Orogenic Au Deposits Research Articles

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

The ubiquity of Au-bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of Au and As in this sulfide occurs under a wide range of physico-chemical conditions. Despite significant advances in the last 20years, fundamental factors controlling Au and As ratios in pyrite from ore deposits remain poorly known. Here we explore these constraints using new and previously published EMPA, LA-ICP-MS, SIMS, and μ-PIXE analyses of As and Au in pyrite from Carlin-type Au, epithermal Au, porphyry Cu, Cu–Au, and orogenic Au deposits, volcanogenic massive sulfide (VHMS), Witwatersrand Au, iron oxide copper gold (IOCG), and coal deposits. Pyrite included in the data compilation formed under temperatures from ∼30 to ∼600°C and in a wide variety of geological environments. The pyrite Au-As data form a wedge-shaped zone in compositional space, and the fact that most data points plot below the solid solubility limit defined by Reich et al. (2005) indicate that Au1+ is the dominant form of Au in arsenian pyrite and that Au-bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native Au. The analytical data also show that the solid solubility limit of Au in arsenian pyrite defined by an Au/As ratio of 0.02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal-chemical properties of pyrite and post-depositional alteration. Compilation of Au–As concentrations and formation temperatures for pyrite indicates that Au and As solubility in pyrite is retrograde; Au and As contents decrease as a function of increasing temperature from ∼200 to ∼500°C. Based on these results, two major Au–As trends for Au-bearing arsenian pyrite from ore deposits are defined. One trend is formed by pyrites from Carlin-type and orogenic Au deposits where compositions are largely controlled by fluid-rock interactions and/or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids. The second trend consists of pyrites from porphyry Cu and epithermal Au deposits, which are characterised by compositions that preserve the Au/As signature of mineralizing magmatic-hydrothermal fluids, confirming the role of this sulfide in controlling metal ratios in ore systems.

Main deposit styles and associated tectonics of the West Junggar region, NW China

The West Junggar region, located in the loci of the Central Asian Orogenic Belt, is a highly endowed metallogenic province with >100 tonnes Au, >0.7 Mt Cu, >0.3 Mt Mo, and >2.3 Mt chromite as well as significant amounts of Be and U. The West Junggar region has three metallogenic belts distributed systematically from north to south: (1) late Paleozoic Saur Au-Cu belt; (2) early Paleozoic Xiemisitai-Sharburt Be-U-Cu-Zn belt; (3) late Paleozoic Barluk-Kelamay Au-Cu-Mo-Cr belt. These belts host a number of deposits belonging to at least eight economically important styles, including epithermal Au, granite-related Be-U, volcanogenic massive sulfide (VMS) Cu-Zn, podiform chromite, porphyry Cu, hydrothermal quartz vein Au, porphyry-greisen Mo(-W), and orogenic Au. These deposit styles are associated with the tectonics prevalent during their formation. Five tectonic-mineralized epochs can be recognized: (1) Ordovician subduction-related VMS Cu-Zn deposit; (2) Devonian ophiolite-related podiform chromite deposit; (3) early Carboniferous subduction-related epithermal Au and porphyry Cu deposits; (4) late Carboniferous subduction-related granite-related Be-U, porphyry Cu, and hydrothermal quartz vein Au deposits; and (5) late Carboniferous to early Permian subduction-related porphyry-greisen Mo(-W) and orogenic Au deposits.

The dilemma of the Jiaodong gold deposits: Are they unique?

The ca. 126–120 Ma Au deposits of the Jiaodong Peninsula, eastern China, define the country's largest gold province with an overall endowment estimated as >3000 t Au. The vein and disseminated ores are hosted by NE- to NNE-trending brittle normal faults that parallel the margins of ca. 165–150 Ma, deeply emplaced, lower crustal melt granites. The deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilatational jogs. Country rocks to the granites are Precambrian high-grade metamorphic rocks located on both sides of a Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous “Yanshanian” intracontinental extensional deformation and associated gold formation.Classification of the gold deposits remains problematic. Many features resemble those typical of orogenic Au including the linear structural distribution of the deposits, mineralization style, ore and alteration assemblages, and ore fluid chemistry. However, Phanerozoic orogenic Au deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong deposits, in contrast, formed within two Precambrian blocks approximately 2 billion years after devolatilization of the country rocks, and thus require a model that involves alternative fluid and metal sources for the ores. A widespread suite of ca. 130–123 Ma granodiorites overlaps temporally with the ores, but shows a poor spatial association with the deposits. Furthermore, the deposit distribution and mineralization style is atypical of ores formed from nearby magmas. The ore concentration requires fluid focusing during some type of sub-crustal thermal event, which could be broadly related to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific plate subduction, and seismicity along the continental-scale Tan-Lu fault. Possible ore genesis scenarios include those where ore fluids were produced directly by the metamorphism of oceanic lithosphere and overlying sediment on the subducting paleo-Pacific slab, or by devolatilization of an enriched mantle wedge above the slab. Both the sulfur and gold could be sourced from either the oceanic sediments or the serpentinized mantle. A better understanding of the architecture of the paleo-Pacific slab during Early Cretaceous below the eastern margin of China is essential to determination of the validity of possible models.

