Au Province Research Articles

The role of an oxidized lithospheric mantle in gold mobilization.

Phanerozoic orogenic gold mineralization at craton margins is related to the metasomatism of the lithospheric mantle by crustal material. Slab subduction transfers Au from the crust to the metasomatized mantle and oxidizes the latter to facilitate the mobilization of Au into mantle melts. The role of volatiles in the mobilization of Au in the mantle is unclear because of the absence of direct geochemical evidence relating the mantle source of Au to Au mineralization in the overlying crust. This study uses lithium isotopes from a large suite of lamprophyres to characterize the mantle beneath the eastern North China Craton, which hosts giant Mesozoic gold deposits. Our results indicate a strong genetic link between carbonate metasomatism in the mantle and Au mineralization in the overlying crust. Although pre-enrichment of Au in the mantle is critical for forming giant Au provinces, the oxidation of the lithospheric mantle controls the mobilization of Au.

Fuzzy c-mean (FCM) integration of geophysical data from an iron-oxide copper gold (IOCG) deposit under thick cover

Geophysical methods depend on a range of physical and chemical mechanisms, and each method has different sensitivities and resolution of Earth parameters, and over different scale lengths. Additionally, such geophysical methods will also have their own statistical characteristics. Thus, it is a significant challenge to combine different methods through a single inversion framework. Rather than attempting to find an optimal and single model for different geophysical responses, an alternative approach is to use FCM clustering to identify clusters of parameters from two or more different geophysical data sets or models that have similar statistical properties. In this paper, we apply the FCM approach to integrate data and model sets for an array of 100 broadband magnetotelluric (MT) and 100 passive seismic receivers spaced 1 km apart on a 10 by 10 grid above the Vulcan IOCG prospect in the Olympic Cu–Au Province, southern Australia. The challenge for exploration of the Vulcan prospect is that it lies beneath 750 m of regolith sedimentary cover, no single geophysical method provides a unique characterization of the deposit geometry, and drilling is very expensive. Fuzzy c-mean cluster analyses are undertaken in 2D for gravity data and shear-wave velocity model data for basement depths beneath cover and in 3D for shear-wave velocity and electrical resistivity model data. Fuzzy c-mean clustering is shown to provide a simple and efficient method of integrating different geophysical measurements to produce a geological framework that can be verified with drilling information.

Magmatic and hydrothermal evolution of the superlarge Dashui goldfield in the West Qinling Orogen, China

Gold (Au) is a precious metal that has played an important role in the development of the global economy. It is therefore important to better define the genesis of Au mineralisation as an aid to focus exploration as the consumption of Au increases. The Dashui goldfield contains a super large Au resource of over 120 t (tonnes) in the West Qinling Orogen, central China. Despite several studies have focused on the genesis of the Dashui goldfield, the original physico–chemical natures of magmatic and hydrothermal evolution remain unclear. Biotite and apatite in the mineralised diorite porphyry of the goldfield are sensitive to the changes in the physico–chemical conditions and the evolution of magmatic and hydrothermal fluids, and thus provide important keys to address these issues. Hence, we conducted detailed in–situ major and trace element studies of biotite and apatite from the ore–bearing diorite porphyry in the Dashui goldfield. Our aim is to elucidate the physico–chemical conditions, magma source, and magmatic and hydrothermal evolution processes during the deposition of the gold, and to enhance the understanding of the Au genesis in the West Qinling Au province. Using the geochemistry of magmatic apatite, and magmatic and hydrothermal biotite, we have found that the crystallisation temperature and pressure along with emplacement depth of diorite porphyry were constrained at ∼ 737–791 °C, 1.08–1.73 kbar, and 3.94–6.30 km. The diorite porphyry has the geochemical affinities with calcium alkaline orogenic suites derived from a mixed crustal and mantle source. During the early emplacement of diorite porphyry, the melt had a high oxygen fugacity and was highly enriched in Au. During the late magma crystallisation of diorite porphyry, the magma was enriched continuously in most elements with the Ba/Rb ratios in the biotite varying from 10 to 5, and most of the Au was hosted by pyrite. Additionally, elements were continuously dissolved into fluids generating Cl–rich hydrothermal fluids during the evolution of the magma. During hydrothermal evolution, Au is migrated with As and Sb in the vapour phases of fluids, whereas Au migrated with Cl in the liquid phase. When temperatures dropped to ∼ 200–400 °C, Au started to combine with S and Si in the liquid phase of fluids. Subsequently, hydrothermal fluids interacted with host carbonate to form the Dashui goldfield, Au during this stage was also transported and hosted in magnetite, hematite, and quartz within the carbonate rocks.

