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ORE(;E()LOC;Y REMFAVS

ELSEVIER Ore Geology Reviews 11 (1996) 175-196

lines ilwill

sub-

Geology and structural evolution of the Muruntau gold deposit, Kyzylkum desert, Uzbekistan

Lawrence J. Drew Byron R. Berger Namik K. Kurbanov " • vs. Geological Survey, MS920, Rennn. VA 22092, USA " U.S. Geological Survey, MS973, Deiwer. CO 80225. USA ' TsNIGRI, 129B. Vamhuuskoyt sli.. 113545 Mosi-nw, Russia

Received 3 April 1995; accepted 10 October 1995

>-sen tech-

lable >asis. ralia, Irica, sues sub-Tel.:

Abstract

The Munmtau gold deposit in the Kyzylkum desert of Uzbekistan is the largest single deposit ( » 1100 tonnes of gold) of the class of low-sulfide syndefonnation/synigenous gold deposits formed in the brittle/ductile transition zone of the cnist within transpressional shear zones. Hosted by the Cambrian to Ordovician Besopan Suite, the ores were deposited in pre-exi.sting ihrust-fault- and raecamorphism-related permeabilities and in synmineralizalion dilational zones created in a large faull-related fold. The Besopan Suite is a 3.000-m-thick sequence of turbiditic siltstones, shales and sandstones. The ore is primarily localized at the base of the Besopan-3 unit, which is a 2,000-m-thick series of carbonaceous shales, siltstones, sandstones and cheits. Initial goid deposition took place within the Sangmntau-Tamdytau shear zone, which was developed along the stratigrs{)hic contact between the Besapan-3 and Besopan-4 units. During the mineralization process, folding of the Besopan Suite and a left-step adjustment in the Sangruntau-Tamdytau shear zone were caused by two concurrent events: (1) the activation of the left-lateral Muruntau-Daugyztau shear zone that developed at° nearly a 90° angle to the preceding shear zone and (2) the intrusion of granitoid plutons. These stmctural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability.

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1. Introduction

SjTKlcformation/synigneous intrusion gold de­posits (cf. low-sulfide; Berg«, 1986) are important sources o f lode and placer gold throughout the world. Well-studied examples include those in the Al­legheny district, California (BChlke. 1989). the Juneau district, Alaska (Goldfarb et al., 1993), the Superior Province, Canada (Kerrich and Feng, 1992). and the Yilgarn block (Groves et al., 1989) and Lachlan fold belt (Sandiford and Keays. 1986). Aus-

' Corresponding author.

tralia. The largest single deposit (» 1100 tonnes of gold) of this lype yet discovered is located at Mumn­tau (Fig. I) on the southeastem flank of the Tamdy Mountains (Tamdytau), Kyzylkum desert, Uzbek­istan. As a mineralization style, the syndeforma­

tion/synigneous intrusion gold deposits are formed in the depth-dependent brittle/ductile transition zone within transpressional shear zones of regional dimen-.sion (Sibson, 1977, 1983; Ho and Groves, 1987). The depo.<iiLS often occur in dilation zones along satellite shears genetically and temporally related to these regional shear zones. Deformation along these

Of frocn

0I69-1368/96/S15.00 Copyright O 1996 Hievier Science B.V. All rights reserved. SSDI 0169-1368(93)0OO33-X

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176 U. Drew el al./Ort Geology Reviews Jl (1996) 175-196

secondary shear zones may be quite brittle, which causes elements within them to be mapped as dis­tinct strike-slip faults. These secondary zones or faults a l so can be regional in scale. The transition from ductile to brittle shear is widely recognized as a depth-dependent condnuum.

The generation of quartz-vein systems, some of which host gold deposits, in collision-related shear zones and strike-slip faults (i.e., compressional oro-gens) is discussed by many authors including Ker­rich and Feng (1992) ; Cox e l al. (1991); Robert (1990) ; H o and Groves (1987) . These vein systems share a conwrion evolution diat includes: (1) forma­

tion during deformation and concurrent magmatism with the consequent f low of large vo lumes of fluids through lithospheric rocks into active shear zones; (2) a late-kinematic timing o f formation in and above the transition zone from ductile to britde shear; (3 ) syndepositional uplift; and (4 ) mineral precipitation predominantly in second- and third-order structures.

Although appreciable volumes o f carbonate alter­ation may not be apparent, this type of alteration, usually dolomitic/ankeritic, is associated with die genesis of these deposits (Colvine et al., 1988). Fluid inclusions are typically rich in C O j - Although the chemistry of the ore-forming fluids is dominated by

EXPLANATION

Central Kizakhstan-North Tleri Shan

Continent (Cilsdonian)

Paltoaotc and younger Mdimemt

Middle CaibooKaroui volcBnic complex (c«le-alkalin«)

f . I Precambrian

t "HeieynJan" - - ^ (old and thrust

Suturr- faiferrad margin ol eontinenls after 'Hercynian" collliion

lOCICIlOMETERS

Fig. 1. Location of inajor plutons and the Hercynian fold and thnist belt of the Tien Shan tystem in centia] Aila (modified from Zonensliain et al.. 1990X

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LJ. Orvwet al/Orr. Cfotcigy Revims 1} (tm) 175-196 177

n Is

*; e 0 •n s. r-y, le d te

•y

lain

•metamorphic-like' fluids, penniss ive isotopic evi­dence indicates that some components may be of magmatic origin (Colvine et al., 1988). The same regional tectonic processes that generate the shear zones and strike-slip faults that host these gold de­posits, also generate the metamorphism and pluton­ism that provide the ore-depositing fluids (Murphy, 1989). Greenschist-grade metamorphism is encoun­tered most commonly in the wall rock o f these low-sulf ide vein deposits. Rocks at this metamorphic grade that arc under shear stress favor the develop­ment o f vein deposits, whereas at a higher tempera­ture and pressure (e.g., amphibolite grade), the for­mation of stockwork deposits is favored (Murphy, 1989).

Given that syndeformation/synigenous gold de­posits are formed in coUisional orogens widiin large fault systems, a wide variety of host rocks can be encountered that include crystalline basement, pas­s ive margin sedimentary, volcanic, obducted ophi­olit ic and syntectonic granitoid rocks. Small intrusive plugs o f granitoid rocks, as well as a variety o f d ikes , usually are associated temporally and spatially wi th the formation of low-sulHde quartz-gold de­posits . Interpretation of the alkaline magmas associ­ated with the gold deposits in the Kirkland Lake area. Canada, indicates that they were emplaced in dilational j o g s along major inter- and intra-subpro-v incc faults (Kerrich and Feng, 1992).

T h e general physical and chemical characteristics o f syndeformation/synigneous intrusion gold de-posite are found at Muruntau. Our purpose in this paper is to present, in light o f these characteristics, a n i n t e r p r e t a t i o n o f t h e i n t e r p l a y o f syndcformation/synintrusion fluids and the struc­tural geologic evolution o f the Muruntau gold de­p o s i t In addition to some data collected by us through original mapping and from analyses o f rock samples, the results and interpretive model presented here include data from published field nuips and pub­lished former Soviet Union literature.

2. Regional geology of Muruntau

2.1. Structural setting

The Muruntau gold deposit occurs at the westem end o f die Tian Shan nnountain system, which ex­

tends from the Gobi desert in Mongol ia westerly dirough China into the Kyzylkum desert of north-central Uzbekistan (Sinitsyn and Sinitsyn, 1958). The fold and fault belt of the Tian Shan system (Fig. I) is extremely complex with various components diat represent different orogenic events that span much o f the Paleozoic and were later affected by the Alpine orogenic event (cf. Zoncn.shain et al., 1990).

