeoplacersderived from older Palaeozoic orogenic gold deposits (e.g.Ballarat, Victoria) are commonly preserved by overlyingvolcanic flows. The most remarkable palaeoplacerdeposits are the giant Witwatersrand deposits, which weredeposited in a retro-arc foreland basin in essentially thesame time period as the giant Late-Archaean orogenicgold provinces formed (Fig. 1a). One reason for theirremarkable preservation is the regional extension leadingto outpouring of Ventersdorp lavas that followed theirdeposition. The Witwatersrand Basin also lies above theoldest known Archaean lithosphere, on which there is themost complete Archaean to early Palaeoproterozoicsedimentary record on earth. Smaller, but significant, goldpalaeoplacers at Tarkwa, Ghana (Fig. 1a), in the giantAshanti gold province, were preserved in Palaeo-proterozoic lithosphere.

Giant iron-oxide CuAu deposits relationship toArchaean lithosphereFinally, the temporal distribution of giant iron-oxideCuAu deposits may also be controlled by the distri-bution of Archaean lithosphere. The earliest knowndeposits formed at ~2550 Ma in the Carajas Province,Brazil,5 whereas others are of Mesoproterozoic age (e.g.Olympic Dam). They formed close to the margins ofArchaean lithosphere, and are probably related toalkaline igneous activity derived from the margins of this



1 Timing of orogenic gold deposits versus periods ofcrustal growth. (a) Distribution of major orogenicgold provinces with time: from Goldfarb et al.3 (b)Temporal evolution of continental crustal growth:from Condie2

The Uralide orogen is one of the major metallogenicprovinces of the world containing world-class Cr,VMS CuZn, orogenic Au and Fe-oxide skarndeposits. The VMS deposits alone contain aconservative 55 Mt of contained metal and the Crreserves at Kempirsai exceed 4000 Mt. The Uralideorogen forms the western part of the Altaid collage, atthe boundary between the East European craton andVendian to Palaeozoic magmatic arcs exposed in theKazakh uplands and Tien Shan.5 Numerous studies of

Uralide tectonics and metallogeny identify major NSfaults including the Main Urals Fault (Fig. 1),proposing that the latter is the main suture separatingthe western and eastern slopes.1,4 The western slope isuniversally recognised as a deformed continentalpassive margin, whereas its eastern, oceanic, slope isa combination of collided and welded magmatic arcs,sutured oceanic basins and microcontinents. Mineraldeposits help better define the tectonic evolution ofthe orogen and point to better regional correlations ofmetallogenic terrains.

Broad domains of the Uralide orogen

Western oceanic complexesThe full section of the oceanic portion of the orogen isexposed only in the Southern Urals. The western-mostSakmara zone consists of Ordovician to Silurianophiolites and immature arc rocks thrust onto deformedpassive margin of the East European craton. These rockshost major Cr and medium-sized VMS deposits. TheMagnitogorsk zone, consisting of Devonian magmaticarc rocks that host major VMS deposits,2 is also thrustwestward along the Main Urals Fault, but in the east it istruncated by a west-dipping structure, which defines itslens-shaped outline in plan view. To the east of this faultis the NS-trending East Uralian linear megazone whichcontains fragments of Precambrian, such as theMugodzhar microcontinent, ophiolite sutures andmagmatic arc fragments.

Syn-collisional axial granitesThe orogen is stitched together along its central axis byMiddle to Late Palaeozoic granites. The pre-granitic

lithosphere, which were metasomatised during post-formational tectonic events. They almost certainly owetheir preservation to their siting in buoyant Archaeanlithosphere.

ConclusionsMineral deposits, because they reflect an anomalousconjunction of processes, are important indicators oftemporal changes in environments and tectonicprocesses. The distribution of orogenic, palaeoplacerand iron-oxide CuAu deposits reflects both theirselective formation and preservation at specific timesin earth history. Of particular importance to theirpreservation is the evolution from equidimensionalcratons of relatively buoyant early Precambrian

lithosphere to elongate belts of negatively buoyantPhanerozoic lithosphere. This, in turn, appears toreflect evolution from episodic, strongly plume-influenced plate tectonics in the early Precambrian tomore continuous, modern-style plate-tectonics post-1750 Ma in response to a cooling earth.

References1. M. E. BARLEY and D. I. GROVES: Geology, 1992, 20,

291294.2. CONDIE: Tectonophysics, 2000, 322, 153162.3. R. J. GOLDFARB et al.: Ore Geol. Rev., 2001, 18, 175.4. W. L. GRIFFIN et al.: Proc. 7th Annu. V. M.

Goldschmidt Conf., 1997, 8283.5. F. H. B. TALLARICO et al.: Econ. Geol., 2003, In press.



