Orogenic Gold Research Articles

Lithological and structural controls of disseminated-type orogenic gold mineralization in high-grade metamorphic turbidites from the Central Zone of the Damara Belt, Namibia

We report the characteristics and controls of low-grade gold-sulfide disseminations in the Twin Hills prospects, the Central Zone of the Damara Belt in Namibia, that combines to a large (>12 km strike length) zone of mineralization. The host rocks are subvertical, tightly folded and transposed, amphibolite facies metaturbidites (cordierite-biotite metapelites and quartz-biotite metapsammites) that form the subvertical limb of a regional-scale syncline. Sulfide and gold grains are finely dispersed in the host rocks or form networks of quartz-sulfide veinlets. Zones of economic-grade gold mineralization are associated with fine-scale quartz-sulfide (pyrrhotite > arsenopyrite ≫ pyrite) vein stockworks. Morphology and deformational textures of the mineralized vein networks suggest their formation during flexural-slip folding and the progressive transposition of the metaturbidites into the regional, ENE trending steep foliation. The more competent metapsammites promoted fracturing while ductile strain during flexural-slip was localized into the schistose metapelites. As a result, economic-grade mineralization is best developed in well-bedded, compositionally heterogeneous parts of the metaturbidites that experienced pronounced strain partitioning. Metapelite-dominated packages, in contrast, show mainly disseminated sulfide mineralization, fewer and variably transposed vein networks and only sub-economic gold grades. The association of disseminated and vein-type mineralization suggests that pervasive fluid flow was channelized within the lithologically heterogeneous packages that provided fracture permeabilities and hydraulic gradients for more focused fluid flow. Higher gold grades correspond to subtle (5-15°) deflections of bedding and the regional foliation that may relate to dilational jog geometries. On a prospect-scale, occurrence of economic-grade gold mineralization coincides with the inflection and change of the vergence of regional-scale first-order folds. The style of mineralization shares similarities with turbidite-hosted orogenic gold deposits. The differences in gold grades and styles of mineralization can be explained by syn-tectonic (D2) fluid flow, the different metamorphic grade (amphibolite versus mid- to lower-greenschist) and ductility of wall rocks that prevented the formation of larger, brittle, high-permeability structures.

Overview of age constraints for gold mineralization in central and western Newfoundland and new 40Ar/39Ar ages for muscovite from selected auriferous zones

This contribution presents new 40Ar/39Ar laser step-heating data for muscovite associated with four significant orogenic gold-mineralized zones in central and western Newfoundland to build upon existing geochronological constraints for orogenic gold mineralization across much of the Newfoundland Appalachians. Additionally, we provide the first 40Ar/39Ar laser step-heating data for detrital muscovite from gold-mineralized sandstone of the Wigwam Formation (Botwood Group) at the Moosehead gold prospect. Most known gold zones on the isIand occur in proximity to reactivated crustal-scale faults and related structures, and are commonly localized within competent, rigid lithological units (e.g., granitoid rocks); although, some vein systems cut less competent, clastic sedimentary rock sequences. Host rocks range in age from Neoproterozoic to Devonian and may extend into the Carboniferous. Robust Pragian to Emsian (ca. 410–375 Ma) rutile, muscovite and zircon ages constrain the timing of gold mineralization in parts of the Exploits Subzone, whereas less precise age estimates for gold mineralization in the Notre Dame Subzone and Laurentian margin range from Wenlock to Emsian (ca. 433–375 Ma). Collectively, the geochronological data suggest that fluid-pressure cycling associated with gold mineralization in central and western Newfoundland occurred during progressive Siluro-Devonian tectonics associated with the waning stages of the Salinic orogenic cycle and spanning the Acadian and Neoacadian orogenic cycles. Multiple, polyphase, overlapping orogenic events allowed for repeated, gold mineralizing fluid flow events, particularly in proximity to long-lived, reactivated crustal-scale fault zones.

Orogenic gold: is a genetic association with magmatism realistic?