In the central to wes tem Tian Shan, a 5- to 6-km-wide stmctural zone strikes over 1,0(X) km north-westerly from Uic Fergana Val ley in eastern­most Uzbekistan, along the northem flank of the Nurata Mountains in the eastem Kyzy lkum desert and thence northeast of the Tamdytau into the east-era Bukantau (Akhber and Mushkin, 1976). This is the suture zone that juxtaposes two cont inenul masses, Karakum and central Kazakhstan-North Ticii Shan (Zonenshain et al„ 1990). A characteristic of the suture zone is the wcst-northwest-striking shear zones that splay to the west off o f the main zone that subdivides the Kyzylkum desert region into a se­quence o f tectonic blocks. The main suture and the splays are offset by transverse northeast-striking shear zones (Mushkin et al., 1975).

In the vicinity of Mumntau, two regional shear zones , the northwesteriy striking Sangruntau-Tamdytau and the transverse MumnUtu-Daugyztau. developed durmg the 'Hercynian* (late Carbonifer­ous to eariy Permian) at the d m e of the continent-to-contincnt collision of the Karakum plate and the central Kazakhstan-North Tien Shan continent (Zonenshain et al., 1990). Evidence for this conti­nent-to-continent collision in Uzbekistan includes the intense calc-alkaline volcanism that forms the Vale-rianovsky volcanic belt on the western and soulhcm margins of the central Kazakhstan-North Tien Shan continent (Fig. 1; Zonenshain et al., 1990) and nappes and fragments of oceanic crast (Sabdyushev, 1971). The Valerianovsky volcanic belt was formed during the middle to late Carboniferous as oceanic c m s t was rapidly consumed between the Karakum plate and the central Kazakhstan-North Tien Shan continent.

The beginning o f continental col l is ion is reflected in the succes-stve deposition of carbonate sequences on the passive margin along with turbidi i ic /o l i s -tostix)mic formations o f the middle and late Car­boniferous. The latest Carboniferous and Eariy Per­mian arc characterized by continental col l is ion with

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178 U. Drfivelai/OreafohsyReiiixn II (im) 175-196

the development of major nappes and the obduction of an ophiolite complex onto the Karakum, Tadjik and Tarim plates, now recognizable as massifs (Hig. 1; Zonenshain et al., 1990). The Hercynian compression led to the formation

of north-dipping nappes. Two of these nappes Iran-sccl Ihe Tamdytau - a regional-scale syncline in the

central part of the range and a regional-scale anti­cline in the south (Fig. 2). A refraction seismic survey (Ivanov and Sabdyushev, 1974) and two U.S.S.R. Dcep-Geodynamic Drilling Program holes located near the center of the Tamdytau confirmed ihe synclinal structure of the central and northem parts of the range (Sabdyushev and Voronov, 1990).

EXPLANATION

Devonian and Caibentferous caibonaie*

Silurian elastics and volcanoclastic MdimenIS

m Ordovician slates and

sandstones

Ptecambriiin basemeni ——Suture

Late Paleoioic (Hercynian) inferred conlineni lo contineni colisior. l o n c

Sansnnlau-TaRuMau CKJI antAaaraone

Nurata Mountalna

Fig. 2. Axes of regional folds (Hercynian), mountain ranges, plutons (cropping out and inrcnred) and location of Heicyniun suture zone in IWielcistan. Central Asia, (modiried from Kotov and Poficskaya, 1992; Tananaeva and Genonlov. 1993).

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U. Drew et al./Ore Geology Review:, I I (1996) 17.1-196 179

lie anti-seismic nd two Tl holes nfirmed lorthem , 1990).

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nncnU

nel

Subsequent Hercynian transpression caused move­ment along the wcst-northwest-striking, lefl-lateral Sangruntau-Tamdytau shear rone and subsequently the southwest-striking, left-lateral Muruntau-Daugyztau shear zone (Fig. 3). The interplay of movement along these shear zones changed the strike of the eastern nose of the antiformal nappe, which resulted in a Z-like-shaped fold near the southeastern tip of the Tamdytau that is best observed in the Besopan-2 unit ( b S 4 ; Fig. 4). The core of this Z-fold is transected by brittle faults of the Muruntau-Daugyztau shear zone and it is within this area that the current Muruntau open pit mine is being devel­oped. Carboniferous-Permian granitoid intrusions were

emplaced into the nappes, regional .shear zones and

transecting the infcmcd sunire zone (Fig. 2). Citing the work of Porshnyakov (1983), Kotov and Porit­skaya (1992) stated that the intrusion of the grani­toids .shown in Fig. 2 was controlled by deep-seated, basement-penetrating faults. The numerous intru­sions shown directly to the west of the Tamdytau indicate that a zone of dilatancy must have existed in the regional synclinorium, presumably created by the same tectonic forces that formed the Z-shaped struc­ture. The Munmtau-Daugyztau shear zone, along

which there has been ductile and brittle deformation, is in the southeastern portion of the Tamdytau (Fig. 2). 'fiiis fault strikes northeast-southwest and has been mapped over a length of 75 km and a width of about 5 km. Sympathedc faults are mapped to the

lent

t l o «i tone

e xone in

EXPLANATION

Tnr foull Thnaliauk

CooacI—Ouhcd wherv mlnraJ

Shear «m(

Highly litcilied a m (Pennian)

Axil ol regional aniidlne

Axil oi regional lyncBnc

Pig. 3. Locaiiun of shear zones, highly silicified txxlies and the Mumntiui mine in southern Tamdytaa. Three ttrandx of lefl-lateral Sangruntau-Tamdytau shear 7*ne ctos. the central and southem parts of the mapped area from nothwest to southeaiU. The left-lateral Muruntau-Daugyztau shear zone is promineatly displayed in the vicinity of the MumnUu mine as a 5-km-wide band of brittle faults that wend* in a nonheau to souUiwest direction (modified from Drew. 1993).

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180 IJ. lirew Kl ol./Ore CenUiay Rtvlmn I I (1996) 175-196

northwest of the main fault zone in the same nappe and further to the northwest in the overlying synfor­mal thrust package of Devonian and Carboniferous carbonate rocks (Fig. 3) . The movement on the Muruntau-Daugyaau faults is left lateral, as may be seen from die movement of the faull-boundcd slices o f Devonian carbonate mcks located to the northeast o f the Muruntau open pit (Fig. 4). The evolution in tectonic stress from the thrusting to the strike-slip regimes is demonstrated by the imposition of steeply dipping left-lateral faulting on the previously devel­oped nappes.

Fig. 4 shows the complex Muruntau-Daugyztau shear-zone system and areas of inlen.se hydrothermal alteration. The orientation of the swarm of fault segments of this shear-zone system in the Muruntau area i s parallel to the axis of the nappe south o f the

Tamdytau (Fig. 2 ) , which suggests that they have been developed along axial fractures and (or) thrust faults formed during the 'Hercynian' continent-to-continent collision.

Besides the large nappe structures noted above , Proterozoic and Paleozoic formations in the Tamdy­tau show several generations o f defonnation that record a history of transition b o m ductile to brittle deformation styles. Alekseyev ( 1 9 7 9 ) identified sev­eral disdnct deformation events in rocks in the Amantaitau region southwest of Muruntau (Fig. 2 ) , the oldest of which consists of small- and large-am­plitude isoclinal folds overturned to the east and north that fold the original bedding. Facing determi­nations indicate that .some of the folds, wh ich are about 2 km wide, were transformed subsequendy into a series of tectonic sl ices (A lekseyev , 1979).

fXPLAtMTION

D , OcoanlM l», B««paniinB I

bi] B<io|i*nunll3

b j ] ^ Baopmiiiit3*Munnl«iLini'

bS] BocpanuMlZ * ^ Thiunhdl

Fig. 4. I^ocation of the 'Munmtau lens' and major straiigraj*ic units in the vicinity of the Muruntau mine. The MnninUu-Daugyztau shear zone spam (he area between the 'structural fault' on the northwest to the unnamed faults south of die Miutenbai deposit The 2^foM is shown by the folding of the bS^ unit (modified fiom Drew, 1993).