Setting and Evolution of the Archean Synvolcanic Mooshla Intrusive Complex, Doyon-Bousquet-LaRonde Mining Camp, Abitibi Greenstone Belt: Emplacement History, Petrogenesis, and Implications for Au Metallogenesis

The synvolcanic Mooshla Intrusive Complex intrudes coeval ~2699 to 2696 Ma volcanic rocks of the Blake River Group within the southern margin of the Archean Abitibi greenstone belt. The upper Blake River Group is host to the Doyon-Bousquet-LaRonde mining camp that contains Au-rich volcanogenic massive sulfide (VMS) deposits, possible subsea-floor epithermal-style deposits, and orogenic Au deposits. In total, the camp contains to date in excess of 28 million ounces (Moz) Au, making it a world-class example of Au-rich paleosea-floor environments. The Mooshla Intrusive Complex is spatially, temporally, and most probably genetically associated with all of the above types of mineralization. It is host to parts of the Doyon (5.5 Moz Au), Mouska (0.8 Moz Au), and Mic Mac (0.11 Moz Au) Au deposits and host to the smaller Mooshla A and B Au occurrences. Host volcanic units to the Mooshla Intrusive Complex are intensely deformed, metamorphosed, altered, and mineralized, as is the intrusion itself. The Mooshla Intrusive Complex was formed by nine distinctive phases of subvolcanic dikes, sills, and stocks. These were emplaced in two stages to form a shallow, multiphase synvolcanic intrusion along the contact between the Hebecourt and Bousquet volcanic formations. The Mouska stage is represented by a preliminary swarm of thin diabase sills, intruded by a well-layered gabbroic sill, a more crudely layered quartz diorite, and tonalite. A period of devolatilization accompanied crystallization of the xenolith-rich top of the tonalite magma chamber, as evidenced by the presence of an aplite dike swarm and associated extensive alteration zones and miarolitic cavities. The younger Doyon stage comprises a series of fine-grained aphyric to porphyritic, tonalite and trondhjemite dikes and sills, which also contain evidence of in situ devolatilization. The geochemical signatures of the Mooshla Intrusive Complex indicate emplacement during formation of an evolved, extensional oceanic island arc-style succession. Primitive mantle-normalized spider plots suggest a common origin for this island-arc intrusive suite that is similar to that of the volcanic succession of the upper member of the Bousquet Formation. Various element ratio plots used to further define magma origin and emplacement history suggest that whereas the Mouska-stage magmatic phases have a relatively straightforward, coexisting fractionation history, the Doyon-stage tonalite-trondhjemite has a more complex interplay of assimilation-fractionation-contamination, suggesting midcrustal partitioning and interaction with both earlier formed, partially hydrated ~2720 Ma oceanic crust and upper Blake River host strata (~2699-2696 Ma). The protracted and mulitphased magmatic evolution of the Mooshla Intrusive Complex led to the generation of volatile-rich phases that contributed to the development of a submarine magmatic-hydrothermal system that is thought to be responsible for the formation of the Doyon Au-Cu deposit. Geologic and timing relationships suggest that this magmatic-hydrothermal system might also have contributed to the generation of Au-rich VMS deposits higher in the host volcanic succession as part of a large Archean magmatic and hydrothermal center.

