Formation Of Orogenic Gold Deposits Research Articles

Coprecipitation of amorphous silica and gold nanoparticles contributes to gold hyperenrichment

Hyperenrichment of Au in orogenic ores occurs overwhelmingly within quartz veins, but the formation pathway of quartz veins in orogenic systems remains enigmatic. We conducted hydrothermal experiments simulating coprecipitation of Au and amorphous silica and subsequent recrystallization to test whether this is a viable mechanism to generate Au nuggets within quartz veins. Within minutes, coprecipitation of amorphous silica and Au nanoparticles occurred, representing an effective Au deposition mechanism. Within one week, amorphous silica had recrystallized to quartz, causing the coarsening of Au particles and their relocation to quartz grain boundaries and fractures. The experimental textures are similar to those observed in high-grade zones of orogenic gold deposits. In addition to trapping Au, amorphous silica may increase competency contrasts that facilitate short-term fracture reactivation during earthquake aftershock periods or swarms, allowing further Au input from fresh fluids. These findings demonstrate that amorphous silica precipitation may be an important transient stage in orogenic gold deposit formation, with significant implications for metal accumulation in quartz veins.

Gold Migration and Precipitation as Collaurum in Orogenic Gold Deposits: Constrains from Microscopic Gold Particles Observed in the Alteration Zone in Shanggong Gold Ore, Henan, China

Aqueous complexation has long been considered the only viable means of transporting gold to depositional sites in hydrothermal ore-forming systems. Here, we present direct evidence supporting an alternative hypothesis, namely, the transport of gold as colloidal particles. We observed nano-scale gold particles adsorbed on halloysite and micro-scale gold particles in altered rocks by TEM and SEM in the Shanggong orogenic gold deposit. Based on this evidence, we propose a feasible model for the origin of microscopic gold particles in alteration zones. In the early stage of ore-forming fluid, gold may migrate in the form of collaurum, which is maintained by supercritical CO2 and colloidal silica. Low salinity and high pressure are conducive to the stable migration of colloidal gold. When the physicochemical conditions change, some collaurum is precipitated and adsorbed by the clay minerals produced by hydrothermal alteration, and some collaurum undergoes growth and evolves into micro-submicrometer-sized gold particles. This study highlighted the significance of collaurum in the formation of orogenic gold deposits.

Lithospheric mantle as a metal storage reservoir for orogenic gold deposits in active continental margins: Evidence from Hg isotopes

Abstract The subcontinental lithospheric mantle (SCLM) is now suggested by many workers to play a role in the formation of orogenic gold deposits in active continental margins, given that the gold and volatiles may be introduced into the SCLM during plate subduction. The giant Cretaceous Jiaodong gold province within the North China block occurs in a convergent margin setting where there is no terrane accretion, thus eliminating from consideration the commonly accepted metal and fluid source reservoir for orogenic gold—the metamorphosed upper crust of an active orogenic belt. Thus, the auriferous fluids were released below the high-grade Archean cratonic rocks from either enriched SCLM or directly from the subducted oceanic slab. Mercury (Hg) isotopes allow fingerprinting of isotopically distinct reservoirs, discriminating between the two possibilities. We present Hg isotope data for a set of pyrite grains, native gold, and lamprophyres from six main gold districts in the Jiaodong gold province. These samples dominantly yielded near-zero Δ199Hg (0‰ ± 0.1‰), within uncertainty of the mantle Δ199Hg value. The results provide novel evidence that the SCLM predominantly controlled the Hg budget of the Jiaodong gold deposits, implying that the SCLM acted as a buffer for the crustal mass-independent fractionation Hg signatures and a storage reservoir for gold and volatiles. Consequently, during slab subduction, gold and volatiles were recycled and stored within the SCLM and were later released from the mantle during uplift and relaxation of compressional tectonics.