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U. Drew el al./Ore Geology Remews II11996) 175-196 181

y have ) thrust icnt-to-

above, famdy-)n that

brittle id scv-in the •ig. 2) . ge-am-st and itermi-ch arc lucntiy 1979).

This isoclinal folding was followed by metamorphic recrystalliyation and, in tum, by more-open folding without significant metamorphism. Several .subse­quent kink-fold events preceded the final shear-zone deformation described above. Because of the direc­tion of isoclinal folding and the transposition of schistosity fabric by the 'Hercynian' comprcs.sion, much o f this deformation is interpreted to be 'Cale­donian.' These deformation events, including the tectonic sl icing, are o f critical importance to inter­preting the sequencing and controls on veining and the stratat)ound nature of ore in the Mumntau and adjacent Miutenbai deposits described below.

2.2. Stratigraphy

Riphean to Vendian ( 1 , 6 5 0 - 5 9 0 Ma) marine sili­ciclastic, mafic volcanic, carbonaceous shale and dolomitic carbonate rocks arc the oldest exposed in the Tamdytau region and are known as the Taskaz­gan Suite (Table 1). The .suite is exposed in a dome along the antiformal axis o f the southem Tamdytau nappe west o f the Mumntau mine (Fig, 3 , R - V ) and appears to be isoclinally folded. Small gabbroic in­tmsions crosscut the sequence. Mirkamalov (1987) described die lower Taskazgan, which is southwest of the Tamdytau, as having been metamorphosed to an assemblage of chlorite-actinolite-epidote-albite schists. Potassium-argon dating o f the mafic volcanic rocks is inconclusive but indicates a middle to late Proterozoic age (Azhgirey, 1991). The likelihood o f an upper age o f early Paleozoic for the Taskazgan Suite, based on microfaunal evidence, is discussed by Abduasimova and Korsakov (1992) .

In the Tamdytau, dismembered ophiolite crops out at (he base of the uppermost thrust sheet between the Precambrian Riphean and Vendian and Cambrian rocks and the underlying Silurian rocks (Fig. 2 ; cf. Drew. 1993). Blocks of pelagic .sedimentary rocks, which commonly are radiolarian chert, varying from a few square meters up to several square kilometers in size witii similar-sized blocks of pyroxenite, dunite, plagiogranitc and serpentinite, crop out in the melange between these uppermost thmst sheets .

The ore host at Mumntau is die Cambrian to Ordovician Besopan Suite, which is about 5 km thick (Table 1. Fig. 4) . Most studies have divided the Besopan into four units, b S , to bS4. The low est unit. b S | , consists of ferroginous sericite-chlorite mica schists derived from predominantly siltstone with some sandstone and clay. Small lenses of s i l iceous volcanic rocks occur near the base of the unit. Large-to small-scale folding is evident. In outcrop. bS , i s light brown to greenish, but in a deep drill hole (SG-10) southea.st of the Mumntau open pit. it i.> carbonaceous.

b S , is predominandy metasandstone with some grit and gravels and is distinguishable from the underlying unit by its darker color, which is due to the presence o f abundant biotite. Although not evi­dent in outcrop, w e observed that the unit was caibonaceous in the deep drill hole (SG-10) near Muruntau (cf. Marakushev and Khokhlov. 1992). The unit is referred to as the 'Gray Besopan' and is the only Besopan unit with n o volcanic or chcrty rocks (Azhgirey. 1991).

The main ore ho.st. b S , . is referred to as the 'Variegated Besopan' because o f its red and green

I shear rotil is

Table 1 Description of the major stratigraphic units in the southem Tamdytau Mountains in the vicinity of the Muruntau mine (see Figs. 3 . 4 and 9 for relations) Map symbol Tliicknest (m) Age Brief description

D-C 1400 - C - O 3900 Differentiated Besopan units * bS^ 1000 bS, 2000

bS, R-V

700 1200 2800

Devonian-Carboniferuiis. Cambrian-Ordovician

Cambrian-Odovician Cambrian-Ordovician Cambrian-Ordovician Carabrian-Ordovic ian Riphean-Vendion

limestones and dolomites undifferentiated Besopan Suite

chlorite xchist, noncaiboniiceous carbonaeeoui and pyritic setidteK:hloriie schist with cheit and tutt terictte-chloriu: schist, mncaitmuKeous sericite-chlorite schist, caibonaceous quartzite, dolomite, greenxtnne and chlorile-amphibole-albttc schist

* In some maps, undifferenUated Besopan units are shown collectively as E-0.

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182 LJ. Difxt Cl ul./Ore Cenhxy Rerieivs II (1996) 175-196

coloi^tton in weathered outcrop. Away from the hydrothermal alteration of the Muruntau deposit, il consists o f a mixture o f phyllitic to schistose car­bonaceous metasiltstone, mclasandslune, calcareous metasandstones and metatuff (cf. Maraku.shev and Khokhlov, 1992) . W c observed minor radiolarian chert and interbedded muscovite schist and chlorite schist in the tuffs.

bS4 is described by Marakushev and Khokhlov (1992) as consist ing predominanrty of quartz-clay sandstone, which is referred to as the 'Green Be­sopan' because o f the metamorphic alteration o f the clay lo chlorite and .sericite, with metasiltstone, argillite and lenses o f metagritstone.

2.3. Igneous petrology

In addition to the volcanic and related intrusive rocks discussed above, substantial volumes of post-suturing granitoid magmas were emplaced into die orogen (Fig. 2) . Approximately 7 km southeast of Muruntau is the Sardarin pluton (Fig. 2), which is a composite porphyritic granodiorite (Kotov and Porit­skaya, 1992) that is hypodiesized by Danskoi (1991) on the basis o f geophysics to extend northwest to­wards and beneath the Muruntau deposit. During seismic surveys in the region, zones of higher veloc­ity were detected in the upper crust that have been interpreted us being intermediate composition intru­sions by Kudrin e t a l . (1990) , In the Tamdytau, a composite granit ic /alaskit ic intrusion is exposed in the nordiwestem part o f the range about 25 km northwest o f MuruniauJChammbacv( l969) reported an average date cf^^0J9 ± 7 . 4 l M a ^ 3 - P,) for these intrusive rocks. In addition to these plutons, swarms o f felsic, syenitic (syenodiorite) and lampro-phyre dikes were emplaced in die regional shear zones, particularly in the southem segment of the Sangruntau-Tamdytau shear zone and in a northeast-em-southwestem-trending zone parallel to and north o f the Muruntau-Daugyztau shear zone (Fig. 3) . The bell o f dikes along the southern margin o f the Tamdytau may be traced for over 4 0 km. Kham­rabaev (1969) reported a lower Permian average date o f 261 .2 ± 5 Ma on a diorite p o r p h y r y ^ k e J r o m the Muruntau area and a biotite dale (^^jCfOS ± 7.4 Ma^ from contact metamorphic h o m f e l T l i d j a c q i r dike.

3. E c o n o m i c geology o f the M u r u n t a u depos i t

Muruntau is one o f die largest individual gold deposits in the worid. Il is local ized at the intersec­tion of the Sungrunlau-Tamdytau and Muruntau-Daugyztau shear zones (Fig. 2) . These regionally extensive shear zones contain many similar-style gold deposits that vary in size from very large ( » 1100 tonnes o f gold at Muruntau) to medium (several at 2 0 0 - 3 0 0 tonnes each at Amantaitau and Daugyztau: Fig. 2 ) to small (50 tonnes at Besopan; Fig. 4 ) and a large number of occurrences.

3.1. Structural control of the Muruntau ore district

3.1.1. General aspects The gold-ore deposits occur in and adjacent to

fractures within regional lateral shear zones wiUiin die Besopan Suite (cf. Drew, 1993) , although some shearing extends into the lower part of the overlying Carboniferous thrust sheet (Figs . 2 and 3). The pre­dominant controls on permeability were schistosity. cleavage planes related lo folding, fraciuring-related active shearing during mineralization, low-angle shear zones interpreted by Azhgirey (1991) as being Caledonian thrust faults and high-angle brittle frac­tures.

