Au Province Research Articles

Hydrothermal mineralization at the Dahu Au–Mo deposit in the Xiaoqinling gold field, Qinling Orogen, central China

The Xiaoqinling gold field in central China is the second largest orogenic Au province in China. In the Xiaoqinling area, the Dahu Au–Mo deposit is typical because it is one of the five early‐discovered large gold deposits, but also unique for its northernmost location and Mo‐association. This study shows that the deposit is a fault‐controlled lode system formed by a three‐stage hydrothermal process. The early‐stage pyrite–quartz veins are structurally deformed and broken. The middle‐stage molybdenite–pyrite–quartz stockworks (mostly coaxial) infill the cracks of the early‐stage veins and minerals. The late‐stage open‐space filling quartz–carbonate veinlets show a comb‐like texture. The early‐ and late‐stage quartz only contains the fluid inclusions of CO2–H2O and H2O–NaCl types, respectively, suggesting an evolution from CO2‐rich to CO2‐poor; whereas the middle‐stage quartz contains fluid inclusions of CO2–H2O, H2O–NaCl, pure CO2 and daughter mineral‐bearing types, supporting a boiling fluid system, because they can occur in a microscopic domain of a single crystal, homogenize divergently at similar temperatures clustering between 293 and 410 °C, and show contrasting salinities of <13.6 wt.% and >26.3 wt.% NaCl equiv. The inclusions in early‐stage quartz are homogenized at 402–503 °C, with salinities of 4.5–10.4 wt.% NaCl equiv.; and those in late‐stage quartz are homogenized at temperatures of <251 °C, with salinities of 14.4–14.8 wt.% NaCl equiv. The trapping pressures of inclusions are estimated to be 138–331 MPa in the early stage and 78–237 MPa in the middle stage, implying that the fluid system alternately fluctuated from lithostatic to hydrostatic, which was controlled by a fault–valve mechanism at depths of >11 km and ~7.8 km, respectively. Hydrogen and oxygen isotope signatures indicate that the fluids originated from metamorphic devolatilization in the early stage, then mixed with meteoric water in the middle stage, and finally replaced by meteoric water in the late stage. Hence, it is concluded, that metal precipitation at the Dahu deposit was mainly caused by fluid boiling, followed by fluid mixing, along with a trans‐compressing at the transitional zone from a magmatic arc to back‐arc basin, which resulted from the Triassic northward subduction of the Mian‐Lue oceanic slab. © 2014 The Authors. Geological Journal published by John Wiley & Sons Ltd.

Prevalence of vision impairment and refractive error in school children in Ba Ria – Vung Tau province, Vietnam

BackgroundTo assess the prevalence of vision impairment and refractive error in school children 12–15 years of age in Ba Ria – Vung Tau province, Vietnam.DesignProspective, cross-sectional study.Participants2238 secondary school children.MethodsSubjects were selected based on stratified multistage cluster sampling of 13 secondary schools from urban, rural and semi-urban areas. The examination included visual acuity measurements, ocular motility evaluation, cycloplegic autorefraction, and examination of the external eye, anterior segment, media and fundus.Main Outcome MeasuresVisual acuity and principal cause of vision impairment.ResultsThe prevalence of uncorrected and presenting visual acuity ≤6/12 in the better eye were 19.4% (95% confidence interval, 12.5–26.3) and 12.2% (95% confidence interval, 8.8–15.6), respectively. Refractive error was the cause of vision impairment in 92.7%, amblyopia in 2.2%, cataract in 0.7%, retinal disorders in 0.4%, other causes in 1.5% and unexplained causes in the remaining 2.6%. The prevalence of vision impairment due to myopia in either eye (–0.50 diopter or greater) was 20.4% (95% confidence interval, 12.8–28.0), hyperopia (≥2.00 D) was 0.4% (95% confidence interval, 0.0–0.7) and emmetropia with astigmatism (≥0.75 D) was 0.7% (95% confidence interval, 0.2–1.2). Vision impairment due to myopia was associated with higher school grade and increased time spent reading and working on a computer.ConclusionsUncorrected refractive error, particularly myopia, among secondary school children in Vietnam is a major public health problem. School-based eye health initiative such as refractive error screening is warranted to reduce vision impairment.

