Orogenic Au Mineralization Research Articles

Late Paleozoic tectonic switching and metallogenic evolution of the southern Yili arc terrane, NW China

Recognition of tectonic switching is of great significance for understanding the formation and evolution of ancient accretionary orogenic belts and can provide insights into metallogenic evolution. The western Tianshan orogenic belt, which lies within the southern Central Asian Orogenic Belt (CAOB), experienced a prolonged tectono-magmatic evolution during the late Paleozoic in response to evolution of the Paleo-Asian Ocean. The aim of this contribution is to constrain the subduction process and timing of tectonic switching from subduction to collision in southern parts of the western Tianshan. To address this, we present the results of an integrated investigation of whole-rock major and trace element and Sr-Nd-Hf isotope data, together with zircon U-Pb ages for late Paleozoic volcanic rocks from the southern Yili arc terrane of the southern western Tianshan. Based on the temporal-spatial distribution of different compositions of volcanic rocks, three distinct episodes of magmatism have been recognized, including: 1) ∼ 380 – ∼340 Ma, this episode of magmatism has high Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and show large variations in εHf(t) (−9.6 to + 15.6) and εNd(t) (−5.2 to + 5.4) values, which is interpreted to be originated from a mantle source with variable input of slab-derived components incorporated in a trench advance setting; 2) ∼ 340 – ∼320 Ma, this episode of magmatism shows low Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and relatively higher εHf(t) (−3.8 to + 14.6) and εNd(t) (−1.5 to + 5) values, reflecting crust-mantle interactions and likely formation in a back-arc basin environment; 3) ∼ 320 – ∼300 Ma, this episode of magmatism has Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and mostly positive εHf(t) (−1.1 to + 14.5) and εNd(t) (+1.6 to + 8.4) values, suggesting continent–continent collision between the northern Tarim Craton and the southern limb of the Kazakhstan composite continent (i.e., Yili and Chinese Central Tianshan Block). Consistently, the two tectonic switching events (ca. 340 Ma and ca. 320 Ma) were temporally associated with skarn Zn-Pb and orogenic Au mineralization in southern Yili arc terrane, respectively. This study presents an example of multiple tectonic switching and metallogenic evolution in hot accretionary orogens.

Control of crustal deformation on orogenic Au mineralization in Himalaya: A case study from Buzhu

The Buzhu Au deposit in the Himalaya orogen, experiencing Cenozoic uplift of the Himalaya, provides a window to study the control of crustal deformation on Au mineralization. The Au mineralization is characterized by quartz veins controlled by an extensional fault system, comprising W- to WNW-trending shear zones/faults and superposed NNE- to N-trending normal faults. The vein system experienced three deformation stages, stages I and II occurred in shear zones, and stage III developed in normal faults. The deformation comprises hydraulic brecciation in stage I and crack-sealing processes during stage II shear deformation in the brittle-ductile transition, followed by matrix-supported breccias progressively crosscut by veins in newly-formed normal faults in stage III. Stages I and II contain invisible-Au-dominated sulfides, while native Au and pyrrhotite formed in stage III.Blocky quartz with oscillatory and sector zoning patterns implies fluid pressure build-up processes in stage I. Pyrite displays coupled Au-As variation, high Au contents (<19 ppm), and restricted δ34S values (−3.6 to −2.4‰), supporting that fluid-rock reaction was responsible for Au precipitation. Elongated quartz of stage II displays contrasting bright and dark cathodoluminescence bands with corresponding high and low Al-Li concentrations, indicating fluctuating fluid pressure. Pyrite shows a negative correlation between Au (<45 ppm) and δ34S values (5.4–1.7‰, from cores to rims), consistent with fluid oxidation associated with a fluid pressure drop. Minor late quartz in open spaces transected earlier quartz, implying the hydrothermal system dropped to near-hydrostatic conditions. The partial replacement textures of pyrite and arsenopyrite from stage III, with varied δ34S values (5.9–9.3‰), and the existence of micro-inclusions and visible Au along the contact, suggest a fluid-mediated dissolution-reprecipitation process. Thus, it is demonstrated that mineralization occurred as the hydrothermal system transitioned from the relatively ductile to brittle domain, with varying Au precipitation mechanisms. This study further summarizes the similarities of Au mineralizing systems in young collisional orogens worldwide, particularly with respect to relationships between structural-hydrothermal system and rapid uplift.