Linking Pacific Plate formation and Early Cretaceous metallogenic response on the circum-Pacific continental margins

Hydrothermal mineralization along the circum-Pacific continental margin is genetically linked to the motions of the Pacific Plate. Compiled geochronological results of ore deposits, coupled with the newly reconstructed geometry of the late Early Cretaceous subduction zones in East China, North America, and the Central Andes using GPlates and I2VIS software, were applied to investigate the relationships between mineralization and Pacific Plate formation. The ∼120-m.y.-old orogenic Au provinces in East China and North America are related to transpression caused by high-rate oblique subduction with intermediate−high dip angles of the Izanagi and Farallon plates, respectively. In contrast, the ∼105-m.y.-old porphyry-epithermal belt in Southeast China was produced by oblique subduction of the Izanagi Plate with low−intermediate subduction rates and intermediate−high dip angles. In the Central Andes, the oblique subduction of the Farallon Plate with low−intermediate rates and low dip angle accounted for iron oxide−copper−gold ore (IOCG) deposit mineralization in South Peru at ca. 110 Ma; whereas the high rate and low dip-angle subduction of the paleo-Phoenix Plate, which caused mild compression, was responsible for Fe, porphyry Cu, and IOCG mineralization in North Chile at ca. 110 Ma. The late Early Cretaceous metallogenic response in the circum-Pacific region coincides with superplume events that triggered the significant growth of the modern Pacific Plate. The forward simulations reveal that different subsequent styles of subduction and associated magmatism are likely responsible for the distinct mineralization types present in the region, including orogenic Au, porphyry-epithermal, Fe, and IOCG deposits. The precise dynamics of the subduction zone determined by this study led to the improved metallogenesis models in the Pacific margin.

Osmium isotopes fingerprint mantle controls on the genesis of an epithermal gold province

Abstract The formation of crustal blocks enriched with gold (Au) deposits above subduction zones is intimately bound to the genesis and evolution of magmatic-hydrothermal systems. A long-standing question, however, is whether the metal fertility of these systems stems from distinct sources that are anomalously enriched in Au or from subsequent processes occurring during crustal magma emplacement and hydrothermal activity. The Deseado Massif auriferous province in southern Patagonia (Argentina) is a unique place to test these contrasting hypotheses because Au-bearing mantle xenoliths indicate the presence of an underlying Au-rich lithospheric mantle reservoir. However, direct geochemical links between the Au-rich mantle source and the formation of the Deseado Massif auriferous province in the overlying crust remain to be established. To address this prominent gap in knowledge, we used sulfide Re-Os geochronology to identify the source of Au at Cerro Vanguardia, the largest low-sulfidation epithermal Au-Ag deposit in the Deseado Massif. Pyrite from high-grade Au quartz veins yielded an isochron age of 147.4 ± 2.9 Ma (mean square of weighted deviates = 1.04, n = 8) and an initial 187Os/188Os ratio of 0.26 ± 0.01, fingerprinting a dominant mantle control for the source of Os and, by inference, the source of Au. Our data provide a unique geochemical linkage between an Au-rich subcontinental lithospheric mantle source and the genesis of epithermal Au deposits, supporting the hypothesis that pre-enriched mantle domains may be a critical factor underpinning the global-scale localization of Au provinces.

Heavy δ26Mg values in carbonate indicate a magmatic-hydrothermal origin of Carlin-type Au deposit

The Carlin-type Au deposits in Youjiang Basin, SW China, (referred to as Guizhou Carlin-type Au deposits) are the second largest Carlin-type Au province in the world after Nevada, USA. To date, the source of ore fluids that formed the Guizhou deposits remains controversial, hampering the formulation of a genetic model. Compared to Nevada Carlin-type Au deposits, a significant difference is that the Guizhou Carlin-type Au deposits contain abundant ore-stage dolomite in the ore. Herein, we present carbonate Mg isotopes combined with C-O isotopes from the giant Shuiyindong deposit to provide new insights into the source of ore fluids and to constrain the ore genesis of the Guizhou Carlin-type Au deposits.Petrographic observation shows that from least altered bioclastic limestone to high-grade ore, dolomite increased significantly, suggesting that dolomite formed as part of the Au mineralization process. Chemical analyses reflect a small amount of Mg in the ore fluids was variably added to ore during Au mineralization, and this added Mg was fixed in the dolomite. Although most ore-stage dolomite formed from the sulfidation of Fe in Fe-dolomite, some ore-stage dolomite formed by the combination of host rock calcite with Mg from ore fluids.The δ26Mg values of carbonate, primarily dolomite, range from −3.49‰ to −0.07‰, with a median value of −1.01‰. The δ18O values range from 21.7‰ to 27.6‰, with a median value of 23.2‰. The δ13C values vary from −14.6‰ to 1.2‰, with a median value of −0.7‰. The δ26Mg values exhibit a positive correlation trend with MgO, S, SiO2, and Carlin-suite elements (Au, As, Hg, Sb, Tl, and Cu), and a negative correlation trend with CaO. Comparison of Mg isotopes data from this study with the major Mg reservoirs indicates that the Shuiyindong samples contain heavier Mg isotopes that drifts towards the heavy Mg reservoirs (magmatic and metamorphic rocks) compared with normal carbonate rocks. Two mechanisms, namely heavy Mg addition from ore fluids and isotopic exchange reaction with heavy Mg-rich ore fluids, could have attributed to heavy Mg in carbonates in the Shuiyindong deposit.The Mg isotopes results, combined with other isotopes (e.g., S, Hg, and He-Ar) and geologic evidence, support a magmatic-hydrothermal origin for the ore fluids. The thick Devonian-Triassic sedimentary sequence and weak extension following sedimentation prevented igneous activity from reaching the surface. However, buoyant ore fluids released from the deep-seated intrusions are interpreted to have ascended along basement-penetrating faults and eventually produced the Guizhou Carlin-type Au deposits at shallow levels. This study implies that Mg isotopes are a novel proxy to infer ore fluids source and evolution, and that they can provide significant constraints on the genesis of hydrothermal deposit.