Uralide orogenic evolution through the Palaeozoic and the link to metallogeny: anupdated model

R. Herrington1, A. Yakubchuk1 and V. Puchkov2

1CERCAMS Group, Dept of Mineralogy, The Natural History Museum, Cromwell Rd, London SW7 5BD, UK2Ufimian Science Centre, RAS Urals Branch, Ufa, Russia

1. Schematic map: structure and key deposit types,Urals

DOI 10.1179/037174503225001668

rocks in this zone form NE-trending rhombic-shapedstructures, truncated in the east by the west-dippingKartala fault. Magmatic arc rocks of the East Uralianmegazone can be tentatively correlated with similarrocks in the Magnitogorsk zone and possibly Tagilzone, which would imply a sinistral offset for up to 300km northward along the Serov-Mauk fault, whichwould, therefore, form a west-dipping strike-slip faultrather than a thrust fault as it is traditionallyinterpreted.

Eastern ocean-arc complexesThe eastern-most zone in the Southern Urals is the Trans-Uralian zone followed to the east by the Valerianov zone.The latter hosts currently unexploited porphyry depositsand world-class Fe-oxide Kiruna-type deposits. Alongstrike, the former consists mostly of Ordovician toDevonian accretionary complexes and some magmaticarc rocks, also sinistrally offset along the Kartala faultwith respect to the East Uralian megazone.

Major structuresThe major faults, such as the Kartala and sub-parallelnetworks, control the distribution of both granitoidsand orogenic Au deposit clusters. Recognition of thesestrike-slip structures in the Urals suggests that pre-strike slip positioning of the Tagil zone in the northernUrals would reconstruct it as an isolated structureframed by Palaeozoic ophiolites and Precambriancrustal fragments at the eastern and western flanks,e.g. it is possible that like the Magnitogorsk zone, theTagil zone may be completely allochthonous, whichmay be supported by recent seismic data. It follows,therefore, that only the western-most zones of theoceanic Urals preserve definite collisional thruststructures, such features controlling distribution of theallocthonous Sakmara and Tagil zones. Some of theeastern zones are bound by inclined sinistral strike-slip faults which may be late-, even post-collisional.

Broader correlation of the Urals with the AltaidsThe relationship of the Uralides to the tectonicframework of the Altaid orogenic collage continues to bethe subject of much debate, mostly due to naturalisolation of this orogen by MesozoicCenozoicsedimentary basins and the lack of competent com-parative studies. Airborne magnetic data help toelucidate the likely continuity of its magmatic arcs andaccretionary complexes under these basins. Based onsuch data, the magmatic arcs of the Urals start underMesozoicCenozoic sediments of the UstYurt plateauto the south of the exposed Urals, continue northwardinto exposed parts of the orogen, towards the PolarUrals, and then must be traced south-eastward underMesozoicCenozoic sediments of the West Siberianbasin towards Rudny Altai. There are problems with thissince there are no analogues of the Tagil zone VMS beltin the Altai-Sayan except for some Cambrian VMSdeposits far to the east in the Tuva republic. The RudnyAltai hosts numerous Devonian VMS deposits; but,since subduction polarity in the Rudny Altai and the

comparative Magnitogorsk zone of the Urals is identical,there are some geometrical problems. There are VMSdeposits in Kazakhstan to the south of the Rudny Altaiwhich may be the time equivalents to the Magnitogorskzone VMS deposits and the Rudny Altai may correlatebetter with deposits in the Sakmara zone. LatePalaeozoic oroclinal deformation events are common tothe Uralides and Altaides, leading to the close structuralconnection and correlation around the arc of theoroclinal closure apexing in the north Urals. Theaccretionary complexes of the Trans-Uralian zone can beconfidently traced into the IrtyshZaissan zone in Altai,defining an extensive MugodzharRudny Altai arcdraped around an internal magmatic arc with arcs of theKazakh uplands and the Tien Shan. Such a suggestiondefines two arc systems in the western part of the Altaids,accreted to the adjacent East European and Siberiancratons which, in turn, assembled with the two distinctLate Proterozoic orogens of the Pre-Uralides (Timanides)and Baikalides.

Implications of the new interpretationsIt follows that the proposed strike-slip faults of theSouth Urals may also represent relatively shortfragments of trans-regional fault systems that mayextend from the Tien Shan to the Polar Urals and Arcticoffshore. Such faults formed during the collision of theUrals and Kazakh coupled with oroclinal bending of



2. Evolution of South Urals during the Palaeozoic links to major metallogenic events. (A) Primitive arc-related mineralisation preserved in allochthons ofOrdo-Silurian oceanic complexes (CuZn VMSdeposits). (B) Devonian oceanic arc complexes(CuZn and polymetallic VMS deposits). (C) LateDevonian continental margin magmatism (porphyryCu). (D) Early-Mid Carboniferous volcanism atrifted continental margin (Fe-oxide, porphyry Cudeposits). (E) Mid Carboniferous collision andgranitoid intrusion (rare metal, orogenic Au deposits)

Phanerozoic orogenic gold deposits are invariablyassociated with translithospheric compressional totranspressional-transtensional shear zones. The depositsare primarily hosted by marine sedimentary rocksaccumulated on continental margins or in arc-trenchsettings. Mineralisation occurred post peak meta-morphic and is associated with the exhumation of theterrain and generally predates the emplacement ofgranitoids.2 The geodynamic settings of these golddeposits corresponds to major accretionary processes,which are typified by transpressive accretion of alloch-thonous terrains to one another or a continentalmargin identifying Cordilleran style tectonics.6