The Fluid Regime of Orogenic Gold Deposit Formation in the Yana-Kolyma Belt

The Fluid Regime of Orogenic Gold Deposit Formation in the Yana-Kolyma Belt

Prograde metamorphism provides gold and base metals to orogenic gold deposits in southern Tibet: Insights from thermodynamic modeling

The metamorphic devolatilization model is presently the most widely accepted model for the formation of orogenic gold deposits. According to this model, Au is likely derived from metamorphic rocks that experienced metamorphism from greenschist to amphibolite facies. Nevertheless, in recent years, some researchers have suggested that Au may have originated from different source regions, such as the mantle. As one of China's most significant orogenic gold provinces, the Indus-Yarlung Zangbo suture zone (IYZSZ) in southern Tibet plays a critical role in this discussion. Orogenic gold deposits within the IYZSZ are typically hosted in Triassic metasedimentary rocks called the Langjiexue group. This paper aims to investigate the suitability of metasedimentary rocks of the Langjiexue group as a source region of Au and base metals. Furthermore, we also examine whether the metamorphic devolatilization model is applicable to the IYZSZ. Our study involves reporting the mineral compositions and evolution of metamorphic zones (chlorite to sillimanite zones) in metasedimentary rocks obtained from the IYZSZ and providing detailed petrographic descriptions of these rocks. Thermodynamic modeling based on metamorphic mineral compositions was used to reveal the behaviors of H2O, Au, S, Cu, Pb, and Zn within prograde metamorphism. Our modeling results proved that the breakdown of chlorite and muscovite, both of which are the main mechanisms of Au and base metals release in metasedimentary rocks, occurs under different geothermal gradients. Given the observed metamorphic mineral compositions, it is the breakdown of chlorite that is the predominant reaction responsible for releasing Au and base metals under the real geothermal gradient of lower crustal metasedimentary rocks (∼25°C/km). This gradient is capable of releasing at least 1.5 ppb Au, 5.2 ppm Cu, 4.3 ppm Pb, 5.0 ppm Zn, and 20 ppm S (as part of the bulk rock) into the ore fluids. Through the mass balance calculations, we found that over 4.05 t of Au, 14040 t of Cu, 11600 t of Pb, and 13500 t of Zn can be mobilized from 1km3 of metasedimentary rocks. Therefore, lower crustal metasedimentary rocks can play a significant role in contributing to Au and base metals in the IYZSZ and other metasediment-hosted orogenic gold deposits worldwide.

Orogenic gold belt overlap in the eastern Peninsular Ranges batholith: La Rumorosa mining area, Baja California, Mexico

Gold deposits in the La Rumorosa mining area (Baja California), located in the eastern Peninsular Ranges batholith (PRB), consist of auriferous quartz-(carbonate) veins hosted by brittle-ductile shear zones within metamorphic and synorogenic plutonic rocks. A combination of two geochronology methods, U–Pb zircon dating of country rocks and 40Ar/39Ar dating of hydrothermal and metamorphic white micas, along with the structural characterization of the main orebodies, has allowed the differentiation of two gold mineralization events in the study area. The oldest deposit (>93 Ma) is related to the main compressive phase of the PRB construction (∼110‒90 Ma) and is part of the yet poorly known Peninsular Ranges orogenic gold belt. Two other gold deposits were dated at ∼77 and 75 Ma, an age span that approximately corresponds to the beginning of the Laramide tectonism instead. Therefore, these deposits can be considered the oldest and westernmost constituents of the Caborca orogenic gold belt (∼69‒36 Ma), formerly recognized in the neighboring states of Sonora, Arizona, and the southeastern tip of California. Furthermore, the Laramide gold deposits at La Rumorosa are contemporaneous with the first generation (∼76‒74 Ma) of copper porphyries in the southern North American Cordillera belt, which are also situated on the opposite margin of the Gulf of California-San Andreas fault system. Both gold and copper deposits show an eastward migration during the early Upper Cretaceous, similar to the well-documented shift in magmatism and deformation in the region, albeit occurring asynchronously. The apparent continuous formation of orogenic gold deposits in southwestern North America from the middle Cretaceous to the Eocene, resulting from the strong convergence between the subducted Farallon plate and the continent, suggests a connection between the Peninsular Ranges and Caborca gold belts. We surmise that both of these orogenic gold belts may form part of a large metallogenic province in western Mexico.