Late Mesozoic tectonic evolution and thermal history in the northeast Xing'an Block, NE China: Implications for gold mineralization and preservation

Late Triassic–Jurassic orogenic and Early Cretaceous epithermal Au deposits are significant producers of Au, Ag, and potentially critical elements (e.g., Te, Se and Sb) in the North Heilongjiang Belt in the Xing'an Block, NE China. However, the preservation mechanism of these Au deposits associated with tectonic evolution is unclear. Here, we conducted zircon U–Pb, sericite Ar–Ar, zircon and apatite (U–Th)/He dating as well as whole‐rock geochemistry in the Xing'an Block. Zircon U–Pb dating shows the ages of 174.1 ± 2.4 Ma for the granodiorite, 169.9 ± 1.6 Ma for the tonalite, 154.0 ± 2.0 Ma for the monzogranite and ca.122 Ma for the diorite. Sericite Ar–Ar dating presents plateau and isochron ages of 163.2 ± 1.8 Ma and 163.4 ± 2.8 Ma. The average apatite (U–Th)/He (AHe) and zircon (U–Th)/He (ZHe) ages from the Jurassic rocks have the ranges of ca. 146–89 Ma and ca. 150–116 Ma and from the Cretaceous are ca. 109–76 Ma and ca. 117–111 Ma, respectively. The Early‐Middle Jurassic intrusions are featured by the tonalite‐trondhjemite‐granodiorite (TTG) rock type. Late Jurassic monzogranite has the geochemical signature of adakitic rocks. The Early Cretaceous granitoids are classified as volcanic arc granites with a part of the Early Cretaceous granodiorite featured by adakitic. These new results reveal five group ages associated with tectonic evolution, Au mineralization and the preservation of the Au deposits. (i) Magmatic events were related to the subduction of the Mongolia‐Okhotsk Ocean during the Early Jurassic (ca. 177–170 Ma) and then the compressional settings with regional deformation (ca. 167–163 Ma). (ii) Thickening crust and orogenic Au mineralization were related to the closure of the Mongolia‐Okhotsk Ocean (ca. 154–145 Ma). (iii) The transformation was from compressional to extensional settings followed by exhumation during the Early Cretaceous (ca. 145–120 Ma), which is the factor of the exhumation of the orogenic Au deposits formed from the Late Triassic to Late Jurassic. (iv) Epithermal Au–Ag mineralization was associated with an extension related to the subduction of the Palaeo‐Pacific Plate (ca. 120–100 Ma). (v) Thermal history shows one thermal event during the Late Cretaceous (ca. 89–76 Ma). The Au deposits have been preserved since the Late Cretaceous.

Gold mineralization related to rifting of the Rodinia surpercontinent: As gauged from the Dongyuan Au deposit, Jiangnan orogen, China

Abundant orogenic gold (Au) deposits, widely distributed in medium to low-grade metamorphic rocks, formed episodically since the Middle Archean. Orogenic Au mineralization waned during the Neoproterozoic era, although Neoproterozoic orogenies were prevalent. The Jiangnan Orogen, South China, as part of the sutures uniting the Rodinia (Meso- and Neoproterozoic) supercontinent, is situated between the Cathaysia and Yangtze blocks. Many orogenic Au deposits, such as the Wan’gu, Huangjindong, Dongyuan, and Jinshan Au deposits, are located in the northeastern section of the Jiangnan Orogen. The orebodies of the recently discovered Dongyuan deposit display a horizontal zonation, as follows: a peripheral quartz–chlorite–sulfide–calcite (ankerite, manganocalcite, and calcite) veinlets/veins and a central sulfide (mainly arsenopyrite) veinlets/veins. The former veinlets/veins also contain a small amount of albite, scheelite, TiO2 minerals, xenotime, and monazite. Compared with the other orogenic Au deposits, arsenopyrite from the Dongyuan Au deposit is characterized by enrichment of Sb–Co–Cd–Ag element assemblages. Based on the electron microprobe and Raman analyses, TiO2 mineral grains, including anatase, rutile, and anatase–rutile intergrowths, are present in the Au veins. In contrast with previous studies, TiO2 minerals (anatase and rutile) have a nearly consistent concentration range of each element detected, which taken together can be used for constraining fluid sources of ores. TiO2 minerals with enriched V–Fe assemblages, combined with initial 187Os/188Os ratio (0.64 ± 0.11) of arsenopyrite, reflect that the Dongyuan Au veins were probably precipitated from a metamorphic fluid. The Liyangdou biotite granodiorite and Donggang granite, located about one kilometer north and six kilometers south of the deposit, have LA-ICP-MS U–Pb zircon ages of 803 ± 3.4 Ma and 814 ± 3.4 Ma, respectively. The analysis of five arsenopyrite samples selected from the Dongyuan Au veins yields an isochron age of 744 ± 36 Ma (with an initial 187Os/188Os ratio of 0.64 ± 0.11 (2σ) and MSWD = 30). We found that the Dongyuan Au mineralization was later than the peripheral Neoproterozoic magmatic activities and metamorphic events of country rocks. Combined with previous studies, the Dongyuan deposit was formed in the tectonic setting of Rodinia supercontinent rifting, after the Neoproterozoic collision event in the Jiangnan Orogen, which likely triggered orogenic Au mineralization.