Gold endowment of the metasomatized lithospheric mantle for giant gold deposits: Insights from lamprophyre dykes

The metasomatized sub-continental lithospheric mantle (SCLM) is increasingly proposed to play a key control in the formation of giant gold (Au) deposits. However, the extent of Au enrichment in metasomatized SCLM and the Au contents of its derivative partial melts remain poorly constrained. Lamprophyres are derived from low-degree melting of metasomatic and fusible components in the SCLM, and could provide fundamental information about the metasomatized source and metal release compared to mantle peridotites and/or pyroxenites. The giant Jiaodong Au province (> 5000 tons, ∼ 120 Ma) in the eastern part of the North China Craton (NCC) is an ideal locality to assess the Au endowment of metasomatized SCLM which may serve as the source of Au, as indicated by the presence of mantle-dominated volatiles in auriferous fluids associated with the deposits. Based on detailed petrological studies, we analyzed Au, S, Cu and platinum-group element (PGE) contents of the SCLM-derived lamprophyres that are temporally (130–121 Ma) and spatially associated with the Jiaodong lode Au deposits. The lamprophyre dykes were emplaced in the deep crust and did not experience significant degassing and sulfide segregation, and the sulfide saturation occurred at a late stage during magma cooling, as demonstrated by sulfide petrography, S contents, Cu/S and Cu/Pd ratios of the lamprophyres. The relatively oxidized (ΔFMQ +1 – +2), hydrous (2–4 wt.% H2O) lamprophyres display variable and generally low Au contents (0.04–3.59 ng/g, mainly 1–3 ng/g, n = 31), as well as low PGE and Cu contents. Their high Au/Cu(N) and Au/Pd(N) ratios together with high Ba/Nb indicate a discernible Au addition to their source, resulting from multi-stage mantle metasomatism related to subduction. However, the average Au contents of the metasomatized SCLM before large-scale Au mineralization are heterogeneous and overall in a range of 0.5–1.5 ng/g. The values are higher than that of depleted SCLM (< 0.5 ng/g) but are still lower than or similar to the fertile upper mantle (1.2–1.7 ng/g). These results indicate that mantle metasomatism related to subduction replenished some amount of Au into previously depleted SCLM, leading to local or regional Au enrichment. However, such processes evidently did not result in remarkable Au enrichment of the mantle source beneath the eastern part of the NCC. We thus propose that the metasomatized SCLM and magmas derived from it, though without anomalously high Au contents, could still provide large amounts of Au for large-scale Au mineralization. Importantly, metasomatic volatiles from the metasomatized SCLM exert a key role, and they enable efficient release of Au from mantle sources, promote Au transportation along trans-lithospheric fault channels and subsequent enrichment and precipitation via magmatic-hydrothermal processes for giant Au deposits.

Tectono-magmatic controls on decratonic gold deposits

Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.

Texture, geochemistry, and geochronology of titanite and pyrite: Fingerprint of magmatic-hydrothermal fertile fluids in the Jiaodong Au province