Genesis of the Au–Bi–Cu–As, Cu–Mo ± W, and base–metal Au–Ag mineralization at the Mountain Freegold (Yukon, Canada): constraints from Ar–Ar and Re–Os geochronology and Pb and stable isotope compositions

The genesis of mineralized systems across the Mountain Freegold area, in the Dawson Range Cu–Au ± Mo Belt of the Tintina Au province was constrained using Pb and stable isotope compositions and Ar–Ar and Re–Os geochronology. Pb isotope compositions of sulfides span a wide compositional range (206Pb/204Pb, 18.669–19.861; 208Pb/204Pb, 38.400–39.238) that overlaps the compositions of the spatially associated igneous rocks, thus indicating a magmatic origin for Pb and probably the other metals. Sulfur isotopic compositions of sulfide minerals are broadly similar and their δ34S (Vienna-Canyon Diablo Troilite (V-CDT)) values range from −1.4 to 3.6 ‰ consistent with the magmatic range, with the exception of stibnite from a Au–Sb–quartz vein, which has δ34S values between −8.1 and −3.1 ‰. The δ34S values of sulfates coexisting with sulfide are between 11.2 and 14.2 ‰; whereas, those from the weathering zone range from 3.7 to 4.3 ‰, indicating supergene sulfates derived from oxidation of hypogene sulfides. The δ13C (Vienna Peedee Belemnite (VPDB)) values of carbonate range from −4.9 to 1.1 ‰ and are higher than magmatic values. The δ18O (V-SMOW) values of magmatic quartz phenocrysts and magmatic least-altered rocks vary between 6.2 and 10.1 ‰ and between 5.0 and 10.1 ‰, respectively, whereas altered magmatic rocks and hydrothermal minerals (quartz and magnetite) are relatively 18O-depleted (4.2 to 7.9 ‰ and −6.3 to 1.5 ‰, respectively). Hydrogen isotope compositions of both least-altered and altered igneous rock samples are D-depleted (from −133 to −161 ‰ Vienna-Standard Mean Ocean Water (V-SMOW)), consistent with differential magma degassing and/or post-crystallization exchange between the rocks and meteoric ground water. Zircon from a chlorite-altered dike has a U–Pb crystallization age of 108.7 ± 0.4 Ma; whereas, the same sample yielded a whole-rock Ar–Ar plateau age of 76.25 ± 0.53 Ma. Likewise, molybdenite Re–Os model ages range from 75.8 to 78.2 Ma, indicating the mineralizing events are genetically related to Late Cretaceous volcano-plutonic intrusions in the area. The molybdenite Re–Os ages difference between the nearby Nucleus (75.9 ± 0.3 to 76.2 ± 0.3 Ma) and Revenue (77.9 ± 0.3 to 78.2 ± 0.3 Ma) mineral occurrences suggests an episodic mineralized system with two pulses of hydrothermal fluids separated by at least 2 Ma. This, in combination with geological features suggest the Nucleus deposit represents the apical and younger portion of the Revenue–Nucleus magmatic-hydrothermal system and may suggest an evolution from the porphyry to the epithermal environments.