The large (> 300 t Au) orogenic Kochkar golddeposit is hosted by a granite-gneiss complex, that isintrusive into supracrustal rocks of the East UralianZone (EUZ). Although the deposit shares manysimilarities with other orogenic gold deposits world-wide, it is not associated with a major shear zone.7

In this contribution, we use geochemical, petrologicaland isotope data to describe the magmatic andtectonometamorphic evolution of the EUZ. The mainaim is to position gold mineralisation in Kochkar into amodel for the orogenic history of the Urals.

Regional geologyThe Urals are a bivergent, linear, NS trending orogenformed during convergence and final EW collisionduring the late-Palaeozoic.3,8,10,11

Geology of the East Uralian Zone (EUZ)The EUZ comprises supracrustal rocks and intrusivegranitegneiss complexes. The granitegneiss complexesare attributed to a long-term orogenic magmatism,4 whichbegan pre-Middle Devonian by the intrusion of TTGseries, coeval with Late Palaeozoic regional meta-morphism, and lasted until the Middle Carboniferous.Younger calc-alkaline and biotite granites intrudedbetween 290250 Ma into shallow crustal levels post-dating regional metamorphism. This suite is considered tobe the product of a large scale melting event in theEUZ.1,4,5

Geology of the Kochkar districtA number of deposits, that are collectively referred to asthe Kochkar district, are hosted by the Plastgranitegneiss massif, which is located to the south ofChelyabinsk. The Plast lithologies are intrusive intoupper-greenschist to mid-amphibolite facies supracrustalrocks. The Borisov massif is formed by granitoid rocks tothe west of the Plast massif. Numerous, steeply dippingmafic dykes describe an overall radial pattern in the Plastmassif. The width of dykes ranges from a few centimetresto > 20 m and strike lengths vary from several tens ofmetres to > 15 km.7

Economic-grade gold mineralisation is hosted bysteeply-inclined gold-quartz veins developed along aNS trending, 15 km long and 5 km wide corridor. Theyare spatially associated with the mafic dykes and occurmostly parallel and directly adjacent to these dykes. The

the arcs during clockwise rotation of the adjacentcratons. Such an architecture was likely to havedeveloped during Late Palaeozoic times when theAltaid/Uralide collage finally assembled and when theexternal arcs of the Kazakh uplands and Tien Shanbecame rotated and pushed against the East Europeancraton.

Tectonic evolution and metallogenyLinkage of metallogeny to tectonics is best explainedin the South Urals where the most complete exposurefrom the western oceanic complexes, oceanic andcontinental arcs and complex collision collage isassembled. The simplified evolution after removingperceived strike-slip complications resulting from theoroclinal bending in the Late Palaeozoic can besummarised as in Fig. 2:

AcknowledgementsPart of this work has been funded by the EuropeanCommunity, Cordis-RTD projects, 5th FrameworkINCO-2, project number ICA2-2000-10011. This is aGEODE publication.

References1. G. S. GUSEV, A. V. GUSCHIN, V. V. ZAYKOV et al.:

213295; 2000, Geodynamics and metallogeny: theoryand implications for applied geology, Moscow, Ministryof Natural Resources of the RF and GEOKART Ltd.

2. R. J. HERRINGTON, R. N. ARMSTRONG, V .V. ZAYKOV etal.: AGU Monograph, 2002, 132, 155182.

3. V. A. KOROTEEV, H. DE BOORDER, V. M. NETCHEUKHINand V. N. SAZONOV: Tectonophysics, 1997, 276, 391300.

4. V. N. PUCHKOV: Spec. Publ. Geol. Soc. Lond., 1997, 121,201236.

5. A. YAKUBCHUK, R. SELTMANN, V. SHATOV and A.COLE: SEG Newslett., 2001, 46, 114.



Orogenic evolution of the East Uralian granitegneiss terrain and timing of goldmineralisation at Kochkar, Russia

J. Kolb, S. Sindern and F. M. Meyer

Institute of Mineralogy and Economic Geology, Aachen University, Wuellnerstrasse 2, D-52056 Aachen, Germany([email protected])

DOI 10.1179/037174503225001677

massive and/or laminated, greyish to milky quartz veinsare, on average, 021 m wide and show distinctlytabular geometries. They contain, on average, 46 g/tAu. In contrast, associated alteration zones show onlysubeconomic gold grades of < 1 g/t Au. Prominent gold-quartz veins can be traced along strike for over 500 mwith similar subvertical down-dip extents. Fluidinfiltration during hydrothermal gold mineralisationwas promoted particularly along the interface betweenmafic dykes and granitoids. Repeated dilation normal todyke walls readily explains the dyke-parallel laminatedor ribbon textures of gold-quartz veins. Mafic dykesacted as weak layers during regional-scale EW directedhorizontal shortening.7

Wall-rock and alteration petrologySchollen- and raft migmatites together with undeformedgranitoids of the Plast massif comprise perthitic feldspar,plagioclase, quartz, microcline, biotite, and muscovite.Accessories are titanite, apatite, zircon, rutile, monazite,and ilmenite. Lithologies of the Borisov massif show agranitic to gneissic texture and comprise microcline,plagioclase, quartz, biotite, and subordinate muscovite,apatite, and zircon. The gneissic fabric and recrystal-lisation textures of feldspars and quartz in both massifsindicate a metamorphic overprint. Feldspars are locallyaffected by saussuritisation and sericitisation, and maficminerals are locally replaced by chlorite and rutile.

