Moz Au Research Articles

Geochemical characteristics of Neoproterozoic metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide deposit, Red Sea hills, North-East Sudan

The Ariab volcanogenic massive sulfide (VMS, ∼118.3 Mt with >2 Moz Au recovered) district is part of the Ariab-Arbaat belt located in the Haya terrane, Red Sea Hills, Northeast Sudan. The gold-bearing gossan and VMS deposits are mainly hosted by Neoproterozoic bimodal volcanic succession. The petrogenesis, age dating, metallogenic evolution, and tectonic environment as well as the relationship between the metals occurrences and hosting materials remain poorly constrained and controversial. The study aims to investigate the geochemical characteristics of metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide. XRF and ICP-MS techniques were employed to analyze major, trace, and rare earth elements content. The metavolcanics composition of Ariab VMS hosting rocks ranging from basalt to rhyolitic lava with variable pyroclastics. The chemical affinity varies from sub-alkaline tholeiitic to calc-alkaline nature, raising an argument against the possibility of fractional crystallization. The Ariab VMS metavolcanics were likely formed in the intra-oceanic arc tectonic environment. Ariab mafic lava is characterized notably by high Cr and Ni concentrations. The negative and positive anomalies in trace element profiles are typically ocean island basalt patterns, implying minimal crustal contamination. The geochemical results of dacitic and rhyolitic rocks reveal an assimilation-fractional-crystallization process, including the possibility that plagioclase-rich restite was formed in the parent magma source with a significant contribution from the old crust. The mobility of elements such as Zn, Cu, Pb, Ag, Au, As, and Co complicates the determination of their primary abundances. The partitioning behavior of REEs, the geochemical signatures of chalcophile elements, and their associations with VMS deposits, underscores the complex interaction between igneous processes and subsequent hydrothermal alteration.

The Snow Lake Deposits in Manitoba, Canada: Formation of Metamorphosed Amphibolite Facies Orogenic Gold Deposits During a Progressive and Prograde Orogenic Event

Abstract The Snow Lake camp is located in the ca. 1.89 Ga Flin Flon-Glennie Complex in the Paleoproterozoic Trans-Hudson orogen, Manitoba, Canada. The Flin Flon-Glennie Complex is bordered by the ca. 1.855 to 1.84 Ga metasedimentary Kisseynew domain to the north and by the Archean Superior craton to the east and is underlain by the Archean Sask microcraton. It hosts several orogenic gold deposits, including the No. 3 zone, Boundary zone, and New Britannia deposit (Toots, Dick, Hogg, Ruttan, and Mine East zones), which formed during thrusting of the Flin Flon-Glennie Complex and Kisseynew domain over the Sask microcraton. The New Britannia deposit produced 1.6 Moz Au and is the largest orogenic gold deposit in the Trans-Hudson orogen in Manitoba. The deposits consist of quartz ± carbonate veins surrounded by alteration zones of biotite, hornblende, plagioclase ± carbonate, diopside, orthoclase, and garnet. The veins and ore zones are folded within the hinge of the synthrusting Nor-Acme anticline, which has an axial plane cleavage defined by biotite and hornblende, but they also cut across the hinge of the anticline and contain foliated fragments of the wall rocks. These mutually overprinting relationships suggest that the veins and ore zones are synfolding. Garnet, diopside, and amphibole porphyroblasts grew during folding because they overgrow the foliation, which also wraps around them. Inclusions of gold and sulfide minerals within the porphyroblasts indicate that mineralization was emplaced early during folding at greenschist or lower amphibolite metamorphic conditions prior to the growth of the porphyroblasts at peak middle amphibolite metamorphic conditions. These observations are corroborated by laser ablation-inductively coupled plasma-mass spectrometry element maps of arsenopyrite grains. These maps reveal a primary internal enrichment of gold in arsenopyrite grains from the Toots, No. 3, and Boundary zones and a lack of primary lattice-bound gold in arsenopyrite grains of the Dick, Ruttan, and Mine East zones. The latter results from the deposition or remobilization of gold during a second hydrothermal event that occurred during shearing of these ore zones along a structure, the Howe Sound fault, which acted as a detachment surface during folding. The Snow Lake deposits are examples of orogenic gold deposits that formed early during a major thrusting and folding event and were later modified and metamorphosed at middle amphibolite facies conditions during the same progressive orogenic event.