Development and application of feature engineered geological layers for ranking magmatic, volcanogenic, and orogenic system components in Archean greenstone belts

Geologically representative feature engineering is a crucial component in geoscientific applications of machine learning. Many commonly applied feature engineering techniques used to produce input variables for machine learning apply geological knowledge to generic data science techniques, which can lead to ambiguity, geological oversimplification, and/or compounding subjective bias. Workflows that utilize minimally processed input variables attempt to overcome these issues, but often lead to convoluted and uninterpretable results. To address these challenges, new and enhanced feature engineering methods were developed by combining geological knowledge, understanding of data limitations, and a variety of data science techniques. These include non-Euclidean fluid pre-deformation path distance, rheological and chemical contrast, geologically constrained interpolation of characteristic host rock geochemistry, interpolation of mobile element gain/loss, assemblages, magnetic intensity, structural complexity, host rock physical properties. These methods were applied to compiled open-source and new field observations from Archean greenstone terranes in the Abitibi and western Wabigoon sub-provinces of the Superior Province near Timmins and Dryden, Ontario, respectively. Resulting feature maps represent conceptually significant components in magmatic, volcanogenic, and orogenic mineral systems. A comparison of ranked feature importance from random forests to conceptual mineral system models show that the feature maps adequately represent system components, with a few exceptions attributed to biased training data or limited constraint data. The study also highlights the shared importance of several highly ranked features for the three mineral systems, indicating that spatially related mineral systems exploit the same features when available. Comparing feature importance when classifying orogenic Au mineralization in Timmins and Dryden provides insights into the possible cause of contrasting endowment being related to fluid source. The study demonstrates that integrative studies leveraging multi-disciplinary data and methodology have the potential to advance geological understanding, maximize data utility, and generate robust exploration targets.

Petrology, mineralogy, and geochemistry of the Carboniferous Katbasu Au-Cu deposit, western Tianshan, Northwest China: Implications for petrogenesis, ore genesis, and tectonic setting

The Katbasu Au-Cu deposit in the Chinese western Tianshan is hosted in the ca. 355 Ma granite intrusion. This paper reports whole-rock geochemical and zircon Hf-O isotopic data for the granites and the mafic enclaves as well as in situ S isotopic data for the sulfides from the Katbasu Au-Cu deposit. Our results provide constraints on the petrogenesis and ore genesis of the Katbasu Au-Cu deposit. Geochemical characteristics suggest that the Katbasu granites are metaluminous rocks, exhibiting mineral assemblages and geochemical characteristics of I-type granites. The granites have positive zircon εHf(t) values of 7.1–10.2 and zircon δ18O values of 6.2–7.2‰, suggesting a juvenile crustal origin. Mixing processes between felsic and mafic magmas were involved in their generation. The mafic enclaves are enriched in LILEs and depleted in HFSEs, and have zircon εHf(t) values of 7.5–15.8 and zircon δ18O values of 5.4–6.9‰, suggesting that they were derived from a subduction-modified mantle and underwent subsequent crustal contamination and/or magma mixing processes. In situ S isotopic analyses show that pyrite grains from the pre-ore stage and post-ore stage have positive δ34S values of 7.0–8.5‰ and 8.2–9.0‰, respectively. Pyrite and chalcopyrite grains from an early magmatic-hydrothermal Cu-Au mineralization stage show δ34S values of 8.8–11.1‰, and pyrite grains from the main orogenic Au mineralization stage have positive δ34S values of 5.9–8.4‰. The heavy S isotope characteristics of magmatic-hydrothermal Cu-Au mineralization indicate that the magma source might be metasomatized by subduction materials, whereas the heavy S isotope characteristics of the late orogenic Au mineralization suggest that the ore-forming materials mainly originated from the metamorphic devolatilization of the source rocks. The ca.352–349 Ma early magmatic-hydrothermal Cu-Au mineralization in the Katbasu deposit formed in a subduction tectonic setting, whereas the ca. 323–311 Ma late orogenic Au mineralization formed in an orogenic environment during the final stage of subduction.