Many workers accept a metamorphic model for orogenic gold ore formation, where a gold-bearing aqueous-carbonic fluid is an inherent product of devolatilization across the greenschist-amphibolite boundary with the majority of deposits formed within the seismogenic zone at depths of 6–12 km. Fertile oceanic rocks that source fluid and metal may be heated through varied tectonic scenarios affecting the deforming upper crust (≤ 20–25 km depth). Less commonly, oceanic cover and crust on a downgoing slab may release an aqueous-carbonic metamorphic fluid at depths of 25–50 km that travels up-dip along a sealed plate boundary until intersecting near-vertical structures that facilitate fluid migration and gold deposition in an upper crustal environment. Nevertheless, numerous world-class orogenic gold deposits are alternatively argued to be products of magmatic-hydrothermal processes based upon equivocal geochemical and mineralogical data or simply a spatial association with an exposed or hypothesized intrusion. Oxidized intrusions may form gold-bearing porphyry and epithermal ores in the upper 3–4 km of the crust, but their ability to form economic gold resources at mesozonal (≈ 6–12 km) and hypozonal (≈ > 12 km) depths is limited. Although volatile saturation may be reached in magmatic systems at depths as deep as 10–15 km, such saturation doesn’t indicate magmatic-hydrothermal fluid release. Volatiles typically will be channeled upward in magma and mush to brittle apical roof zones at epizonal levels (≈ < 6 km) before large pressure gradients are reached to rapidly release a focused fluid. Furthermore, gold and sulfur solubility relationships favor relatively shallow formation of magmatic-hydrothermal gold systems; although aqueous-carbonic fluid release from a magmatic system below 6 km would generally be diffuse, even if in cases where it was somehow better focused, it is unlikely to contain substantial gold. Where reduced intrusions form through assimilation of carbonaceous crustal material, subsequent high fluid pressures and hydrofracturing have been shown to lead to development of sheeted veins and greisens at depths of 3–6 km. These products of reduced magmatic-hydrothermal systems, however, typically form Sn and or W ores, with economic low grade gold occurrences (< 1 g/t Au) being formed in rare cases. Thus, whereas most moderate- to high-T orogens host orogenic gold and intrusions, there is no genetic association.

A TRIASSIC OROGENIC GOLD MINERALIZATION EVENT IN THE PALEOPROTEROZOIC METAMORPHIC ROCKS: EVIDENCE FROM TWO TYPES OF RUTILE IN THE BAIYUN GOLD DEPOSIT, LIAODONG PENINSULA, NORTH CHINA CRATON

Abstract Rutile grains often occur in different types of gold deposits, and their U-Pb ages have been widely used to determine the formation time of gold mineralization. However, the origin of rutile grains in the gold deposits remains controversial. In this paper, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of U-Pb ages and trace elements on rutile grains were applied to investigate the metamorphic and hydrothermal processes of the Baiyun gold deposit (70 t Au, avg grade: 3 g/t) in the Liaodong Peninsula in the northeastern part of the North China craton. Rutile grains in the hydrothermal altered gold schist from the Baiyun deposit yielded two group ages of 1924 ± 18 and 237.0 ± 1.8 Ma, respectively. Combined with our systematic U-Pb zircon geochronological results of the ore-hosting schists and post-ore dikes, we suggest these rutile ages record a Paleoproterozoic metamorphic event and a Triassic hydrothermal gold mineralization event, respectively. The metamorphic and hydrothermal rutile grains have no obvious textural differences, but they show distinct trace element contents of Zr, W, Nb, and Ta. Combined with previous published data, we propose that high W (&amp;gt;1,000 ppm) and low Zr (&amp;lt;200 ppm) contents in rutile can be used to distinguish hydrothermal rutile from metamorphic and magmatic rutile. The newly identified ca. 237 Ma hydrothermal event is much older than the ca. 227 to 210 Ma Triassic magmatic rocks in the region, which precludes a temporal and genetic link between the Baiyun gold mineralization and the regional Mesozoic magmatism. Rather, the ca. 237 Ma gold mineralization may be associated with the Triassic orogenic metamorphism, and Baiyun is an orogenic gold deposit. The Triassic gold deposits in the northern margin of the North China craton formed by orogenesis between the Siberian craton and the North China craton. After a hiatus, the large-scale gold deposits formed during the Early Cretaceous in the North China craton due to a westward subduction of the paleo-Pacific plate beneath the craton since the Early Jurassic. Our study highlights that rutile in gold deposits may be inherited from the host rocks and/or formed by hydrothermal fluids. Distinguishing between these two different rutile generations requires a combination of in situ age dating and trace element geochemistry in petrogenetic context.

Paleoproterozoic gold deposits at Alta Floresta Mineral Province, Brazil: two overprinted mineralizing events?