Petrogenetic of Igneous Complex of Ilota Nanasi Gold Deposit, Gunung Pani, Gorontalo, Indonesia

Ilota Nanasi Au-Ag deposit located in the western flank of the Gn. Baganite, a part rhyodacite dome in Gn. Pani district, Gorontalo Province, where the extensive exploration programs were carried out by PT Gorontalo Sejahtera Mining (a subsidiary of PT J Resources Nusantara) from 2012 – 2020. As the result, a total of 72.7 Mt @ 0.98 g/t Au and 0.85 g/t Ag (2.3 Moz Au and 2.6 Moz Ag) mineral resource was delineated in 2019. This paper mainly aimed to the petrogenesis association of granitoid basement and rhyodacite unit as the host rock in the Ilota Nanasi gold deposit. The geology of Ilota Nanasi is overlain by Late Miocene granodiorite, andesite and diorite basement, Pliocene volcanic complex predominantly dacite – rhyodacite, tuff, breccia, and quaternary deposit as alluvial and surface breccia. Hydrothermal alteration and gold – silver mineralization centered in the porphyritic rhyodacite host rocks where the intense silicification forming a zone around the hydrothermal crackle breccia unit and/or high-density quartz vein, veinlets and stockworks. Mineralization in Ilota Nanasi is interpreted as a low sulfidation epithermal system dominated by a large volume of hydrothermal crackle breccia, intense quartz veining, veinlets, and high-density fracturing. A combined analytical result of petrography and whole-rock geochemistry has been used to assess the petrogenetic association of the rhyodacite and granitoid basement in the Ilota Nanasi. The result of the AFM diagram plot shows sample trends in calc-alkaline magma field. In contrast, the SiO2 – K2O plot distributed in two trends: the older unit was associated with high-K calc-alkaline and the shoshonite series for the younger unit. The changing of magma series is considered as product of crystallization differentiation of evolved parental magma or derived from the melting of mantle material after the mantle or lower crustal was metasomatized during a former episode of subduction.

The Sukari Gold Deposit, Egypt: Geochemical and Geochronological Constraints on the Ore Genesis and Implications for Regional Exploration

AbstractThe Sukari gold deposit (>15 Moz Au) in the Eastern Desert of Egypt is hosted by a deformed granitoid stock (Sukari tonalite-trondhjemite intrusion) and mainly occurs as a network of crosscutting sulfide-bearing quartz (± carbonate) veins and intensely sulfidized-silicified-sericitized wall rock. Emplacement of the Sukari intrusion into a tectonized Neoproterozoic accretionary complex was controlled by a system of NE- to NNE-trending oblique faults that are related to a deep-seated positive flower structure. A robust genetic model has been hampered by the poorly understood relationships between gold mineralization and host rocks. In this study, zircon U-Pb ages of three samples from the Sukari intrusion define a crystallization age of ~695 ± 2 Ma. In contrast, hydrothermal sericite from the ore zone yields an 40Ar/39Ar age of ~625 ± 3 Ma, which coincides with the onset of major sinistral transpression in the region.Features including sigmoidal morphology of gold quartz veins and abundant subhorizontal tension gashes alongside widespread brecciation and recrystallization suggest that quartz veining occurred during renewed shortening and exhumation through the brittle-ductile transition. Petrographic and micro-X-ray fluorescence (µXRF) studies indicate that disseminated gold and sulfides, commonly associated with sericite and carbonate alteration, are mostly confined to stylolitic bands in the quartz veins. Oscillatory and sector zoning patterns, irregular As-rich bands, and truncations between early- and late-genetic pyrites reflect variations in temperature and mechanism of ore deposition, demonstrated by variable As/S and Co/Ni ratios in the different pyrite generations. Laser ablation-inductively coupled plasma-mass spectrometry analysis pinpoints the covariance of gold and arsenic contents in pyrite, but free milling gold inclusions in microfractures consistently have a mercury-bearing electrum composition, depicting different ore formation stages.Ore fluids with δ34SH2s values of –1.9 to –3.0‰, modeled from gold-associated pyrite and arsenopyrite assemblages with nearly identical δ34S values, suggest a likely single source of sulfur. Alternatively, multisourced sulfur could have extensively mixed and equilibrated by fluid reaction with carbonaceous wall rock. Gold deposition was triggered by abrupt changes in fluid pH and fO2. As an implication for future exploration, sites of maximized strain gradients adjacent to granitoid bodies along extensive transpression zones in the district could be highly prospective targets, particularly where imprinted by sericite-carbonate ± graphite alteration.