Using pyrite composition to track the multi-stage fluids superimposed on a porphyry Cu system

Abstract The Yulekenhalasu porphyry Cu-Mo deposit (0.2 Mt Cu at 1.04 wt% and 0.012 Mt Mo at 0.06 wt%) is located in the Devonian Halasu copper belt, East Junggar block, northwest China. At Yulekenhalasu, Cu and Mo mineralization commonly occurs as disseminated sulfides or veinlets in porphyry-related alteration zones. Five alteration stages have been identified, including porphyry-type alteration, i.e., sodic-calcic (stage I), potassic (stage IIa), propylitic (stage IIb), and phyllic (stage III) alteration, and widespread late Cu sulfide-bearing veins (stage IV) cross-cutting porphyry-type alteration, plus a post-ore fault-controlled argillic alteration (stage V). Stages IV and V have overprinted porphyry-type alteration (stages I–III). Anomalous concentrations of trace elements in stage IIb pyrite (e.g., Ti, Zr, Gd, and Hf) are due to the presence of micro-inclusions (e.g., zircon and rutile) in the low-temperature (~200 °C) propylitic zone. Cu, Ag, Co, and Mn, occurring as stoichiometric substitutions or as tetrahedrite inclusions in overprinting stage IV pyrite, were sourced directly from the primary hydrothermal fluid. The enrichment of distal pathfinder elements (e.g., Cr, Au, and Tl) in overprinting stage V pyrite was caused by a low-temperature (~160 °C) hydrothermal event related to regional orogenic Au mineralization. The spatial variation of Se/S in pyrite among various paragenetic stages were influenced by changes in the hydrothermal fluid composition and temperature, with the latter having the effect of decreasing pyrite Se/S. Lower Se concentrations in pyrite of stages IIb and III close to the orebody are explained by relatively higher temperatures in the locus of mineralization. This may provide a potential vectoring tool to mineralization using pyrite geochemistry in porphyry deposits. Systematic thermodynamic calibrations were applied to pyrite compositions to fingerprint the corresponding Se/S and Co/Ni ratios of fluids and further to develop a complete metallogenic model for Yulekenhalasu. The Devonian diorite porphyry generated fluids that produced the early porphyry-type alteration. High Co/Fe (average ~1 × 10–4) and Ni/Fe (average ~3 × 10–6) ratios of fluid for late Cu sulfide-bearing veins, combined with higher Se/S (average ~6 × 10–7) than orogenic Au deposits (average ~3 × 10–8), indicate that the fluids possibly derived from a Late Devonian-Carboniferous mafic intrusion. Argillic alteration assemblages forming at ca. 280 Ma host pyrite relatively enriched in Au (average 0.1 ppm, with native gold inclusions). Therefore it is likely related to a regional orogenic gold mineralizing event in the Early Permian that overprinted Devonian mineralization. Although spatially contiguous, hydrothermal alteration and hypogene mineralizing stages identified herein represent discrete episodes of hydrothermal activities at Yulekenhalasu. The multi-stage alteration features observed at Yulekenhalasu may provide insights into the complete evolutionary history of Paleozoic porphyry Cu deposit systems in the Central Asian orogenic belt. This study contributes to a better understanding of the metallogenic and exploration models of porphyry Cu deposits overprinted by multi-stage hydrothermal events, which is economically important in Phanerozoic orogenic belts.

Permian ridge subduction-related magmatism in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Permian large-scale transcurrent tectonics and syn -kinematic ultramafic–felsic magmatism are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval Ni Cu mineralized mafic-ultramafic intrusions and felsic intrusions, and the geodynamics of the region remains equivocal. To assess these topics and improve our understanding of this geologically complex and economically significant region, zircon Hf isotope and bulk-rock geochemical data from Permian granitoids along the Kanggur–Huangshan Shear Zone (KHSZ) in the Eastern Tianshan are determined and interpreted in the context of previous geochronological–geochemical data. The Kungurian Longdong (ca. 278 Ma) and Hongshi (ca. 273 Ma) granitoids are high-K, calc-alkaline, metaluminous to weakly peraluminous I-type granitoids, whereas the Capitanian Shuangchagoubei (ca. 262 Ma) granitoids are calc-alkaline, peraluminous, adakitic I-type granitoids. All of the granitoid samples are enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, U, and K), and depleted in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, indicating that subduction-related magmatism was still active in the late Permian. Taking into account the regional geology, and the presence of Permian high-temperature metamorphism, adakites, I-type granites, Alaskan-type Ni Cu sulfide and orogenic Au mineralization, and EW-trending N-MORB type gabbro and mélange along the KHSZ, we suggest that subduction of a spreading center during the Permian generated a slab window, allowing the upwelling of asthenospheric mantle. This permitted the generation and migration of mantle-derived magmas to shallower crustal levels along translithospheric faults (e.g., Kanggur and Yamansu faults), forming the mafic–ultramafic intrusions and associated Ni Cu sulfide deposits. The continuous migration of magma provided the necessary heat to induce partial melting of juvenile crust, generating the Permian high-K to calc-alkaline I-type granitoids. Ridge subduction-induced juvenile crustal reworking likely played a central role in the formation of the granitoids during the late Paleozoic accretionary orogenesis of the southern Altaids. • Subduction-related magmatism was active in the Eastern Tianshan in the late Permian. • Crustal growth and reworking processes must have occurred simultaneously in the Eastern Tianshan during the Permian. • Ridge subduction-induced juvenile crustal reworking likely played an important role in the formation of the granitoids in the southern Altaids.