Mafic dykes display highly variable textures rangingfrom fine-grained, massive, to porphyritic or stronglyschistose dykes. The peak metamorphic hornblende-plagioclase assemblage was used to calculate temper-atures to 635 40C. This metamorphic assemblage isalmost invariably replaced by alteration mineralscomprising biotite, actinolite, albite, K-feldspar, quartz,epidote, tourmaline, and sericite. Gold-quartz veinscomprise quartz with minor amounts of calcite, sericite,scheelite, biotite, tourmaline, actinolite as well as pyrite,arsenopyrite, subordinate chalcopyrite, sphalerite,fahlores, galena, and bismuthinite. Gold is fine grained(< 20 mm) and mainly occurs as specks of free gold inquartz.7 Biotite and tourmaline of the alterationparagenesis were used to calculate the temperature ofgold mineralisation to 500 20C. A second retrogradeoverprint is indicated by the replacement of formermineral assemblages by chlorite, green biotite, andsericite, which reveals lower-to-mid greenschist faciesconditions.

Wall-rock geochemistryThe modal variation of the Plast gneisses are alsoreflected by their major element composition. SiO2ranges between 6072 wt% and peralominosity between1114. Na2O/K2O ratios (wt%) are highly variablebetween 03 and 42. As a consequence, some samplesare trondhjemitic whereas the other Plast samples aregranitic to granodioritic.

The Plast gneisses have low HREE-contents (Yb03416 ppm), relatively low Y-concentrations (4719ppm) which, together with the relatively high Al2O3,

gives them an adakitic character. However, Sr-concentrations are variable (259508 ppm) and Sr/Y-ratios vary from 285 to 1081. This deviates fromadakitic signatures which might be due to hydrothermaleffects as well as the variation of Na2O/K2O ratios.

The mafic dykes are heterogeneous with respect totheir major element composition. Trace elementsignatures are most consistent with a subductionrelated origin.

GeochronologyIn order to obtain information on the timing ofmagmatism and the tectonometamorphic evolution,four samples were chosen for geochronology. Zircons ofa Borisov massif sample were dated at 362 24 Ma,which is interpreted as the intrusion age of the granitoids.RbSr analyses of a least altered and an altered Plastmassif sample indicate isotopic disequilibria for thevarious mineral separates. This can be interpreted byopen system behaviour during the retrograde meta-morphic evolution. RbSr analysis of separates from aquartzsericite veinlet, which formed during theretrograde greenschist facies event, yields an isochrondefining an age of 265 3 Ma.

Model for the orogenic evolutionDating of the Borisov (362 24 Ma) and Plast massifs(341 20 Ma)9 indicate that these arc-related granitoidsintruded the supracrustal rocks of the EUZ in the EarlyCarboniferous. Field observations, however, show thatthe intrusion of the Borisov massif caused radialfractures in the Plast massif, suggesting that intrusionof the Borisov postdated that of the Plast massif. Theradial fractures were intruded by mafic, arc-relatedmelts in the Carboniferous.7 Amphibolite faciesmineral parageneses and fabric development in thegranite-gneiss complexes as well as in the mafic dykesdeveloped during doming and sinistral shearing alongNS trending shear zones resulted from regional EWcompression. Continuous compression caused shearingand gold mineralisation along the mafic dykePlastgranitoid contact in the lower amphibolite facies. Thetectonometamorphic evolution is finalised by greenschistfacies veining at 265 3 Ma. Disequilibria in the RbSrisotopic signatures of altered Plast gneisses at Kochkarsuggest that the retrograde metamorphic evolutionoccurred between 320265 Ma.

ConclusionsThe EUZ represents a mid-crustal section of an islandarc and shows a complex orogenic evolution in a longlasting EW compressional environment. Peakmetamorphic doming of the granitegneiss complexeswas followed by a retrograde exhumation and goldmineralisation at Kochkar, which coincides with theintrusion of Permian granites. This and the fact thatthe hypozonal Kochkar deposit is not hosted by amajor shear zone contradicts a Cordilleran tectonicsetting. Instead, a post-collisional slab rollback or



The Caledonian hinterland of northern Britain com-prises mainly Late Proterozoic metasediments andmetavolcanics (locally termed Moinian and DalradianSupergroups), and Lower Palaeozoic (Cambrian toLower Devonian) sediments and volcanics which havebeen amalgamated during the Caledonian Orogeny. TheOrogeny resulted from the complex closure of theIapetus Ocean mostly over the period 470390 Ma, theclosure lineament, the Iapetus Suture Zone extending toNewfoundland, mainland Canada, and easternGreenland. On the basis mainly of key faunal,structural, radiometric and geophysical parametersthese Caledonian rocks have been divided up into anumber of discrete terrains. Our work relates to fourterrains in particular which are best known as (fromNS) the North Highland, Grampian, SouthernUpland an Lakesman terrains.