High-Grade Copper and Gold Deposited During Postpotassic Chlorite-White Mica-Albite Stage in the Far Southeast Porphyry Deposit, Philippines

Abstract Ninety-eight underground diamond holes (~102 km) drilled by Far Southeast Gold Resources Inc. at the Far Southeast porphyry Cu-Au deposit, Philippines, from 2011 to mid-2013, provide a three-dimensional exposure of the deposit between 700- and –750-m elevation, with surface at ~1,400-m elevation. Far Southeast contains an inferred resource of 891.7 million tonnes (Mt) averaging 0.7 g/t Au and 0.5 wt % Cu, equivalent to 19.8 Moz Au and 4.5 Mt Cu. This contribution reports the spatial and temporal distribution of alteration and mineralization at Far Southeast, notably a white-mica–chlorite-albite assemblage that formed after early secondary biotite and before late quartz–white-mica–pyrite alteration and that is associated with the highest copper and gold grades. Alteration assemblages were determined by drill core logging, short-wavelength infrared (SWIR) spectral analysis, petrographic examination, and a quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) study. Alteration is limited around sparse veins or pervasive where vein density is high and the alteration halos coalesce. The alteration and mineralization zones with increasing depth are as follows: (1) the lithocap of quartz-alunite–dominated advanced argillic-silicic alteration that hosts part of the Lepanto high-sulfidation Cu-Au epithermal deposit (mostly above ~700-m elevation), (2) an aluminosilicate-dominated zone with coexisting pyrophyllite-diaspore ± kandite ± alunite and white mica (~700- to ~100-m elevation), (3) porphyry-style assemblages characterized by stockwork veins (below ~500-m elevation), (4) the 1 wt % Cu equivalent ore shell (~400- to –300-m elevation), and (5) an underlying subeconomic zone (about –300- to –750-m elevation, the base of drilling). The ore shells have a typical bell shape centered on a dioritic intrusive complex. The paragenetic sequence of the porphyry deposit includes stage 1 granular gray to white quartz-rich (± anhydrite ± magnetite ± biotite) veins with biotite-magnetite alteration. These were cut by stage 2 lavender-colored euhedral quartz-rich (± anhydrite ± sulfides) veins, with halos of greenish white-mica–chlorite-albite alteration. The white mica is largely illite, with an average 2,203-nm Al-OH wavelength position. The albite may reflect the mafic nature of the diorite magmatism. The quartz veins of this stage are associated with the bulk of copper deposited as chalcopyrite and bornite, as well as gold. Thin Cu sulfide (chalcopyrite, minor bornite) veins with minor quartz and/or anhydrite (paint veins), with or without a white-mica halo, also occur. These veins were followed by stage 3 anhydrite-rich pyrite-quartz veins with white-mica (avg 2,197 nm, illite)–pyrite alteration halos. Combined with previous studies, we conclude that this porphyry system, including the Far Southeast porphyry and Lepanto high-sulfidation Cu-Au deposits, evolved over a period of 0.1–0.2 m.y. Three diorite porphyry stocks were emplaced, and by ~1.4 Ma biotite-magnetite–style alteration formed with quartz-anhydrite veins and deposition of ≤0.5% Cu and ≤0.5 g/t Au (stage 1); coupled with this alteration style, a barren lithocap of residual quartz with quartz-alunite halo plus kandite ± pyrophyllite and/or diaspore formed at shallower depth (>700-m elevation). Subsequently, lavender quartz and anhydrite veins with bornite and chalcopyrite (high-grade stage, avg ~1 wt % Cu and ~1 g/t Au) and white-mica–chlorite-albite halos formed below ~400-m elevation (stage 2). They were accompanied by local pyrite replacement, the formation of hydrothermal breccias and Cu sulfide (paint) veins. Stage 2 was followed at ~1.3 Ma by the formation of igneous breccias largely along the margins of the high-grade zones and stage 3 pyrite-quartz-anhydrite ± chalcopyrite veins with white-mica (mostly illitic) halos. At shallower depths in the transition to the base of the lithocap, cooling led to the formation of aluminosilicate minerals (mainly pyrophyllite ± diaspore ± dickite) with anhydrite plus high-sulfidation-state sulfides and pyrite veinlets. Consistent with previous studies, it is likely that the lithocap-hosted enargite-Au mineralization formed during this later period.

