Greenstone-hosted Gold Deposits Research Articles

Multistage gold mineralization events in the Archean Tati greenstone Belt, northeast Botswana: Constraints from integrative white mica Ar/Ar, garnet U-Pb and sulfides Pb/Pb geochronology

The Tati Greenstone Belt (TGB) in northeastern Botswana hosts numerous Au deposits (Shashe, Mupane and Signal Hill) associated with sulfides, garnet and white mica. In this study, integrative white mica Ar/Ar, garnet U-Pb and sulfides Pb/Pb dating techniques were combined with whole rock and sulfide Pb isotope characteristics to track the source(s) of gold and constrain the timeframes of gold mineralization events. All sulfides and arsenopyrite samples from the TGB yielded overlapping Pb/Pb errorchron ages of 2227 ± 66 Ma and 2220 ± 73 Ma, respectively, which coincide with the Shashe sulfides Pb/Pb errorchron age of 2250 ± 110 Ma. These relatively imprecise Pb/Pb dates suggest heterogeneity in the initial Pb isotope ratios of sulfides. At Mupane, whereas Au mineralization-associated hydrothermal almandine garnet yielded overlapping Tera-Wasserburg lower intercept 206Pb/238U age and a concordia age of 2119 ± 18 Ma (MSWD = 1.03) and of 2105 ± 24 Ma (MSWD of concordance = 1.10), respectively, sulfides produced an errorchron Pb/Pb age of 2873 ± 140 Ma, which despite the large error remains geologically meaningful as it coincides within error with the first Neoarchean Limpopo-Liberian Orogeny (2.70–2.65 Ga), granitoids intrusion emplacement (2.65–2.73 Ga) and deposition of banded iron formation (2.73 ± 0.15 Ga). Ore-related white mica from Signal Hill yielded an overlapping Ar/Ar plateau age of 1987 ± 24 Ma and a weighted mean Ar/Ar age of 1987 ± 13 Ma (n = 15, MSWD = 4.3), which coincide within error with the 2.05–1.95 Ga second Limpopo-Liberian tectonic cycle, herein considered to have possibly triggered the Au mineralization in this area. The obtained radiometric dates point to at least three well constrained gold mineralization events, with two of them possibly triggered by two different regional tectonic events. Lead isotope compositions of most of the sulfides overlap with those of spatially associated schists and granitoids, thus suggesting these units possibly represent Pb and by inference Au rock sources. The genetic model of the TGB Au deposits is consistent with many greenstone-hosted gold deposits worldwide, suggesting our results are not solely of local/regional interest, but can be used to characterize greenstone-hosted gold deposits worldwide.