A wide range of sulphur isotope studies have beenpublished for the Palaeozoic and older terrains ofNorthern Britain over the last 20 years. A large datasetof d34S (in excess of 600 analyses) now exists, fromsulphide disseminations in Caledonian intrusions andtheir related mineralisations,3,9,12,13 base-metal vein,stratiform and stratabound deposits,1,11,14,16,18 and pyritein metasediments (e.g. Hall et al.8,9 and referencestherein). Our analysis of this dataset highlightsdistinctive isotopic variations which not only providefurther evidence for the delineation of the putativeterrain boundaries, but also hints at deep crustal andpossibly subcrustal variations across the region.

Here we discuss the data for four terrains from NSand assess the environment of formation of thesulphides and sulphates and the role of sedimentarysulphur in subsequent mineral deposit formation.

Northern Highland Terrain and ForelandThe foreland sequence consists of cratonic basementoverlain by unmetamorphosed Neoproterozoic andPalaeozoic sediments. Pyrite segregations in the LateArchaean Lewisian gneiss basement of the forelandhave values from 1 to +5. Lower Proterozoicmetasedimentary units of the Loch Maree Supergroupin the cratonic sequence of the foreland are host toboth disseminated and stratiform sulphides with d34Sbetween 3 and 1. A value of 3 has also beenrecorded for sulphide in the terrestrial Torridoniansediments overlying the cratonic sequence.

The Northern Highland cratonic margin terraincontains inliers of cratonic basement directly beneath theMoinian succession, and this shallow basement providesmuch of the sulphur seen within mineralisation of theterrain. The Moinian sequence of generally coastalsediments contains only minor primary sulphides withvalues around +3 to +5.10 The Hebridean forelandlithologies and their extension beneath the NorthernHighland Terrain are thus a source of sulphur in therange 3 to +5.

Grampian TerrainThe Dalradian Supergroup, with its wide range oflithologies representing changing environments ofdeposition has the widest range of sulphide sulphurd34S of these terrains ranging from 15 to +43.

The shallow shelf sequence of the lower AppinGroup is characterised by d34S values ranging from+12 to +16.4,10

The middle Dalradian Argyll Group is notable forthe onset of strong tectonic controls on sedimentation

delamination of the lithosphere possibly provides theheat source for the large scale Carboniferous toPermian melting event in the EUZ and the fluidplumbing system for gold mineralisation at Kochkar.

References1. F. BEA et al.: Tectonophysics, 1997, 276, 103117.2. F. P. BIERLEIN and D. E. CROWE: Rev. Econ. Geol., 2000, 13,

103139.3. H. P. ECHTLER et al.: Science, 1996, 274, 224226.

4. G. B. FERSHTATER et al.: Tectonophysics, 1997, 276,87103.

5. A. GERDES et al.: Int. J. Earth Sci., 2002, 91, 319.6. R. KERRICH et al.: Rev. Econ. Geol., 2000, 13, 501551.7. A. F. M. KISTERS et al.: Mineral. Deposita, 2000, 53,

157168.8. V. N. PUCHKOV: Geol. Soc. Lond. Spec. Publ., 1997, 121,

201237.9. V. N. SAZONOV et al.: Econ. Geol., 2001, 96, 685703.10. L. P. ZONENSHAIN et al.: Tectonophysics, 1984, 109,

95135.11. L. P. ZONENSHAIN et al.: Am. Geophys. Union Geodynamics

Ser., 1990, 21, 2754.



Sulphur isotope signatures of Neoproterozoic and Palaeozoic terrains of NorthernBritain environments of formation

D. Lowry1, A. J. Boyce2, A. E. Fallick2, W. E. Stephens3, A. J. Hall4 and N. V. Grassineau1

1Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK ([email protected])2Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK3School of Geography and Geosciences, St Andrews University, St Andrews, Fife KY16 9AL, UK4Archaeology Department, Glasgow University, Glasgow G12 8QQ, UK

DOI 10.1179/037174503225001686

and it shows the widest d34S variations and generallyrepresents the widening, deepening and eventuallyfailed rifting of the Dalradian marginal basin. Thebase of the sequence is marked by the Port Askaigglacial tillite. This was followed by a shift to a muchwarmer climate for the overlying Bonahaven dolomitewhich has exceptional pyrite values of up to +43.8