Timing of Magmatism and Skarn Formation at the Limon, Guajes, and Media Luna Gold ± Copper Skarn Deposits at Morelos, Guerrero State, Mexico

Abstract The Morelos district, located in the center of the Guerrero gold belt, Guerrero State, southern Mexico, has been the site of several recent gold ± copper skarn discoveries (total of ~8.5 Moz Au) associated with Upper Cretaceous-Paleocene granodiorite intrusions. The Limon, Guajes, and Media Luna skarn deposits developed along the contact of the El Limon granodiorite and the sedimentary host rocks. Native gold is associated with retrograde alteration of garnet-pyroxene skarn and is accompanied by native bismuth as well as telluride and sulfide minerals. The Media Luna deposit is characterized by a distinct magnesium-rich skarn mineralogy and contains an average of ~1 wt % Cu, in addition to gold and silver. This distinguishes it from the other gold-only skarn deposits in the Guerrero gold belt. This observation suggests that skarns in the Guerrero gold belt, which replaced magnesium-rich sedimentary protoliths, may host significant copper resources in addition to gold. Uranium-lead laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of the El Limon granodiorite range from 66.86 ± 0.40 to 65.88 ± 0.79 Ma. Preskarn molybdenite-quartz veins cut through the El Limon granodiorite and yield Re-Os molybdenite ages between 66.63 ± 0.22 and 65.55 ± 0.25 Ma. A series of pre-, syn-, and postskarn porphyritic dikes intruded the El Limon granodiorite, having zircon ages ranging from 65.9 ± 1.1 to 64.68 ± 0.38 Ma. The timing of skarn formation at Limon and Media Luna was constrained to 64.6 ± 1.2 to 63.2 ± 1.0 Ma, applying U-Pb LA-ICP-MS to garnet. The ubiquitous presence of garnet in skarn deposits and the time- and cost-efficiency of in situ geochronology are striking advantages of this method to constrain the timing of mineralization.

Geology and Controls on Gold Enrichment at the Horne 5 Deposit and Implications for the Architecture of the Gold-Rich Horne Volcanogenic Massive Sulfide Complex, Abitibi Greenstone Belt, Canada

Abstract The Archean Horne 5 deposit, located in the Rouyn-Noranda district in the southern Abitibi greenstone belt, Canada, contains a total resource of 172.4 t Au (5.6 Moz) from 112.7 Mt of ore grading at 1.53 g/t Au. The deposit is part of the Au-rich Horne volcanogenic massive sulfide (VMS) complex that also includes the past-producing Horne mine (i.e., the Upper and Lower H zones plus small subsidiary lenses) that yielded 325.4 t Au (10.5 Moz Au) from 53.7 Mt of ore grading at 6.06 g/t Au. Combined, the Horne mine and Horne 5 deposit contain ~500 t Au (16 Moz), making them the world’s single largest accumulation of VMS-related Au. The Horne 5 deposit consists of stacked lenses of massive to semimassive sulfides alternating with extensive zones of disseminated and stringer sulfides. The mineralization is hosted within thick accumulations of steeply dipping dacitic to rhyodacitic volcaniclastic units of transitional to calc-alkaline magmatic affinity. Dacitic-rhyodacitic synvolcanic units (lobes, sills, and/or domes) intrude the host succession, which is also crosscut by a series of post-ore mafic and younger intermediate to felsic feldspar ± quartz porphyry dikes. A broad and diffuse halo of distal sericite-chlorite-epidote alteration extends outboard of intensely sericite-altered zones proximal to the sulfide lenses. Gold is interpreted to be synvolcanic on the basis of Au-rich massive sulfide clasts in the volcaniclastic units, the presence of preserved Au-rich primary pyrite, Au zones limited to the sulfide envelope, crosscutting deformed but unaltered and barren dikes, and the absence of typical syndeformation, orogenic-style alteration and mineralization despite overprinting high-strain corridors and faults. Gold is spatially associated with pyrite, sphalerite, and chalcopyrite, and its distribution is largely controlled by the higher porosity and permeability of the volcaniclastic host rocks, which are interpreted to have facilitated hydrothermal fluid circulation in the subseafloor environment. Synvolcanic intrusions and fine-grained tuffs overlying auriferous zones also influenced the distribution of the mineralization by acting as cap rocks to ascending fluids. Evidence suggests that Au enrichment at the Horne 5 deposit is due to efficient transport and precipitation of Au in the subseafloor environment, a favorable geodynamic setting (transitional to calc-alkaline magmatism over thick crust), and possible input of magmatic fluids as suggested by high Te and Cu in the mineralization. Minor and very local remobilization of metals occurred in response to regional deformation and associated greenschist facies metamorphism. The detailed study of the Horne 5 deposit geology and a review of the available information on the Horne mine and recent 3-D modeling indicate that the Horne 5 deposit may have formed higher in the stratigraphy than the Upper and Lower H orebodies of the former Horne mine, which originally formed a single lens. Therefore, the Horne Au-rich VMS complex originally formed as a stacked system in which the Horne 5 deposit was deposited above the Upper and Lower H zones and not in a distal or lateral position as previously proposed, indicating that a robust hydrothermal system was responsible for the formation of the world’s largest Au-rich VMS complex.