Supergene gold transformation: Biogenic secondary and nano-particulate gold from arid Australia

Biofilms on Au grains from sub-tropical and tropical sites promote the dispersion of gold (Au) by continuously recycling coarse Au and releasing nano-particulate Au. In this study we investigated the influences of (bio)geochemical processes on the transformation of Au grains in arid environments. At these sites water and nutrient availability are limited and episodic, hence abiogenic evaporative mechanisms were thought to control the formation of secondary and especially nano-particulate Au. Gold grains were collected from eight arid sites in three Australian Au provinces, i.e., Lawlers (Western Australia), Tanami (Northern Territory), and Flinders Ranges (South Australia). Sites were chosen based on contrasting deposit styles, i.e., primary underground and epithermal deposits as well as secondary eluvial-, colluvial- and alluvial placers at increasing distances from primary mineralization. Gold grains were studied using optical microscopy, FEG-SEM, FIB–SEM–EDXA, EPMA and PIXE. Gold grains from all surface environments displayed supergene transformation features, i.e., morphotypes indicative of Au and Ag dissolution, Au aggregation and Au neoformation. The latter included spheroidal and bacteriomorphic Au, which increased in abundance with distance to source. Viable biofilms containing abundant Au nano-particles and spheroidal Au μ-crystals were detected on all grains from the Flinders Ranges. Gold grains from the Lawlers and the Tanami provinces are covered by polymorphic layers containing abundant nano-particulate, spheroidal and bacteriomorphic Au. The polymorphic layers consist of vermiform clays and organic matter, suggestive of remnant biofilms. These results demonstrate that biofilms capable of transforming Au grains develop episodically on Au grains in arid environments. In conclusion, this study shows that microbial processes play a critical role for the transformation of Au grains and contribute to the dispersion of Au and the formation of geochemical anomalies in arid environments.

Australia through time: a summary of its tectonic and metallogenic evolution

The geological evolution of Australia is closely linked to supercontinent cycles that have characterised the tectonic evolution of Earth, with most geological and metallogenic events relating to supercontinent/supercraton assembly and breakup. Australia mainly grew from W-E, with two major Archean cratons, the Yilgarn and Pilbara cratons, forming the oldest part of the continent in the West Australian Element. The centre of the continent consists of the mainly Paleoproterozoic-Mesoproterozoic North and South Australian elements, whereas the E is dominated by the Neoproterozoic-Mesozoic Tasman Element. The West, North and South Australian elements initially assembled during the Paleoproterozoic amalgamation of Nuna, and the Tasman Element formed mostly as a Paleozoic accretionary margin during the assembly of Gondwana-Pangea. Australia's present position as a relatively stable continent resulted from the breakup of Gondwana. Australia is currently moving northward toward SE Asia, probably reflecting the earliest stages of the assembly of the next supercontinent, Amasia. Australia's resources, both mineral and energy, are linked to its tectonic evolution and the supercontinent cycle. Australia's most important Au province is the product of the assembly of Kenorland, whereas its major Zn-Pb-Ag deposits and iron oxide-Cu-Au deposits formed as Nuna broke up. The diverse metallogeny of the Tasman Element is a product of Pangea-Gondwana assembly and most of Australia's hydrocarbon resources are a consequence of the breakup of this supercontinent.

Mineralogical and Geochemical Controls on the Formation of the Woods Point Dike Swarm, Victoria, Australia: Evidence from the Morning Star Dike and Implications for Sourcing of Au Within Orogenic Gold Systems

The Woods Point dike swarm of eastern Victoria, Australia, has produced over 133 tonnes (t) of Au. Here, as in a number of other orogenic Au provinces, a close spatial relationship is observed between orogenic Au mineralization and intrusive rocks; this study focuses on the well-endowed Morning Star dike, a gabbro to gabbro-diorite dike that has produced ~28 t of hydrothermal quartz-carbonate-pyrite vein-hosted Au and a further 24 t of placer gold derived from primary rocks in the area immediately around Morning Star. The fractionated magma that formed the dikes within the Woods Point dike swarm was sourced from a staging magma chamber at a depth of 16 to 23 km; here, mantle-derived magmas were driven to S saturation by assimilation of crustal material, leading to the formation of Cu-Ni-platinum group element (PGE)-Au–bearing sulfides, most of which segregated and settled toward the bottom of the staging chamber. Emplacement of the dike initiated with tapping of the staging chamber and release of the magmas within the chamber, forming the Woods Point dike swarm; flow differentiation and the formation of dike bulges enabled the segregation of sulfide-bearing mafic units and intermediate sulfide-barren units within these dikes. This is exemplified by primitive amphibole-rich, sulfide-bearing gabbroic sections and more evolved sulfide-barren gabbro-diorite portions of the Morning Star dike. We suggest that the source of Au within the Morning Star dike as well as that in other members of the Woods Point dike swarm was the Cu-PGE-Au sulfides left at depth within the magma conduit system, for example within the Thomson River Copper Mine dike. Mass balance calculations undertaken suggest that ~104 million tonnes (Mt) of magmatic sulfide may exist within the magma conduit system; these magmatic sulfides, if they are the equivalent of the sulfides preserved in the Morning Star dike, probably contain around 3% Ni and 10% Cu. During postmagmatic metamorphism, we infer that hydrothermal fluids flowed along dikes, interacting with magmatic sulfides and leaching some of the Au, which was subsequently deposited in quartz-carbonate veins at higher levels in the system.