Globally, the seawater sulphate reached its highestd34S value during this Snowball Earth period ataround +35 and the associated fall in sea level mayhave marginalised the basins in this region cutting offthe recharge of seawater sulphate. The following onsetof rapid subsidence and localised basin developmentresulted in contemporaneous deep and shallow watersedimentation, such as the Easdale Slates of the westcoast5 and the Ben Eagach Schist which is at a similarstratigraphic level further east15,16 with d34S of +12 to+22. The d34S values of seawater sulphate remainedheavy as seen in the schist-hosted syn-sedimentaryAberfeldy barite and the smaller BaPbZn Loch Lyonhorizon with sulphide d34S values range from +16 to+26 and barite values from +27 to +40.16,18

After a period of basin filling, the major period ofbasin opening and failed rifting commenced with shelfmuds, now represented by the Ardrishaig phylliteswhich have d34S of 15 to 1.7,18 This is followed byan enrichment in 34S again, through the Ben LawersSchist (4 to +4),15 the Crinan Grit (+1 to+8)10,18 and Ben Challum Quartzite units (+8 to+15).15 The Southern Highland Group of theDalradian includes pillow lavas, limestones and gritswhich have d34S values between 2 and +4.18

Within the Dalradian sequence as a whole, only theArdrishaig/Craignish phyllite units have values typical ofopen system bacterial reduction (fractionation of 3550from seawater sulphate values) suggesting that there wasrecharge of seawater sulphate and possibly an opening toIapetus. The other units are dominated by fractionationsrelative to seawater of 1030, representing closed- orsemi-closed-system bacterial reduction.

Southern Upland TerrainThe main sulphur isotopic characteristic of this terrain isa depletion in 34S. Ordovician diagenetic pyrite in theMoffat Shale Unit has d34S of 17 to 0, representingopen system reduction of Ordovician seawater sulphate(+28)2 to H2S and then pyrite.

1

Lower Carboniferous PbZn-rich vein systems hostedby the Lower Palaeozoic sediments at Wanlockhead andLeadhills (and additionally the Salterstown mineralisationin NE Ireland) have d34S of 10 to 3.1,14 TheSouthern Uplands Terrain and its continuation intoLongford Down, Ireland, therefore represents a 34S-depleted reservoir.

Lakesman TerrainThe main sulphur isotopic characteristic of this terrain11

is an enrichment in 34S. The dominantly argillaceaousOrdovician Skiddaw Group formed in an arc basinwhich was closed to, or only periodically recharged with,

Ordovician seawater sulphate resulting in diageneticpyrite d34S values of +11 to +28 formed by closedsystem bacterial reduction of H2S.

Devonian and Carboniferous veins hosted by theOrdovician volcano-sedimentary sequence with d34S of+13 to +23 represent partially homogenised sulphurfrom a Skiddaw Group (or similar) source.

SummaryThe d34S data in these terrains highlight manyimportant features, but two are particularly relevantto this discussion. First is the distinct variation intenor across the Iapetus Suture Zone, in lithologies ofthe same age, implying a radical difference in thehistory of diagenetic sulphide on either side of thepalaeo-ocean. The lower Palaeozoic Lakesman andSouthern Uplands terrains represent sources of 34S-enriched and 34S-depleted sulphur, respectively, whichhave been related to different modes of bacterialseawater sulphur reduction either side of the IapetusSuture.11

Second is the large within-terrain variation across theDalradian Supergroup which reflects basin developmentand climate controls. Where estimates of seawatersulphate d34S are possible in the Argyll Group, they areconsistent with the global record for this period of theNeoproterozoic reaching all time highest values by thePrecambrian-Cambrian boundary.17 Proponents ofSnowball Earth hypotheses surmise that globalglaciation would cause massive fluctuations in seawaterd34S. This was followed by tectonically-controlled basins,rifting and basic magmatism with large fluctuations inwater depth and rates of sulphate recharge.

The features of the d34S data in these terrains can all berelated to the upper crustal sulphur, with the exception ofmineralisation hosted-by sub-crustally sourced intrusives.The disseminated diagenetic and metamorphic sulphidescan be related to their environment of formation,controlled by the d34S of seawater sulphate (possiblyinfluenced by global glaciation) and basin tectonicscontrolling the sedimentation depths and the continued orrestricted supply of sulphate. These changes over theperiod from 750450 Ma have given the terrains their ownpersonal d34S tenor, often traceable laterally for hundredsof kilometres. This tenor is the primary control on d34S ofsubsequent SilurianCarboniferous vein mineralisation.The dominant upper crustal units in each terrain providethe sulphur for local copper, lead and zinc veinmineralisation, being partly homogenised during theprocesses of scavenging and transport.

References1. I. K. ANDERSON et al.: J. Geol. Soc. Lond., 1989, 146,

715720.2. G. E. CLAYPOOL et al.: Chem. Geol., 1980, 28, 199260.3. T. A. FLETCHER et al.: J. Geol. Soc. Lond., 1989, 146,

675684.4. A. J. HALL et al.: Chem. Geol., 1987, 65, 305310.5. A. J. HALL et al.: Mineral. Mag., 1988, 52, 483490.6. A. J. HALL et al.: Chem. Geol., 1991, 87, 99114.7. A. J. HALL et al.: Scott. J. Geol., 1994, 30, 6371.