Structural and geochronological constraints on orogenic gold mineralization in the western Wabigoon subprovince, Canada

Some of the world’s largest orogenic gold deposits are hosted within granite-greenstone subprovinces of the Superior Province of Canada. The known gold endowment of the western Wabigoon subprovince is much less than that of the Abitibi subprovince, despite both Neoarchean subprovinces sharing similar structural and metallogenic histories. Further, the relationship between regional deformation and orogenic gold mineralizing events in the western Wabigoon subprovince is poorly constrained in comparison with other greenstone belts within the Superior Province. New structural mapping of the Dryden area of the western Wabigoon subprovince indicates that it experienced a N–S shortening event (D1), circa 2705–2695 Ma, which reactivated, or initiated the formation of, the E–W-trending Wabigoon and Mosher Bay – Washeibemaga deformation zones (Wdz and MBWdz, respectively). Following the D1 event, after 2695 Ma, a protracted period of NNW–SSE shortening (D2) localized transpression along the Manitou Dinorwic deformation zone (MDdz), which is spatially associated with orogenic gold systems, including the Goldlund deposit (∼1.7 Moz Au) and Kenwest prospect (∼0.3 Moz Au). During D2, the MDdz experienced NW-over-SE sinistral transpression, while the Wdz and MBWdz experienced N-over-S dextral-sense movement. U–Pb geochronology of vein-hosted xenotime from the Goldlund deposit and Kenwest prospect indicate that the main hydrothermal event, and likely gold mineralization, occurred syn- to late-D2 deformation around 2664 ± 8.3 Ma. A second population of xenotime, and overgrowths, recorded a younger hydrothermal overprint between ∼2590 and ∼2580 Ma. These ages are comparable with other absolute geochronology data on hydrothermal events in the gold-rich Abitibi subprovince. The structural evolution and timing of orogenic hydrothermal events in the western Wabigoon subprovince are broadly similar to the Abitibi subprovince. Therefore, we propose that the structural evolution and timing of individual greenstone belts is not the primary controlling factor on their bulk gold endowment. Nevertheless, orogenic gold deposits are spatially associated with major D2 deformation zones and linked to widespread hydrothermal event(s), spanning 2680–2660 Ma across the southern Superior Province.

Ore Mineralogy, Geochemistry, and Dating (Re-Os, Ar-Ar) of the El Volcán Porphyry/Epithermal System, Maricunga Gold Belt, Northern Chile