The Hercynian Sangruntau-Tamdytau fault sys ­tem is observable as a zone within which there is both brittlely fractured rock and linear zones of sheared, phyllitic to cataclasized rock. The zone , though roughly parallel to the strike o f the Besopan Suite bedding, crosscuts bedding. Alteration minerals also readily delimit die shear zone , the earliest as­semblage being "s i lky fi lms o f biotite, muscovite , chlorite and carbonaceous material. . ." (Marakushev and Khokhlov, 1992, p. 71 ) . This alteration and subsequent hydrothermal assemblages may tw traced in the two major strands of the Sangruntau-Tamdytau system (Fig. 3) along the entire southem flank o f the Tamdytay. The Sangruntau-Tamdytau system both crosscuts und encloses imbricate breccia zones re­lated to the older Caledonian thrusting.

In addition to the prc-Hercynian i.soclinal folds and the broad anticlinal nappe in the southern Tamdytau, the Muruntau deposit occurs within a Hercynian fault-propagated fold. There are wel l -de­veloped cleavage fractures associated with each o f

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U. Dreweiai/OrtCicotngyRtiiewx II (19%) 175-196 18.1

it

gold srsec-ntau->nally :gold 1100

rai at /ztau; and a

Ida

nt to vithin some •lying ; pre-osity, :lated angle being frac-

sys-!re is is of zone, iopan icrals st as-3vite, isiiev

and raced jytau >f the bodi s re­folds them lin a 1-dc-h of

these folding events. Within the open pit. Mukhin et al. (1988) mapped northeast-striking axial traces of i.soclinal folds. The.se folds arc particularly evident in the northem part of the pit. In the southeastern part of die pit. the axial traces of several isoclinal folds strike southeast

The youngest fracturing events are high-angle faults delimited by breccia and gouge that crosscut all oUier fracture types. In cross section, these faults ore interpreted by Azhgirey (1991) to form the vari­ous parts of upward branching flower stmctures.

.?. L2. Structural control of veins ITiere are myriad, complex vein shapes and rela­

tions at Mumntau. The coarsest level of classifica­tion is to divide the veins into duee groups: (1) layer-like zones of many millimeter- to several cen­timeter-wide veins, frequently boudinaged or folded; (2) micrometer- to centimeter-wide pervasive veins with no continuity on the meter scale in any direc­tion; and (3) centimeter- to meter-wide veins with meters nf strike length and continuity down dip. The vast majority of the Mumntau ore is made up of the first and second group of veins. They are also para­genetically earlier and lower grade, on average, than the larger veins of die third group.

The earliest group of ore-bearing veins occurs widiin the Sangmntau-Tamdytau shear zone in sub-parallel, imbricate, interformational thmst planes of Caledonian age (Fig. 5). These mineralized planes extend in a colinear manner from the Miutenbai deposit to the Besopan deposit (cf. Kurbanov et al., 1991). Importantiy, the layer-like zones occur over a broader widUi of the shear zone than those planes tiiat constitute ore. Quartz veins within die.se layer-like sequences are described by Kurbanov ct al. (1991) as being veins and 'stringers' of synkinematic quartz. Where we observed them, millimeter- to several centimeter-width veins are often folded or boudinaged, and nearly meter-thick flat veins occur along dilatant stmctures parallel to partings and arc commonly boudinaged. All of these veins display crack-seal growth textures in fliin .section as evi­denced by the growth habit of the quartz and trains of minor minerals and fiuid inclusions (cf. Cox and EUieridge, 1983).

The second group of veins is less ductility de­formed. These veins are micrometers to I to 2

centimeters thick and may refiect a structural evolu­tion of the permeabilities that controlled the first group of veins, and the two groups are intermixed within the Mumntau orebody. This second group occurs in several forms. Micrometer- to millimelcr-widc veins occur parallel lo the prc-Hercynian .schis­tosity and Hercynian-transposed cleavage and along post-cleavage fractures of various orientations. Tiny folds and crenulations are commonly observed, and veins typically occur in short, stepped, linear se­quences. The.se veins arc referred to as 'stockworks' by mine geologists. Millimeter- to centimeter-wide veins occur along closely spaced parallel fracmres that appear to be axial-plane cleavages and are re­ferred to as 'banded veins' by mine geologists. The reference to banding stems from the close-spaced parallel repetition of these quartz veins and not any 'ribbon'-like stmcture widiin die veins themselves (cf. Lindgren, 1933). The relation of these 'banded veins* to stmctural elements is best observed in oxidized rocks al the Besopan deposit. Crack-seal textures are commonly present in thin section in all veins of this group.

The third group is typical 'Mother Lode'-style, more-continuous veins found in most mining dis­tricts of this type deposit woridwide. These veins formed along high-angle faults (the flowcr-stmcturc faults) that crosscut all of die vein styles of the first two groups. These veins have classic 'me.sothermar (Lindgren, 1933) ribbon suiicture and are referred to by mine geologists as die 'Central Veins.'

The veins of groups 1 and 2 above make up complexly shaped orebodies. At a small scale, the zone of orebodies appears elongated northea.st to southwest (Fig. 4). Within die Muruntau open pit, the orebodies are sinuous and in the cenU I and northern parts of the pit are individually elongated northeast to southwest (Rg. 5). In die southem part of die pit the sinuous orebodies are elongated nortii-we.st to southeast.

3.2. Hydrothermal alteration and gold mineraliza­tion

The hydrothermal alteration elucidates die details of die stmctural evolution of the gold deposit (Table 2). A zone of spotted schist occurs widiin and sur­rounding the Mumntau open pit Inside of diis spot-

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B«id«»*rrtIfi«lor««*hloeel"ftoeliv«H<-

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Tattle 2 Sequence of alteration and veining associated witfi the Murunuu hydrothennal system

Sequence' Scale" I5e«cription

1 I. spotted schist; biotite-chlorite-plagioclase spots at shallow depths, with cordicrtie and sillinranite in spots at greater depths

2 R quartz-albite-biotite-chlorite-oligoclase alteration and subpnrallel zones of quartz veinit and veinlets as incipient 'Mother Txsde*-style veins within the northem strand of the SangranUu-Tamdytau shear zone

y R phlogopitc-pyritc-arscDopyrite veinlets with muscovite, Mg dilorite. quartz. K-spar and FcMnCOj selvages 4 L quartz-K-spar-FeMnCO]-sulfide& veinleu with apatite, monazite and brookite 5 L quartz veins with FbMnCO, and sulfides (Central Veins) with quart2-K-.spar-FeMnCO, alteration 6 R intrusion of silicic dikes 7 L quartz vcinleti with K-spar, FeMnCO), muscovite, tourmaline and pyrite 8 L calcite veinlets and pervasive alteration of rock matrix: some pyrite, brookite (?) and rare-eanh-eletnent minerals

* Sequence established by crosscuuing relations. *" R >- regional and local scale distribution of alteration and (or) veining.

ting is a bulbous-shaped zone of hydrothennal alter­ation surrounding the Muruntau ores (Fig. 4 ) and occurring more narrowly to the west along several strands o f Uie Sangruntau-Tamdytau shear zone (cf. Mukhin et al., 1988; Marakushev and Khokhlov, 1992; Drew, 1993) and also to the southeast along this zone dirough the Miutenbai depos i t The bulbous part of the alteration pattern occurs toward the soudi-wes t within and adjacent to the Muruntau open pit along the Muruntau-Daugyztau shear zone.