Recent advances in understanding the processes thatform Mississippi Valley-type (MVT) PbZn-(FBa)deposits clearly point to specific environments andperiods of mineralisation. These new advances, inconjunction with knowledge of the regional geology,may help to identify new areas for the exploration ofmineral resources. The primary objective of thisabstract is to present the explorationist with a shortsynopsis of the key factors necessary to form aMississippi Valley-type PbZn deposit/district.

Geochronology

Host rocksIt has long been recognised that MVT deposits occur inplatformal carbonate successions. While the age ofmineralisation is difficult to determine, the age of thehost rocks can be established with a fair degree ofcertainty. Fig. 1 illustrates that MVT district are hostedby carbonate rocks ranging in age from the Neoarchaeanto Tertiary. A histogram of host rock ages reveals that~60% of MVT deposits are hosted by Palaeozoic rocks,while 25% are hosted by Precambrian rocks, and 15% byMesozoic and Cenozoic rocks (Fig. 1). As illustrated inFig. 1, the Palaeozoic is marked by maxima in theCambro-Ordovician and Devono-Carboniferous, yetdeposits are less common in rocks of Silurian andPermian age.4 When compared with palaeogeographicreconstructions of the earth,7 it becomes clear that duringthe Cambro-Ordovician and Devono-Carboniferous thesupercontinent Laurentia was located in equatorialregions, thereby allowing for the formation of thickcarbonate sequences.

MineralisationRecent advances in age-dating techniques have had aprofound impact on our understanding of MVT deposits.However, only a short summary will be provided. For amore detailed analysis, the reader is referred to Leach et

al.5 MVT deposits world-wide appear to have formedduring four periods Palaeo- and Mesoproterozoic,Devonian to Permian, and Cretaceous-Tertiary (Fig. 2).The most important periods of MVT genesis appear to bein the Palaeozoic Era.5 They note that this is exemplifiedby more than 75% of the combined metal produced arefrom deposits that have dates that correspondtoDevonian through Permian time. Indeed, if the ageof mineralisation is compared with palaeogeographicreconstructions, it becomes quite clear that thesemineralising events closely correspond in time withconvergent orogenic events, thereby indicating that theformation MVTs is not simply a passive process, butrather a dynamic one.



New insights into the exploration for Mississippi Valley-type leadzinc deposits

C. R. McClung1, D. L. Leach2, J. Gutzmer1, D. Bradley3 and S. Gardoll4

1Paleoproterozoic Mineralization Research Group, Department of Geology, Rand Afrikaans University,

PO Box 524, Auckland Park 2006, South Africa ([email protected])2US Geological Survey, PO Box 25046, DFC MS 973, Denver, CO 80225, USA3US Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA4Centre for Global Mineralogy, School of Earth and Geographical Sciences, University of Western Australia,

35 Stirling Highway, Crawley, WA 6009, Australia

1 Age distribution of MVT host rocks. Scale at leftrefers to number of districts. Here we describe adistrict as a group of deposits that share a closegeographic association. Therefore a single deposit(e.g. Pering) is classified as a district, as well as agroup of geographically close deposits (e.g. SEMissouri). Modified from Sangster6

8. A. J. HALL et al.: Mineral. Mag., 1994, 58, 486490.9. R. LAOUAR et al.: Geol. J., 1990, 25, 359369.10. D. LOWRY: Unpublished PhD thesis, St Andrews

University, 1991, 1625.11. D. LOWRY et al.: J. Geol. Soc. Lond., 1991, 148, 9931004.12. D. LOWRY et al.: Trans. R. Soc. Edinb., 1995, 87, 221237.13. D. LOWRY et al.: Appl. Earth Sci. (Trans. Inst. Min. Metall.

B), 1997, 106, 157168.

14. R. A. D. PATTRICK and M. J. RUSSELL: Mineral. Deposita,1989, 24, 148153.

15. R. A. SCOTT et al.: BGS Stable Isotope Report, 1987, 130,140.

16. R. A. SCOTT et al.: Trans. R. Soc. Edinb., 1991, 82, 9198.17. G. A. SHIELDS: Geophys. Res. Abs., 2003, 5, 10859.18. R. C. R. WILLAN and M. L. COLEMAN: Econ. Geol., 1983,

78, 16191656.

DOI 10.1179/037174503225001695

Tectonic environmentAs recently noted by Bradley and Leach,1 it has onlybeen within the last 20 years that the connectionbetween MVT mineralisation and plate tectonics wasrealised. Leach and Rowan3 and Bradley and Leach1

illustrated that most, but not all, MVT depositsappear to form in foreland basins or in rocks thatformed in a foreland environment. Despite the factthat MVTs may form in foreland basins or in forelandbasin rocks, one thing is clear, locally within thedeposit or district, most MVTs are controlled by deep-seated structures. Bradley and Leach1 describe somecharacteristics of these structures. Regardless of howthese structures form, it is important to note that theydo localise mineralisation (i.e. Irish Midlands, USMid-continent, Metaline, Cracow-Silesian, UpperMississippi Valley district, and many others).

