Porfidos Hypotheses

7/30/2019 Porfidos Hypotheses

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Some Hypotheses Regarding the Formation of GiantHydrothermal Ore Deposits

These are thumb-nail sketches only. The hypotheses are discussed in more detail in later sections of the paper.

1. The super-giant to behemothian porphyry Cu-Au deposits

EXAMPLES : Chuiquicamata

El Teniente

Grasberg


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Commonly these deposits form by a two stage process (Zentilli et al, 1995)

1. Formation of high temperature assemblages :M veins:A/B veins

2. Remobilizing and upgrading of this proto-ore by lower temperature fluids to form quartz-poor sulfide veins(C veins).

The bulk of the resources form during the second stage.

Keys to deposit formation are:

q Development of a sealed magma chamber which acts as a fluid reservoir.q Focused release of the fluids (brine, vapor, melt) along some major conduit and commonly old basement

structures utilized.q High fluid pressures during stage 1 to maintain sulfide saturation at high temperatures.q Condensation of acid volatiles, coupled with a drop in fluid pressure drives the leach process in the deep

parts of the system, during stage 2.q Same structures utilized during stages 1 and 2.q Quiescent phase of volcanism.q A decrease in fluid pressure between stages 1 and 2 maybe related to rapid uplift.q Limited movement on the structures which act as major conduits coupled with limited deformation of fluid

reservoirs.q Relatively stable tectonic setting, during stages 1 and 2.

In the Northern Chile porphyry province these conditions have been sustained along the west fissure andmaintained for approximately 10 Ma from the Late Eocene to Early Oligocene. This system was most probably

driven by a semi-continuous magma chamber beneath the whole belt. Domains of fluid focusing were controlledby crosscutting NE-trending basement structures.


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2. Giant and high grade lode Au deposits

EXAMPLES :

q Ashanti

q Golden Mileq Porgeraq Lihir?q Telferq Northern Nevada Au province

These deposits developed at many different times in earth history and in a range of geological settings. They:

q May be associated temporally and/or spatially with intrusive rocks.q May be hosted by sequences of black shales.q Commonly occur within large fault systems or related to crustal lineaments.q

Commonly evidence of hypogene remobilization of gold and release of refractory gold.q Appear to have formed a various levels in the crust. A common theme is formation at a late stage in a

tectonic cycle.q Fluids commonly CO2 and/or CH4 rich aqueous fluids. Some are notably depleted in H2O and may be

nitrogen rich.q Commonly evidence of strong redox contrasts at the site of ore deposition. In such cases Au appears to

have been transported by the reduced fluid and deposited by oxidation.

It is suggested that:

q Au is mobilized from deep-crust/mantle in the upper crust by variously reduced volatile-rich fluids. Thefluids are of composite origins and may include surface/high level fluids that recharge aquifers in the lowercrust during periods of extension.

q Depositional mechanisms are variable but a significant process is the mixing of a reduced Au-bearing fluidwith an oxidized fluid.

In the Ashanti deposit the redox contrast was generated by mixing a deep-crustal CH4-N2- rich fluid with a CO2 -

rich fluid at mid-crustal levels. Significant hyogene reworking also occurred.

Zone 7 of the Porgera deposit resulted from the condensation of SO2-rich magmatic volatiles into a reduced fluid

within the Romane Fault. A significant proportion of the Au was probably reworked from the earlier A-vein veins(Cameron et al., 1997)

The Golden Mile developed at a high level in the crust by mixing of a reduced, gold-bearing CO2-rich fluid (the

generic Archean gold fluid of Phillips et al., 1996 that deposited the thousands of other Archean lode depositsthroughout the Yilgarn) with a SO2-rich magmatic fluid. The chemical integrity of the two fluids up to the point of

mixing was maintained by the Golden Mile Dolerite which behaved as an aquiclud at the local to regional scale.

At Kidston, the Lochaber - Bagstowe complex was a reservoir of mildly oxidized, CO2-rich gases and the

Gilberton structural corridor acted as a flow zone for the reduced fluid. At the regional scale, the Kidston pipe,was a domain of low pressure drawing in fluid from both the Gilberton corridor and the Lochaber - Bagstowecomplex. The pipe effectively operated as a valve for both fluid reservoirs.