The protracted hydrothermal evolution of the Mount Isa Eastern Succession: A review and tectonic implications

Protracted metal and sulfur contributions to the Eastern Succession iron-oxide–Cu–Au (IOCG) province of the Proterozoic Mount Isa Block occurred primarily as a consequence of long-lived fluid fluxes, stimulated by repeated emplacement of voluminous magmas during rifting and thin-skinned convergence cycles. Although there is a direct role for felsic intrusions of the ca. 1530 Ma Williams–Naraku Batholith in hydrothermal ore genesis, these intrusions came at the culmination of protracted metal reorganization in the crust, not as the sole cause, as indicated by geochronology, mineral paragenesis, and the shapes of some orebodies relative to pre-1530 Ma structures. Spatial and geochemical data on mafic rocks suggests that the concentration of copper and gold into some of the mineral deposits involved a significant component of m- to 1000 m-scale remobilization and reworking of early enrichments, formed during basin evolution and initial inversion, by later regional metamorphic and magmatic–hydrothermal fluids. Osborne (eastern domain) and Eloise-type ores (or ore precursors) initially formed during or before the 1600 Ma regional metamorphic peak, by interaction of basinal or early metamorphic fluids with mafic rocks and ironstones, whereas younger oxidised brines released by the Williams/Naraku intrusions at ∼1530 Ma overprinted magnetite ± sulfides at Osborne (western domain) and Starra to produce the presently mined hematite–chalcopyrite ores. CO 2 with mantle-like stable isotope character is abundant at all stages of the hydrothermal evolution and is present in high concentrations even in felsic magmas. We thus infer that CO 2 was released directly from enriched mantle, or indirectly from mafic magmas, contaminating the process of volatile release from the top of felsic magma chambers and contributing to production of carbonate gangue in orebodies. Ernest Henry, the largest IOCG deposit in the district, remains the best candidate for a true syn-granite magmatic–hydrothermal orebody. We infer that ore deposition occurred when mantle- or mafic-derived H–C–O–S fluid mixed with saline, oxidised brine derived from the Williams/Naraku Batholith, stripping some ore components (Fe, Sr, Cu) from the local wallrocks, in particular mafic rocks. The protracted hydrothermal evolution is reminiscent of modern back-arcs but the position of the arc during the post-1800 Ma history was hundreds of kilometres east. We propose that mantle enrichment in volatiles occurred around a pre-1840 Ma plate boundary leaving the Kalkadoon–Leichhardt belt as a magmatic arc remnant. This metasomatised mantle was subsequently re-tapped during prolonged distal back-arc spreading and periodic shortening accompanying ongoing magmatism.