Abstract Large gold provinces commonly show complicated mineralization histories, and the Paleoproterozoic Alta Floresta, one of Brazil's most exciting Au–Cu mineral provinces, is a good example. The current models defined four deposit types, all connected to a single (1.88–1.75 Ga) magmatic–hydrothermal event. However, long Province history, diverse geodynamic environment and older ages of Type-1mineralization weaken the single metallogenic event and enable the hypothesis of overprinted mineral events. By scale-integrated analyses, we revise the tectonic–geological context, structural–hydrothermal alterations and chlorite–white mica geothermobarometry and propose the Type-1 as an older, granitoid-hosted orogenic mineralization, with subsequent overprinting by the magmatic–hydrothermal event. The older orogenic gold event developed orogenic gold deposits on WNW-trending shear zones in the Peixoto de Azevedo domain granitic–gneiss rocks. Phengite, biotite and chlorite–carbonate phyllonites (3.3–6.1 kbar, 300–420°C) host fault-fill quartz veins (pyrite–chalcopyrite–magnetite–pyrrhotite–gold–Bi–Ag tellurides). Mg-rich chlorite–phengite is the main alteration footprint for this mineralization type. A younger magmatic–hydrothermal event in the Juruena magmatic-arc rocks produced Fe-rich chlorite–white mica alteration zones (0.6–4.6 kbar, 120–380°C) and disseminated and stockwork–breccia ore (pyrite–chalcopyrite–gold–molybdenite–Ti minerals–allanite) in porphyry–epithermal deposits. Where the younger mineralization overprints the older, phyllic alteration destroyed the phengite orogenic gold phyllonite S n +1 foliation. The ages of two pyrite populations (1979 and 1841 Ma) in the older fault-fill veins and molybdenite in late fractures (1805–1782 Ma) or disseminated in the c. 1.79 Ga syenogranite porphyry suggest more than two episodes of mineralization. These two events differ in their alteration styles, P–T conditions and structural, mineralogical and textural ore styles. The multiscale approach sheds light on the relationships between the various mineralization events, allowing a new explorational potential within the province. Supplementary material: Chlorite and white mica complete chemical composition data and other characteristics of the studied deposits are available at https://doi.org/10.6084/m9.figshare.c.6056324

An introduction to Recent Advances in Understanding Gold Deposits: from Orogeny to Alluvium: the importance of multi-method approaches and developing a characterization

Abstract Gold occurs in many settings but the dynamic nature of Earth's crust means overlapping and overprinting deposit styles are common. Characterization of mineralization from an early stage becomes important particularly where the mineralization is complex, in order to maximize exploration and project development success, and mining productivity. Various techniques are used at different stages of a project to characterize gold deposits. This Special Publication Recent Advances in Understanding Gold Deposits: from Orogeny to Alluvium offers a cross-section of some specific techniques to investigate a variety of gold deposit types. In this Introduction, we briefly compare the most common gold deposit types, summarize the techniques used to investigate them and discuss some of the most important outstanding questions regarding understanding gold deposits. This is followed by a summary of each paper in this Special Publication. Whilst the 15 papers in this book can only showcase some of the many approaches available, they do highlight both the breadth of the available techniques and their utility in deposit characterization. Several papers include suggestions for avenues for fruitful further research, including a paper suggesting a new approach to classifying orogenic gold deposits, and a paper describing archaeological applications of natural gold analyses.

New Insights into the Evolution and Footprints of the Paraíba Au-Cu-Mo Deposit, Alta Floresta Mineral Province (Brazil), through Integration of Spectral and Conventional Methods

The Paraíba is an Au-Cu-Mo deposit located in the southern part of the Amazon Craton, in the Alta Floresta Mineral Province, Mato Grosso State, Brazil. It is composed of Au-Cu-bearing quartz veins and Cu-Mo-rich hydrothermal breccias and stockworks, both associated with several hydrothermal alteration zones. The integration of spectral (reflectance and imaging spectroscopy) and conventional techniques (core logging, petrography, mineral chemistry, and scanning electron microscopy) was applied to map alteration zones, identify mineral parageneses, and determine the evolution of the deposit to generate a solid proxy for mineralized zones. This study shows an overlapping of two different mineralization systems in the AFMP, referenced hereafter as alteration Groups 1 and 2. Group 1 is an Orogenic Gold system related to Au-Cu-rich quartz veins in phyllonites and mylonites. Group 2 is a Cu-Mo Porphyry system related to a syenogranite intrusion and the subsequent generation of several hydrothermal alteration zones, Cu and Mo hydrothermal breccias, and stockwork mineralization. This study reveals several alteration footprints, guides, and vectors for the mineral exploration of these deposits in the Alta Floresta Mineral Province.

Formation of orogenic gold deposits by progressive movement of a fault-fracture mesh through the upper crustal brittle-ductile transition zone

Orogenic gold deposits are comprised of complex quartz vein arrays that form as a result of fluid flow along transcrustal fault zones in active orogenic belts. Mineral precipitation in these deposits occurs under variable pressure conditions, but a mechanism explaining how the pressure regimes evolve through time has not previously been proposed. Here we show that extensional quartz veins at the Garrcon deposit in the Abitibi greenstone belt of Canada preserve petrographic characteristics suggesting that the three recognized paragenetic stages formed within different pressure regimes. The first stage involved the growth of interlocking quartz grains competing for space in fractures held open by hydrothermal fluids at supralithostatic pressures. Subsequent fluid flow at fluctuating pressure conditions caused recrystallization of the vein quartz and the precipitation of sulfide minerals through wall-rock sulfidation, with some of the sulfide minerals containing microscopic gold. These pressure fluctuations between supralithostatic to near-hydrostatic conditions resulted in the post-entrapment modification of the fluid inclusion inventory of the quartz. Late fluid flow occurred at near-hydrostatic conditions and resulted in the formation of fluid inclusions that have not been affected by post-entrapment modification as pressure conditions never returned to supralithostatic conditions. This late fluid flow is interpreted to have formed the texturally late, coarse native gold that occurs along quartz grain boundaries and in open spaces. The systematic evolution of the pressure regimes in orogenic gold deposits such as Garrcon can be explained by relative movement of fault-fracture meshes across the base of the upper crustal brittle-ductile transition zone. We conclude that early vein quartz in orogenic deposits is precipitated at near-lithostatic conditions whereas the paragenetically late gold is introduced at distinctly lower pressure.