The spotted schist is considered to be due to contact metamorphism (Maraku.shev and Khokhlov, 1992). The spots , overprinted by the later shear-zone controlled mineralization, originally consisted pre­dominantly of biotite and chlorite widi smaller quan­tities o f oUter minerals. Accompanying plagioclase in the unaltered spots is distinctiy more calcic andesine to labradorite, than the metamorphosed or altered Besopan, biotile is more dtaniferous and sillimanite and cordierite are found at depth.

Hydrothennal alteration related to several stages o f the mineralizing process occurs extensively along t h e northern and southern strands o f die Stmgruntau-Tamdytau shear zone. In general the alteration consists o f the formadon o f shert silicates, quartz and feldspar in carbtmaceous, sheared and

brecciated rocks widiin the zone. The alteradon is distinguishable from regional metamorphism by the higher magnesium content of the biodte and chlorite, albite to albite-oligoclase rather dian more calcic plagioclase, higher whole-rock potassium contents and the proportions o f the individual minerals when compared to unmineralized, metamorphosed rocks (Marakushev and Khokhlov, 1992). Above back­ground concentrations of arsenic and gold also occur within these rocks (Fig. 6) .

The earliest hydrodiermal alteration assemblage occurs along the northern strand of the Sangruntau-Tamdytau shear zone continuously between the Miutenbai and Besopan deposits and is known lo ­cally as die 'Muruntau Lens ' (Mukhin et al., 1988) and (or) 'biotitc-two feldspar alteration' (cf. Kur­banov et al., 1991; Marakushev and Khokhlov, 1992). This alteration is overprinted on die spotted schist and consists o f quartz + albite -1- biotite + chlorite -t-oligoclase with linear, subparallel zones of quartz veins and veinlets. W e refer to the subparallel zones quartz veining as incipient 'Mother Lode' - type veins (Fig. 7) . Gold concentrations throughout the zone between Miutenbai and Besopan generally exceed crustal abundance.

Widiin the Muruntau open pit, several paragenetic

Rg. 5. Geology of the Munmtau mine area. (A) l^tcation of gold nuneralization, major faults and t>cdding in the Mnniutau mine area, Uzbekistaa (B) East-west cross section through the Murunuiu mine area. (C) Northwest-southeast cross section through the Muruntau mine area (modined fiom Kurbanov et al., 1991).

ANOMALOUS

GoldandAnerIc

^ » HtoMyslclflalarea ffrequtnlKi ore twuingl

' • ^ • 1 ^ - Shear lone

MuruniM . . . / .

Hg. 6. Ucation of gold. «cnic «.d 8old/«se„i« « « m a U « in relaii.M, «> toe .hear zone., in the .southern TanKlyiau Monnurins, Uzbelcistan (modified fwn, Khamnibaev. 1969).

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U. Drewel oL/Ort Genlngy Reriews 11 (1996) 175-196 187

sluges of hydrothermal alteration were recognized on the basis of cross cutting or replacement relations. The earliest alteration assemblage consists of quartz + albite + phlogopite ± oligoclase, which we con­sider to be equivalent to the earliest ore-stage alter­ation assemblage noted above in the Muruntau Lens of Mukhin et al. (1988). The oligoclase occurs as only remnant grains, having been replaced by albite und (or) later K-feldspar. Alteration commenced along planes of schistosity and cleavage related to prc-orc folding. Quartz and albite occur primarily within the rock matrix and are elongated in the plane of the schistosity. Phlogopite is fine-grained where it grew along grain boundaries and is coarser along the cleavage planes.

The Muruntau open pit ores within metasedimen­tary rock are confined wholly to bS,. A swarm of latc-stage intramineral felsic dikes strikes northwest in the northeastem pan of the open pit and adjacent gmund (Fig. 3). Similar dikes occur in the central part of the deposit and along and adjacent to north­east-striking faults that make up part of the Mumn-lau-Daugyztau shear zone in the southeastem part of the pit (Nekrasov, 1975; Mukhin et al.. 1988). Mukhin et al. (1988) depicted the dikes as dipping soudicast Soudicasl of the Soudi Fault (Fig. 4), dikes strike nonhwest along tlie Sangruntau-Tamdytau shear zone.

The Mumntau Lens-equivalent, albite-bcaring as-.scmblage was followed by the widespread formation

of phlogopite widi pyrite ± arsenopyrite in en eche­lon microveins with .selvages of muscovite, magne­sian chlorite, quartz, phlogopite. K-feldspar and mi­nor iron-magnesium carbonate. Crack-seal and step-fractured textures arc common. The veinlets crosscut the schistosity and axial plane cleavage, which indicates diat a new regime of permeability was developing. Some of die microveins arc filled widi pyrite-I-arsenopyrite, which appears to have filled portions of originally phlogopite microveins that were intermittently reopened. The phlogopite. however, forms selvages around the sulfide, which indicates that the sulfide deposition is not entirely a later alteration event Analytical data indicate that this assemblage may contain at least several hunilred parts per billion gold.

The third stage of veining consists predominantly of quartz, K-feldspar and muscovite with ankeritic cartwnate and sulfides. It crosscuts the phlogopite veins and locally contains abundant carbonate that also pervades and replaces the rock manix. This same alteration is associated widi die Central Veins (cf. Khamrabaev, 1969). These veins contain the highest grades of gold in the deposit widi average grades varying from 3.5 to 11 g/t Au (Kurbanov et al., 1991); locally, grades are much higher.

Silicic dikes intmde all of the above alteration stages after die formation of the Central Veins, either concurrendy or immediately before the next alter­ation stage. The fourth stage of veins and alteration

Fig. 7. Location of the Sangruntau-Tamdytau shear zone in relation to the axis of a regional anticlinal structure in the Tamdytau Mountains, U2bekisttn.

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188 U. Drew el al./Ore Geolngy Reviews ll (I'm) 175-196

consists o f K-feldspar -I- dolomitic carbonate + tourmaline ± pyrite. This assemblage crosscuts the si l icic dikes and appears to have been formed along brittle fractures of a similar orientation to those that control the emplacement of die Central Veins. The tourmaline in this assemblage is dravite and occurs as segregated clots within the quartz vein.s and as discrete microveins of tourmaline. In addition, tour­maline forms the matrix in breccias, which include coarse auriferous quartz-vein fragments. Kham­rabaev (1969) showed that the tourmaline veins oc­cur m eas t -wes t - to ea.st-northea.st-elongatcd ellipti­cal zones of en echelon fractures, with zones that occur intermittendy across Ihe breadth o f the open pit from north south.

Calcite veining with sparse pyrite, brookite(?), monazite and baslnacsite(?), with associated perva­sive calcite replacement of the rock matrix, is the last stage o f veining and it occurs diroughout the mine.

A later stage of alteration consists of quartz-sericite. Although w e did not observe this alteration, quite intense zones appear to occur along brittle faults that offset Central Veins and other types of ores ['sericitolites' in Kotov and Poritskaya (1992)].

3.3. Ore mineralogy

The principal ore nuneral is nadve gold, which occurs in the megascopic to microscopic quartz veins. The most abundant sulfide mineral is pyrite with significant arsenopyrite and some marcasite and pyrrhotite. Other minerals include scheelite, gold and bismuth tellurides and selenides, galena, sphalerite, chalcopyrite, molybdenite, wolframite, magnetite and i lmenite (Khamrabaev, 1969). W e observed small quantities o f galena, sphalerite and chalcopyrite in Central Vein material.

Khamrabaev (1969) reported a paragenetically late silver-enriched ore event with sulfosalLs and tel­lurides. Ore minerals from this event include miar­gyrite, tetrahedrite, tennantite, pyrargyrite, polybasite and native silver.

4. Disciusion

T w o questions for which w e sought answers dur­ing our field work at Muruntau deposit were "What

is die relation of the deposit to its lithotectonic environment?" and "What are the physical at­tributes of the depos i t?"