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Key to size and grade:

q Regional scale focusing of fluids from two reservoirs into a domain of low pressure. This may occur atvarious levels in the crust.

q The bonanza grades reflect the strong redox gradients generated by simultaneously focusing two fluids ofstrongly contrasting redox conditions.

3. The origins of the super-giant Bingham Cu-Au deposit


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The Key: Regional scale focusing of two fluids??

q The size and Au tenor of the Bingham Cu-Au deposit is unique in the southwestern US porphyry province.q It is hypothesized that this super-giant porphyry deposit resulted from the effective focusing and mixing of

two discrete fluids : a Cu-rich but sulfur-depleted magmatic brine and: a reduced, H2S-rich and Au-richfluid.

q The later fluid is akin to the fluids that formed the sediment-hosted Au deposits of northern Nevada.q The productive period of Cu-Au deposit formation within the Great Basin reflects this interaction between

magmatic fluids and deep-seated reduced Au-bearing fluids at around 40 Ma across Northern Utah andNevada.

Crustal-scale fractures and distribution of major deposits

Empirical evidence supports an association of significant deposits and mineral provinces with major faults and/orlineaments.

The origins of lineaments remain controversial, particularly as such features appear in part to be independent ofupper crustal geology. Here, it is assumed that they image an array of fractures within the mantle and that thesemantle fractures have repeatedly influenced patterns of deformation in the crust.

A first-order set of mantle fractures is estimated from the outline of the Australian continent. The rather geometric

shape of the western two-thirds of the continental is taken to reflect the influence of an array of mantle fractureson the break-up of Gondwanaland during the Mesozoic and Cenozoic.


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The fracture sets also played a significant role during phases of convergence. The active set appears to relate tothe angle of collision.

The north-east trending set was active during collision of the Australian and Pacific plates during the last 10 ma,acting as transfer structures and controlling the distribution of major mineral deposits in Papua New Guinea andIrian Jaya (Hill and Mason, 1997; Hall, 1995; Corbett, 1994).

The west-northwest trending set exerted a fundamental control on the distribution of major deposits and mineralprovinces in the Tasman Orogen during Paleozoic convergence events (Walshe et al., 1995; Walshe and Glen,1996). Greg Hall (1996) has suggested that the gold deposits in Kalimantan relate to a north-northwest set oflineaments. This trend correlates with a similar trend in the Australian continent.

The Issues

q Fluid reservoirsq Fluid mixing, particularly liquids and gasesq Hypogene enrichment of primary mineralizationq Controlling basement structuresq Pressure evolution with depth, temperature and time

The construction of giant, high grade hydrothermal deposits requires effective methods of storing large volumesof fluids within the earths crust while sustaining metal and sulfur solubility within those fluids. There must also beeffective ways of delivering the fluids to sites of ore formation, whilst ensuring the chemical gradients necessary


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to produce high grade mineral deposits are maintained.

Metal solubility is essentially controlled by a relatively small number of parameters: temperature, acidity of fluid,redox state of fluid, salinity of fluid and concentration of volatile species, particularly sulfur. A fundamentalconstraint on the development of most base metal deposits is the need to deliver sufficient reduced sulfur, as wellas, metals to the site of ore formation.

High metal grades will occur when large gradients in one or more of these solution parameters can be sustained.The mechanisms for sustaining these gradients are limited. Common processes include reactions of fluids withspecific host rocks, phase separation at particular sites and mixing of fluids (liquids and/or gases). Fluid-rockreactions and phase-separation mechanisms have limited capacity to maintain gradients and generate both largetonnages and high grades (Heinrich, 1996). Processes involving the mixing of large volumes of fluid with stronglycontrasting properties or processes involving the recycling and upgrading of initially low grade deposits arepotentially the most effective mechanisms for generating both high grade and large tonnage deposits.

It is important to understand the processes operating at the regional to crustal scales that give rise to reservoirsof heat, salinity, acidity, sulfur and redox capacity within the crust and mantle. These are effectively the metalreservoirs or the reservoirs of the critical reagents (oxidant, reductant, etc.) required for metal precipitation.Knowledge of the mechanisms of fluid release from these reservoirs and the pathways which allowed fluids to befocused into the upper crust while maintaining their chemical integrity is also significant. It is at this point that thebulk composition/lithology of rock sequences in the upper crust are likely to play a major role in enhancing ordegrading the ore forming potential of fluids generated in the lower or middle crust.