Crustal conductivity footprint of the orogenic gold district in the Red Lake greenstone belt, western Superior craton, Canada

AbstractA magnetotelluric (MT) study across the Red Lake greenstone belt of the western Superior craton, Canada, images a 50-km-long north-dipping conductor (<20 Ω·m) at 20–25 km depth and subvertical conductors spatially correlated with crustal-scale shear zones and large orogenic gold deposits. The conductors are interpreted to be the conductivity signature of the deep crustal source of the auriferous fluids and pathways of the orogenic gold system. The geophysical results, supported by existing geochemical and fluid inclusion studies, suggest that the Au- and CO2-rich fluids responsible for gold mineralization were released by devolatilization of supracrustal rocks underthrust to mid- to lower-crustal levels during subduction. This MT study links shallow gold mineralization to a deep crustal source region, demonstrating the connection between a crustal suture zone and the formation of orogenic gold deposits in an Archean greenstone belt.

Lithosphere architecture characterized by crust-mantle decoupling controls the formation of orogenic gold deposits.

This study, via combined analysis of geophysical and geochemical data, reveals a lithospheric architecture characterized by crust-mantle decoupling and vertical heat-flow conduits that control orogenic gold mineralization in the Ailaoshan gold belt on the southeastern margin of Tibet. The mantle seismic tomography indicates that the crust-mantle decoupled deformation, defined from previous seismic anisotropy analysis, was formed by upwelling and lateral flow of the asthenosphere, driven by deep subduction of the Indian continent. Our magnetotelluric and seismic images show both a vertical conductor across the Moho and high Vp/Vs anomalies both in the uppermost mantle and lowest crust, suggesting that crust-mantle decoupling promotes ponding of mantle-derived basic melts at the base of the crust via a heat-flow conduit. Noble gas isotope and halogen ratios of gold-related ore minerals indicate a mantle source of ore fluid. A rapid decrease in Cl/F ratios of lamprophyres under conditions of 1.2 GPa and 1050°C suggests that the ore fluid was derived from degassing of the basic melts. Similar lithospheric architecture is recognized in other orogenic gold provinces, implying analogous formational controls.

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.

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.

Upper temperature limits of orogenic gold deposit formation: Constraints from TiO2 polymorphs in the Dongyuan Au deposit, Jiangnan Orogen, China

Abstract Gold (Au) deposits have formed in orogenic belts throughout Earth’s history. However, the upper temperature limits of orogenic Au vein formation are difficult to constrain because measurements made on fluid inclusions focus on intermediate to late-stage minerals (e.g., quartz and calcite) or are based on P-T estimates for the metamorphic mineral assemblages of the host rocks. We conducted a study of TiO2 polymorphs that are among the earliest minerals that grew in Au-bearing veins of the Dongyuan deposit, Jiangnan orogenic Au belt, South China. Based on Raman analyzes, we identified TiO2 polymorphs of anatase (with Raman peaks at 396, 515, and 638 cm−1), rutile (with Raman peaks at 235, 447, and 613 cm−1), and anatase–rutile intergrowths. Transmission electron microscope (TEM) confirmed the polymorphs identifying the [111] zone axis of anatase, [110] zone axis of rutile, and [111] and [111] zone axes of rutile–anatase intergrowths. The TiO2 polymorphs in the Dongyuan Au veins constrain a temperature range for early mineral precipitation in the veins of 450–550 °C. The results show that ore-forming fluids for this orogenic Au deposit emplaced in the shallow crust originated from deeper and hotter crustal levels (e.g., high-grade metamorphic rocks in the middle to lower crust).