Wide-spaced sampling for delineation of geochemical provinces in desert terrains, northwestern China

Desert terrains are widely distributed in northwestern and northern China, but remain largely unexplored or under-explored. In the past, lag materials (coarse fraction of regolith) have been sampled for regional geochemical surveys. This sampling method was successful in mountainous and hilly regions, but it failed in areas of lower relief, such as slopes, plains, alluvial valleys and basins. Studies conducted since 1999 were aimed at understanding the environment and the nature of geochemical dispersion in order to find a cost-effective, wide-spaced sampling method for delineation of geochemical provinces. The results show that many elements related to mineral deposits are concentrated in the fine fraction (<120 mesh, 125 μm) in the weakly cemented horizon in the upper part of the vertical profile. Wide-spaced sampling was carried out in an area of c. 150 000 km 2 at a density of one sample per 100 km 2 . Samples were collected from the weakly cemented sandy clay-rich horizon at a depth of 15–30 cm and sieved to <120 mesh in the field. The sieved samples were ground to 200 mesh (75 μm). Thirty-four elements were determined by ICP-MS, GF-AAS and AFS. A geochemical atlas and interpreted geochemical maps were generated using a GIS. Geochemical provinces of Au, Cu and U have been recognized. Five Au provinces, with threshold values of more than 2.0 ppb Au, have been delineated. Three of these are consistent with known large Au deposits, and preliminary follow-up surveys indicate that the other two are prospective for undiscovered Au deposits. Seven Cu provinces have been delineated, of which three are endowed with hydrothermal Cu deposits, porphyry Cu deposits and Cu-Ni deposits, respectively. Two provinces of U associated with Mo were delineated in the basin. One of them contains a large sandstone-hosted U deposit and in the other, a large new U deposit was discovered by follow-up drilling.

Exploration under regolith cover, and the problem of remanent magnetism: an example from the Tanami Au province

Evidence of remanent magnetism was observed in aeromagnetic images fromThe Granites/Tanami Inlier in the Northern Territory. An investigation was undertaken to verify whether remanent magnetism existed, and if so, what mineral and/or system caused it. Alternating field demagnetisation and Curie Temperature analyses was employed to calculate the magnetic properties of whole rocks. Optical petrology and electron microscopy were then used to determine the minerals present and their chemical compositions. The two dominant magnetic minerals in this region are multidomain magnetite and monoclinic pyrrhotite, confirmed by petrological analysis. Chemical analysis determined the magnetite and pyrrhotite were pure with no substitution of elements. Alternating field demagnetisation and Curie Temperature analyses show monoclinic pyrrhotite holds the remanent magnetism, but has a low magnetic susceptibility. The study indicates there is remanent magnetism in the Tanami region. If there is inadequate understanding of magnetic data, misinterpretation of magnetic anomalies could occur.

Geology, fluid inclusions, isotope geochemistry, and geochronology of the Paishanlou shear zone-hosted Gold Deposit, North China Craton