Maintaining fluid pressure as well as temperature within fluid reservoirs is likely to be of paramount importance.Commonly, the partial pressure of the acid volatile species (CO2, H2S, SO2, HCl) determines acidity andoxidation state; sulfur levels and maintains a balance of both metal and the sulfur components within the fluid.Loss of fluid pressure is likely to lead to a degradation of the reservoir both in terms of volume of available fluidand its chemical potential for deposit formation. However, in a pre-existing ore zone a loss of fluid pressure canlead to sulfide dissolution and a significant upgrading of the resource.

There is a need to recognize the existence and extent of regional seals, be it a plug in the top of a magmachamber or a clay horizon within a sedimentary basin, and to learn to recognize when one or several reservoirshave released fluids in a controlled and focused way. These are likely to be times of great potential for largetonnage and high grade hydrothermal deposits.

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Part A

Mostly about Porphyry Deposits

Magmatic Hydrothermal Systems

A1: Diversity and complexity

It is suggested that the diversity of processeswithin the clan of magmatic hydrothermal deposits may be represented interms of the following end-member subsystems:

1. Ortho-magmatic subsystems: - straight magmatic fluids; metal budget of the fluid is a product of supra-solidusprocesses in the magma chamber.

2. Para-magmatic subsystems: - evolved magmatic fluids which have been substantially modified by fluid-rockreactions and/or mixing with external fluids. Metals are largely derived from the intrusive complex but metalbudgets reflect the extensive subsolidus history of these fluids.

3. Supra-magmatic subsystems: - dominantly external fluids and metal sources; the magma chamber is a source of

heat and possibly acid-volatiles, oxidant and/or some key metals.4. Xeno-magmatic subsystems: - metal-bearing fluids sourced from similar depths to the magmas; interplay of

magmatic and xeno-hydrothermal fluids may be critical to deposit formation in the upper crust.

These model systems summarize the dominant processes by which metals may be transported and deposited in fluids, inand around intrusive complexes at about the time of complex formation. These subdivisions do not replace the welldefined and accepted classes of deposits which are based on descriptive criteria. They are based on the recognition thatthe processes operating in and around an intrusive complex can be broadly grouped. To some extent these subsystemsmay be considered substages or subdomains since subdivisions 1, 2 and 3 could be considered stages or domainswithin a single evolving system. However, there is no requirement that all of these subsystems be present in a givensystem just as there is no requirement that an epithermal deposit should sit above a porphyry deposit. Conversely, fewdeposits are likely to have formed through operation of just one of the dominant processes summarized by these

subsystems. The genesis of most deposits will reflect some complex interplay between these end-member systems.

Systems are commonly open; open at the top, the sides and the bottom. We have a good understanding of theopenness of these systems at the top but a very poor understanding of their openness at the bottom. Model 4addresses this issue and permits consideration of the interaction of fluids from a magma chamber with fluids from non-magmatic reservoirs that might reside in the middle or lower crust.

A2: Synopsis of the proposed subsystems

q The ortho-magmatic subsystem: The ortho-magmatic hydrothermal fluid is a direct product of the silicate melt. Itseems that it is possible to recognize deposits which formed from fluids that contained a dominant component of

this fluid. The best examples are found amongst the Cu and Au rich porphyry deposits. Typically the fluids are highT; oxidized, super saline and sodic at least in their early stages. Such deposits are commonly associated with anunderlying magmatic chamber which supplied the juice. The Burnham view (Burnham, 1979) of how porphyry


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deposits form probably applies more to the chamber than the overlying deposit. In these systems the nature of thesilicate melt is extremely important. Another key factor seems to be the ability of the high level chamber to releasestored fluids in a focused manner. A critical problem is how to recognize when and how sub-volcanic chambers actin this manner. An understanding of these processes in particular should yield more robust conceptual models

q The para-magmatic subsystem: D and O isotope studies of porphyry Cu deposits and Sn deposits through the1970s and 80s brought recognition that the phyllic alteration in many deposits reflects an ingress of meteoric waterinto the magmatic hydrothermal system, commonly at a late stage. This led to a two-fluids model of these systems