Abstract The El Volcán gold project (8.9 Moz Au @ 0.71 g/t Au) is located in the Maricunga gold belt in northern Chile, on the flank of the large Cenozoic Copiapó Volcanic Complex. Precious metal mineralization is hosted in two zones (Dorado and Ojo de Agua) of (pervasively) altered Miocene porphyry intrusions and lava flows of andesitic to rhyolitic composition, and in breccias. The ore zones reflect an evolving magmatic-hydrothermal system with mineral assemblages of magnetite-ilmenite-pyrite-molybdenite (early), bornite-chalcopyrite-pyrite-rutile (stage I), chalcocite-chalcopyrite-enargite-fahlore-pyrite (stage II), and chalcopyrite-covellite-pyrite (stage III). Alteration is dominantly of Maricunga-style (illite-smectite-chlorite ± kaolinite), partly obscured by quartz-kaolinite-alunite ± illite ± smectite alteration. Powdery quartz-alunite-kaolinite alteration with native sulfur and cinnabar forms shallow steam-heated zones. Early K-feldspar ± biotite alteration is preserved only in small porphyry cores and in deep drill holes. Most gold is submicrometer size and is in banded quartz veinlets, which are characteristic of the Maricunga gold belt. However, some gold is disseminated in zones of pervasive quartz-kaolinite-alunite alteration, with and without banded quartz veinlets. Minor visible gold is related to disseminated chalcocite-chalcopyrite-enargite-fahlore-pyrite. The lithogeochemical database identifies a pronounced Au-Te-Re signature (>100× bulk crust) of the hydrothermal system. Molybdenum-rich bulk rock (100–400 ppm Mo) has an Re-Os age of 10.94 ± 0.17 Ma (2σ). 40Ar-39Ar ages on deep K-feldspar alteration and on alunite altered rock have the same age within error and yield a combined age of 11.20 ± 0.25 Ma (2σ). The formation of the El Volcán gold deposit took place during the establishment of the Chilean flat-slab setting in a time of increasing crustal thickness when hydrous magmas were formed in a mature arc setting. The vigorous nature of the hydrothermal system is expressed by abundant one-phase vapor fluid inclusions recording magmatic vapor streaming through a large rock column with a vertical extent of ≥1,500 m.

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part I. Volcanic Architecture, Deformation, and Metamorphism

Abstract The Westwood deposit (4.5 Moz Au) is hosted in the 2699–2695 Ma Bousquet Formation volcanic and intrusive rocks, in the eastern part of the Blake River Group, southern Abitibi greenstone belt. The Bousquet Formation is divided in two geochemically distinct members: a mafic to intermediate, tholeiitic to transitional lower member and an intermediate to felsic, transitional to calc-alkaline upper member. The Bousquet Formation is cut by the synvolcanic (2699–2696 Ma) polyphase Mooshla Intrusive Complex, which is cogenetic with the Bousquet Formation. The deposit contains three strongly deformed (D2 flattening and stretching), steeply S-dipping mineralized corridors that are stacked from north to south: Zone 2 Extension, North Corridor, and Westwood Corridor. The North and Westwood corridors are composed of Au-rich polymetallic sulfide veins and stratabound to stratiform disseminated to massive sulfide ore zones that are spatially and genetically associated with the calcalkaline, intermediate to felsic volcanic rocks of the upper Bousquet Formation. The formation of the disseminated to semimassive ore zones is interpreted as strongly controlled by the replacement of porous volcaniclastic rocks at the contact with more impermeable massive cap rocks that helped confine the upflow of mineralizing fluids. The massive sulfide lenses are spatially associated with dacitic to rhyolitic domes and are interpreted as being formed, at least in part, on the paleoseafloor. The epizonal, sulfide-quartz vein-type ore zones of the Zone 2 Extension are associated with the injection of subvolcanic, calc-alkaline felsic sills and dikes within the lower Bousquet Formation. These subvolcanic intrusive rocks, previously interpreted as lava flows, are cogenetic and coeval with the intermediate to felsic lava flows and domes of the upper Bousquet Formation. The change from fractional crystallization to assimilation- and fractional crystallization-dominated processes and transitional to calc-alkaline magmatism is interpreted to be responsible for the development of the auriferous ore-forming system. The Westwood deposit is similar to some Phanerozoic Au ± base metal-rich magmatic-hydrothermal systems, both in terms of local volcano-plutonic architecture and inferred petrogenetic context. The complex volcanic evolution of the host sequence at Westwood, combined with its proximity to a polyphase synvolcanic intrusive complex, led to the development of one of the few known large Archean subaqueous Au-rich magmatic-hydrothermal systems.