The Paishanlou gold deposit lies along the northern margin of North China Craton gold province, the third largest Au province in China. The deposit is mainly hosted in Archean metamorphic rocks of the Jianping Group and is structurally controlled by two sets of ductile shear zones. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones, with a typical greenschist facies alteration assemblage of sericite + chlorite + calcite + biotite + quartz and a distinct alteration zoning of inner pyrite-sericite zone, middle carbonate zone, and outer epidote-biotite-chlorite zone from orebody to wall rock. Fluid inclusion petrography and microthermometric results suggest that four types of fluid inclusions are present at the deposit. Type 1 CO 2-rich inclusions have homogenization temperatures of 22.4–27.9 °C. Type 2 H 2O–CO 2 inclusions show salinities of 4.6–12.3 wt.% NaCl equivalent and Th-tot of 242 to 337 °C. Type 3a primary aqueous inclusions have salinities of 5.9–11.2 wt.% NaCl equivalent and Th-tot of 187–289 °C, while Type 3b secondary aqueous inclusions have salinities of 3.1–8.3 wt.% NaCl equivalent and Th-tot of 128–239 °C. Type 4 multiphase inclusions have salinities ranging from 31.4 to 35.2 wt.% NaCl equivalent and Th-tot of 370–447 °C. The close association of CO 2-rich inclusions and H 2O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. Estimated P– T values range from 1.4 to 1.9 kbar at 322–417 °C. The δ 18O values of the ore-forming fluids are calculated to range from 3.8‰ to 7.4‰ and the δD values from − 87.3 to − 116.2‰, indicating a mixed source of deep-seated magmatic water and shallower meteoric water. The δ 34S values (0.3–7.5‰) of sulfide ore minerals are similar to those of both pyrite from metamorphic country rocks and whole rock sulfur of metamorphic country rocks. The 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb of sulfide ores range within 16.40–17.0, 15.21–15.37, and 36.69–37.38, respectively, with the model ages of 397–727 Ma, consistent with those of Late Mesozoic granites and lying between Archean metamorphic rock lead and the Proterozoic dolomitic marble lead. These sulfur and lead features reflect the fact that the ore-forming materials originated mainly from multiple wall-rock sources. The well-defined Ar/Ar plateau age (126.6 ± 1.1 Ma) of biotite from the NNE ductile shear zone provides a possible temporal link between Au mineralization, magmatism and deformation at the Paishanlou deposit. The concordance of the biotite age with other well-constrained mineralization ages for the deposit, and the intrusion age of the granite, suggests that Au mineralization was essentially contemporaneous with late Mesozoic granitic magmatism and the second stage of mylonitization. In terms of its genetic linkage with the subduction process at the leading northern edge of the North China Craton, the Paishanlou deposit can be favored as an orogenic gold deposit. Its formation benefited from the combined action of protracted activity of ductile fault zones, hydrothermal circulation associated with granitoid intrusion and preferred structural setting favoring gold deposition. The major geodynamic switch towards generalized extensional tectonics that occurred during uplift, and thinning of the upper crust after the collisional stages in the northern edge of the North China Craton, may be the key factors controlling Au concentration and enrichment in the Paishanlou deposit.

Magmatic sulphide/volatile interaction as a mechanism for producing chalcophile element enriched, Archean Au-quartz, epithermal AuAg and Au skarn hydrothermal ore fluids