Stratigraphy and U–Pb detrital zircon provenance, Crixás Greenstone Belt, Goiás-Brasil: Gold-bearing vs barren siliciclastic rocks

The Crixás Greenstone Belt, located in an Archean-Paleoproterozoic terrane within the Neoproterozoic Tocantins Province, hosts an important gold deposit in Brazil with an endowment of 7 Moz currently operated by AngloGold Ashanti. Its stratigraphy is composed of an Archean komatiite and basalt basal sequence followed by a Paleoproterozoic sedimentary sequence composed of carbonaceous phyllites, dolomite-rich marbles with minor siliciclastic lenses (Ribeirão das Antas Formation) followed by a uniform siliciclastic sequence (Córrego Geral Formation). At the mining area, the entire sequence is stratigraphically overturned and metamorphosed under greenschist to lower amphibolite facies conditions. The orebodies are structurally controlled by thin-skinned low-angle thrust faults, and are mainly hosted by the sedimentary sequence in dilation zones named locally, from the shallowest to the deepest, as Structure Palmeiras, Structure IV and Structure III. The gold mineralization occurs in association with disseminated sulfides, massive sulfide lenses or quartz veins. Structure III is the deepest mineralized structure with gold mostly occurring in quartz veins cutting carbonaceous phyllites. Siliciclastic lenses occurring within the Structure III interval show variable gold grades, while siliciclastic rocks from the uniform deeper sequence just below Structure III are barren. The chronology of the sedimentary deposition, metamorphism and mineralization is still poorly understood. Detailed drill core logging, petrography and U–Pb geochronology of detrital zircons from siliciclastic rocks were carried out in order to better understand the relationship between the two metasedimentary packages and gold distribution. Petrographically there are no relevant differences between Au-bearing siliciclastic rocks (Ribeirão das Antas Formation) and barren siliciclastic rocks (Córrego Geral Formation), except for the presence of sulfides in the former. The new dataset of detrital zircon U–Pb ages revealed a ubiquitous Paleoproterozoic peak in all samples, with nearly no Archean contribution. K-feldspar and lithic angular fragments, zircons with low to medium roundness, and the U–Pb results indicate a Rhyacian provenance from a proximal source. Neoproterozoic overgrowths are found on detrital zircons from the barren siliciclastic interval of the Córrego Geral Formation, providing unequivocal evidence of a Neoproterozoic event around 600 Ma that affected the rocks. The Neoproterozoic partial reset, possibly related to the overriding of the Mara Rosa Magmatic Arc onto the Crixás Greenstone Belt, is unlikely to be linked to the mineralization considering that zircons from the barren rocks are the most thermally affected by this event. The Au mineralization is more likely related to Paleoproterozoic events, a time period known as a major global episode of orogenic gold deposit formation.

Structural setting and fluid composition of gold mineralization along the central segment of the Keraf suture, Neoproterozoic Nubian Shield, Sudan: implications for the source of gold

ABSTRACT We provide the first large field investigation of more than 20 gold deposits and sites along the Keraf suture zone, covering an area of ~15,000 km2. The area shows diverse structural settings for gold mineralization in greenschist, amphibolite, and hornfels metamorphosed rocks. The reverse fault and dyke-hosted settings appear to be the most favourable for forming gold deposits. Two major N-S-trending, ~300-km-long corridors of deformation are delineated (West and East) but only the West seems important for gold. From fluid inclusions, 6 types of fluids are distinguished based on specific volatile contents and proportions. Most types can be related to fluid evolution by hydrothermal reactions and phase separations from a primitive fluid of metamorphic origin, containing H2O, CO2, N2, CH4, C2H6, and H2. Regional hydrothermal fields are identified based on the combinations of structural settings and fluid types. The northern field is defined as a single large hydrothermal system accounting for the mineralization of the 3 gold deposits (WG-03, UTM and Central). For sites along the West Corridor, such as Toubi, Anas, NW and Shereik, multiple and overlapping hydrothermal systems are necessary to explain the various fluid types and structural settings, even at the deposit scale (Shereik). Conversely, other sites (SW, Yasmine, Korup, Saijd, Dardora, Negeim, and SE) formed by localized individual hydrothermal systems. The documentation of fluids containing ethane at numerous sites and deposits is the most significant outcome. Such ethane-bearing fluids are generated by the metamorphism of carbonaceous-pyritic sedimentary rocks. These rocks are considered one of the best sources for providing ligands and gold for the formation of orogenic gold deposits. Consequently, the Keraf suture zone, composed mainly of carbonaceous turbidites, is interpreted to have provided such source rocks in the suture zone and below the adjacent cratons during the collision, forming orogenic gold deposits (graphical abstract).