AbstractThe Au mineralization in the giant Jiaodong Au province is enigmatic and difficult to fit current classic mineralization models, primarily because of uncertainties as to the sources of ore-forming fluids and metals. The ca. 120 Ma Au mineralization has been previously proposed to have occurred during a magmatic lull, which would negate a magmatic-hydrothermal genetic model. However, recent drilling has revealed a buried mineralized monzonite equivalent in age to the Au mineralization in the Linglong goldfield. Here, we present comprehensive textural, geochemical [LA-(MC)-ICP-MS trace element, Nd and S isotopes] and geochronological (LA-ICP-MS U-Pb dating) analyses of titanite and pyrite from this previously unrecognized monzonite. Three types of titanite were distinguished, including magmatic Ttn1 and hydrothermal Ttn2 and Ttn3, which show indistinguishable U-Pb ages (120.7 ± 3.1 and 120.9 ± 2.6 Ma), REE patterns and Nd isotopes [εNd(t) = –14.7 to –12.9], implying that hydrothermal fluids were directly exsolved from the monzonitic magma, contemporaneous with the large-scale Au mineralization at ca. 120 Ma. The Nd isotopes of titanite potentially indicate a lower crustal source mixed with mantle materials for the monzonite. Four types of pyrite were analyzed, including magmatic Py1 from fresh biotite monzonite, hydrothermal Py2 from altered biotite monzonite, hydrothermal Py3 from quartz-pyrite veins with a monazite U-Pb age of 118.2 ± 4.6 Ma, and magmatic Py4 from mafic enclaves of the Gushan granite at ca.120 Ma. The δ34S values of magmatic Py1 and Py4 (+1.9 to +6.3‰, and +5.0 to +6.4‰, respectively) and hydrothermal Py2 and Py3 (+6.4 to +9.5‰ and +6.5 to +7.6‰, respectively) are consistent with sulfur isotopic fractionation between melt and fluid. Hydrothermal Py2 and Py3 also have higher Co, As, Ag, Sb, and Bi contents and submicrometer gold inclusions, implying that the magmatic-hydrothermal fluids were fertile for mineralization. This study highlights the importance of monzonite magmatism and exsolved fertile fluids in regional Au mineralization. Hydrous magmas at ca.120 Ma probably extracted Au efficiently from the lower crustal-mantle sources and released auriferous fluids at the late magmatic stage, leading to the formation of Au deposits in the Jiaodong province.

The Qiyugou Au orefield — An intrusion-related gold system in the Eastern Qinling ore belt, China: Constraints from SIMS zircon U-Pb, molybdenite Re-Os, sericite 40Ar-39Ar geochronology, in-situ S-Pb isotopes, and mineralogy

The Eastern Qinling ore belt is the second largest Au province in China and is estimated to contain more than 1300 metric tons of Au. Most of the gold deposits in the belt are vein-type (fracture-hosted Au). Whether they are of metamorphic origin or magmatic-hydrothermal origin is controversial. The Qiyugou Au orefield developed granite-hosted Au, breccia pipe-hosted Au, and fracture-hosted Au, and the present study investigates these three types of gold mineralization. New zircon U-Pb and molybdenite Re-Os ages, along with previous published data, are used to constrain the ages of the three types of Au at about 131 Ma, corresponding to the emplacement age of hornblende monzogranite. S-Pb isotope compositions, telluride and Bi-sulfosalt minerals, hematite, alteration and metal zonation in the three types of Au suggest a common oxidized magmatic-hydrothermal origin. These results suggest an intrusion-related Au system in the Qiyugou Au orefield. This model confirms the magmatic-hydrothermal origin for the fracture-hosted Au, and provides important implications for further regional exploration.

Triassic gold-silver metallogenesis in Qingchengzi orefield, North China Craton: Perspective from fluid inclusions, REE and H–O–S–Pb isotope systematics

The Qingchengzi Au–Ag orefield is a key Au province in the North China Craton. The fault-controlled orebodies are hosted in the Paleoproterozoic meta-sedimentary rocks of the Dashiqiao and Gaixian Formations. We present new bulk-ore REE, fluid inclusion (FI) microthermometric and mineral H–O–S–Pb isotopic data of these Au–Ag deposits, and discuss their ore fluid/material source and metallogenesis. Our FI microthermometric data indicate that the ore fluids are of medium temperatures (260–310 °C), low salinity (≤10 wt% NaCl eq.), and belonged to a H2O–CO2–NaCl system. Hydrogen (δD = –6.5 to 9.1‰) and oxygen (δ18O = –139.4 to –90.1‰) isotopes of the hydrothermal quartz indicate that the early-stage ore-forming fluids were mainly magmatic sourced. The δ34S values (–12.5 to 18.7‰) of the ore sulfides overlap with the Paleoproterozoic ore host (δ34S = –0.5 to 13.2‰) and the Late Triassic granites (δ34S = 5.6–7.6‰). Lead isotope compositions of the ore sulfides (206Pb/204Pb = 17.269–18.334; 207Pb/204Pb = 15.335–15.883; 208Pb/204Pb = 37.254–38.784) also support a metallogenic link with the granites and wall-rocks. Therefore, we suggest that the Qingchengzi Au–Ag deposits are best classified as magmatic-related medium-temperature hydrothermal lode type, albeit spatial correlations with magmatic intrusions are not ubiquitous. Closer to the hydrothermal centers, the higher-temperature, magmatic-dominated fluids were responsible to generate the Au(-Ag) mineralization, whereas the more distal, lower-temperature magmatic-meteoric-mixed fluids may have formed the Ag and Pb–Zn mineralization. The ore-forming fluids and materials were likely sourced mainly from the Triassic granitoids, although significant contributions of ore metals and sulfur from the Au-/Ag-rich Dashiqiao and Gaixian Formations should not be neglected. Considering the regional tectonic setting, the Late Triassic Qingchengzi Au–Ag polymetallic mineralization may have occurred during the post-collisional extension after the Yangtze-North China continent–continent collision.