4.1. Regional tectonics

The geologic relations and physical attributes of the deposit cleariy imply that Muruntau was formed within die britt le/ducUle transition zone during the later stages o f processes related to the coll is ion of two tectonic plates. Thnist faulting of a col lage o f platform cartmnates, siliciclastic rocks, melange and ophiolite fragments over older metamorphosed, folded and thrust-faulted siliciclastic and volcanic rocks was fol lowed by transpressional ductile shear­ing. During shearing, many late orogenic (early Per­mian) plutons, stocks and dikes o f varying compos i ­tion intruded the previously developed Tamdytau dirust plates (Kotov and Poritskaya, 1990). Hy­drothermal activity occurred in association with in­trusive activity and was confined predominantiy to die ductile shear zones. Subsequently, a transition from ductile to brittie shear occurred, as evidenced by early-stage folded auriferous and (or) boudinaged veins and crack-.seal and ribbon vein strucmres, all followed by brittie fracture and associated veining. The morphology of microscopic fracture veins imply diat die deposit was under .shear stress during forma­tion. Some late-orogenic granitic composit ion dikes were emplaced during the mineralization associated with brittie fracture veins.

4.2. Theories on the timing of gold mineralization

Considerable attention has been given in the Rus­sian language literature to die hypothesis that there have been more than one episode o f mineralization occurring at different times at Muruntau beginning widi Ordovician-Silurian lenticular accumulatioas of gold-bearing zones , fol lowed by Carboniferous-Per­mian stratiform gold ores and culminating in the Permian-Triassic widi the formation o f the high-grade CenU'al Veins (Migachev, 1993; also Be l 'kova and Ognev, 1971; Mukhin et al., 1988; Marakushev and Khokhlov. 1992). Kuibanov ct aL ( 1 9 9 1 ) argued that Muruntau was fonned from the superposition o f high-temperature alteration and metallization on synkinematic metamorphic/meta.somatic products of

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tectonic ;,n earlier stage (Ordovician-Silurian). "The metal leal at- ,ffu,st have been derived from gold sulfide accumula-

tioivs...formed at the earlier, volcanosedimcntary [and related] formadon stages" (Kurbanov et al., 1991, p. 18). Dynamothermal metamorphism and postmag-inaiic activity served as a mechanism to concentrate

•utes of the metals with the ' ' bulk of commercial gold-quartz formed ores related to the late stages...following the em-ing the placement of the orogenic granite intrusions..." sion of (Kurtjanov et al., 1991, p. 19) at «=278 Ma. Kur-lage of hjuiov refers to this as ' polychronic' mineralization, ige and The proposition that there are several temporally Jhosed, distinct episodes of mincralizadon is generally sup-olcanic ported by an interpretation of layers of angular frag-.shear- ments of Besopan, comminuted shale and quartz

ly Per- cemented with carbonate at the contact of the Be-mposi- .sopan and die Devonian carbonate rocks (cf. Kotov ndytau and Poritskaya, 1992). Instead of a dirust contact the ). Hy- muldple-epLsodcs models consider the contact to be idi in- an unconformity and the 'conglomerate' to be a ntly to .sedimentary unit. Our field observations and die nsition results of drilling through the carbonate and silici-ienccd clastic sequences convince us diat die contact is inaged tectonic and that the conglomerate is not a sedimen-les, all tary unit. Kotov and Poritskaya (1992) noted diat lining. quartz veinlets crosscut the contact between die Be-imply sopan and the Devonian carbonate rocks widi small

forma- areas of oxidized gold mineralizadon occurring up dikes ° into the carbonate rocks. A second line of evidence

•dated that mitigates against a pre-Carboniferous gold-mineralizing episode is Rb-Sr isochron data from a pre-gold ore hornfels assemblage of 273. ±2.8 Ma

tion sampled within die Munintau open pit (Kosdtsyn, 1994).

jRus-dicre 4.3. A model

zation inning Our model shares some aspects in common with ons of other interpretations, but we rely on the interplay of :-Per- structure and fluid chemistry in our explanation of n the die origin and physical character of Muruntau. Our high- model does not require a gold-enriched protolidi. *kova The geometries of the alterations and ores are a ushev function of the prelateral faulting permeabilities, die rgued sequence of evolution of various shear zones, mag-on of raatic activity as a diermal-focusing mechanism and 1 on fluid flow and metal enrichment owing to seismic :ts of pumping.

4.3.1. Fault geometry, permeability and fluid focus­ing

The lateral faulting in the Tamdytau is con­strained by geologic relations to have originated during the later stages of the upper Carboniferous continent-to-continent collision. Further, the clear control of these structures on the distribution of earliest stage alteration in the Muruntau ore field and the intramineral alteration of dated Eariy Permian granitic dikes associate and link all aspects of the mineralization to the late-accretionary structural pro­cesses and pre-existing .structural elements.

Critical to unraveling the relation of fault system evolution and hydrothermal alteration geometry is the constraint of how fluids move and arc focused in deep, compressional, continental collision environ­ments. Fluid flow in such environments should be by repetitious crack opening, flow and sealing (Cox and Elheridge, 1983; Wall ct al.. 1983; Sibson, 1987) and towards zones of lower mean stress (Ridley, 1993). Thus, in a generally compositionally homogeneous rock unit such as die Besopan Suite, shear zones arc the most continuous zones of high permeability, and fluids will be focused into diem. Aldiough some cnick-seal microveining and alteration effects occur in rocks outside of the large lateral shear zones at Muruntau, most of these are confined to the shear zones.

The earliest alteration that we have recognized petrographically in samples that we collected at Mu­runtau is quartz-albite-phlogophite and die distribu­tion of this assemblage is predominandy along the northem sbiand of the Sangmntau-Tamdytau shear zone. Widiin diis alteration are subparallel layers of quartz veinlets frequendy manifested as linear siliceous masses within the shear zone (Fig. S). Because of the compositional homogeneity of bSj widiin the Sangmntau-Tamdytau shear zone, it is likely that fluid-phase viscosity contrasts in bS, were die result of pre-existing permeability contra.st5, such as die Caledonian thmst planes. This is evidenced by the subparallel vein zones within the shear zones. Variations in mean stress in layered sequences de­pend upon layer diicknesses and the relative compe­tency of die layers. These variations are an effective mechanism for focusing fluids in a regional stress field (Ridley, 1993).

The restriction of the quartr-albite-phlogopite

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alteration to a long, linear zone that extends from the Besopan deposit to the Miutenbai deposit (cf. Mukhin et al., 1988; Marakushev and Khokhlov, 1992; Ko­tov and Poritskaya, 1992) implies diat other strands o f die Sangruntau-Tamdytau shear zone cither did not exist at dial time or were hydraulically noncon-ductive at the time diis alteration and the subparallel microveinlets were formed.

W e bel ieve that the critical geologic evidence regarding hydraulic conductivity versus nonconduc-dvity of a pre-cxisdng structure is in the interpreta­tion of aeromagnetic data regarding buried intrusions in conjunction with die paragentic relations between alteration as.semblages and dike emplacement de­scribed above. Regional magnetic surveys of the Tamdytau indicate that the largest pluton in the vicinity o f Muruntau is die Sardarin stock that crops out soudieast of MurunUiu and projects to the north­west in the subsurface to Muruntau. In addition, there is a much smaller, northeast-trending, subsur­face positive anomaly approximately 5 km w e s t -soutiiwest o f the Muruntau mine. Our interpretation o f data in Khamrabaev ( 1 9 6 9 ) is that this small anomaly west of Muruntau is highly correlated with dikes throughout the suuUiern Tamdytau with a prob­able source region beneath the alluvium south o f die range. The upper surface of the anomaly consists of a number of linear, dike-like magnetic highs, most of which are coincident widi surface exposures of dikes. A s w e know the paragenetic position of the dikes vis-a-vis the alteration s u g e s (Table 2) , tiie implica­tion is Uiat this western intrusive activity is younger than the Sardarin intrusion beneath Muruntau, which produced prehydrothermal contact /metamorphic spotting but only in the immediate vicinity o f the Muruntau deposit. Furthermore, the gold-bearing, hydrodiermally derived Muruntau Lens is restricted to die northem strand o f die Sangruntau-Tamdytau shear zone, with subsequent alterations and deforma­tion superimposed upon this lens. All of these data taken togedier imply that as shear zones developed, they were hydraulically conductive, and. therefore, the lack o f evidence o f earliest stage alteration out­s ide o f die northem strand o f the Sangrunuiu-Tamdytau shear zone is itself evidence diat the other shear zones did not exist in the initial stages of die development o f die Muruntau deposit.