Black shales and mesozonal quartz vein‐hosted Au: The Truchas Syncline, Spain and the Harlech Dome, Wales, a comparative study

A comparative study of quartz vein‐hosted gold occurrences associated with Palaeozoic metapelites in two areas of Wales and Spain combines new, previously unpublished and published data. Metamorphic grade is greenschist in both areas, but very low‐grade indicators in the host metapelites distance the environment from the greenschist/amphibolite transition zone required for some orogenic gold occurrences. Basin fertility for Au is indicated by the presence of auriferous pyrites in the protolith black shales in Spain. Only minor igneous activity has taken place in both study areas. Mineral parageneses are similar, with early sulphide phases characterized by As/Co and later auriferous phases by Cu/Pb/Zn sulphides. Mesozonal P–T conditions apply at deposition in both terranes. In Spain, mineralisation typically occurs in quartzites near to the metapelites, but not where the veins are in contact with them, and extensional faulting appears to be a stronger control over mineralisation than geochemical interaction with metapelite wall‐rocks. In Wales, both structural and geochemical factors (C content of the wall‐rocks and coupled oxidation of NH4 ions substituted in wall‐rock phyllosilicates to produce CH4 and N2) could have a role in Au deposition. In both areas, minor cross‐fault systems between larger faults are typical hosts of the mineralisation. Assignment is made to different subtypes of the orogenic gold model but these subtypes share the characteristic of a local source. This has implications for exploration methodology in epizonal/anchizonal metapelite‐dominated terranes, where indicators of basin fertility for Au within the protolith itself assume importance.

The Quartz Veins, Hydrothermal Alteration and Ore Mineralization of Orogenic Gold Deposit at Mendoke Mountains, Southeast Sulawesi, Indonesia

The research at Mendoke Mountains, Southeast Sulawesi Province, particularly for the study of gold deposits, becomes a new challenge because there is no previous detailed study. This study aims to analyze the characteristics of orogenic gold deposits, which include the type and texture of quartz veins, alteration and hydrothermal mineralization with hosted metamorphic rocks. The research methods used include fieldwork and laboratory analysis. The type and texture of quartz veins were identified based on the field observations. The hydrothermal alteration was analyzed using X-ray diffraction analysis, and hydrothermal mineralization which analyzed by ore microscopy. The results showed that there are three types of quartz veins parallel and crosscut to the direction of foliation, and laminated quartz veins. The quartz veins’ texture is deformed, segmented, brittle, sheared, laminated, sheeted, irregular veins, brecciated, massive and sigmoidal. The hydrothermal alteration consists of sericitization, argillic, propylitic and carbonization alterations. The hydrothermal mineralization consists of native gold, chalcopyrite, pyrite, stibnite, covellite, cinnabar, galena, arsenopyrite, chrysocolla, magnetite, hematite and goethite. The host rock of gold mineralization in the study area is classified into the greenschist facies. Based on these characteristics, it shows that gold deposits in the Mendoke Mountains are orogenic gold deposits.

Large Amount of Excess Argon in Hydrothermal Quartz from the Vangtat Orogenic Gold Belt, Southern Laos: New In-Sight from K-Ar and Noble Gas Isotope Analyses

K-Ar dating and 3He/4He and 40Ar/36Ar analyses were conducted on samples of hydrothermal quartz from the Thongkai-Ok Au deposit in the Vangtat orogenic Au belt of southern Laos to study the presence of excess argon in hydrothermal quartz and to better understand the origin of the ore-forming fluids. The K-Ar age of two hydrothermal quartz separates yielded apparent dates of 1040 and 1385 Ma. These ages are significantly older than the estimated age of Vangtat Au mineralization (~200 Ma) and other regional geologic events, indicating the presence of an extraordinary amount of excess argon in the hydrothermal quartz crystals. In vacuo crushing analyses of the Thongkai-Ok hydrothermal vein quartz delivered 3He/4He ≈ 0.2–0.3 R/Ra and 40Ar/36Ar ≈ 455–725, suggesting the ore-forming fluid is mainly derived from a crustal component with a minor contribution of mantle and meteoric fluids.