- a magmatic ore fluid which essentially formed the deposit and a late-stage fluid of meteoric origin which variouslyoverprinted and modified the pre-existing sulfide deposit as the magmatic hydrothermal system waned. What isless well recognized is the ore fluid which is transitional between ortho-magmatic fluids (as defined above) andexternal (near surface) fluids which are readily recognizable from their D and O isotopic signatures. These para-magmatic fluids may be evolved ortho-magmatic fluids which derived their metal budgets through re-equilibrationwith the host igneous body at subsolidus conditions; they may be external fluids which re-equilibrated with igneoushost rocks or perhaps most commonly they are fluids derived by a combination of these processes. The term para-magmatic is adopted here to describe these fluids of diverse origin but which retain some definable link with themagmatic system; either by direct derivation of some components from the silicate melts or through exchange ofcomponents with the crystallized igneous body at subsolidus conditions. The prefixes ortho- and para- contrast theidea of the 'straight' magmatic fluid with that of an evolved magmatic fluid which still retains some of its magmaticheritage. Making the distinction between these two fluid types is important to our understanding of the complexarray of processes involved in the transition from the purely magmatic domain of the magmatic hydrothermalsystem to the open domain of the system where external fluids play a highly significant role. It is within thistransitional domain that ore fluids may be generated and dispersed; metals concentrated and diluted; ore bodieswon and lost. Many porphyry Cu deposits may have formed entirely from para-magmatic fluids which had theirmetal and sulfur budgets determined by subsolidus processes within the intrusions.

q Supra-magmatic systems: Epithermal, mesothermal and other deposit types which may be related to underlyingintrusions (known or inferred). The ore fluid is largely an external fluid with the metals being largely derived fromthe country rocks but with key components (acid volatiles and some metals) being derived from the underlyingintrusion.

q Xeno-magmatic systems: Encompasses the concept that a hydrothermal fluid may come from a deep-seatedsource(s) in the crust and/or mantle and may ascend similar pathways as the magmas. The fluids would need tobe CO2 or CH4-rich at depth so as not to dissolve in the melts and may be highly reduced. Mineral deposition may

occur through mixing with surface or magmatic fluids, or through reaction with specific host rocks. Chances are weare not very good at differentiating 4 from 1, 2 and 3 in high-level geological settings. Deep-seated reduced fluidswhich find their way into the upper crust may generate reduced Au-skarn deposits for example. Such deposits arecommonly spatially associated with oxidized magmatic systems (e.g. Sheahan Grants, Fortitude, Ladolam) and theswitch from high-T oxidized fluids to low-T reduced fluids seen in these systems could be explained by a switchfrom oxidized magmatic fluids to deep-seated reduced fluids.

A3: Ortho-magmatic versus para-magmatic subsystems

In brief

It seems possible to make a first-order distinction between porphyry deposits which formed by ortho-magmatic processes

in the main and those deposits which formed by para-magmatic processes from the nature of veining, the alterationassemblages and the fluid inclusion compositions.

The salient characteristics of deposits in which ortho-magmatic processes played a dominant role are:

q Early high temperature Fe - Na Ca metasomatism characterized by plagioclase biotite and magnetiteq This early assemblage is commonly overprinted by a potassic assemblage of K-feldspar - biotite magnetite -

sulfide anhydrite.q Irregular, discontinuous quartz veins which predate laminated straight-walled veins.q Highly saline (> 50 wt. %) to supersaline fluid inclusions (up to 70 to 90 wt %); commonly the most saline fluids are

the most sodic.

Experimental studies and thermodynamic calculations indicate that the high salinity, sodic fluids were most likely inequilibrium with a silicate melt at suprasolidus conditions, prior to the saturation of alkali feldspar and quartz, at hightemperatures and low pressure.

The porphyry deposits most likely to have formed by para-magmatic processes are those in which the deposition ofsulfides is relatively late in the paragenesis. In these deposits an early sulfide-poor quartz stockwork is commonlyassociated with potassic alteration (K-feldspar - biotite). Sulfides are deposited from low salinity fluids at relatively lowtemperatures.