W e found petrographically that die early perva­

s ive alteration within the northern strand o f the Sangmntau-Tamdytau shear zone w a s initially con­trolled by schistosity, and other c leavages related lo different scales of folding within the Besopan Suite. Secondary albite and quartz are e longated in the plane of schistosity and the textures imply piutially annealed crack-seal growth (cf. S ibson, 1990). The various orientations o f schistosity and other c leavage within die rock in the shear zone result in locally inhomogeneous permeability and, therefore, inhomo-geneous stress. Because the shear zone is the main focus of fluid flow, fluid pressures wil l increase within the zone, dilatancy will be local and die fluid-flow within the shear zone wil l be focused further into these local zones of dilatancy; that is. zones of lowest mean stress (cf. Ridley. 199.3). Thus, die bulk permeability of the Sangruntau-Tamdytau shear zone relative to the surrounding rock fucu.sed fluid into die zone, and the dilation of die schistosity and other c leavage wi lh in the zone focused fluid flow locally in the absence o f otiicr fractures. During active transpression, continued high fluid pressures witiiin the shear z o n e led to addi­tional microfracturing. and these fractures, therefore, supplanted the schistosity as the zone o f l owes t mean stress.

The homogeneity and pervasiveness o f the early-stage alteration is the result o f the generally .uniform chemical composition of the Besopan Suite. Through seismic pumping in die large v o l u m e o f homoge­neous rock,, die fluid composit ion is effectively buffered to die regional-scale chemical commonal i ty ob.served. The crack-seal-style textures indicate that increased permeability occurred by means of pene­trative fracnire propagation (Wall et al., 1983) and that permeability probably increased more rapidly in the zones of Caledonian thrust-related shearing. W e interpret the subparallel zone.s o f quartz veiidets to be the incipient development of large vein systems witiiin the shear zone o f the variety c o m m o n m such mining districts as the Modier Lode , California, and Juneau. Alaska.

4.3.2. Association of intrusions with shear zones A s discussed above, mineralogical ev idence indi­

cates that tiie highest temperatures in the southern Tamdytau most probably were initially in the M u m n ­tau mine area, where the development o f high-tem-

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>f the perature contact /metamorphic and metasomatic ef-y con- fects are obscn-cd. The prealteration spotted schist, ited to which is restricted to the area within and surrounding Suite. the Muruntau mine, changes witti depth from a in the creenschist facies metamorphic rock to amphitiolite irtially and gamet-bearing calc-s i l icate facies rocks. With ). The time, however, mineralizadon developed over a much lavage broader area, and dikes were emplaced along many locally structures in die southem Tamdytau. W e .speculate homo- that as the stress regime evolved in this orogen after

main collision, a zone o f dilatancy formed west o f die crease Tamdytau (Fig. 2) that al lowed igneous magmas to id the be emplaced. A.s this emplacement proc&ss evolved, )cused the britde-acting Mumntau-Daugyztau shear zone hat i s , formed. Adjustment on this z o n e was left-lateral 1993). resuldng in the formadon of a fault-related fold (the

untau- Z-fold, Fig. 4). As diis fold formed, slippage along unding the fault .sy.stem developed (Fig. 4). An equant shaped lion of zone of permeability (on the order of 9 km* in plan J zone v iew) opened forming the large bulb-shaped mass in • other the p r e v i o u s l y d e v e l o p e d Muruntau L e n s :d high (Sangruntau-Tamdytau shear zone) . With each suc-I addi- ccs-sivc movement on this britde fault system, aurif-refore, erous, silica-bearing hydrothermal solutions invaded t m e a n the near-vertical brittle stmctures depositing the

high-grade gold-bearing quartz veins (the Central Veins, Fig. 5) .

4..?.J. Effects of additional shear zones On the basis on how fluids f low in compression-

ally stressed regimes (Ridley, 1993), the geometry of the orebodies and stmctural control and strike of late-.stage dikes imply sequential changes in the stmctural environment at Mumntau. These changes are after the formation of the quartz-albi te-bi iui te alteration zone and its enclosed subparallel zones of quartz microveining. Stmctural elements formed sub-.sequently to the quartz-albite-biot i te alteration in­clude the Z-shape fold, folds in the subparallel quartz veinlet zones (s i l iceous masses), northeast-striking orebodies. e a s t - w e s t Central Veins, quartz-tourma­line veinlets and calcite veining.

Within the mine, the main individual orebodies strike northeast parallel to the axial traces o f folds in bSj (Fig. 5 A ) and oblique to the displacement direc­tion on the northwest-trending Sangmntau-Tatndyiau shear zone. Within the Sangruntau-Tamdytau shear zone, diis strike o f tensional opening is not s tmc­turally consistent widi all movement being o n the northem strand and thus implies that some change

eariy-niform hrough imoge-ct ive ly onality ite that •' pene-3) and •idly in ig. W e ilets to .ystems in .such lia, and

nes : e i n d i -oudiem Mumn-gh-tem-

Muruntau-Oaugyitau shearlone

^ S ; ^ Temporal relation between the- iliear zone in the southem Tamdytau. Movement on the Munintau-Dangyztau shear zone is last major shearing event in the foimation of the gold ore zones in the area of the MurunUu mine.

3

B) FonnacionofthaZ-foktandmpantian of t lw Muruntau Lena

Fig. 9. Structural elements associated with the mineralization at Muninuiu. (A) Location of sevend shear zotiex and faults in the southem Tamdytau, U/Aclcistan. (B) Creation of the fault-related fold (Z-fold) and the Munmtau Lens. (C) Initiation of the formadon of the 'stockwork' ore in relation to Ihe movement of die nuuor faults. (D) Formation of the Central Veins in relation to rouvcnwot oo the maiot faults.

s

4

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had taken place. We suggest that the left-lateral Muruntau-Daugyztau shear zone (Fig. K) became acdve before the formadon of the.se orebodies. thus resuUing in the Z-shape fold (Fig. 9 ) in the Besopan where the two major shear zones intersect.

There were two major effects o f die acdvadon of the Murantau-Daugyztau shear zone. One was the folding to varying extents o f the prior quartz-albite-biotite alteration and subparallel gold-bearing quartz veinlet zones, which brought the axial traces o f the Caledonian isoclinal folds into an orientadon subparallel to the Muruntau-Daugyztau shear zone. Second was its effect on die trace of and fluid flow within the Sangruntau-Tamdytau system.

The rotation of the prior structural elements and subparallel quartz veinlet zones is evident within the Muruntau mine. The zones of quartz veinlets within the quartz-albite-biot i te alteration are folded to varying extents within the Muruntau deposit (Fig. 5B and C) . Kurbanov et al. (1991) indicated that the amplitude and wave length of die folding of diese surfaces were not uniform and that the greatest am­plitudes and shortest wave length folds are south of the Northeast Fault (Fig. 5A) . The greatest vertical thickness o f ore is within die fold structures along the southem, or hanging wall , s ide of the Northeast Fault (Fig . 5C) . Mukhin et al. (1988) showed die isoclinal folds in the Besopan to strike northeast in the northem and centrai parts o f die Mumntau mine and northwest in the southem part of the mine. Thus, in the northern and central parts of the Mumntau open pit, the axial cleavage of these folds has been rotated into die plane of extension within the San­gmntau-Tamdytau shear zone.