Barren ground depressions, natural H2 and orogenic gold deposits: Spatial link and geochemical model

A review of the localities in continental rocks where H2-rich gases have been reported, showed that they are mainly located near orogenic gold deposits. Two types of geomorphological features known as markers of gas venting in sedimentary basins were also systematically observed near orogenic gold deposits on satellite images. They consist in both barren ground depressions and high densities of small (<20 m in diameter) circular- and comet-shaped white spots in 32 and 7 localities, respectively. Point pattern analysis revealed that the white spots are self-organized, and similar to previously described vegetation patterns associated with termite mounds and fairy circles. We proposed a geochemical model to account for this relationship between orogenic gold deposits, H2 emanations and geomorphological features. Fe‑carbonates are ubiquitous mineral products associated with gold mineralization. They can further dissolve in the presence of aqueous fluid due to their high reactivity below 200 °C to produce magnetite and up to ∼1 mol H2 per kg of rock along with ∼3 mol/kg CO2. This process induces a solid volume decrease of 50 %. Therefore, we propose that Fe‑carbonate dissolution is (1) the primary source of H2 in orogenic gold deposit areas, and (2) involved in the formation of the geomorphological structures reported here, providing a new framework to understand their seemingly complex formation. Ground depressions and white spots are possible tools for gold exploration. Actually, we identified four new areas where we suspect possible orogenic gold deposits. The association between H2-rich gas and ground depressions was also made near other formations containing Fe‑carbonates such as iron formations and carbonatites. This suggests that H2 production through Fe‑carbonate dissolution is not restricted to gold deposits. The global H2 production in crustal rocks associated with Fe‑carbonate alteration is estimated to 3 × 105 mol/yr.

Lithium isotopic behaviour during high-temperature fluid-rock reactions of metapelites (>200 °C): A case study from the Baiyun orogenic gold deposit, Liaodong Peninsula, North China Craton

Mixing of low δ7Li sediment components (δ7Li(‰) = 1000 × [(7Li/6Li)sample/(7Li /6Li)L-SVEC − 1]) with high δ7Li altered oceanic crusts can explain the MORB-like Li isotopic signature of arc lavas. However, marine sediments with low δ7Li values have rarely been reported. The metasomatism of metasedimentary rocks in mélange zones may induce large Li isotopic fractionation; however, the Li isotopic behaviour of metasedimentary rocks during high-temperature fluid-rock reactions remains poorly understood. In this study, an integrated Li-O-Sr-Nd isotopic investigation was conducted on the Baiyun orogenic gold deposit, which represents a relatively simplified high-T hydrothermal system, to explore the Li isotopic behaviour during high-T metasomatism (>200 °C) of metapelites. The main host rocks in Baiyun are lithologically monotonous schists, with alterations including K-feldspathization, sericitization, and carbonation. The Nd isotopes identified host rock samples that could best represent the protolith of the altered schists in Baiyun. The Sr and O isotopic modelling yielded consistent water/rock ratios of 0.1–7.6 and 0.6–5.9, respectively, suggesting that relatively small volumes of fluids were infiltrated. There is no correlation among Li, Sr, and O isotopes; instead, a negative Li-K2O correlation and a positive δ7Li-K2O correlation of the altered schists are observed, suggesting that potassic metasomatism controlled Li concentrations and Li isotopes. The δ7Li values of altered schists in Baiyun can be classified into Group I, having lower Li concentrations (3.2–22.2 ppm) and δ7Li values (−3.8–0.4‰) than the protolith (31.6–47.9 ppm, 0.9–2.8‰), and Group II, having significantly lower Li concentrations (0.9–2.5 ppm) and overlapped or higher δ7Li values (1.0–4.5‰) than the protolith. Fluid leaching during high-T alteration removes Li and more 7Li than 6Li, which accounts for the lower Li concentrations and δ7Li values of Group I. When there is little Li remaining in rocks owing to progressive fluid leaching, the Li isotopes of the altered schists are equilibrated with heavy fluid Li (fluid buffering), which leads to higher δ7Li values observed in Group II. Based on a case study in Baiyun, this study proposes that similar high-T metasomatism of metasedimentary rocks in mélange zones could result in low δ7Li sediment inputs. Mixing of flux-modified isotopically light sediment inputs with isotopically heavy, altered oceanic crusts could account for the MORB-like Li isotopes of arc lavas.