A4: The evolutionary stages of porphyry systems:

Beginning at the mesoscale: vein paragenesis and alteration assemblages:

q The evolution of porphyry systems can be described in terms of the generalized paragenetic scheme:


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r - early M and A veinsr - transitional B veinsr - late C and D veins

q This is simple retrogressive paragenetic scheme.q The histories of deposits show infinite complexity but most (all?) seem to be some variation on this basic scheme.

Vein Paragenesis

Understanding the complex interplay between spatial relationships, highlighted by Lowell and Guilbert (1970) and thetemporal evolution, emphasized by Gustafson and Hunt (1975) in their study of the El Salvador deposit, remainsfundamental to unraveling the histories of magmatic-hydrothermal systems. Gustafson and Hunt (1975) used a schemeof A, B and D veins and related alteration assemblages which reflected the temporal evolution observed in that deposit.The scheme has been extended by Dilles and Einaudi (1992) to include C veins and by Clark (1993) and Clark andArancibia (1995) to include M veins to give an overall evolutionary scheme for porphyry systems of M veins (commonlyearliest) through A, B, C and D veins (commonly latest).

This generalized paragenesis, represents the evolution from early high-temperature alteration processes (M and Astages) to later lower temperature processes (C and D stages) with B representing a transitional stage. The significanceof the M stage has only recently been recognized. It may be that in many instances it has been overlooked as a

hydrothermal event or confused with other sodic and calcic alteration events which may develop as cooler fluids on themargins of systems are drawn in and heated (Carten, 1986, Dilles et al., 1992, Dilles and Einaudi, 1992).

Few deposits exhibit the full spectrum of vein types and alteration assemblages.

Examples of deposits with early M and/or A stages:

q Endeavour 26N, Goonumbla Volcanic Complex, NSWq Island Copper, Cu-Mo-Au deposit, British Columbiaq Park Premier, Central Wasatch Mountains, Utahq Yerington, Yerington Batholith, Nevadaq El Salvador, Atacama Province, Chileq Grasberg, Grasberg Igneous Complex, Irian Jayaq Koloula Porphyry Copper Prospect, Guadalcanalq Panguna Cu deposit, Bougainville, Papua New Guineaq Bajo de La Alumbrera, Farallon-Negro Complex, Catamarca, Argentina

High temperature and commonly early

M veins

Magnetite or quartz-magnetite veins associated with sodic calcic alteration assemblage of plagioclase (andesinethrough albite) biotite calcic amphibole (actinolite to magnesio - hornblende) pyroxene.

A veins

Irregular, discontinuous veins of quartz magnetite bornite chalcopyrite anhydrite associated with a potassicalteration assemblage of K-feldspar biotite.

B Veins (transitional)

Straight-walled, continuous, laminated, gray vitreous quartz veins, centimeters thick commonly with molybdenite chalcopyrite bornite magnetite anhydrite. Sericite may develop in the potassic alteration assemblages in this stageas well as tourmaline veins.

C veins

Veins of quartz biotite chlorite epidote chalcopyrite bornite pyrite molybdenite.

D veins

Veins of quartz pyrite chalcopyrite anhydrite sphalerite galena associated with an alteration assemblage ofquartz - sericite - chlorite.

Low temperature and commonly late

In porphyry deposits associated with shoshonitic complexes (Grasberg, Endeavour 26N) the early stage mineralogyappears to be limited to magnetite-biotite-plagioclase whereas for deposits hosted by calc-alkaline complexes (KoloulaPorphyry Copper Prospect, Park Premier, Island Copper Cu-Mo-Au deposit) the early alteration mineralogy commonly


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includes amphibole and pyroxene.

In many deposits the early M and A veins are not developed and the earliest veining is a barren quartz stock (mostprobably equivalent to B) with minor sulfides, most commonly molybenite.