In situ apatite U-Pb dating for the ophiolite-hosted Nianzha orogenic gold deposit, Southern Tibet

Mineralization age dating is essential for understanding orogenic-type gold metallogeny, but suitable minerals for dating are not always available. Apatite can incorporate considerable amount of U and Th, making it a potential U-Pb geochronometer to study hydrothermal alteration and mineralization processes. The newly-discovered Nianzha is a large orogenic Au deposit (∼25 t @ 3.08 g/t Au) and located in the contact fault zone between ultramafic rocks and diorite in the Renbu tectonic mélange of southern Tibet. Two different types of apatite were identified in the Nianzha gold deposit: type I magmatic apatite hosted in diorite and syenite, and intergrown with other magmatic minerals; type II hydrothermal apatite hosted in mineralized diorite. These apatite grains are coarse euhedral granular and closely associated with auriferous sulfides (e.g., pyrite, chalcopyrite, galena). Type I apatite is F- and SO3-rich, whereas type II apatite is distinct from type I apatite by its significantly higher Cl, Mn, rare earth elements (REE), U, Th, and As contents, indicative of a hydrothermal origin. Thus, formation age of type II apatite reflects the timing of gold mineralization. Two type I magmatic apatite samples yielded similar discordia U-Pb ages of 80.35 ± 1.56 Ma (MSWD = 1.3; n = 86) and 79.53 ± 1.27 Ma (MSWD = 0.91; n = 72), respectively, whilst type II hydrothermal apatite yielded a discordia age of 44.60 ± 1.45 Ma (MSWD = 1.2; n = 64). The gold mineralization age is consistent with that of nearby orogenic gold deposits (e.g., Mayum, Bangbu, Zhemulang, and Juqu) in the region. Therefore, we suggest that hydrothermal apatite U-Pb dating can effectively constrain the timing of orogenic Au mineralization events.

Trace elements in pyrite and pyrrhotite in the Pitangui Orogenic Au deposit, Pitangui greenstone belt, São Francisco Craton: Implications for the ore-forming fluids and metal sources

Orogenic Au deposits represent one of the largest sources of gold in the world and in Brazil, particularly at the Quadrilátero Ferrífero, southeastern portion of the São Francisco Craton. In the last decades, advances in in-situ analytical techniques allowed a detailed investigation of the chemistry of sulfides. This can reveal information about the origin and composition of the mineralizing fluids, potential sources of metals and sulfur and the fluid-rock interaction during the ore-forming processes. In this paper we combine Electron Microprobe Analysis and LA-ICP-MS analyses to put constraints on the nature and evolution of the Pitangui gold deposit. The Pitangui deposit represents an important Orogenic Au deposit hosted in clastic metasedimentary rocks of the Pitangui greenstone belt, northwestern portion of the Quadrilátero Ferrífero. Detailed drill hole, petrographic observations and in-situ chemical analyses of the hydrothermal alteration zones show that pyrite is the dominant ore mineral in the deposit and two possible origins are suggest including (i) syngenetic pyrite development at the early stages of the greenstone belt evolution and (ii) epigenetic pyrite, which is associated with hydrothermal processes during the Orogenic Au event. Syngenetic pyrite (PyI) has high Co, Bi, Pb, Sb, Te, Se, Ag and Au concentrations. The PyI could represent an important source of metals, a set of trace elements and likely sulfur for the later Orogenic Au mineralization. Epigenetic pyrites (PyII, PyIII and PyIV), conversely, are mainly found in association with hydrothermal minerals and within the mineralized carbonate-quartz-vein. The variations at Co, Ni, Pb, Te, Sn, Se, As, Bi, Sb, Ag, Mo and W in the epigenetic pyrite indicate that the dehydration PGB sequences during orogenesis constitutes the main fluids sources. The PyV, in turn, only appears at metamafic dyke, but its origin remains unclear.