The roles of hydrocarbons on the mineralization of Carlin-type gold deposits, Nanpanjiang Basin, South China

The spatial association of hydrocarbons with metalliferous ore deposits, like MVT lead–zinc deposits, sandstone uranium deposits and various type gold deposits (epithermal Au-Ag (-Hg) deposits, orogenic gold deposits and Carlin-type gold deposits) is found worldwide. However, the genetic relationship between hydrocarbon evolution and mineralization is still under controversy. Integrating isotope dates related to timing of petroleum evolution and gold mineralization from published data, as well as high-resolution transmission electron microscopy (TEM) analysis of pyrobitumen, electron probe microanalysis (EPMA) on both pyrobitumen and pyrite, the In-situ S isotope analyses of the gold-bearing pyrite in this study, we consider different type hydrocarbons (liquid oil and gas) played different roles on the mineralization of Carlin type gold deposits. As the residue of liquid oil cracking, the widespread high maturity pyrobitumen within the reservoir indicate both oil and gas has ever accumulated in the Nanpanjiang Basin. The high gold abundance (EPMA: ∼120 ppm; TEM: ∼1500 ppm) within the pyrobitumen in agreement with the metal solubility experiment suggest liquid oils could have acted as gold carrier before metal precipitation. The late Triassic timing overlapping between gas generation and gold mineralization and the δ34S features with gold bearing pyrite ∼ 10 ‰ lighter than the coeval seawater sulfate indicate gas dominated TSR process may play an important role on the supply of gold bisulfide complex (Au(HS)2−) in the ore-forming fluid and directly involved the precipitation of the Carlin-type gold deposit.

Genesis of the Longtougou gold deposit, South Qinling Orogen, China: Constraints from ore geology, trace element, and S–Pb–H–O isotopes

Orogenic gold deposits are a significant source of gold production, although the geological processes and metallogenic material sources remain somewhat controversial. The Longtougou deposit is located in the South Qinling Orogen, China, and is a newly discovered moderate tonnage gold deposit. The deposit can be split into four stages of mineralization and contains three types of pyrite (Py0, Py1, and Py2). The Py0 is fine-grained, present as subhedral–anhedral aggregates, and formed during diagenetic stage of sandstones of the Devonian Dafenggou Formation. Coarse- to medium-grained euhedral Py1 is hosted by early-ore stage quartz–pyrite veins. The last pyrite to form was fine-grained anhedral Py2, which coexists with barite within main-ore stage quartz–barite–polymetallic sulfide veins. The final stage is associated with the formation of quartz–barite–carbonate veins and stockworks. Early-ore stage samples yield δ18OH2O values from 9.8‰ to 13.9‰ and δDV-SMOW values from −75.4‰ to −65.3‰, indicative of fluids may be derived from a mixing of metamorphic and magmatic fluids. Main-ore stage samples yield δ18OH2O and δDV-SMOW values of 5.4‰ and −70.6‰, respectively, whereas late-ore stage samples yield δ18OH2O values from −9.0‰ to 3.3‰ and δDV-SMOW values from −74.5‰ to −56.7‰. These data indicate that the main-ore stage involved meteoric and organic water, whereas the late-ore stage was dominated by meteoric water. The Py0 has significantly negative δ34S values (−15.1‰ to −14.5‰), suggesting that the sulfur in this pyrite was derived from bacterial sulfate reduction. Early-ore stage sulfides have restricted ranges of δ34S values (−5.7‰ to 5.6‰) that are indicative of a magmatic–hydrothermal fluid origin. The Py2 yields lower positive δ34S values (2.4‰ to 5.3‰) and coexisting barite has higher positive δ34S values (15.3‰ to 17.5‰), indicative of a transition into an oxidizing environment. The Δ34SA–B−δ34SB diagram yields a δ34SΣS value of 8.2‰, indicative of sulfur derived from a mix of sedimentary and magmatic sources. All of the sulfides have Pb isotopic ratios that plot within the field of underlying Paleozoic strata and are close to the Indosinian and Yanshanian granites, suggesting that the metals were derived from a combination of underlying Paleozoic sedimentary and granitic sources. Most of the hydrothermal pyrites have Co/Ni and S/Se ratios that are indicative of a magmatic-hydrothermal origin, suggesting that the Au was derived predominantly from magma but with possible contributions from sedimentary and metamorphic fluid sources. In addition, hydrothermal pyrites have trace-element variations that differ from Py0, suggesting that the gold was not derived from the local host rocks. An underlying Paleozoic sedimentary sequence enriched Au that underwent greenschist facies metamorphism may be another source of the gold. The majority of invisible Au is present as structurally bound, with visible gold present as inclusions or as fracture fills within Py2. The gold was precipitated as a result of fluid mixing, a process that caused an increase in fO2 values and a change from reduced to oxidized conditions during the main-ore stage. This study suggests that the Longtougou deposit is an orogenic gold deposit that formed during the transition between compression and extension in an intracontinental orogeny.