Carlin-style gold province linked to the extinct Emeishan plume

Carlin-style gold deposits constitute one of the most important types of Au resources in the world. Previous studies on these deposits correlated the source of Au with metal-enriched sedimentary formations or metasomatized subcontinental lithospheric mantle (SCLM) associated with oceanic subduction. The role of mantle plumes in generating Carlin-style deposits has remained enigmatic due to lack of adequate evidence. Here we investigate the second largest Carlin-style Au province in the world, after Nevada in USA, within the Youjiang basin in the eastern part of the ∼260 Ma Emeishan large igneous province (LIP) of southwestern China. We present a novel model for the genesis of this ore province based on thermodynamic modeling, which envisages that cumulates formed by high degree (over ∼80 wt.%) of crystal fractionation from the Emeishan plume-derived magmas under low oxygen fugacity conditions generated an enriched juvenile lower crust. Our new results from integrated in situ zircon geochronology and oxygen isotopes of mineralized tuffs from the Au deposits hosted in the Emeishan volcanic rocks reveal zircon growth at ∼129–136 Ma and ∼239 Ma with δ18OVSMOW values of +5.9‰ to +6.9‰, suggesting that the gold mineralization events in the Youjiang basin followed the Emeishan plume activity and that the ore-forming fluids were of magmatic origin. In conjunction with evidence from the Emeishan LIP for native gold grains that precipitated in the early fractionation stage under low oxygen fugacity, we propose a model involving juvenile lower crust beneath the eastern Emeishan province as a potential Au reservoir, the reactivation of which by later magmatic-hydrothermal activity associated with oceanic subduction events generated the large auriferous province in the Youjiang basin. We therefore conclude that the crustal magmatic underplate generated by mantle plume has the potential to contribute to the formation of Carlin-style Au deposits, thus offering new insights into the possible link between mantle plumes and various types of Au deposits such as porphyry, sediment-hosted and orogenic Au deposits associated with LIPs.

Geological interpretation of the 2008 seismic reflection, refraction and magnetotelluric data from the northern Eyre Peninsula, Gawler Craton

A deep seismic reflection profile and coincident refraction and magnetotelluric data were acquired across the northern Eyre Peninsula, Gawler Craton in 2008 to enhance the prospectivity of this region for uranium and geothermal energy, by establishing the crustal architecture and regional geodynamic framework. The seismic line crossed several tectonic domain boundaries, including the Kalinjala Mylonite Zone, which is a key structure in the southern and central Gawler Craton. In the vicinity of the seismic section, this mylonite zone dips moderately to the east and appears to be a major crustal-scale fault which cuts through to the Moho; it separates middle to lower crust of differing seismic reflectivity. The changes in seismic character correspond with the boundary between the Olympic Fe-oxide Cu- Au-U Province (host of Olympic Dam and Prominent Hill deposits) in the east, and the Central Gawler Au Province in the west, and also are suggested to limit the extent of the South Australian Heat Flow Anomaly.

Supergene gold enrichment in the Castromil-Serra da Quinta gold deposit, NW Portugal

ABSTRACTSeveral gold deposits hosted mainly by Variscan granites and Precambrian to Palaeozoic metasediments occur in the northwestern part of Portugal. Most of these deposits were mined by the Romans (in the period I BC to II AD) as open pits and surface galleries. The Castromil-Serra da Quinta gold deposit is an important example of such a mined site; it occurs in the Dúrico-Beirã Au province located in the Central Iberian Zone (CIZ) in the western branch of European Variscan belt, mainly on the eastern flank of the Valongo anticline. Open pits and underground galleries at Castromil-Serra da Quinta exploited the gossan formed from the weathering of primary mineralization. The gossan is composed essentially of goethite, scorodite and clay minerals. A recent drilling campaign at Castromil-Serra da Quinta has provided samples of the primary mineralization below the oxidation level. Different modes of gold occurrence are defined based on metallographic studies of both the gossan and drill cores. Gold I occurs encapsulated in primary sulfide minerals, mainly arsenopyrite and pyrite; Gold II is also associated with the main primary sulfides, but occurs along grain boundaries and in microfractures of the sulfides or in associated quartz veins; and Gold III occurs as free gold particles in iron oxides within the gossan. In the gossan samples, it is difficult to distinguish whether the gold particles hosted in oxides correspond to Gold I, Gold II, or both, so these particles are described as Gold I–II and they are commonly surrounded by very much smaller particles of Gold III. Scanning electron microscope (SEM) and electron probe microanalysis (EPMA) data for the different gold particles reveal that Gold I is poorer in Ag (~15.5–39.76%) than Gold II (37.46–51.45%), whereas Gold III corresponds to native gold (&lt;16.11% Ag). Gold III is thought to reflect gold enrichment in the upper level of the deposit, resulting from weathering processes that affected the primary Au (Bi) mineralization.