Examples of deposits without early M and/or A stages:

q Ann-Mason Cu deposit, Yerington Batholith, Nevadaq Bingham Cu deposit, Utahq Ok Tedi Cu-Au deposit, Western Provinceq Santa Rita Cu deposit, New Mexicoq Sierrita Cu deposit, Arizona

Taking account of the microscale data:

A5: Insights from the fluid inclusions

Summary comments

q In the ortho-magmatic systems fluid compositions converge with increasing temperature to extremely saline andsodic compositions.

q These compositions are to be anticipated for a saline brine coexisting with a silicate melt, a magmatic vapor andplagioclase alkali-feldspar.

q M-stage veins with their characteristic sodic calcic alteration assemblage of plagioclase (andesine through albite) biotite calcic amphibole (actinolite to magnesio-hornblende) pyroxene are generated by ortho-magmaticfluids which coexisted with silicate melt prior to saturation of alkali-feldspar in the melt and most probably prior tosaturation of quartz.

q The characteristic potassic alteration of A-vein stage occurs after alkali-feldspar saturation in the melt. Cooling offluids from a melt saturated in the two feldspars leads to the precipitation of K-feldspar at the expense ofplagioclase.

Compositions of the saline fluid inclusions

Fluid inclusion studies of magmatic hydrothermal deposits have shown that the total salinity of the hydrothermal fluidsmay vary from near zero through to eighty or ninety weight percent NaCl equivalent. The high salinity inclusions are

generally taken to be magmatic brines and a first order view, consistent with other data (geological setting and stableisotopes), is that variation in fluid salinity is commonly a result of dilution of high temperature magmatic brines with dilutesurface or ground waters. However other sources of salt certainly cannot be discounted nor other mechanisms of dilution.


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Inclusion compositions for porphyry deposits also show a wide range in NaCl/KCl ratio (data for a selection of depositsare shown in Figure A.2). An important feature noted from numerous studies of porphyry deposits is the halite trend ofCloke and Kesler (1979) - linear to sub-linear trends in the data that converge towards the NaCl corner of the NaCl - KCl- H2O ternary (Eastoe, 1978, 1982; Cloke and Kesler, 1979; Wilson et al., 1980; Roedder, 1984; Quan et al., 1987) with

increasing temperature. The origin of the halite trends was much discussed in the literature of the 1970s but asatisfactory explanation remained elusive. The following interpretations developed from the study of the Endeavour 26N

deposit in the Goonumbla Volcanic Complex (Heithersay and Walshe, 1995).

Not all porphyry deposits show these trends. They appear to be associated with porphyry deposits which have the earlyM and A stages of mineralization (Goonumbla, Panguna, Park Premier, Granisle Bell). Deposits which only contain B, C,D stages (Bingham, Santa Rita and Sierrita) do not show the halite trend; inclusion salinities in these deposits rangebetween about 5 and 50 weight percent NaCl equivalent.

The composition of the ortho-magmatic brine

The salinity of a magmatic fluid may be controlled by the extent of magma fractionation but once the two phase region(coexisting vapor and brine) is intersected (commonly at high temperature and low pressure) the salinity of the magmaticbrine will depend only on temperature and pressure. Hence at the P/T conditions most probably encountered in high level

magma chambers (say pressures around 500-750 bars and temperatures of the order of 700-1000 C) the total salinity ofthe fluid will lie in a relatively restricted range from about 70 to 90 weight percent (Figure A.3).


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If a concentrated brine coexists with a silicate melt, NaCl is strongly partitioned into the brine relative to KCl, i.e. the ratioof (mK/mNa)aq to (mK/mNa)melt is less than 0.1 (Figure A.4 and the experimental work of Kravchuk et al., 1993). Hence,

the composition of very saline fluids, separating from a silicate melt must lie close to the NaCl - H2O side of the ternary

almost irrespective of melt composition.

These two observations suggest that the commonly observed convergence of fluid inclusion compositions at hightemperature to high salinities and sodic compositions reflects a convergence to a common fluid composition buffered bysilicate melt and magmatic vapor.

Such fluids will precipitate plagioclase on cooling. Once the melt is saturated in two feldspars, plagioclase and alkalifeldspar, the NaCl/KCl ratio of the brine, vapor and melt will be approximately buffered by these minerals (approximatelybut not uniquely because the compositions of the two minerals also evolve). These fluids will precipitate alkali feldspar oncooling.


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Compositions of rock-buffered fluids within igneous complexes

As most igneous rocks contain two feldspars at subsolidus conditions, the conditions of fluid-rock equilibrium areequivalent to the domain in which the two feldspars are in equilibrium with the fluid in Figure A.5. This domain is based inpart on experimental work and in part on observations from natural systems. At high temperatures the two feldspars willcoexist with a silicate melt, brine and vapor, this condition is shown as a separate subdomain in Figure A.5. At subsolidusconditions the two feldspars coexist with brine and vapor only. For deposits such as Panguna it appears from the fluidinclusion compositions that the fluids were close to equilibrium with the two feldspars (compare Figures A.2 and A.5). Formost deposits the fluid inclusion data indicates significant fluid-rock disequilibrium.