Re-Os Molybdenite, Pyrite, and Chalcopyrite Geochronology, Lupa Goldfield, Southwestern Tanzania: Tracing Metallogenic Time Scales at Midcrustal Shear Zones Hosting Orogenic Au Deposits

A detailed Re-Os molybdenite, pyrite, and chalcopyrite geochronology at five shear-hosted Au occurrences (Kenge, Mbenge, Porcupine, Konokono, and Dubwana) in the Lupa goldfield, southwestern Tanzania, is reported in this paper. Au occurrences within the Lupa goldfield share many geologic similarities with the orogenic Au deposit type and are situated within a Paleoproterozoic magmatic arc that intruded the Archean Tanzanian cratonic margin. Pyrite ± chalcopyrite ± molybdenite-bearing fault-fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest Au grades. Re-Os sulfide ages are deemed a suitable proxy to constrain the timing of Au based on the occurrence of Au-bearing minerals as inclusions within pyrite and chalcopyrite, whereas Au-bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event. Molybdenite at Kenge occurs as ultrafine disseminations within fault-fill veins (1953 ± 6 Ma; n = 3) that possess nominally older weighted average Re-Os ages than molybdenite hosted by stylolite-like veins (1937 ± 8 Ma; n = 7). Both sample sets are ca. 70 m.y. older than a weighted average Re-Os pyrite age from the mylonitic shear zones at Kenge and Mbenge (1876 ± 10 Ma; n = 13), which contain fault-fill veins and record the timing of mylonitization. Molybdenite at Porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples (1886 ± 6 Ma; n = 4) that are broadly equivalent in age to weighted average Re-Os ages of molybdenite occurring as stylolite-like veins (1873 ± 5 Ma; n = 6) and pyrite within oblique-extension veins (1894 ± 45 Ma; n = 2). Weighted average Re-Os pyrite model ages at Konokono (1880 ± 14 Ma; n = 9) and Dubwana (1905 ± 25 Ma; n = 2) are also consistent with the ca. 1.88 Ga event observed at Kenge, Mbenge, and Porcupine. Gold occurrences in the Lupa goldfield therefore record a protracted hydrothermal history (1.95–1.87 Ga) comprising at least three temporally distinct hydrothermal events (ca. 1.95, 1.94, and 1.88 Ga), which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the Re-Os method. The sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca. 1.88 Ga that permitted the transportation and deposition of gold. Comparison between Re-Os sulfide and high-precision U-Pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism. However, the range of Re-Os ages argues against an intrusion-related deposit model whereby metallogenic fluids are solely derived from an individual intrusion. The regional ca. 1.88 Ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the Ubendian orogenic cycle and provides one of Earth’s earliest temporal links between subduction zone processes and orogenic Au deposit formation during the Paleoproterozoic.

Geology, mineralogy, and geochemistry of magnetite-associated Au mineralization of the ultramafic–basalt greenstone hosted Crusader Complex, Agnew Gold Camp, Eastern Yilgarn Craton, Western Australia; a Late Archean intrusion-related Au deposit?

The Crusader Complex, part of the Agnew gold camp of the Lawlers Anticline of the Yilgarn Craton, Western Australia, is located close to or along the contact between the Lawlers Basalt and Agnew Ultramafics units. Au mineralization within the four orebodies that form the Crusader Complex is dominated by very pure Au, containing less than 1wt.% Ag, with Au variably associated with scheelite, Bi-tellurides and minor chalcopyrite within a magnetite and titanite gangue assemblage. Hydrothermal alteration associated with this style of mineralization is characterized by increasing concentrations of Mo, Be, Li, Sn and Fe and depletions in Na, Cu, Ba, Pb, Mn, Zn, Si, and K relative to protolith concentrations; these enrichments are more typical in orebodies associated with felsic intrusive-related mineralizing systems rather than the more well-known orogenic Au deposits found elsewhere within the Lawlers Anticline (e.g. at Waroonga) and within the greater Yilgarn Craton.We propose that flexures of the contact between the Lawlers Basalt and Agnew Ultramafic units acted as conduits for Au-bearing felsic intrusive-derived fluids and generated structural traps that enhanced fluid flow. The mineralizing fluids that formed the Crusader deposits were derived from the Lawlers granitoid pluton that intruded into the study area. Enhanced fluid flow promoted interaction between hydrothermal fluids and the reactive mafic–ultramafic rock sequence, augmenting the amount of Au that was precipitated during formation of the orebodies at Crusader. The magnetite-dominated quartz- and sulfide-poor intrusion-related mineralization at Crusader contrasts sharply with other late Archean intrusion-related deposits of the Yilgarn Craton that are usually sulfide- and/or quartz-rich. This may in turn suggest that the Crusader deposit represents a new class of under-explored intrusion-derived deposits, possibly opening new mineral exploration opportunities for the Agnew region, and potentially the wider Eastern Goldfields Superterrane. Enrichments in Mo and Sn and significant depletions in Cu suggest that other parts of the Lawlers batholith may also be prospective for base metal mineralization.Integration of stratigraphic interpretation with the identification of key structural fluid pathways and the presence of felsic intrusive bodies, as presented in this study, enables the delineation of the key elements that underlie mineralization at the Crusader Complex. We propose that these key elements provide vital information for future gold exploration models that can be used within other Archean terranes and within the Eastern Yilgarn Craton in particular.

Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO2 of 55–60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S2−-rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ∼80° (from SW to NW) at approximately 125–122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ˜120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.

A unified model for gold mineralisation in accretionary orogens and implications for regional-scale exploration targeting methods

Accretionary orogens are the sites of long-lived convergent margin tectonics, both compressional and extensional. They are also the hosts to the majority of the world’s important gold deposits. A very diverse range of deposit types occurs within accretionary orogens, commonly in close proximity in space and time to each other. These include porphyry and associated high-sulphidation Au–Cu–Ag deposits, classic low-sulphidation Au–Ag deposits, low-sulphidation Au deposits centred on alkalic intrusive complexes, Carlin-type Au deposits, Au-rich volcanic-hosted massive sulphide deposits, orogenic Au deposits, intrusion-related Au deposits and iron oxide Cu–Au deposits. Empirical patterns of spatial distribution of these deposits suggest there must be fundamental generic controls on gold metallogeny. Various lines of evidence lead to the proposal that the underlying key generic factor controlling accretionary orogen gold metallogeny is regional-scale, long-term, pre- and syn-subduction heterogeneous fertilisation of the lithospheric mantle that becomes a source of mineralisation-associated arc magma or hydrothermal fluid components. This process provides a gold-enriched reservoir that can be accessed later in a diverse range of tectonomagmatic settings. Based on this concept, a unified model is proposed in which the formation of a major gold deposit of any type requires the conjunction in time and space of three essential factors: a fertile upper-mantle source region, a favourable transient remobilisation event, and favourable lithospheric-scale plumbing structure. This framework provides the basis for a practical regional-scale targeting methodology that is applicable to data-poor regions.

Are Iron-Rich Sedimentary Rocks the Key to the Spike in Orogenic Gold Mineralization in the Paleoproterozoic?

Paleoproterozoic rocks with ages from 2100 to 1800 Ma are recognized as belonging to one of the three main periods in Earth history that host orogenic gold deposits. In contrast to older Archean orogenic gold deposits, which are hosted predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Paleoproterozoic. Unusually iron rich 1840 Ma marine mudstones in the Tanami region host one world-class gold deposit and many other large gold deposits. Fluid-rock modelling at 300° and 325°C suggests a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region. These northern Australian Paleoproterozoic iron-rich sedimentary rocks could be linked globally to iron-rich and gold-bearing sedimentary rocks deposited between 2100 to 1800 Ma in Homestake, South Dakota (United States), and in Ghana, and Brazil. Worldwide, from about 2400 to 1800 Ma, the Paleoproterozoic is also marked by the deposition of mainly Superior style BIFs, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in northern Australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic Au deposits hosted in rocks aged from 2100 to 1800 Ma.

Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt

The Hercynian Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum–Tarim and the Kazakhstan paleo-continents. In order to constrain timing of Hercynian post-collisional magmatism, 27 intrusions were sampled for U–Pb zircon dating along a ca. 2000 km – long profile in Uzbekistan and Kyrgyzstan. The samples were dated utilizing sensitive high resolution ion microprobe (SHRIMP-II). The obtained ages, together with previously published age data, allowed the timing of Hercynian post-collisional magmatism to be constrained and interpreted in the context of the Paleozoic magmatic evolution of the region. Apart from Hercynian post-collisional magmatism, two older magmatic episodes have been recognized, and the following sequence of events has been established: (1) approximately 10 Ma after cessation of continuous Caledonian magmatism a number of Late Silurian–Early Devonian intrusions were emplaced in the Middle and Northern Tien Shan terranes between 420 and 390 Ma. The intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of Caledonian Paleo-Kazakhstan continent; (2) the next relatively short Late Carboniferous episode of subduction under Paleo-Kazakhstan was registered in the Kurama range of the Middle Tien Shan. Calc-alkaline volcanics and granitoids with ages 315–300 Ma have distinct metallogenic affinities typical for subduction-related rocks and are not found anywhere outside the Middle Tien Shan terrane west of the Talas–Farghona fault; (3) the Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries. The post-collisional intrusions formed within a relatively short time span between 295 and 280 Ma. The model for Hercynian post-collisional evolution suggests that after collision the Tien Shan was affected by trans-crustal strike-slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust. This produced both granitoid magmas and hydrothermal fluid flow. As a result post-collisional intrusions and orogenic Au deposits, known in the region, formed coevally and were tectonically controlled; (4) between 240 and 220 Ma a Triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale. Probably the influx of heat into the crust during the Triassic was tectonically focused and varied significantly in different terranes. In the region under investigation the Triassic thermal event was not accompanied by any significant magmatic activity. Thus, after cessation of Hercynian post-collisional magmatism ca. 280 Ma ago there was a long magmatically quiet period in the Tien Shan.

Arsenic speciation in fluid inclusions using micro-beam X-ray absorption spectroscopy

Synchrotron radiation X-ray fluorescence (SR-XRF) was used to characterize As speciation within natural fluid inclusions from three deposits with different hydrogeochemical and geological settings. The studied samples represent different compositions of Au-bearing fluids: typical orogenic Au deposit (low-salinity, ~6 mol% CO2 ± CH4; Brusson, Western Italian Alps); brines from a Proterozoic (Fe)- Cu-Au deposit (Starra, Queensland, Australia); and an As-rich magmatic fluid with a bulk composition similar to that typical of orogenic gold (Muiane pegmatite, Mozambique). Arsenic K-edge X-ray absorption spectra (XAS) were obtained from fluid inclusions at temperatures ranging from 25 to 200°C, and compared with spectra of aqueous As(III) and As(V) solutions and minerals. X-ray absorption near edge structure (XANES) data show that initially the fluid inclusions from all three regions contain some As in reduced form [As(III) at Brusson and Muiane; As-sulfide or possibly As(0) at Starra]. However, this reduced As is readily oxidized under the beam to As(V). Therefore, extended X-ray absorption fine structure (EXAFS) spectra for the As(III) aqueous complex could be collected only on the sample from the Muiane pegmatite containing large fluid inclusions with high As concentrations (>>1000 ppm). Analysis of these EXAFS data shows that As(OH)3(aq) (coordination number of 3.0 ± 0.2 atoms, bond length of 1.76 ± 0.01 Å) is the dominant arsenic aqueous species in the Muiane fluid inclusions at 100 °C, in accordance with predictions based on studies conducted using autoclaves. The As(V) complex resulting from photooxidation in the Muiane inclusions was characterized at 200 °C; the As-O bond distance (1.711 ± 0.025 Å) corresponds to that found in the arsenate group in minerals, and to that measured for the (HAsO4)2- complex at room temperature (1.700 ± 0.023 Å).

Sulfur isotopes in sediment-hosted orogenic gold deposits: Evidence for an early timing and a seawater sulfur source

We report sulfur isotopic compositions of sulfides of various paragenetic stages in the giant Sukhoi Log sediment-hosted orogenic Au deposit in Russia. The overall mean value and the significant variability in early pyrite indicate that the sulfur was from the reduction of seawater sulfate. The later generations of sulfide have δ34S values in successively smaller ranges, coincident with the mode that is around the median value of the whole data set. Together with textural evidence, sulfide trace element data, and gold occurrence, the data demonstrate that metamorphism has gradually homogenized the early sulfur, accompanied by the segregation of quartz and the release of Au from the lattice of early pyrite and its reprecipitation as inclusions in later pyrite. The S isotopic compositions of sulfides in Sukhoi Log, and many other major orogenic Au deposits hosted in sedimentary rocks of various ages, show a pattern generally parallel to the seawater sulfate curve through geologic time, indicating that the sulfur in most sediment-hosted orogenic Au deposits was probably also originally from the reduction of seawater sulfate. We conclude that sulfidation and gold mineralization in many sediment-hosted orogenic gold deposits was early during basin evolution when seawater was the principal active fluid, rather than later, during or after basin inversion, as proposed in current models.