Geology and geochemistry-based metallogenic exploration model for the eastern Tethys Himalayan metallogenic belt, Tibet

The Tethyan Himalaya is the fourth metallogenic belt discovered to date within the Tibetan Plateau and contains chromite, gold and Sb–polymetallic mineralization. The belt records a series of complex tectonic–magmatic events that include the break-up, subduction, collision, detachment, and obduction of the Eastern Gondwana continent, all of which generated ideal conditions for ore formation. However, this area remains under-explored, with only limited knowledge of the deposit types present in this area and their relationship to tectonic and magmatic events. This has hampered mineral exploration in the region. This study uses previous research and mineral exploration in this region, combined with knowledge of mineralization in adjacent regions, to suggest the presence of three main deposit types (ophiolitic chromite, orogenic Au, and hydrothermal vein Sb–polymetallic deposits), and highlights the potential for this area to host as yet undiscovered island arc-type porphyry Cu–Au and leucogranite-related Sn–(Be) mineralization. This study summaries the relationship between the tectonic evolution and magmatism within the region and the mineralization, establishing a temporal and spatial framework for mineralization within the eastern Tethys Himalayan metallogenic belt.Improved knowledge of the geochemical signatures of mineralization, obtained using compositional data analysis (CoDA) combined with geological information, enabled the identification of nine compositional balances, including the presence of sedimentary units, accretionary wedge structures, leucogranites, mafic–ultramafic rocks, chromite mineralization, Au mineralization, and Sb–polymetallic mineralization. This study establishes a predictive mineral exploration model for the eastern Tethys Himalayan metallogenic belt and combines this model with geological and geochemical data to identify areas for future exploration. These targets include a 50-km-long ophiolitic belt in the Nedong area that is highly prospective for chromite mineralization; a positive Au–Cu–Mo geochemical anomaly in the Qonggyai–Qusum area that is prospective for orogenic Au mineralization; and positive Au–Cu–Mo geochemical anomalies in the Nianzha, Sangri, and Gyaca areas that are prospective for exploration for Xiongcun-type island arc-associated porphyry Cu–Au mineralization, as well as deep-seated extensions to known Sb–polymetallic mineralization and the inner and outer contacts of the Lhozhag and Cuonadong leucogranite, all of which are prospective for exploration for Sn–(Be) mineralization.

Archean controls on basin development and mineralisation in the Southern Capricorn Orogen

Basins along the northern margin of the Yilgarn Craton developed in response to extensional and compressional processes during the Paleoproterozoic. Early extension resulted in the formation of the Yerrida Basin as a large single basin over the northern Yilgarn Craton. Subsequent rifting led to voluminous volcanism in the northern part of the basin, within two depositional centres - the c. 2.03 to 1.96 Ga Bryah and Mooloogool Sub-basins. Cu-Au VMS deposits formed in the Bryah Sub-basin. Yilgarn Craton crust can be mapped using gravity and magnetic data beneath the Yerrida Basin, and Mooloogool Sub-basin. However, the Yerrida Basin can’t be mapped below much of the Bryah Basin, implying the formation of an ocean in this region. The degree of rifting of the Yilgarn Craton, and resulting architecture influenced subsequent basin development, and deformation in the region. For example, in areas where Yilgarn Craton crust can’t be mapped beneath basin sediments deformation is pronounced with the formation of disharmonic folds, refolded folds, and anastomosing shear zones. The southern part of the Yerrida Basin and the Earaheedy Basin formed shallow depositional centres over the Yilgarn Craton, and subsequent deformation in these regions is less intense. Base metal mineralisation in the region can, in part be related to the presence of deep crustal scale structures that initially developed in the Archean, and were re-activated during the Proterozoic. However, the location of c. 1800 Ma orogenic Au mineralization in the Bryah Sub-basin may not have been influenced by localisation of fluid flow around deep crustal-scale faults that formed as 200 Ma earlier, or even as early as the Archean.

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

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

The Tongon Au Deposit, Northern Côte d’Ivoire: An Example of Paleoproterozoic Au Skarn Mineralization☼