Gold on the Kaapvaal Craton, outside the Witwatersrand Basin, South Africa

Gold occurs in a variety of ancient geological environments in South Africa, the most well-known being the unique Witwatersrand Basin in the central part of the Kaapvaal Craton. Outside of the Witwatersrand Basin significant quantities of gold have been recovered from Archaean greenstone belts of the 3500 to 2700 Ma Kaapvaal Craton as well as from various occurrences in the 2600 to 2200 Ma Transvaal Supergroup. Very little gold has been found in younger formations. Archaean greenstone belts in South Africa are characterised by substantial sequences of ultramafic (komatiitic) volcanic rocks generally overlain by mafic volcanic successions which, in turn, are overlain in some cases by greywacke-shale sequences and younger arenaceous lithologies. Highly sheared and carbonate altered ultramafic zones, together with the immediately surrounding country rock, host some of the most important gold deposits on the Kaapvaal Craton. In the northern part of the Barberton Greenstone Belt, shearing tends to be most intense along the contact of the ultramafic schists of the Zwartkoppie Formation and the overlying greywacke-shales, the contact being marked by the presence of a chert “bar” or banded iron formation (BIF) horizon. Gold mineralisation occurs below, above and/or within the more competent and fractured chert/BIF marker horizon. The locus of gold mineralisation is typically controlled by the rheological characteristics of the wallrocks in the proximity of the shear systems, e.g. the ductile greywacke-shale strata are tightly folded with sub-parallel mineralised shear zones, while competent arenaceous rocks have yielded in a brittle manner resulting in high-angle mineralised fractures. Extensive carbonate alteration in ultramafic rocks is often present along, and in the vicinity of, shear zones where potassium alteration results in a vivid green coloration due to the formation of fuchsite (Cr-mica) and where zones of silicification and quartz veining are closely associated with gold, arsenic and antimony mineralisation. Where mafic rocks act as more competent and fractured hosts to mineralisation, carbonate alteration results in the breakdown of ferromagnesian minerals and the liberation of considerable quantities of silica, manifested as silicification and as mineralised quartz veins. The breakdown of mafic minerals results in “bleaching” of the rocks as carbonate minerals replace ferromagnesian minerals, but may also result in the development of green, brown or black schists depending on the composition of the host rocks. One or more of the above alteration rock types and mineralisation styles is present in virtually all of the greenstone-hosted gold deposits of the Kaapvaal Craton. Wherever available, geochemical data is consistent with alteration fluids of relatively uniform composition but the style of the resultant mineralisation is ultimately dependent on the nature of the host rock. Greenstone gold deposits are irregularly distributed, both in terms of their size and distribution. Only about 1% of greenstone deposits can be regarded as successful mines, while the vast majority remain as showings. Geographically, these gold deposits are preferentially located along litho-structural features where both fluid flow and wallrock conditions were suitable for mineralisation. These criteria are restricted to major structural nodes, significant lineaments, or lithological units, with the result that the stronger mineralising systems tend to be concentrated in finite gold “camps” which host the larger deposits and numerous styles of mineralisation, e.g., the Sheba-Fairview Complex on the refolded arcuate Eureka and Ulundi synclines. Gold hosted in BIFs is a widespread association, but rarely economic except where the nature of the mineralisation allows for economic extraction, e.g., in the Kraaipan Greenstone Belt, where the volume of BIF is large enough to contain the mineralised fracture system. Gold in the Paleoproterozoic Transvaal Supergroup occurs in the lowermost Black Reef Formation quartzites as detrital accumulations in conglomerate channels, but more abundantly as epithermal veins in the dolomite of the overlying Malmani Subgroup and shale of the Pretoria Group as vertical cross-cutting veins and sub-horizontal reefs emplaced along parting planes. The quartz-carbonate-sulphide veins differ from Archaean ores in that gold rarely impregnates the wallrocks; they contain significant carbon which impedes gold recovery, and they contain silver, copper, arsenic, bismuth, lead and zinc in addition to gold.

Genesis of greenstone-hosted gold deposits of India

Genesis of greenstone-hosted gold deposits of India

CRATONIC EXTENSION AND ARCHAEAN GOLD MINERALISATION IN THE SHEBA-FAIRVIEW MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

Gold mineralisation in the world-class Sheba-Fairview mining district in the Barberton greenstone belt, South Africa, occurs along a fracture network that is arranged in overlapping Riedel, P-shear and anti-Riedel arrays on 10m and 100m scales. The mineralised fracture arrays overprint and fold the Sheba shear zone interpreted by previous workers as a major, strike-parallel thrust thought to have controlled mineralisation in the area. Our results indicate that mineralised structures did not result from movement on the Sheba shear zone, which is not a major thrust. Instead the Sheba shear zone is locally overprinted by the mineralised structures and reactivated as a sinistral normal fault. Mineralisation was accompanied by silicification and the emplacement of porphyries, and occurred after the ductile geometry of the greenstone belt was fully established, i.e. after the emplacement of late-tectonic, 3105 Ma potassic granites and associated doming and constrictional folding. Kinematic analysis shows that the network of mineralising fractures formed in an extensional environment with 1 vertical and 3 orientated in a horizontal northwest to southeast direction. The stress regime was predominantly radial extensional, with 3 orientated at right angles to the long axes of the greenstone belt. Greenstone hosted gold deposits such as the Sheba-Fairview deposits have been classified as orogenic gold deposits invoking a genetic link between mineralisation and compressional or transpressional, accretionary thrusts in a late tectonic environment likened to modern volcanic arcs. Our results suggest that mineralisation in the Sheba area formed in an extensional environment, related to structures that developed after cratonisation of the Kaapvaal Craton had occurred; i.e. the Archaean lode gold deposits at Sheba-Fairview mine are probably not related to orogenic processes linked to formation of the greenstone belt.

Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Mesothermal, greenstone-hosted gold deposits are typically products of complex hydrothermal systems that involved multiple fluids at various times throughout their histories. The Giant mine is an example of such an extremely complicated system, including (1) multiple, local and regional gold-depositing events; (2) at least two styles of gold ore introduction in the mine area, including both refractory, sulfide-hosted and free-milling, vein-hosted ores, whose relative timing is enigmatic; (3) fluid overprinting associated with deposition of multiple generations of postore vein- and vug-filling minerals; and (4) postore deformation and recrystallization of ore veins, especially along faults. Three main stages of quartz-carbonate mineralization are recognized in the Giant mine. Stage I encompasses deposition of dominant refractory, sulfide-hosted ores and subordinate free-milling, vein-hosted gold ores. Refractory, metavolcanic rock-hosted orebodies are connected to metasedimentary rocks east of the mine by an east-dipping alteration zone characterized by a depletion in Na and enrichments in K, Ag, As, S, and Sb. Quartz veins within the wall-rock alteration zone have δ 18 O values that decrease systematically from 14.7 per mil in deeper metasedimentary rocks toward 11.6 per mil in shallower metabasalts in the mine. This decrease is interpreted to indicate that 18 O-enriched ore fluids originated in deeper, metasedimentary rocks and reacted extensively with wall rocks along the entire extent of their flow paths before depositing dominantly refractory gold ores within more 16 O-enriched, shallower, Ti-rich tholeiitic metabasalts. Quartz ± carbonate veins related to these gold ores were deposited from H 2 O-CO 2 -NaCl fluids with T h values of 180° to 360°C and salinities of 4 to 9 wt percent NaCl equiv. Evidence of sporadic fluid unmixing indicates that gold was deposited at temperatures near 350°C and pressures of 1 to 2 kbars. The δ 18 O values (SMOW) of vein quartz (11.6–14.7‰) and calcite (8.6–14.1‰) within the ore-related wall-rock alteration zone indicate deposition from fluids ( δ 18 O water = 4.4–9.8‰) that equilibrated with metasedimentary and metavolcanic rocks during greenschist metamorphism. The δ 18 O values (8.6–11.4‰) of vein quartz that locally contains free gold, hosted in metavolcanic rocks outside of the ore-related alteration zone, indicate deposition from fluids with lower δ 8 O water values of 2.8 to 5.6 per mil. The difference in ranges of δ 18 O water values may indicate that multiple events were responsible for gold-bearing quartz veining, or alternatively, that fluids depositing veins within and outside of the alteration zone represented distinct fluid reservoirs that evolved through reaction with isotopically distinct rocks in their source regions (i.e., 18 O-enriched metasedimentary vs. 16 O-enriched metavolcanic rocks). Postore (stage II) carbonate veins associated with minor Pb-Zn-Sb-Ag mineralization were deposited from highly saline NaCl-CaCl 2 brines with T m values of –32° to –36°C and T h values of 72° to 273°C. The δ 18 O values of these carbonates (20.6–26.1‰) indicate that their parent brines ( δ 18 O water = 9–14‰) also equilibrated with metasedimentary and metavolcanic rocks. Subsequent dissolution of these carbonate veins created abundant vuggy porosity. Late (stage III), primarily vug-filling dolomite ± stibnite, overgrown by scalenohedral calcite, was deposited from dilute fluids ( h values of 95° to 115°C. The δ 18 O values of dolomite (17.4 toward 13.4‰) and latest calcite (10.9‰) indicate deposition from progressively less evolved meteoric waters with decreasing δ 18 O values from 1.1 toward –6.9 per mil. Refractory ores in the main alteration zone and gold-bearing quartz veins contained therein formed from chemically similar CO 2 -H 2 O-NaCl ore fluids whose oxygen isotope compositions are consistent with a metasedimentary source. These two styles of gold mineralization appear to be part of the same mineralizing event in which the style of mineralization was dictated by the mechanism of ore deposition. Refractory, sulfide-hosted gold mineralization resulted from reaction of mineralizing fluids with metavolcanic wall-rock Fe 2+ . Free-milling, gold-bearing quartz vein ores were deposited as a result of fluid unmixing, with loss of H 2 S to the vapor phase. Gold-bearing quartz veins outside of the main wall-rock alteration zone in the Giant mine were also deposited by unmixing of similar H 2 O-CO 2 -NaCl fluids. However, these fluids were isotopically distinct from those within the alteration zone and may not be part of the refractory, sulfide-hosted gold-depositing event. They may instead represent a separate mineralizing event whose fluids and ore-forming constituents were derived solely from within metavolcanic rocks. This overprinting of one event on the other may have been a necessary condition for the development of world-class gold deposits in the Yellowknife district. The important roles of both metavolcanic and metasedimentary source rocks may explain why some smaller greenstone belts, with limited volumes of metavolcanic rocks, can host substantial economic gold mineralization and has important implications for regional resource evaluation and exploration for gold deposits in greenstone belts.

Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Application of airborne geophysical data to mineral exploration in the uneven exposed terrains of the Rio das Velhas greenstone belt

The highly prospective nature of the Rio das Velhas Greenstone Belt is known since the end of the last century. Archean greenstone-hosted gold deposits represent a distinctive group with many common characteristics. The geophysics approach employed concentrated on processing and enhancing the geophysical data to accurate positioning of geological boundaries. The rationale was that in exploration, we should not be looking for geophysical anomalies, but the responses related to mineralization, lithology, and structure that may have economic importance. Aeromagnetics identifies magnetic greenstone units, important structures related to mineralization and allowed a better understanding of sctrutural geophysical pattern. The radiometric data was an excellent tool for mapping and trace individual lithological units in the in areas of outcrop. The most important host rocks or mineralized domains show high conductivity response. There is a strong correlation of these domains with the mapped Morro Vermelho, Quebra Osso and Ouro Fino Formations. Such domains host the known gold mineralization, illustrating the utility of the EM data to improve the geologic map.

APPLICATION OF AIRBORNE GEOPHYSICAL DATA TO MINERAL EXPLORATION IN THE UNEVEN EXPOSED TERRAINS OF THE RIO DAS VELHAS GREENSTONE BELT

The highly prospective nature of the Rio das Velhas Greenstone Belt is known since the end of the last century. Archean greenstone-hosted gold deposits represent a distinctive group with many common characteristics. The geophysics approach employed concentrated on processing and enhancing the geophysical data to accurate positioning of geological boundaries. The rationale was that in exploration, we should not be looking for geophysical anomalies, but the responses related to mineralization, lithology, and structure that may have economic importance. Aeromagnetics identifies magnetic greenstone units, important structures related to mineralization and allowed a better understanding of sctrutural geophysical pattern. The radiometric data was an excellent tool for mapping and trace individual lithological units in the in areas of outcrop. The most important host rocks or mineralized domains show high conductivity response. There is a strong correlation of these domains with the mapped Morro Vermelho, Quebra Osso and Ouro Fino Formations. Such domains host the known gold mineralization, illustrating the utility of the EM data to improve the geologic map.

Alteration geochemistry and fluid inclusion characteristics of the greenstone-hosted gold deposit of Hutti, Eastern Dharwar Craton, India

Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350–477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9–13.5 wt% NaCl equivalent) H2O–CO2 rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P–T values range from 1.0 to 1.7 kbar at 280–320 °C. These data, along with the alteration assemblages and the characteristic gold–sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P–T conditions of formation of gold–quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45–1.7 kbar/240–270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton.

Geological setting and SHRIMP U-Pb geochronological evidence for ca. 2680-2660 Ma lode-gold mineralization at Jundee-Nimary in the Yilgarn Craton, Western Australia