Fluid and metal sources in the F\xe4boliden hypozonal orogenic gold deposit, Sweden

To model the formation of orogenic gold deposits, in a global perspective, it is important to understand the ore-forming conditions not only for deposits hosted in greenschist facies rocks but also in amphibolite facies. The Paleoproterozoic Fäboliden deposit in northern Sweden belongs to the globally rare hypozonal group of orogenic gold deposits and, as such, constitutes a key addition to the understanding of amphibolite facies orogenic gold deposits. The Fäboliden deposit is characterized by auriferous arsenopyrite-rich quartz veins, hosted by amphibolite facies supracrustal rocks and controlled by a roughly N-striking shear zone. Gold is closely associated with arsenopyrite-löllingite and stibnite, and commonly found in fractures and as inclusions in the arsenopyrite-löllingite grains. The timing of mineralization is estimated from geothermometric data and field relations at c. 1.8 Ga. In order to constrain the origin of gold-bearing fluids in the Fäboliden deposit, oxygen, hydrogen, and sulfur isotope studies were undertaken. δ18O from quartz in veins shows a narrow range of + 10.6 to + 13.1‰. δD from biotite ranges between − 120 and − 67‰, with most data between − 95 and − 67‰. δ34S in arsenopyrite and pyrrhotite ranges from − 0.9 and + 3.6‰ and from − 1.5 and + 1.9‰, respectively. These stable isotope data, interpreted in the context of the regional and local geology and the estimated timing of mineralization, suggest that the sulfur- and gold-bearing fluid was generated from deep-crustal sedimentary rocks during decompressional uplift, late in the orogenic evolution of the area. At the site of gold ore formation, an 18O-enriched magmatic fluid possibly interacted with the auriferous fluid, causing precipitation of Au and the formation of the Fäboliden hypozonal orogenic gold deposit.

40Ar/39Ar geochronology constraints on formation of the Tuwaishan orogenic gold deposit, Hainan Island, China

The Tuwaishan deposit is an orogenic gold deposit that is structurally controlled by the NE-trending Gezhen shear zone in the western part of Hainan Island, south China. It is part of a belt of numerous similar deposits hosted in Mesoproterozoic granitoids and lesser Paleoproterozoic to Mesoproterozoic schists. The Tuwaishan gold orebodies comprise auriferous quartz veins and disseminated gold in sulfidized granitic mylonite. The veins consist of early quartz-pyrite-arsenopyrite-pyrrhotite with minor gold, middle stage quartz-gold-base metal sulfides, and barren late quartz-carbonate. New 40Ar/39Ar dating of muscovite from wallrock schists yielded a minimum age of 356.8 ± 0.9 Ma, suggesting the youngest possible age estimate for amphibolite facies regional metamorphism of the Baoban Group country rocks. The 40Ar/39Ar dating of sericite from auriferous quartz veins yielded a weighted mean age of 243.4 ± 2.3 Ma, representing the ore-forming age of the Tuwaishan gold deposit. The formation of the shear-hosted gold deposits along the Gezhen gold belt was likely related to the Early to Middle Triassic convergence between the Indochina and South China Blocks. Based on the widely accepted metamorphic devolatilization model for orogenic gold deposit formation, most Au tends to be released into the ore-forming fluid in the source region when country rocks were firstly metamorphosed from greenschist to amphibolite facies. Therefore, it is highly unlikely that the immediate country rocks of the Baoban Group, which were at amphibolite facies by early Carboniferous, were the source for fluids and metals along the Gezhen shear zone.