Does Tungsten Availability Control the Presence of Tungsten In Turbidite-Hosted Orogenic Gold Mineralization? Evidence From the Meguma and Bendigo-Ballarat Terranes

Turbidite-hosted orogenic Au deposits are commonly enriched in W, along with a variety of other trace elements. A mineralogical source for W has recently been shown in the Otago Schist of southern New Zealand (Cave et al. 2016), with detrital rutile in the metasedimentary rocks recrystallizing to metamorphic titanite and making W available to be mobilized from the rock mass. In this study, we investigate the availability of W through prograde metamorphic mineral recrystallization in two additional turbidite-hosted orogenic Au provinces, one containing orogenic Au mineralization with associated subordinate W (Meguma Terrane, Canada), and the other containing orogenic Au mineralization without associated W (Bendigo-Ballarat Terrane, Australia). This was undertaken to assess whether W availability during prograde metamorphism is a key process in controlling the presence of W in turbidite-hosted orogenic Au mineralization. Like the Otago Schist, in both terranes detrital rutile is identified as being the most important host mineral for W in the lowest metamorphic grade rocks, and its prograde metamorphic recrystallization (to ilmenite) makes significant amounts of W available for mobilization (0.65 and 1.85 g of W per tonne of rock from the Goldenville and Halifax groups of the Meguma Terrane, respectively, and 0.16 g of W per tonne of rock from the Castlemaine Group of the Bendigo-Ballarat Terrane). This release of W in the Meguma Terrane is likely the source of W in these orogenic Au deposits. The lack of W in the orogenic Au deposits of the Bendigo-Ballarat Terrane suggests that W availability is not the only process controlling the presence of W minerals in turbidite-hosted orogenic Au mineralization. Alternatively, it might reflect a lower greenschist facies metasedimentary (Castlemaine Group) source for these deposits (i.e., a lower metamorphic grade source than the rutile to ilmenite conversion), as has been previously suggested.

Arsenic and mercury contamination related to historical gold mining in the Sierra Nevada, California

Arsenic (As) is a naturally occurring constituent in low-sulphide gold-quartz vein deposits, the dominant deposit type for lode mines in the Sierra Nevada Foothills (SNFH) gold (Au) province of California. Concentrations of naturally occurring mercury (Hg) in the SNFH Au province are low, but extensive use and loss of elemental Hg during amalgamation processing of ore from lode and placer Au deposits led to widespread contamination of Hg in the Sierra Nevada foothills and downstream areas, such as the Sacramento–San Joaquin Delta and San Francisco Bay. This review paper provides an overview of As and Hg contamination related to historical Au mining in the Sierra Nevada of California. It summarizes the geology, mineralogy, and geochemistry of the Au deposits, and provides information on specific areas where detailed studies have been done in association with past, ongoing, and planned remediation activities related to the environmental As and Hg contamination.

Mesoproterozoic fluid events affecting Archean crust in the northern Olympic Cu–Au Province, Gawler Craton: insights from 40Ar/39Ar thermochronology