Fluid flowAlthough several fluid flow mechanisms have beenproposed, most of these can be grouped into either: (i)sediment diagenesis and compaction; or (ii) tectonicforce.4 Sediment diagenesis and compaction have beenproposed as a viable mechanism to generate the fluidflow required to move MVT ore fluids. However, (asargued in Leach and Sangster4) this method is unlikelyto provide adequate discharge rates and heat neededto form MVT mineralisation.2 In the alternativemodel, during plate convergence, uplift along theflank of a basin will cause groundwaters, recharged inthe highlands to migrate through the deep portions ofthe basin, collecting heat and metals, and discharge atthe opposite side of the basin.1,2,4

Contained metalsAs stated above, over half of all known MVT depositsoccur in the Palaeozoic with maxima during the early andlate Palaeozoic. Similar to Fig. 1, a plot of host-rock agesversus contained MVT Zn + Pb metal indicates maximain the Cambro-Ordovician (~35% of known MVT metal)and Devono-Carboniferous (~45% of known MVTmetal). In comparison, the Precambrian contains ~5% ofknown MVT metal, while the Mesozoic contains ~15% of

known MVT metal (Fig. 3). All contained metal valueswere calculated from published grade and tonnage values;however, not all known deposits/districts have publishedgrade and tonnage values. Therefore, not all MVTdeposits/districts are included here.

Implications for explorationIt has become clear that MVT formation is mainlyrestricted to two windows of geological time Devono-Permian and Cretaceous-Tertiary. In theirevaluation of MVT deposits world-wide, Leach et al.5

determined that these windows of time correlate withthe assimilation of Pangea (Devono-Permian) and theNorthern Hemisphere AlpineLaramide orogenies ofthe Cretaceous-Tertiary. Based on this and other linesof evidence, Leach et al.5 and Bradley and Leach1

concluded that MVT mineralisation is directly relatedto major fluid flow events associated with global-scalecollisional orogenies.

As illustrated in Figs. 1 and 3, over half of all knowMVT occurrences and ~80% of the combined metal occursin Palaeozoic carbonate host rocks. Therefore, whenexploring for major MVT deposits or districts one shouldconcentrate on Palaeozoic platformal carbonate succes-sions that have been later affected by a collisional orogeny(preferably Pangean or Cretaceous-Tertiary in age).

References1. D. C. BRADLEY and D. L. LEACH: Mineral. Deposita, 2003,

DOI 10.1007/s00126-003-0355-2.2. S. GE and G. GARVEN: J. Geophys. Res., 1992, 97,

91199114.3. Leach, D. L. and Rowan, E. L: Geology, 1986, 14, 932935.4. D. L. LEACH and D. L. SANGSTER: Geol Soc Can. Spec.

Paper 1993, 40, 289314.5. D. L. LEACH et al.: Mineral. Deposita, 2001, 36, 711740.6. D. L. SANGSTER: Appl. Earth Sci. (Trans. Inst. Min.

Metall. B), 1990, 2142.7. C. R. SCOTESE: Atlas of Earth history, vol. 1, Paleo-

geography, PALEOMAP Project, 2000, Arlington, Texas.



2 Age distribution of MVT mineralisation. Scale atleft refers to number of districts. Modified fromLeach et al.5

3 Age distribution of MVT host rocks versus theamount of contained Zn+Pb metal

The hydrothermal systems responsible for the generationof orogenic gold deposits5 are associated with sub-duction related accretion.7 Under these conditions,major thermal anomalies occur involving fluid liberationand widespread granitoid intrusions. These granitoidscan, therefore, be correlated with both the fluid liber-ation and subsequent gold mineralisation.

This paper describes the tectono-magmatic evolutionof the Hutti-Maski Greenstone Belt (HMGB), DharwarCraton, India. It was here that the key environment wasobtained for the development of Indias largest goldmine (Hutti Gold Mine). New UPb zircon SHRIMPages of the syn-tectonic Kavital granitoid in addition tofelsic metavolcanic host rocks, add to the structuralinterpretation of the belt and, therefore, the timing of thegold mineralisation.

Geological settingThe Dharwar Craton can be subdivided into easternand western blocks, due to lithological variations,differences in volcano-sedimentary environments,magmatism and the grade of metamorphism. Thecontact is formed by an elongate body of granitetrending NS to NWSE, the Closepet granite.9

The basement of the western block is characterisedby 2933 Ga10 tonalitetrondhjemitegranodioriteintrusives referred to as the Peninsular Gneiss.13

Volcanic and sedimentary schistose units in thePeninsular Gneiss, the Sargur Group,13 have beendated between 29633 Ga.11 Younger, volcanic andsedimentary rocks collectively termed the DharwarSupergroup13 were deposited between 2926 Ga.13

In c

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Earth History and Mineralisation