Metamorphic devolatilization of basalts across the greenschist-amphibolite facies transition zone: insights from isograd mapping, petrography and thermodynamic modelling

ABSTRACT The greenschist-amphibolite facies transition zone within metabasites is of great importance due to the common occurrence of this transition within greenstone belts, and the proposition that fluids released across the transition may be involved in orogenic gold deposit formation. In this work, the nature of devolatilization reactions occurring across the greenschist-amphibolite facies transition zone is assessed from petrological examination and thermodynamic modelling of an exceptionally exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan). The sequence at Flin Flon comprises an intact metamorphic sequence of ~10 km length spanning the greenschist and lower amphibolite facies, the latter of which can be subdivided into three zones (from S to N): the hornblende-actinolite zone, the hornblende-actinolite-oligoclase zone, and the hornblende-oligoclase zone, demarcated by the hornblende-in, oligoclase-in and actinolite-out isograds. The crossing of the hornblende-in isograd is associated with small amounts of hornblende growth, relatively little change in the proportions of chlorite, and negligible fluid loss. By contrast, significant changes in the modal mineralogy occur across the 1500 m wide hornblende-actinolite-oligoclase zone, with the breakdown of approximately 75% of the chlorite and the loss of approximately 1-2 wt% H2O. There is no textural or modal evidence for significant loss or gain of carbonate or sulphide minerals going through this interval. Petrological estimates of devolatilization across the greenschist-amphibolite facies transition zone are compared with predictions from thermodynamic modelling. Although there are differences between the two, both indicate that the fluid loss is strongly tied to variation in bulk composition, the latter predominantly the result of pre-metamorphic alteration processes. High-Mg and high-Ca basalts, representative of pillow rim and core material, undergo an average of 1.8 wt% (modelling estimate: 2.2 wt%) and 1.1 wt% H2O loss (modelling estimate: 1.8 wt%), respectively, across the hornblende-actinolite-oligoclase zone. T-XCO2 modelling predicts that the XCO2 content of fluid produced from the hornblende- and oligoclase-producing reactions is low ( 0.2) above the greenschist-amphibolite facies transition zone within samples containing high carbonate contents (>5%). However, in contrast to the modelling, the majority of chlorite (>75%) breaks down at the main greenschist-amphibolite facies transition zone in the Flin Flon sequence, and most samples have low carbonate contents (1.5% average), limiting the volumes of fluids with higher XCO2 compositions that can be generated at higher grades. Thus, whilst the greenschist-amphibolite facies transition zone at Flin Flon is the site of significant devolatilization (1-2 wt%) over a small interval of P-T space, is does not appear to have been accompanied by significant carbonate and sulphide breakdown, and the fluids generated across this interval were CO2-poor. If the Flin Flon sequence is representative of other metamorphosed greenstone belts, it may be that metabasites metamorphosed across the greenschist-amphibolite facies transition zone do not, in general, release the CO2– and gold hydrosulphide-bearing fluids characteristic of orogenic gold deposits.