Fluid inclusion and in situ sulfur isotope studies of the carbonaceous strata-hosted Eryiling gold deposit in the Beishan orogen, NW China

The Eryiling gold deposit in the Jinwozi orefield is a carbonaceous strata-hosted gold deposit in the Beishan Orogen, NW China. The gold orebodies are mainly layered, lenticular, and vein hosted in Jinwozi Formation, different from the Jinwozi gold deposit. In this contribution, based on a comprehensive study including field investigation of ore geology, fluid inclusion microthermometry, and in situ sulfur isotopic composition, we discuss the origin and source of the ore fluids and the gold precipitation mechanism at the Eryiling deposit. The ore-forming process includes the early milky white quartz-pyrite vein, the main stage smokey gray quartz-sulfide vein, and the late-stage quartz-calcite vein. The auriferous vein characterizes the main-stage veins. By micro-thermometry, CO2-bearing(C-), Pure CO2(PC-), Water(W), and daughter crystal(S) types of fluid inclusions were identified and investigated. The early-stage quartz vein contains C-type FIs with a salinity of 3.5 to 7.7 wt% NaCl eqv., and total homogenization temperature (Th) of 243 and 321 °C. It shows that the ore fluids possess characters of low salinity, CO2-rich, and mesothermal. These characters resemble to those of orogenic-type gold deposits. The C-type FIs in the main stage quartz show salinity of 3.5 to 8.9 wt% NaCl eqv., and Th of 200 and 350 °C. The W-type FIs are coexisting with C-type FIs which they yield salinity of 5.5 to 10.2 wt% NaCl eqv., and Th of 158 – 287 °C. A large amount of immiscibility fluid inclusion assemblages found in the main stage quartz provide clear evidence for gold precipitation caused by fluid immiscibility. Trapping pressures of 61–233 MPa for the early stage and 48–182 MPa for the main stage are estimated, equivalent to mineralization depths of ∼ 8.1 km and ∼ 6.4 km, respectively. The δ34S value of sulfide increased from early-stage (average of 3.8 ‰) to main-stage (average of 6.4 ‰), indicating sulfur contributed by the wall rocks through fluid-rock interaction as the wall-rock sulfide display value of average of 16.2 ‰. In summary, the Eryiling gold deposit is situated in the altered carbonaceous rock of the Jinwozi Formation and regulated by a fractured zone, which could be considered a typical carbonaceous strata-bound orogenic gold deposit. Moreover, the similarity of ore-forming fluids between Jinwozi and Eryiling gold deposits indicates that they are the products of the same ore-forming fluid in different wall rocks.

Clumped isotope geothermometry in Archean mesothermal hydrothermal systems (Augmitto-Bouzan orogenic gold deposit, Abitibi, Québec, Canada): A note of caution and a look forward

This study evaluates the applicability of the clumped isotope thermometry to mesothermal hydrothermal systems (5–10 km depth; 250–450 °C). We measured Δ47, δ18O and δ13C of calcite as well as δ18O of cogenetic minerals from typical quartz-calcite ± tourmaline±chlorite orogenic veins from the Neoarchean Augmitto-Bouzan orogenic gold deposits (Abitibi, Canada). Our findings show that caution is required when utilizing the clumped isotope thermometry in the study of old mesothermal deposits. Temperatures calculated from Δ47 values are systematically and significantly shifted to low temperature, i.e., ∼150 °C rather than the ∼350 ± 50 °C expected for orogenic gold deposit formation and documented using oxygen isotope equilibrium between mineral pairs. We show that the low temperatures estimates resulted from solid-state reordering that occurred in calcite grains during the cooling history of the vein-hosting rocks. Because the fast cooling rates required to preserve the initial temperature of calcite formation in mesothermal hydrothermal systems are geologically unrealistic, we suggest that refractory minerals (i.e., minerals with higher blocking temperature such as magnesite, dolomite, ankerite) should be investigated to apply clumped isotope thermometry in such context.

Co-precipitation of gold and base metal sulfides during fluid boiling triggered by fault-valve processes in orogenic gold deposits

The extreme fluctuations in pressure during earthquakes are widely regarded as responsible for gold mineralization in quartz-vein-hosted gold deposits. However, it is barely noticed that base metal sulfides can precipitate together with gold during these processes. Here we present the phenomenon and unravel the mechanism of the co-precipitation of Au and base metal sulfides during the fluid boiling via geological study and thermodynamic modeling, respectively. The Bangbu deposit, a lode-type orogenic gold deposit in Tibet shows two mineralization substages of pyrite. The pyrite formed in the later substage is hosted in the wall rock selvages of laminated crack-seal quartz veins formed by the fault-valve processes. The pyrite grains are characterized by anhedral small crystals with porous textures and abundant native gold, chalcopyrite, galena, and sphalerite inclusions. They also contain higher concentrations of Cu, Pb, Ag, Sb, and Au, and lower concentrations of Co and Ni, compared to the euhedral large-grained pyrite formed in the initial substage. The textural and trace element characteristics of the pyrite indicate that vigorous boiling occurred during fault-valve behavior, which decreased the solubilities of Au, Cu, Pb, and Zn in the ore fluid in Bangbu. The co-precipitation of Au and base metal sulfides triggered by fluid boiling has also been reported in other orogenic gold deposits worldwide.Thermodynamic models are designed to acquire the predominant species and solubility of Au, Cu, Pb, and Zn in orogenic ore fluids under varied chemical conditions. The results show that Au, Cu, Pb, and Zn are dominantly transported as hydrosulfide complexes under lower mesozonal to epizonal deposit conditions. At temperatures of about above 350 °C, Au hydrosulfide species still predominates, but the predominant Cu, Pb, and Zn species change from hydrosulfide to chloride complexes. A sudden decrease in the reduced sulfur concentration during fluid boiling may be the most important mechanism controlling the precipitation of Au, Cu, Pb, and Zn in lower mesozonal to epizonal deposits. The decrease of logfS2 during fluid boiling can increase the fluid pH, resulting in the decrease of solubilities of base metal chloride complexes and thus explain the co-precipitation of Au and base metal sulfides in hypozonal deposits.