The Tongon deposit in northern Cote d’Ivoire is the first reported occurrence of Au skarn mineralization hosted in Paleoproterozoic greenstone rocks of the West African craton. Tongon is an unusually large skarn (3.8 Moz at 2.5 g/t Au) replacing noncarbonate host rocks. The deposit is hosted exclusively in basaltic-andesitic crystal tuffs of the Senoufo greenstone belt. All other rock types in the local stratigraphy, including carbonaceous shales, siltstones, and dacitic crystal and lapilli tuffs, are devoid of mineralization. This observation implies a strong bulk compositional control on the distribution of the skarn. At the district scale, the Tongon hydrothermal system is confined to a corridor of ENE-striking orogen-oblique transfer faults, which link orogen-parallel N-to NNE-striking reverse faults to the north and south. Ferroan A-type granites and lamprophyric dikes also occur along these orogen-oblique faults. To date, no Au skarn mineralization has been discovered outside the corridor of ENE-striking faults elsewhere in the Senoufo belt. The alteration haloes surrounding the deposit formed during three main stages: stage 1, early/distal potassic (biotite-K-feldspar-actinolite) metasomatism; stage 2, prograde clinopyroxene ± grossularitic garnet skarn; and stage 3, retrograde skarn consisting of epidote-clinozoisite-prehnite-albite and slightly later quartz-sulfide-Au. Garnets contain high Al and Ti, and the skarns have elevated Ni and Cr, reflecting the mafic composition of the host rocks. Proximal to peripheral mineral zonation patterns are defined by decreases in the ratio of garnet to pyroxene, increasing Fe in pyroxene, higher Au concentrations, and an increase in the intensity of retrograde alteration, seen as a change from white-to green-colored skarn. Spatially, gold-rich stage 3 alteration coincides with stage 2 intermediate diopside-rich skarn and distal hedenbergite-ferroactinolite skarn, whereas the proximal garnet-bearing skarn typically has much lower Au grades. Fluid inclusions studies in the prograde skarn assemblages show primary, high temperature (>400°C), moderately saline (8–14 wt % NaCl equiv) H2O-NaCl-CaCl2 inclusions, with pressures estimated at 2 ± 0.5 kbar. The source of these fluids remains unclear, as plutonic rocks immediately to the west of Tongon intrude the skarn. U-Pb dating of hydrothermal titanite indicates the Au skarn formed between 2139 ± 21 and 2128 ± 21 Ma, 20 to 70 m.y. before the major episode of orogenic Au mineralization in southwest Ghana and western Mali. Dating provides further evidence that Au mineralization within the Paleoproterozoic rocks of West Africa craton occurred over several tens of millions of years through a variety of different processes.

Paleogene magmatism and gold metallogeny of the Jinping terrane in the Ailaoshan ore belt, Sanjiang Tethyan Orogen (SW China): Geology, deposit type and tectonic setting

The Jinping terrane is situated in the southern segment of the Ailaoshan ore belt, Sanjiang Tethyan Orogen (SW China). The Paleogene intrusions in Jinping consist of syenite porphyry, fine-grained syenite and biotite granite stocks/dikes, and contain relatively low TiO2 (0.21–0.38wt%), P2O5 (0.01–0.35wt%), and high Na2O (2.00–4.62wt%) and K2O (4.48–7.06wt%), belonging to high-K alkaline series. Paleogene gold mineralization in Jinping comprises four genetic types, i.e., orogenic, alkali-rich intrusion-related, porphyry and supergene laterite. The NW–NNW-trending faults and their subsidiaries are the major ore-controlling structures. The orogenic Au mineralization, dominated by polymetallic sulfide-quartz veins, occurs in the diorite and minor in Silurian-Devonian sedimentary rocks. It contains a CO2-rich mesothermal fluid system generated from the mixing of mantle-derived fluids with crustal-derived metamorphic fluids, and the ore-forming materials were upper crustal- or orogenic-derived. The alkali-rich intrusion-related Au mineralization is hosted in the Ordovician-Silurian sedimentary rocks and minor in the Paleogene alkaline intrusions, and the Au orebodies occur predominantly in the alteration halos. It contains a CO2-bearing, largely metamorphic-sourced mesothermal fluid system, and the ore-forming materials were derived from the ore-hosting rocks and minor from the alkali-rich intrusions. The porphyry Cu-Mo-Au mineralization occurs in the granite/syenite porphyries and/or along their contact skarn, with the mineralizing fluids being magmatic-hydrothermal in origin. The former two hypogene Au mineralization types in Jinping were mainly formed in the late Eocene (ca. 34–33Ma) and slightly after the porphyry Cu-Mo-Au mineralization (ca. 35–34Ma), which is coeval with the regional Himalayan orogenic event. Subsequent weathering produced the laterite Au mineralization above or near the hypogene Au orebodies.