Karakteristik Fluida Hidrotermal Endapan Emas Orogenik di Pegunungan Rumbia, Kabupaten Bombana, Provinsi Sulawesi Tenggara

Recently, gold exploration activities are not only focused along volcanic-magmatic belt but also starting to shift along metamorphicand sedimentary terrains. The purpose of this study is to analyses the characteristics hydrothermal fluids gold deposits t in the Rumbia Mountains, Bombana Regency, Southeast Sulawesi. There are three generations of veins identified including the first is parallel to the foliations, the second crosscuts the first generation of veins/foliations, and the third is of laminated deformed quartz+calcite veins at the late stage. Temperature of homogenization (Th) and salinity at Rumbia Mountain of the first vein vary from 220 to 355.30oC and 6.74 to 10.11 wt. % NaCl eq., respectively. The second generation vein was originated at Th of 157 to 255.50oC and salinity of 3.39 to 6.88 wt.%NaCl eq., whereas the third generation vein formed at lowest Th varying from 104.40 to 265.90oC and less saline fluid at salinity range between 0.18 and 6.30 wt.% NaCl eq. The result of temperature formation value correlation to the depth of the formation of orogenic gold deposits in Rumbia Mountain is indicated to form on sub-greenschist to greenschist facies at depth of about 4-8 kilometers and formation temperature between 104.40 - 355.30oC at zone epizonal and mesozonal. Based on characteristics fluids inclusion discussed above, the primary metamorphic-hosted gold mineralization type at Rumbia Mountain tends to meet the criteria of orogenic gold type. Keyword : fluid iclusion, quartz veins, Rumbia mountain, orogenic gold deposits.

Karakteristik Fluida Hidrotermal Endapan Emas Orogenik di Pegunungan Rumbia, Kabupaten Bombana, Provinsi Sulawesi Tenggara

Recently, gold exploration activities are not only focused along volcanic-magmatic belt but also starting to shift along metamorphicand sedimentary terrains. The purpose of this study is to analyses the characteristics hydrothermal fluids gold deposits t in the Rumbia Mountains, Bombana Regency, Southeast Sulawesi. There are three generations of veins identified including the first is parallel to the foliations, the second crosscuts the first generation of veins/foliations, and the third is of laminated deformed quartz+calcite veins at the late stage. Temperature of homogenization (Th) and salinity at Rumbia Mountain of the first vein vary from 220 to 355.30oC and 6.74 to 10.11 wt. % NaCl eq., respectively. The second generation vein was originated at Th of 157 to 255.50oC and salinity of 3.39 to 6.88 wt.%NaCl eq., whereas the third generation vein formed at lowest Th varying from 104.40 to 265.90oC and less saline fluid at salinity range between 0.18 and 6.30 wt.% NaCl eq. The result of temperature formation value correlation to the depth of the formation of orogenic gold deposits in Rumbia Mountain is indicated to form on sub-greenschist to greenschist facies at depth of about 4-8 kilometers and formation temperature between 104.40 - 355.30oC at zone epizonal and mesozonal. Based on characteristics fluids inclusion discussed above, the primary metamorphic-hosted gold mineralization type at Rumbia Mountain tends to meet the criteria of orogenic gold type. Keyword : fluid iclusion, quartz veins, Rumbia mountain, orogenic gold deposits.

Karakteristik Fluida Hidrotermal Endapan Emas Orogenik di Pegunungan Rumbia, Kabupaten Bombana, Provinsi Sulawesi Tenggara

Recently, gold exploration activities are not only focused along volcanic-magmatic belt but also starting to shift along metamorphicand sedimentary terrains. The purpose of this study is to analyses the characteristics hydrothermal fluids gold deposits t in the Rumbia Mountains, Bombana Regency, Southeast Sulawesi. There are three generations of veins identified including the first is parallel to the foliations, the second crosscuts the first generation of veins/foliations, and the third is of laminated deformed quartz+calcite veins at the late stage. Temperature of homogenization (Th) and salinity at Rumbia Mountain of the first vein vary from 220 to 355.30oC and 6.74 to 10.11 wt. % NaCl eq., respectively. The second generation vein was originated at Th of 157 to 255.50oC and salinity of 3.39 to 6.88 wt.%NaCl eq., whereas the third generation vein formed at lowest Th varying from 104.40 to 265.90oC and less saline fluid at salinity range between 0.18 and 6.30 wt.% NaCl eq. The result of temperature formation value correlation to the depth of the formation of orogenic gold deposits in Rumbia Mountain is indicated to form on sub-greenschist to greenschist facies at depth of about 4-8 kilometers and formation temperature between 104.40 - 355.30oC at zone epizonal and mesozonal. Based on characteristics fluids inclusion discussed above, the primary metamorphic-hosted gold mineralization type at Rumbia Mountain tends to meet the criteria of orogenic gold type. Keyword : fluid iclusion, quartz veins, Rumbia mountain, orogenic gold deposits.