ABSTRACTThe Olympic Cu–Au Province, Gawler Craton, is host to the Olympic Dam and Prominent Hill iron oxide–copper–gold (IOCG) deposits. Both of these deposits and the region between the two are covered by Neoproterozoic to Cenozoic sediment, making inferences about prospectivity in this portion of the Olympic Domain reliant on geophysical interpretation and sparse drill hole information. We present new U–Pb zircon sensitive high resolution ion microprobe (SHRIMP) dates from two basement intersecting drill holes in the region between Olympic Dam and Prominent Hill that show bimodal volcanism occurred at 2555 ± 5 Ma, and was followed by intrusion of tonalite at 2529 ± 6 Ma. Laser 40Ar/39Ar dating of biotite and muscovite from the tonalite yields ages around ca 2000 Ma, consistent with slow cooling trends observed in Archean rocks elsewhere in the northern Gawler Craton. Step heating experiments on K-feldspar from the same tonalite yields an age spectrum with older ages around 1740 Ma from the highest temperature steps becoming progressively younger to a minimum of 1565 Ma in the lowest temperature heating steps; this is consistent with either Paleoproterozic cooling to final closure of K-feldspar by 1565 Ma or a reheating event at ca 1565 Ma, with the latter more likely, given the evidence for sub-solidus alteration of the K-feldspar. Sericite within hematite–sericite–chlorite altered portions of the tonalite yield a poorly defined age of ca 1.6 Ga. Taken together the 40Ar/39Ar data providing evidence for a fluid event affecting this region between Olympic Dam and Prominent Hill during the early Mesoproterozoic. Low temperature quartz–carbonate–adularia veins occur in <10 cm wide fractures within basalt in one drill hole in this region. Adularia from these veins yields 40Ar/39Ar ages that span from ca 1.3–1.1 Ga. This age range is interpreted to approximate either the timing of adularia formation during a hydrothermal event or the timing of resetting of the 40Ar/39Ar systematics within the adularia as a result of fluid flow in this sample. This is evidence for a mid-Mesoproterozoic fluid event in the Gawler Craton and necessitates a reconsideration of the long-term stability of the craton, as it appears to have been affected, at least locally, by fluid flow related to a much larger event within the Australian continent, the Musgrave Orogeny.

Age constraints on the hydrothermal history of the Prominent Hill iron oxide copper-gold deposit, South Australia

The Mesoproterozoic Prominent Hill iron-oxide copper–gold deposit lies on the fault-bound southern edge of the Mt Woods Domain, Gawler Craton, South Australia. Chalcocite–bornite–chalcopyrite ores occur in a hematitic breccia complex that has similarities to the Olympic Dam deposit, but were emplaced in a shallow water clastic–carbonate package overlying a thick andesite–dacite pile. The sequence has been overturned against the major, steep, east–west, Hangingwall Fault, beyond which lies the clastic to potentially evaporitic Blue Duck Metasediments. Immediately north of the deposit, these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite–calc–silicate skarn ± pyrite–chalcopyrite. The hematitic breccia complex is strongly sericitised and silicified, has a large sericite ± chlorite halo, and was intruded by dykes during and after sericitisation. This paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation. The breccia complex contains fragments of granitoid and porphyry that are found here to be part of the Gawler Range Volcanics/Hiltaba Suite magmatic event at 1600–1570 Ma. This indicates that some breccia formation post-dated granitoid intrusion. Monazite and apatite in Fe-P-REE-albite metasomatised granitoid, paragenetically linked with magnetite skarn formation north of the Hangingwall Fault, grew soon after granitoid intrusion, although the apatite experienced U–Pb–LREE loss during later fluid–mineral interaction; this accounts for its calculated age of 1544 ± 39 Ma. To the south of the fault, within the breccia, 40Ar–39Ar ages yield a minimum age of sericitisation (+Cu+Fe+REE) of dykes and volcanics of ∼1575 Ma, firmly placing Prominent Hill ore formation as part of the Gawler Range Volcanics/Hiltaba Suite magmatic event within the Olympic Cu–Au province of the Gawler Craton.

The Nucleus Deposit: Superposed Au–Ag–Bi–Cu Mineralization Systems at Freegold Mountain, Yukon, Canada

AbstractIn this paper we present titanite U–Pb (both single crystal CA ID‐TIMS and in situ LA ICP‐MS) data, coupled with ore and gangue mineralogy and geochemical (both lithogeochemistry and microanalysis) data from the Nucleus Au–Ag–Bi–Cu deposit, in the Yukon (Canada) portion of the Tintina Au province. Arsenic‐bearing Au–Ag–Bi–Cu mineralization at Nucleus consists of two distinct styles of mineralization including: (i) reduced Au skarn and sulfide replacement; and (ii) a relatively shallow‐emplaced (as supported by textures and temperature of formation), vein‐controlled mineralization occurring mainly as veins and veinlets of various shapes (sheeted, single, stockworks, and crustiform), breccias, and disseminations. Whereas Au, Bi, and Cu mineralization from skarn is associated with hydrous retrograde alteration phases (actinolite, ferro‐actinolite, hastingsite, cannilloite, and hornblende), numerous alteration types are associated with the vein‐controlled style of mineralization and these include: biotite, phyllic, argillic, propylitic, carbonate, and quartz (silicification) alterations. The mineralization–alteration processes took place over a wide temperature range that is bracketed between 340 and 568°C, as indicated by chlorite and arsenopyrite geothermometers. The Au‐rich Nucleus deposit is characterized by anomalously high content of As and Bi (as much as 1 %), and whereas Au moderately correlates with Bi (r = 0.40) in the skarn mineralization style (where native Au is spatially associated with native Bi and Bi‐bearing sulfides), the two elements correlate poorly (r = 0.14) in the vein‐controlled type, in which native Bi‐ and Bi‐sulfide‐bearing veins are locally observed. Sphalerite from the vein‐controlled mineralized type is Fe‐rich (9.92–10.54 mol % FeS) indicative of low sulfidation conditions, as well as high temperature, with the latter further supported by arsenopyrite geothermometry (up to 491°C), low Ag content (3–7 wt.%) in Au, and the high gold fineness (926–964). Whereas molybdenite Re–Os ages from quartz‐molybdenite veins range from 75.8 to 76.2 ± 0.3 Ma, titanite from the skarn type mineralization recorded CA ID‐TIMS and LA ICP‐MS U–Pb ages of 182.6 ± 2.4 Ma and 191.0 ± 1.5 Ma, respectively, thus precluding any genetic link between the two spatially associated styles of mineralization from the Nucleus deposit area. The Au–Ag–Bi–Cu Nucleus deposit is therefore regarded as a superposed system in which two mineralization types, without any petrogenetic relationship, overlapped, possibly with remobilization of early‐formed mineralization.