PLATE TECTONICS AS A TOOL FOR GLOBAL SCREENING OF MAGMATIC ARCS AND PREDICTIONS FOR RELATED PORPHYRY DEPOSITS

Abstract The formation of most mineral deposits is closely linked to a geodynamic context—for example, the association of porphyry Cu-Au-Mo deposits with subduction and development of volcanic arcs. This paper proposes a new approach to the global screening of volcanic arcs and their duration, as a predictive method for a mineral systems-based approach (e.g., discovery of potential porphyry occurrences). The method utilizes geospatial and temporal analytics run on a combination of large global data sets and a global plate tectonic model (covering the time period 600 Ma to present) containing plate boundaries. The method involves (1) calculating present-day arc volcano-to-trench distances and obtaining average volcanic-arc widths in both continental and intraoceanic settings, (2) applying these values to the paleotrenches contained in the plate tectonic model on 53 time intervals spread throughout the Phanerozoic, (3) unreconstructing the results back to present day, and (4) summing up all magmatic arc occurrences using their cumulative durations. This results in a spatiotemporal model of the total cumulative duration of magmatic arc activity at the global scale, back to 600 Ma, that is updatable and can serve as a proxy to predict porphyry deposit likelihood. The model output is tested against a porphyry copper occurrence data set to validate the approach as a predictive proxy for arc-related porphyry deposits. The alignment of the model results with data control is high for most geologic time periods throughout the Phanerozoic—up to 90% in the case of buffered (1σ) magmatic arcs and up to 100% in the case of buffered magmatic arcs with an additional search distance (2σ). Recent advances in plate tectonic model quality and detail now offer a higher level of precision and confidence than ever before and enable tools for the prediction and screening of porphyry deposit locations, as well as opening the potential to screen for other geodynamic context-dependent commodities (e.g., orogenic gold, volcanogenic massive sulfide, or Ni and platinum group element-sulfide deposits), particularly in the search for poorly exposed or subsurface orebodies.

FAULT-INDUCED GOLD SATURATION OF A SINGLE AURIFEROUS FLUID IS A KEY PROCESS FOR OROGENIC GOLD DEPOSIT FORMATION

Abstract In orogenic gold deposits, the mechanism by which a hydrothermal fluid precipitates gold in laminated quartz veins remains elusive. The Kanowna Belle deposit hosts gold mineralization in structurally controlled quartz-pyrite laminated veins that cut volcano-sedimentary and granitic rocks of the Kalgoorlie terrane, Australia. Veins show microtextural evidence for multiphased emplacement, corresponding to three distinct pyrite growth zones in which Au enrichment in the pyrite is attributed to high fluid influx. To monitor the chemical and isotopic evolution of the auriferous fluid leading to gold precipitation, we combine in situ multiple sulfur isotope analyses with trace element composition of gold-bearing pyrite growth zones: cores (Au ≤149 ppm; δ34S –3.3 to +4.2‰; As ≤2.5 wt %; Ni ≤4,022 ppm; Te ≤416 ppm), Au-rich oscillatory zoned mantles (Au ≤2,251 ppm; δ34S –8.4 to +0.1‰; As ≤4.5 wt %; Ni ≤1,111 ppm; Te ≤829 ppm), and rims (Au ≤264 ppm; δ34S –6.0 to +2.8‰; As ≤1.4 wt %; Ni ≤2,113 ppm; Te ≤229 ppm). The positive and consistent Δ33S of each zone (Δ33S = +0.3 ± 0.2‰; n = 160) indicates that one single reservoir was tapped during the evolution of the mineralizing system. The gold-rich pyrite mantle zones precipitated from a fluid undergoing SO42−/H2S fluctuations associated with phase separation due to rapid and repeated releases in fluid pressure. This study demonstrates that the “fault-valve” process applies a first-order control on the precipitation of gold from a single auriferous fluid reservoir.