Multi-stage and multi-sourced fluid and gold in the formation of orogenic gold deposits in the world-class Mana district of Burkina Faso – Revealed by LA-ICP-MS analysis of pyrites and arsenopyrites

Mana is a world-class Paleoproterozoic orogenic gold district (∼8.0 Moz) that includes five gold deposits (Fofina, Nyafé, Siou, Wona-Kona and Yaho) hosted in various rocks in the northern part of the Birimian Houndé greenstone belt, western Burkina Faso. The gold deposits recorded different styles of mineralization associated with sulfides during polyphase deformation events. Detailed petrographic and LA-ICP-MS analyses of pyrites and arsenopyrites from the main mineralization of each deposit provide new information on the potential gold and arsenic sources and timing of fluids responsible for mineralization at Mana. Four hydrothermal fluid events associated with two distinct source reservoirs and at least two hydrothermal systems were recognized. The first hydrothermal fluid event was generated during the Eoeburnean D1MD (MD for Mana district) shortening event (∼2172 Ma), evidence of which is preserved in the Fofina and Siou deposits, and occurred under greenschist metamorphic facies conditions. Economic gold deposits at Fofina are associated with metamorphic disseminated pyrite (Py1F) in black shale, zoned pyrite (Py2F) enriched in As, Ni, Cu, Ag, Sb, W, Au and Pb, and arsenopyrite in folded V1F quartz-carbonate veins. In contrast, the Siou deposit exhibits Au in porous pyrite cores (Py1S), which are associated with Ag, Bi, Te and W (±Sb) in V1S quartz veins hosted in the Siou granodiorite. These two trace element suites are interpreted as having been induced by a metamorphic source in the Fofina deposit and by a local contribution of a magmatic source in the Siou deposit. The second hydrothermal fluid event occurred during the Eburnean D3MD transpressional event (2113–2090 Ma) in the Nyafé deposit. This event is defined by Au-As-rich rims in hydrothermal pyrite (Py3N) and by the co-precipitation of Au-rich arsenopyrite in the V3N quartz veinlets (V3N) controlled by a second-order dextral shear zone. The third and main hydrothermal fluid event during the D4MD transpressional event (∼2090 Ma) is characterized by the development of strong silicification and by zoned pyrites (Py3WK and Py2Y) and Au-rich arsenopyrite in the Wona-Kona and Yaho deposits. However, the same late Au-As-rich rims in hydrothermal pyrite are observed in both deposits and in the Nyafé deposit, and these rims are interpreted to be related to the same economic gold source related to the Lower Birimian metamorphic reservoir, composed of black shale (diagenetic framboidal pyrite) and tholeiitic basalts. The diagenetic framboidal Py1WK in black shale (Lower Birimian group) is enriched in Au (median of 0.8 ppm), low in As (median of 70 ppm), with other trace elements (Ni, Cu, Zn, Ag, Sb and Pb). The last hydrothermal event is associated with free gold remobilization in late microfractures in the Wona-Kona and Siou deposit during the reactivation of major transcurrent shear zones during the D4MD event or the D5MD shortening event (as late as 2022 Ma). A binary diagram of As/Ag vs. Sb/Bi appears to be effective at separating the diagenetic, magmatic and metamorphic pyrites and could be used in orogenic gold exploration to ascertain potential gold endowment at the district scale.