Albitization and redistribution of REE and Y in IOCG systems: Insights from Moonta-Wallaroo, Yorke Peninsula, South Australia

Abstract Trace element concentrations, particularly rare earth elements and yttrium (REY) in feldspars and accessory minerals, have been determined in a suite of albitized igneous, metasedimentary and metasomatite rocks from the Moonta-Wallaroo district, Olympic Cu–Au Province, South Australia. Results show that changes in REY-fractionation trends and concentrations in feldspars and common accessories are associated with key textures in albite-bearing associations from different lithologies. In granitic rocks, pseudomorphic replacement of pre-existing feldspars is typified by porous albite with cleavage-oriented intergrowths of sericite and pore-attached hematite. These observations are comparable with albitization features of granitic terranes elsewhere. A mineral association (albite-sericite ± chlorite), similar to that from granitoids, is observed as pervasive spots in limestone, inferring prograde skarnoid reactions at low fluid/rock ratio in an impure carbonate. In metasedimentary and metasomatite rocks with comparable Na2O content (~ 5–6 wt.%), fine-grained granoblastic albite suggests growth under high fluid/rock ratios irrespective of lithology. In such cases, albite with the highest REY content (ΣREY ~ 200 ppm) accounts for the entire REY budget, e.g., in albite–biotite-schist with the lowest abundance of accessory minerals. Nanoscale investigation confirms this albite to be a REY carrier (elements incorporated within the crystal lattice); no pore-attached inclusions are observed. In contrast, albite with the lowest REY-concentration (~ 14 ppm) is encountered in the metasomatite. In such rocks, recording the highest ΣREY (~ 1000 ppm) in whole-rock, partitioning of REY is favoured among the abundant accessories (titanite, apatite) and calc-silicates (actinolite, clinozoisite) rather than albite. Comparable low-REY albite is also found in granitoid-derived albitite (Na2O ~ 5 wt.%), in which abundant accessories and discrete REY-minerals formed during albitization account for the high ΣREY content (~ 700 ppm) in whole rock. The role of coupled dissolution–reprecipitation reactions (CDRR) is critical for REY (re)distribution within albitized igneous rocks, where REY-release from early magmatic accessories and/or feldspars assists REY-enrichment into late albite. The presence of abundant nanopore-attached inclusions in plagioclase demonstrates the nanoscale nature of CDRR-driven albitization in granitoids, consistent with published experimental work on altered granites. Such porosity offers sites for REY entrapment seen within discrete REY-minerals in new-formed K-feldspar. Similarly, release and uptake of REY, concurrent with albitization, is seen in formation of coarser REY-minerals (xenotime, bastnasite, synchysite) during CDRR-driven replacement of accessory Fe–Ti-oxides by symplectites of chlorite and hematite. Based on the differences identified between the albitization pathways in igneous and metasedimentary rocks, we discuss how albitization proceeds via a series of complex fluid–mineral reactions, each involving the redistribution, accumulation and retention of REY. These reactions are critical for defining the endowment and deportment of REY in rocks that have undergone sodic alteration. Contrary to previous models, albitization appears controlled by pH rather than redox conditions. Despite regional differences in local geological environment and alteration style across the Olympic Cu–Au Province, albitization, the initiation of hydrothermal alteration, is a pre-requisite stage for REY-enrichment in Iron-Oxide–Copper–Gold (IOCG) systems. REY distribution patterns in feldspars may thus have value in mineral exploration as criteria enabling alteration associated with mineralization to be distinguished from the regional background. Strong albitization without superposition of later potassic alteration may not, however, be automatically linked to the formation of giant IOCG deposits. Albitization enhances rock permeability and in a strongly faulted structural environment without a suitable trap, hydrothermal fluids may be more readily lost from the system.