The 7 million oz. Jundee–Nimary lode-gold deposit occurs in the northern portion of the Yandal greenstone belt in the northeastern part of the Archean Yilgarn Craton of Western Australia. Gold mineralization at Jundee–Nimary is similar in structural style, mineralogy, geochemistry and relative timing with respect to deformation and metamorphism, to other Western Australian Archean greenstone-hosted gold deposits, but is unusual in the fact that mineralized structures are crosscut by structurally late intermediate to felsic dykes. Within the Deakin South open cut, gold mineralization is hosted in brittle–ductile shear zones primarily developed within the dacitic Mitchell Porphyry. The Moore Porphyry, a broad dyke of porphyritic granodiorite, intrudes the Mitchell Porphyry, crosscutting and post-dating gold mineralization. Analytically indistinguishable SHRIMP U–Pb zircon ages of 2678 ± 5 Ma for the Mitchell Porphyry and 2669 ± 7 Ma for the Moore Porphyry require that gold mineralization at Jundee–Nimary occurred at ca. 2680–2660 Ma, approximately 40 million years earlier than the majority of published robust ages for gold mineralization in the Yilgarn Craton, which mostly overlap at ca. 2640–2630 Ma. The close spatial and temporal relationship between gold mineralization and felsic to intermediate magmatism at Jundee–Nimary also raises the possibility of a genetic link between hydrothermal and igneous activity. However, additional work is required to establish a firm connection. Current research on lode-gold mineralization in Archean, Paleozoic and Phanerozoic terranes suggests a model which postulates that these deposits formed during transpressional to compressional deformation in accretionary and collisional orogens and that their formation is intimately related to orogenic processes. Consequently, mineralization and regional metamorphism are expected to be diachronous, as terranes are accreted and the front of orogenesis migrates. Consideration of the new data presented in this paper in conjunction with previously published dates supports the hypothesis that gold mineralization, along with regional metamorphism, was generally diachronous from northeast to southwest across the Yilgarn Craton, over a period of approximately 40 million years from ca. 2680–2660 Ma to ca. 2640–2630 Ma. This is directly analogous to the accepted model for the timing of orogenic lode-gold mineralization in other provinces and therefore provides further support for a unified model for this style of deposit through geological time.

Metasedimentary influence on metavolcanic-rock–hosted greenstone gold deposits: Geochemistry of the Giant mine, Yellowknife, Northwest Territories, Canada

Research Article| January 01, 1999 Metasedimentary influence on metavolcanic-rock–hosted greenstone gold deposits: Geochemistry of the Giant mine, Yellowknife, Northwest Territories, Canada Edmond H. P. van Hees; Edmond H. P. van Hees 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Kevin L. Shelton; Kevin L. Shelton 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Todd A. McMenamy; Todd A. McMenamy 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Louis M. Ross, Jr; Louis M. Ross, Jr 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Brian L. Cousens; Brian L. Cousens 2Department of Geological Sciences, Carleton University, Ottawa, Ontario K1S 5B6, Canada Search for other works by this author on: GSW Google Scholar Hendrik Falck; Hendrik Falck 3Royal Oak Mines Inc., P.O. Bag 3000, Yellowknife, Northwest Territories X1A 2M2, Canada Search for other works by this author on: GSW Google Scholar Malcolm E. Robb; Malcolm E. Robb 3Royal Oak Mines Inc., P.O. Bag 3000, Yellowknife, Northwest Territories X1A 2M2, Canada Search for other works by this author on: GSW Google Scholar Tim W. Canam Tim W. Canam 3Royal Oak Mines Inc., P.O. Bag 3000, Yellowknife, Northwest Territories X1A 2M2, Canada Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (1): 71–74. https://doi.org/10.1130/0091-7613(1999)027<0071:MIOMRH>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edmond H. P. van Hees, Kevin L. Shelton, Todd A. McMenamy, Louis M. Ross, Brian L. Cousens, Hendrik Falck, Malcolm E. Robb, Tim W. Canam; Metasedimentary influence on metavolcanic-rock–hosted greenstone gold deposits: Geochemistry of the Giant mine, Yellowknife, Northwest Territories, Canada. Geology 1999;; 27 (1): 71–74. doi: https://doi.org/10.1130/0091-7613(1999)027<0071:MIOMRH>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Giant mine is a mesothermal, greenstone-hosted gold deposit that has produced ∼250 metric tons of gold, principally from sulfide ores in altered metavolcanic rocks. Previous studies concluded that mineralizing fluids acquired metals and other ore-forming components from within the ore-hosting metavolcanic rocks and ascended a steep-dipping shear zone to the site of ore deposition. Our studies indicate that although the metavolcanic host rocks were important geochemically in the precipitation of gold, extensive metasedimentary rocks to the east were a more important conduit and/or source of fluids, metals, and ore-forming constituents. Geochemical analyses reveal an east-dipping Na depletion zone extending from the ore zone to within the metasedimentary sequence that coincides with enrichments in Ag, As, S, and Sb and with δ180quartz values of 11.7‰ to 14.1‰. These data indicate that wall-rock–hosted gold mineralization was deposited where fluids emerging from metasedimentary rocks encountered highly reactive Ti-rich tholeiitic basalts. From a geochemical standpoint, this ore system represents a metasedimentary-type gold deposit hosted in metavolcanic rocks. Documentation of a metasedimentary influence on formation of the minerals of the Giant mine helps explain why smaller greenstone belts can host substantial economic gold mineralization and has important implications for exploration for giant (>150 t) gold deposits. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The classification of Western Australian greenstone-hosted gold deposits according to wallrock-alteration mineral assemblages