It is possible that the very sodic fluids are of straight magmatic origin. However, this is not possible for the many fluidswhich fall on the more potassic side of the two feldspar domain shown in Figure A.5. These fluids must be the products ofsubsolidus fluid-rock reaction and may be generated by an acid leach (perhaps driven by acid volatiles condensing frommagmatic gases) of the host rocks. Their compositions will depend on the composition and mineralogy of the host rocksas well as the initial fluid conditions.

Ortho-magmatic versus para-magmatic saline fluids

Revisiting the concept

Within porphyry deposits, it seems possible to identify saline fluids, or at least components thereof, which are of straightmagmatic origin (the ortho-magmatic component) and fluid components (the para-magmatic component) which haveundergone reaction and exchange with the host rocks and/or mixed with external fluids.

The significance of differentiating between these fluids lies in the potential controls on their metal carrying capacity. Thecapacity of the para-magmatic fluids may be largely a function of their subsolidus history while the ore forming potentialof the ortho-magmatic fluids will reflect the properties of the magma.


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Indicative compositional ranges for these two fluid types on the NaCl - KCl - H2O ternary are given in Figures A.5 and

A.6.

The domain of ortho-magmatic fluids is drawn assuming:

q Saline fluids of around 70 to 80 weight percent may coexist with two feldspars plus silicate melt and vapor at hightemperature and low pressure,

and thatq Lower salinity ortho-magmatic fluids may coexist with plagioclase only at still higher temperatures and pressures.

These are potentially the most primitive magmatic fluids seen in porphyry deposits.

The domain of para-magmatic fluids encompasses fluid conditions which approximate equilibrium with two feldspars as

well as fluids with higher ratios of KCl to NaCl. These higher ratios are indicative of fluids modified by subsolidus fluid-rock reaction.

Subsolidus reactions and para-magmatic fluids

The mechanisms for achieving values of mKCl/mNaCl greater than the ratio given by the albite-K-feldspar buffer arelimited. If K-feldspar remains stable, as is commonly the case in the inner zones of porphyry systems, it may occur onceall the albite is removed and if the zone remains closed to external waters. In this way the mNaCl of the fluid will beconstant as it is neither added nor removed from the fluid. The mKCl content of the fluid will be increased by thedissolution of K-feldspar in the potassic zone to form sericite and this reaction can be driven by the partial pressure ofHCl in the system. In this situation the governing fluid-rock reaction would be

3KAlSi3O8 + 2HCl v + 2H2Ofld -> KAl3Si3O10(OH)2 + 2KClfld + 6SiO2 + 2H2Ov

for which

(mKCl/H2O)fld = (fHCl /fH2O)vap . K

Hence the mKCl/mH2O of the brine will increase with increasing partial pressure of HCl in the system. This will yield anincrease in mKCl/mNaCl providing mNaCl is constant.

This argument suggests that the high KCl/NaCl and KCl/H2O ratios of the saline fluid inclusions in deposits such as

Endeavour 26N (type D), and Granisle Bell indicate extensive brine-vapor-rock interaction at high partial pressure of HCl.It is possible that gas pressures were generally high in these systems. The linear to sub-linear trends on the NaCl - KCl -H2O ternary for these deposits suggests mixing of ortho-magmatic fluids with brines that evolved within the zone of


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potassic alteration. Mixing of these ortho- and para-components may be an important process in sulfide deposition.

It may be that systems which apparently evolved within the two-feldspar domain with falling temperature and salinity weresystems which lost pressure with cooling. It is likely that an influx of external rock-buffered fluids was associated with anysuch pressure drop and this would also promote evolution within the two-feldspar domain with falling temperature.