The amount of offset of the Besopan Suite and die over-dirust carbonate rocks along faults related to die Mumntau-Daugyztau shear system is not signifi­cant. Nevertheless, its tectonic importance to die .stmctural control of the Mumntau mineralizadon is evident not only from die folding of die Besopan rocks, but also from the nearly orthogonal left-.step-ping (rather than oblique) o f the Sangmntau-Tamdytau shear zone (Figs. 8 and 9) from Miutenbai across die Mumntau ore zone. W c suggest that die predominant effect o f die Mumntau-Daugyztau zone up to this t ime was ducdle shear with later move­ment being of a britde nature.

The effects o f the Mumntau-Daugyztau shear

zone on the strike of the Sangmntau-Tamdytau shear zone may be inferred from the fracture trends that control post-Z-fold mineralization in the Mumntau deposit and those shears diat controlled the emplace­ment of intramineral dikes. The fol lowing t w o pieces of evidence are crirical to the evolv ing structural relations: (1) the northeast-striking orebodies and (2 ) the emplacement of late-stage intramineral granidc dikes. The dikes are along die Sangmntau-Tamdytau shear zone southeast of the Mumntau mine und the soudiem strand o f this shear zone west o f Murantau (Fig. 3 ) . The critical argument, as presented above, is diat dikes respond to the same mean stress con-.straints as hydrothermal fluids (Ridley, 1993). The dikes imply that the lowest mean stress had been transferred to the s o u t h e m strand o f the Sangmntau-Tamdytau system. T h e shear zones that crosscut up into the upper plate rocks in the Tamdy­tau (Figs. 3 and 8) were probably formed al this time because diey localize no alteration and shearing is much less extensive tlian on the early .strand (Fig. 5) . These factors, therefore, further imply that there is a step in die Sangmntau-Tamdytau shear zone from southeast of Mumntau across the pit area to die west-striking soudicm shrand (Fig. 9) . The northeast­e m strike o f the orebodies is the expected posit ion­ing of tensional opening in die system between the two steps. The formation of the Central Ve ins and the later tourmaline-bearing veins along more c a s t -wcst-trending structures rather than die northeastern strike o f the earlier banded and stockwork ores is consistent with the ultimate regional predominance of the Mumntau-Daugyztau shear system and atten­dant stresses.

Geochemically anomalous concentrations o f gold ( > SO ppb to > 1 ppm Au) in the a lb i te -o l igoc lase cariy-stage alteration between Muruntau and B e ­sopan are convincing evidence that gold was being deposited from the beginning o f hydrodiemuil activ­ity. That most o f die altered ground with 0.5 to 2 ppm Au is in immediate proximity to ore-grade bodies implies that the eariy stage contains only a small proportion o f die total gold w e .

During die later stages of die p h l o g o p i t e - m u s c o -vite-K-feldspar alteration, die amount o f sulfide, primarily in the form of pyrite and arsenopyrite, increases as does the amount o f carbonate alteration. Clearly, the amount of C O j in solution has in-

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194 U. Ortwetal/Ort CenloKy Rm-iews ll (19%) 175-196

Fig. 10. Location of the fonnation of the calcite veins as mapped in the Munmtau pit

creased. The overall effect on the hydrodiermal sys ­tem, therefore, was an apparent increase in the re­duced-gas content. Depending on the temperature, the increased amount of sulfide in solution also would have increased the solubility o f gold. One possible effect is that in carbonaceous and (or) fer­rous-iron-beating lithologies, C O j reacts with the rock to form methane. With methane in excess , hydrogen is relea.sed, and metal precipitation, includ­ing gold, i s catalyzed. Khamrabaev (1969) presented analyses o f pyrite and arsenopyrite that show vein and veinlet (stockwork veining) pyrites contain l o w to moderate amounts o f gold and that coarse- and fine-grained arsenopyrites contain from low to high amounts o f gold. The high-grade nature of die Cen­tral Veins c losely associated with our third-stage assemblage of hydrodiermal alteration further im­plies dial higher grades of gold go along with tiic main period of potassium-carbonate meta.somatism. According to Khamrabaev (1969) , die ensuing cross-cutting, tourmaline-bearing veins contain only minor amounts o f go ld in pyrite. The lower concentrations o f go ld associated with the tourmaline- and carbon­ate-bearing veins may reflect the paucity o f reactive ferrous iron and carbonaceous matter that remain in the immediate host rocks or some other process, which Includes lower gold concentrations in solution.

The latest stage o f vein formation in die Muruntau deposit is calcite. These veins crosscut all preceding alteration types, as well as igneous dikes. Pyrite and nuc-carth-bearing minerals, which possibly include baesnaesite, are found in these veins at some locali­ties. T h e s e rdations and the occurrence o f veins

along north-southerly .striking brittie faults (Fig. 10) indicate that tiicy were deposited during the waning stages of the mineralization process.

5. Conclusions

Muruntau Ls a syndeformat ion/synigenous , or low-sulfide shear-zone-hosted, gold deposit , the sub­stantial size o f which is a result o f its structural evolution. Before the Carboniferous to Permian pe­riod (Hercynian) of tectonism. a schistosity was de­veloped in the Cambrian lo Ordovician host Besopan Suite, and intraformational imbricate thrusting re­lated shear zones were developed. During the initial stage of mineralization, quartz veinlet zones with gold were formed as fluids wen; refocused into 7ones of pre-existing permeability witiiin the San­gruntau-Tamdytau left-lateral wrench-fault system. T h e minera l i za t ion l o o k p lace w i t h i n the hrittle/duc-tilc transition zone in the crust and within the thermal influence of concurrent granitic pluton­ism. The development o f these incipient major quartz veins was interrupted by changes in the regional stress regime and the development o f a competing left-lateral wrench-fault system, die Muruntau-Daugyztau shear zone. The changed stress regime refocu.sed die hydrodiermal fluids to form the second stage of veinlets in the northeast-striking zones o f tension that are related to the interaction of tiie.se fault systems and the left-stepping of the Sangrun­tau-Tamdytau shear zone. A s the initial wrench-fault system was succeeded by the Muruntau-Daugyztau system, a third stage of mineralization occurred that is represented by major, more dut)ugh-going aurifer­ous quartz veins along brittle fractures. The final ore-bearing stage fol lowed the emplacement of granitic dikes and consists of tourmaline-bearing quartz veinlets. The waning stage of mineralization consists of calcite veinlets.

Acknowledgements

This work is a product o f a cooperative scientific agreement between the U.S . Geological Survey and the Ministry o f Geo logy in die Former Soviet Union signed in 1989. W e gratefully acknowledge the assis-

From: Interiibrary Loan To: DDX Date: 2/18«7 Time: 23:42:57 Page 23 of 24

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G o l d f a r b w h o a c c o m p a n i e d us t o M u r u n t a u , as w e l l

as o u r coun te r -pa r t s i n th is ag reement . T s N I G R I and

the i n v a l u a b l e t r ans la t i ng capab i l i t i es o f I r i n a

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n ized i t i n c o o p e r a t i o n w i t h T u l k i n S h y a k u b o v . G a n y

A b d u r a h m a n o v and R e m i r T s o y o f the State C o m ­

mi t tee o n G e o l o g y and M i n e r a l Resources o f U z b e k ­

is tan. T h e M u r u n t a u M i n i n g C o m b i n e , the K y z y l k u m

E x p e d i t i o n , I g o r L e v c h e n k o a n d V i c t o r D a n s k o i ,

S A I G I M S . T a s h k e n t , a n d D m i t r i A z h g i r e y , T s N I ­

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i n g o u r f i e l d w o r k . T h e au tho rs w i s h t o thank M i c h a e l

Foose, R i c h a r d H e n l e y a n d P a u l Ba r ton f o r the i r

very h e l p f u l r e v i e w s o f t he m a n u s c r i p t .

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