The epigenetic, greenstone-hosted gold deposits in the Archaean Yilgarn Block of Western Australia can be classified according to their silicate-carbonate gangue, which forms the innermost wallrock-alteration zone of the structurally controlled hydrothermal systems. The proposed classification is descriptive, and emphasizes associations of prominent, macroscopically visible minerals. The gold deposits are divided into two large groups: high-temperature deposits (400°–700°C) formed at pressures of 3–5 kbars in the lower parts of the greenstone belts (10–15 km depth), and medium-temperature deposits (250°–400°C) formed at pressures of 1 to 2 kbars in the upper parts of these belts (3–7 km depth). The high-temperature deposits are usually located in metamorphic terranes of medium grade (amphibolite facies), and are characterized by either microcline-muscovite-andalusite, garnet-pyroxene-biotite or amphibole-biotite-calcite alteration in the ore zones. Deposits showing the latter two alteration styles may also be classified as skarns in the descriptive sense of Einaudi and Burt (1982). The classic medium-temperature (mesothermal) deposits, studies of which form the basis of all current models on Archaean gold metallogenesis, represent only about one half of the total population of deposits in the Yilgarn Block, although they dominate by total gold production, as they include the only giant deposit in Western Australia, i.e., the Golden Mile at Kalgoorlie. The medium-temperature deposits are usually located in metamorphic terranes of low grade (greenschist facies), and are characterized by biotite-ankerite-albite or sericite-ankerite-albite alteration in the ore zones. It is possible that some of these deposits grade into amphibole-biotite-calcite skarns at depth.

Strontium isotope systematics of hydrothermal minerals from epigenetic Archean gold deposits in the Yilgarn Block, Western Australia

Strontium isotope compositions are presented for samples of hydrothermal scheelite and tourmaline from one granitoid-hosted and 14 greenstone-hosted Archean gold deposits in the Yilgarn block, Western Australia. The direct measurement of the 87 Sr/ 86 Sr ratios of Rb-poor minerals such as scheelite and tourmaline, which show an intimate paragenetic relationship to gold, allows an accurate estimate of the local strontium isotope composition of the Archean gold-bearing hydrothermal fluids. The population of greenstone-hosted gold deposits is sub-divided according to gangue mineralogy into gold skarns with garnet-pyroxene-biotite and amphibole-biotite-calcite wall-rock alteration, which formed in the lower parts of the green-stone belts at depths of 10 to 15 km, and medium-temperature gold deposits with biotite or sericite-ankerite-albite wall-rock alteration, which formed in the upper parts of the belts at depths of 3 to 7 km.The 87 Sr/ 86 Sr ratios of the fluids vary from 0.7020 to 0.7035 in skarns with garnet-pyroxene-biotite alteration, from 0.7020 to 0.7043 in skarn-quartz orebodies with amphibole-biotite-calcite alteration, and from 0.7014 to 0.7028 in the medium-temperature gold deposits. The variations in the strontium isotope composition of scheelite from the same mining district are interpreted to reflect mixing between radiogenic strontium, introduced by the hydrothermal fluid, and nonradiogenic strontium released from the wall rocks during interaction with this fluid. The greenstone host rocks of all deposits, mainly metamorphosed ultramafic or mafic volcanic rocks and their intrusive equivalents, are characterized by the low initial 87 Sr/ 86 Sr ratio (0.7007-0.7012) and low Rb/Sr ratio of the Archean mantle. On the other hand, the initial 87 Sr/ 86 Sr ratios of granodiorite and granite plutons in the batholiths, which underlie and border the greenstone belts, vary from 0.7023 to 0.7044. The strontium isotope data indicate that the deep-seated gold skarns formed predominantly from fluids which evolved from or interacted with anatectic magmas in the granitoid batholiths. The data do not as clearly constrain the origin of the fluids in the higher level, medium-temperature hydrothermal systems. They are, however, inconsistent with models involving predominantly metamorphic fluids generated by the devolatilization of the lower parts of the greenstone belts.