Integrating the microscale and the mesoscale data

A6: Summary correlation of

q vein paragenesisq alteration mineralogyq fluid inclusion data

Ortho-magmatic systems

It is possible to correlate the fluid inclusion data with the vein paragenesis and alteration assemblages. Deposits whichhave the M-vein stage (i.e. show early plagioclase alteration with magnetite, biotite, amphibole pyroxene ) transitionalto A/B include Park Premier, Panguna, Endeavour 26N, and Granisle Bell. The fluid inclusion data for these deposits alsoshow the characteristic convergence to sodic, saline fluids at high temperature.

It may be inferred that the saline brines which formed the M-vein stage separated from the silicate melt prior to saturationin alkali-feldspar and most probably prior to quartz saturation also. Quartz phenocrysts in porphyries associated with thisstage are uncommon in the deposits quoted. The primitive brines were probably cooled and de-pressurized prior toformation of the quartz veins; their high salinities may reflect the P/T conditions of fluid-melt equilibrium rather than P/Tconditions of trapping of the fluid in quartz.

The potassic alteration assemblage (K-feldspar biotite) associated with A and B veins developed by cooling ofhydrothermal fluids separated from melt saturated in both plagioclase and alkali-feldspar. Cooling promotes the


17/17

precipitation of K-feldspar at the expense of plagioclase.

The extent of subsolidus reaction at the A/B stage within potassic zones may be largely controlled by gas pressure. Highgas pressure, or at least a high partial pressure of HCl, will enhance reactivity. Maintaining a high partial pressure of H2Sor SO2 may also be important in stabilizing the sulfides at high temperature and in particular promoting the stability ofbornite over chalcopyrite.

Barren stockworks and para-magmatic systems

A loss of gas pressure in a magmatic hydrothermal system is likely to limit reactions of the brine component with therocks of the igneous complex and/or country rocks and may lead to brines becoming undersaturated in sulfides. Systemsin which the pH and the temperature are effectively buffered by the rocks are prone to sulfide undersaturation as theactivity of sulfur in the system drops without a compensating drop in temperature or increase in pH which can occur in anopen vein.

This phenomenon results in sulfide dissolution if it occurs in a column of rock which contains a significant amount of pre-existing sulfide. The textures indicative of sulfide dissolution are not widely recognized but have been noted in cassiterite-sulfide deposits (Halley and Walshe, 1995) and at Porgera (Cameron and Walshe, 1996). The pre-existing sulfide in therock column ensures that the fluids remain saturated in sulfide despite the loss of sulfur from the system. If a drop in gas

pressure occurs prior to sulfide saturation or in rocks that contain only minor amounts of sulfides then the fluids maybecome undersaturated in sulfide. The dispersal of magmatic brine and vapor, rather than the focused release of the twofluids, can lead to the brine becoming unsaturated in sulfides. This phenomenon is probably quite common and largelyunrecognized.

Brines undersaturated in sulfides may give rise to the early barren stockworks (most likely equivalent to B) which occur ina significant number of deposits. Fluid inclusion studies at Bingham, Santa Rita and Sierrita have shown that the barrenstockworks which are associated with potassic alteration contain the highest temperature (Reynolds and Beane, 1985)and highest salinity fluids (Preece and Beane, 1982) within the deposits and work at Bingham (Anderson et al, 1989) hasshown these fluids are metal rich. However, sulfide precipitation is associated with later veins which formed at lowertemperature, from lower salinity fluids (equivalent to C and D veins in the generalized paragenetic scheme). In thesedeposits the key to sulfide precipitation could be the decline in temperature or these degassed brines might be recharged

by in-mixing of sulfur-rich external fluids or by in-mixing of magmatic gases being discharged from deeper parts of themagmatic-hydrothermal system.

The rationale for correlating barren stockworks with B (transitional) veins

A7: Wriggly veins versus the straight-walled veins

Some preliminary observations and interpretations

q A very general indicator of pressure regime.q Wriggly veins are indicative of fluid pressures just equal to confining pressure - probably above hydrostatic but

sublithostatic.q Straight walled veins are indicative of lower confining pressures and veins remain open. Although they are

laminated, they are not crack seal veins and therefore were opened a limited number of times.q The transition from wriggly to straight occurs in the ortho-magmatic systems at the level of the deposit.q In the para-magmatic system this transition occurs somewhere below the level of the deposit.q Differing vein styles may be used as a rough P/T delimiter in magmatic hydrothermal systems as the vein style is

dependent on P/T and independent of chemical reactions within the system.

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