Tectonism and Exhumation in Convergent Margin Orogens: Insights from Ore Deposits

Three widespread and abundant types of ore deposits, epithermal silver‐gold (Ag‐Au), porphyry copper (Cu), and orogenic gold (Au), form by hydrothermal activity during tectonic convergence along continental margins, and their preservation provides new insights into rates of those processes that form and destroy ore deposits as well as overlying rock. Ages for each of the three deposit types define lognormal age‐frequency distributions that are nearly identical in form but differ in their modal ages. Epithermal Ag‐Au, porphyry Cu, and orogenic Au deposits exhibit age modes at ∼3, 11, and 199 Ma, respectively. These ore types form at depths of ∼0.5, 1.9, and 10 km, respectively, such that modal age increases with depth of ore emplacement. Qualitatively, the dearth of deposits younger than their modal age reflects the fact that ore deposits form at depth within the crust, and time is required for exhumation to expose them. Continued erosion serves to destroy ore bodies, giving rise to fewer deposits with increasing time beyond modal ages. These relations suggest that ore deposit age frequencies should serve as monitors of rates of tectonism and exhumation along convergent continental margins. Quantitatively, the lognormal aspect of age frequencies is closely approximated by a steady state model of ore formation and crustal deformation. The model presumes that the population of exposed deposits of each type results from ore emplacement at a constant rate and depth and that, after emplacement, these deposits are dispersed vertically (upward or downward) by tectonic uplift and/or subsidence. The aggregate paths of all deposits of some particular type behave as a two‐dimensional random walk in vertical (depth) and lateral (time) space. Moreover, the lognormal nature of ore deposit age‐frequency distributions requires that amounts of uplift and subsidence are generally subequal (there is no bias to the random walk) and necessitates that tectonic “steps” taken during this random walk are on the order of a few hundreds of meters per million years. Modal rate of ore deposit exhumation (emplacement depth/modal age) decreases as a power function with increasing modal age (exhumation distance $$[ \mathrm{mm}\,] =5.12\times \mathrm{time}\,$$ interval0.799 [yr]). Lower rates of exhumation with greater emplacement depth and modal age, however, merely reflect greater random‐walk path lengths (and durations) with greater emplacement depth and are no less than rates that would be anticipated for a steady state system with invariant rates of ore mineralization and tectonic deformation. In fact, exhumation histories of epithermal Ag‐Au, porphyry Cu, and orogenic Au deposits scale in a similar manner and therefore record largely indistinguishable styles of tectonic uplift and subsidence among the ∼700 ore deposits considered here. Agreement between model and observed ore deposit age‐frequency distributions also allows the formulation of a general inventory of ore body abundances in continental crust. For example, if currently known occurrences of epithermal Ag‐Au reasonably approximate the actual number of these deposits that now exist at the Earth’s surface, then less than 1% of all such deposits are now exposed; tectonic uplift and erosion has served to remove ∼93% of all such epithermal Ag‐Au deposits that have formed over Phanerozoic time.

In situ sulfur isotope analysis by laser ablation MC-ICPMS

A new method for in situ S isotopic analysis was tested using a laser ablation system together with a multi-collector (MC)-ICPMS. The method was tested for the analysis of pyrite, pyrrhotite, chalcopyrite and pentlandite using a large pyrite crystal as an in-house standard. Repeated measurements of Py, Pn, Po and Cpy provide an average internal precision of less than 0.1‰ (2 σ). The method was also applied to a pyrite-bearing orogenic Au deposit to display the ability of the method to resolve minor variations in δ 34S across growth zoning in pyrite.

A reassessment of the tectonic zonation of the Uralides: implications for metallogeny