A combination of available geological, geophysical and geochemical evidence indicates that significant parts, or components, of Archean Au-quartz vein, epithermal AuAg and Au skarn ore fluids were magmatically derived, and that Au, Ag and associated, enriched elements were also magmatically derived (e.g., Giggenbach, 1986; Henley and Hoffmann, 1987; Sillitoe, 1989; Sillitoe and Bonham, 1990; Spooner, 1991 a,b). There is a significant core of commonality in element occurrence in Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn systems which suggests a shared genetic process. For instance, 44% ( n=12) of 27 elements occur in more than seven of ten ore deposit subdivisions: Au, Ag, Fe, Cu, Pb, Zn, Hg, Mo, Sb, As, Te and S (nine metals; two metalloids: one non-metal). Addition of B and W gives 52% ( n=14) in more than five subdivisions. The frequency of occurrence of enriched chalcophile/siderophile elements is high at 85%; 23 out of 27. The converse is also high at 74% (23 of 31): all significant chalcophile/siderophile elements occur enriched in Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn deposits. However, if magma volatile saturation occurred after gravitational loss of magmatic sulphide melt/suspended solids, then Au, Ag and associated chalcophile elements would have been effectively removed from the magma because of known high sulphide-melt/silicate-magma partition coefficients of ∼ 10 2–10 4 (e.g., Au at ∼ 3×10 4; Se at ∼ 10 3). Hence, it is proposed that Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn ore fluids derive their characteristic element enrichments by high-temperature fluid/vapour dissolution of magmatic sulphide-liquid droplets/solids ( T<∼ 1010–1050°C) enriched in Au and associated elements (e.g., 1–13 ppm Au in MORB sulphide globules; Mathez and Peach, 1989) in mafic/intermediate igneous compositions before sulphide loss. The key components of the mineralization enrichment process are sulphide melt/silicate magma partition, during which ore-grade Au concentrations can be produced from non-anomalous background Au abundances of ∼ 1 ppb (enrichment factor=∼ 10 4), high aqueous-phase sulphide solubilities at high temperatures (e.g., Hemley et al., 1986; to 600°C), and selective Au precipitation. Support for this hypothesis is provided by: (a) the element association in Au skarns which shows 81% and 75% overlaps with associations in epithermal AuAg and Archean Au-quartz vein systems, respectively; (b) being able to explain enrichments of a variety of low-abundance elements simply because they are chalcophile (e.g., Se, Tl; Sb, Hg, Bi, As, Te; Ni, Co, Cd, Pd, Pt; Mo, W as scheelite); and (c) being able to explain a tendency for certain large AuAg systems to occur associated with volatile-rich (e.g., subduction zone magmas, 3–7 wt% H 2O; sodic alkaline magmas, ∼ 0.5–1 wt% H 2O; c.f., ∼ 0.1 wt% H 2O in MORB), more mafic/intermediate magma compositions (e.g., Au skarns, Meinert, 1989; Hollinger-McIntyre, ∼ 995 tonnes Au; Ashanti, ∼ 710 tonnes Au; Cripple Creek, ∼ 590 tonnes Au; Ladolam, ∼ 570 tonnes Au; Porgera, ∼ 410 tonnes Au; Kerr Addison-Chesterville, ∼ 335 tonnes Au; Emperor, Fiji, ∼ 120 tonnes Au). On the basis of well-defined igneous intrusive associations, the typical presence of H 2OCO 2 low-salinity ore fluids, and particularly the presence of carbonate ( δ 13C=−4.5±0.6‰) and a chalcophile element association (AuAgSFeCuPbZnMoWAsSbBiTe) which is very similar to those of Archean Au-quartz vein, epithermal AuAg and Au skarn ore systems, a magmatic origin for Cordilleran Au-quartz vein ore systems may be assessed (e.g., Sierra Nevada Au province, California; ∼ 3,150 tonnes Au; Clarke, 1970). This proposal can explain why Archean Au-quartz and, particularly, epithermal AuAg systems occur associated with several different magma series such as calc-alkaline, shoshonitic, sodic alkaline with and without TiNbTa subduction depletions, and components of the Archean tonalite-trondhjemite-granodiorite (TTG) association. These series share a common property of being relatively volatile-rich, thus causing magmatic volatile saturation in more mafic compositions before loss of magmatic sulphide. This process is essentially unavoidable, and should have repeated in space and time. Implications for mineral exploration include a justification for large-tonnage, low-grade Au targets in, or related to, mafic/intermediate intrusions in Archean terrains (e.g., Moss Lake, N.W. Ontario; diorite hosted, >65 mtonnes at ∼ 1 g/tonne Au; >65 tonnes Au) and a justification for evaluating mafic, sodic and potassic alkaline complexes.

Geodynamic setting of mesothermal gold deposits: An association with accretionary tectonic regimes

Mesothermal gold provinces of Phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs. A recurring sequence of transpressive deformation, uplift, late kinematic mineralization, and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust. Mesothermal gold camps in the Superior province are spatially associated with large-scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes: these boundary structures are characterized by the sequence of significant horizontal shortening, uplift, late-kinematic mineralization, and shoshonitic lamprophyres and therefore may have the same geodynamic significance as Phanerozoic counterparts. In this model, thermal re-equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime, over time scales of 10 to 40 m.y., is a necessary precursor to gold mineralization. Hydrothermal fluids are released along boundary faults and their splays during uplift: the uniform temperature, low salinity and mole% CO2 signify uniform source conditions, whereas the variable O, C, Sr, and Pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits. Ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction-related crustal underplating and deep, late metamorphism, rather than magmatic or metamorphic events in the supracrustal rocks. Secular variations in the generation of Archean, Proterozoic, and Phanerozoic mesothermal Au provinces reflect the timing of collisional orogenies within terranes of these eras.