Identifying potential Au-Pb-Ag mineralization in SE Shuangkoushan, North Qaidam, Western China: Combined log-ratio approach and singularity mapping

Recognition of weak geochemical anomalies from background is an overlooked issue in the interpretation of regional geochemical data used to define the exploration targets in the areas covered by deserts, regolith, or vegetation. In this study, the log-ratio transformation methods were used to account the compositional character of the geochemical data. A robust compositional factor analysis (CoDa-FA) was utilized to identify the ensembles of elements related to mineralization. Multifractal singularity analysis and the mean±2 standard deviation (mean±2STD) method were used to identify the Au mineralization-related geochemical anomalies in North Qaidam Gobi Desert landscape area. The results show that CoDa-FA for the centered logratio-transformed geochemical data can accurately recognize the geochemical compositional assemblages related to Au and Pb-Ag mineralization, representing the orogenic Au mineralization and hydrothermal-type Pb-Ag mineralization, respectively. The geochemical anomalies delineated by the mean±2STD method showed weak signals in SE Shuangkoushan. However, singularity mapping is efficient for enhancing and outlining the weak anomalies in a low background relative to the mean±2STD method. The anomaly maps determined by singularity mapping show good ore-potentiality in SE Shuangkoushan, and many Au and Pb-Ag ore bodies have been found in the anomalous areas. The results would not only be useful to guide anomaly verification and potential exploration targets in the North Qaidam metallogenic belt, but also raise the awareness of reanalyzing those weak anomalies in a covered area.

The evolution of a Precambrian arc-related granulite facies gold deposit: Evidence from the Glenburgh deposit, Western Australia

Gold deposits are rare in upper-amphibolite to granulite facies environments. Known examples commonly attract debate about whether they formed under these conditions or instead represent metamorphosed, metasomatic, or superimposed (retrograde) mineralization. The Glenburgh gold deposit is located in the Paleoproterozoic upper-amphibolite to granulite facies Glenburgh Terrane in the southern Gascoyne Province of Western Australia. Gold at the Glenburgh deposit is free and disseminated within quartz–biotite–garnet gneiss, amphibolite, and (post-gold) quartz–chlorite veins. No clear association with a specific host lithology has been identified and mineralization does not have a visually distinct proximal alteration assemblage.The rocks hosting the deposit represent a distinct sedimentary package that was deposited, mineralized, buried, and metamorphosed, all during arc magmatism. Features within the internal structure of gold grains, such as high-purity gold veinlets, incoherent twinning, and low silver content, suggest the gold has been through post-depositional processes such as metamorphism and deformation. Abundant sulfide minerals are interpreted to have formed by sulfidation of the host rock contemporaneously with gold mineralization, and the presence of rounded sulfide inclusions within garnet porphyroblasts illustrates the presence of a sulfide phase prior to peak metamorphism. Geochronology of zircon and monazite constrains the timing of mineralization to be younger than c.2035Ma—the maximum depositional age of the metasedimentary host rocks—but older than c.1991Ma—the peak of M1 metamorphism during the Glenburgh Orogeny; these events were synchronous with arc magmatism. Rocks at the Glenburgh deposit were likely deposited in a fore-arc or accretionary wedge, a favourable setting for porphyry Cu–Mo–Au, epithermal Au, polymetallic (Sn, W) skarn, and orogenic Au mineralization. Phase equilibria modelling of a pelitic migmatite constrains peak P–T conditions to be 865–885°C, 6.8–7.6kbar, consistent with elevated thermal gradients within the arc, followed by conductive cooling of arc magmas. Partial melting during peak M1 metamorphism possible caused gold remobilization. The lack of an alteration assemblage further suggests that the alteration assemblage and mineralization were recrystallized during deformation and metamorphism. However, increases in Ca and K abundance and magnetic susceptibility decreases toward mineralization, suggesting that they may constitute ore vectors.

OROGENIC GOLD PROSPECTIVITY MAPPING USING GEOSPATIAL DATA INTEGRATION, REGION OF SAQEZ, NW OF IRAN

The aim of this study is to map orogenic gold prospecting areas in the region of Saqez, NW of Iran. In order to achieve this task geological, geochemical and airborne geophysical data are analyzed and integrated using index overlay and fuzzy logic methods. Geological map of Saqez (1:100000 scale) is used to assign lithological weights based on their favo- rability for hosting orogenic Au mineralization. Also a fault density map is produced and assigned based on the structural map which is included in the geological map. For prepa- ring geochemical evidence maps, data from 535 stream sediment samples are examined using Number-Size multifractal method for Au, As, Bi and Hg. The detected thresholds are used to assign the catchment basins of the stream sediment samples. Aeromagnetic data is employed to detect the edges of magnetic anomalies based on an enhanced edge detection method. Extracted lineaments are then converted to a density map and assigned properly. Airborne radiometric data is also used to produce two evidence maps. Potassium count grid independently and K/Th ratio map are employed to distinguish locations with hydrother- mal activity. Finally after integrating evidence maps, new locations with high potentials of Au mineralization are identified considering that the gold indications of the study area (Qolqoleh, Kervian and Ghabaghloujeh) are placed in the first priority of the fuzzy logic prospectivity map.

Structural Controls on Orogenic Au Mineralization During Transpression: Lupa Goldfield, Southwestern Tanzania

Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW– to E–W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.

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.