Abstract A review of the structural zonation of the ‘oceanic’ Urals shows that only its westernmost Sakamara, Tagil and Magnitogorsk zones reveal the presence of thrust structures, whereas in the East Uralian megazone and Trans-Uralian zone, the classic zonation rather reflects late- or post-collisional granitic welding and strike-slip displacement of the orogen for 100–300 km. This sinistral strike-slip displacement is responsible for the lens-shapes structure of the individual zones in the Urals. Metallogenically, these orogen-parallel faults and the eastern boundary of the East European craton control the distribution of the orogenic Au deposits. Restoration of the individual zones into their pre-strike-slip fault positions suggests that the Urals contains only two magmatic arcs, one in the west and one in the east. The western Tagil-Magnitogorsk immature arc hosts a variety of chromite, Alaska-type PGE and major VMS deposits. The eastern Valerianovka arc effectively stitches together the Kazakh-Tien Shan structures and is host to important copper-gold and giant iron(-copper) skarn deposits. The geodynamic evolution of the Urals can be observed with the generation of the immature Tagil-Magnitogorsk magmatic arc in the Late Ordovician. Metallogenic zoning of the VMS deposits supports the petrological data that the arc developed due to eastward subduction inside the oceanic back-arc basin that existed in the rear of the Kazakhstan-Tien Shan arcs. In the late Palaeozoic, these arcs collided with each other and were together thrust onto the East European craton. Syncollisional granitoid intrusions welded the magmatic arcs, which were soon displaced into presently observed fragments along the post-collisional orogen-parallel strike-slip faults.

Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model

The Altaids are an orogenic collage of Neoproterozoic–Paleozoic rocks located in the center of Eurasia. This collage consists of only three oroclinally bent Neoproterozoic–Early Paleozoic magmatic arcs (Kipchak, Tuva–Mongol, and Mugodzhar–Rudny Altai), separated by sutures of their former backarc basins, which were stitched by new generations of overlapping magmatic arcs. In addition, the Altaids host accreted fragments of the Neoproterozoic to Early Paleozoic oceanic island chains and Neoproterozoic to Cenozoic plume-related magmatic rocks superimposed on the accreted fragments. All these assemblages host important, many world-class, Late Proterozoic to Early Mesozoic gold, copper–molybdenum, lead–zinc, nickel and other deposits of various types. In the Late Proterozoic, during breakup of the supercontinent Rodinia, the Kipchak and Tuva–Mongol magmatic arcs were rifted off Eastern Europe–Siberia and Laurentia to produce oceanic backarc basins. In the Late Ordovician, the Siberian craton began its clockwise rotation with respect to Eastern Europe and this coincides with the beginning of formation of the Mugodzhar–Rudny Altai arc behind the Kipchak arc. These earlier arcs produced mostly Cu–Pb–Zn VMS deposits, although some important intrusion-related orogenic Au deposits formed during arc–arc collision events in the Middle Cambrian and Late Ordovician. The clockwise rotation of Siberia continued through the Paleozoic until the Early Permian producing several episodes of oroclinal bending, strike–slip duplication and reorganization of the magmatic arcs to produce the overlapping Kazakh–Mongol and Zharma-Saur–Valerianov–Beltau-Kurama arcs that welded the extinct Kipchak and Tuva–Mongol arcs. This resulted in amalgamation of the western portion of the Altaid orogenic collage in the Late Paleozoic. Its eastern portion amalgamated only in the early Mesozoic and was overlapped by the Transbaikal magmatic arc, which developed in response to subduction of the oceanic crust of the Paleo-Pacific Ocean. Several world-class Cu–(Mo)-porphyry, Cu–Pb–Zn VMS and intrusion-related Au mineral camps, which formed in the Altaids at this stage, coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs. Major Pb–Zn and Cu sedimentary rock-hosted deposits of Kazakhstan and Central Asia formed in backarc rifts, which developed on the earlier amalgamated fragments. Major orogenic gold deposits are intrusion-related deposits, often occurring within black shale-bearing sutured backarc basins with oceanic crust. After amalgamation of the western Altaids, this part of the collage and adjacent cratons were affected by the Siberian superplume, which ascended at the Permian–Triassic transition. This plume-related magmatism produced various deposits, such as famous Ni–Cu–PGE deposits of Norilsk in the northwest of the Siberian craton. In the early Mesozoic, the eastern Altaids were oroclinally bent together with the overlapping Transbaikal magmatic arc in response to the northward migration and anti-clockwise rotation of the North China craton. The following collision of the eastern portion of the Altaid collage with the Siberian craton formed the Mongol–Okhotsk suture zone, which still links the accretionary wedges of central Mongolia and Circum-Pacific belts. In the late Mesozoic, a system of continent-scale conjugate northwest-trending and northeast-trending strike–slip faults developed in response to the southward propagation of the Siberian craton with subsequent post-mineral offset of some metallogenic belts for as much as 70–400 km, possibly in response to spreading in the Canadian basin. India–Asia collision rejuvenated some of these faults and generated a system of impact rifts.