Gold in Paleoproterozoic (2.1 to 1.77 Ga) Continental Magmatic Arcs at the Tapajós and Juruena Mineral Provinces (Amazonian Craton, Brazil): A New Frontier for the Exploration of Epithermal–Porphyry and Related Deposits

This review paper aims to integrate geological, tectonic and metallogenetic data, including new data, and propose a regional model for the gold (and base metal) mineralization in the south Amazonian Craton to support the mineral exploration concerning magmatic–hydrothermal deposits. The Proterozoic evolution of the Amazonian Craton comprises the accretion of terrains to the Archean Carajás Mineral Province. In the Tapajós and Juruena mineral provinces, located at the south part of the Amazonian craton, a long-lived ocean–continent subduction event produced ca. 2.0 to 1.77 Ga continental magmatic arcs. Extensive lava flows, volcaniclastic, sedimentary, and plutonic rocks were originated during at least four major orogenic magmatic events (ca. 2.1, 1.9, 1.88, and 1.80 Ga) and two post- to anorogenic events (ca. 1.87 and 1.77 Ga). Gold mineralization occurs in: (i) alluvial/colluvial occurrences, (ii) orogenic carbonate–sulfide-rich quartz veins in shear zones, (iii) stockworks, veins, and dissemination in granites, (iv) contact of basic dikes, (v) well-preserved high-, intermediate- and low-sulfidation epithermal mineralization, and (vi) porphyry-like and intrusion-related gold systems associated with late- to post-orogenic epizonal granites. The estimated historical gold production, mainly in secondary deposits, is over 27 Moz at the Tapajós and 6 Moz at the Juruena provinces. A total resource of over 5 Moz Au is currently defined in several small to large primary gold deposits. Andesite to rhyolite, volcaniclastic, and clastic sedimentary rocks (1.96–1.88 Ga) host epithermal (high-, intermediate-, and low-sulfidation) Au–(Ag–Pb–Zn) mineralization, whereas Au–Cu and Cu–Mo–Au mineralization is hosted in sub-volcanic tonalitic to granitic plutons. Advanced argillic alteration (alunite, pyrophyllite, enargite) associated with high-sulfidation mineralization occurs in ring volcanoes around nested volcanic calderas. This zone grades outward to propylitic or chlorite alteration, often covered by silica caps with vuggy silica. Lava flows and volcaniclastic rocks within faults or associated with volcanic edifices and rhyolitic domes host low- and intermediate-sulfidation mineralization. Low-sulfidation alteration zones typically have adularia and illite or sericite. Chalcopyrite, sphalerite, galena, pyrite, digenite, and manganiferous calcite are related to intermediate-sulfidation gold mineralization. Late- to post-orogenic evolved oxidized I-type granitoids host alkalic-type epithermal and porphyry-like gold mineralization. Porphyry-style hydrothermal alteration is analogous to those of modern systems, with inner sodic and potassic (potassic feldspar ± biotite or biotite) alterations grading to propylitic, muscovite-sericite, chlorite–sericite, and chlorite alterations. Potassic alteration zones are the locus of Cu–Mo mineralization, and gold-rich zones occur in muscovite/sericite–quartz–pyrite alteration. The Paleoproterozoic epithermal and porphyry-like mineralization in these large provinces defines a new frontier for the exploration of world-class gold deposits in the worldwide Proterozoic arc-related magmatic terrains.

Temporal–Spatial Separation of Cu from Mo in the Jiama Porphyry Copper–Polymetallic Deposit, Southern Tibet, China

Jiama is a super-large porphyry copper–polymetallic deposit located in the Gangdese metallogenic belt of southern Tibet. The deposit consists of a combination of a polymetallic skarn, Cu–Mo mineralization at the contact between the Jiama Porphyry and hornfels, and distal Au mineralization in fault. The current metal reserves are 7.4 Mt Cu, 0.6 Mt Mo, 1.8 Mt Pb–Zn, 6.65 Moz Au, and 360.32 Moz Ag, with a skarn to porphyry–hornfels host-rock ratio of ~3:1. Based on detailed field and laboratory investigations, this paper indicates that: (i) the skarn and porphyry–hornfels orebodies are almost entirely preserved; (ii) the emplacement age of the Cu-bearing plutonic rocks is earlier than the plutons containing elevated Mo assays; (iii) the permeability of the wall rocks gradually decreases in an upward direction; (iv) the fluid temperature during the precipitation of Cu was higher than that of the Mo mineralization; (v) the lithospheric pressure during the precipitation of Cu and Mo was the same; (vi) the laser Raman spectroscopy shows that the fluid carrying the Cu was rich in magnetite, hematite, and anhydrite, and the fluid carrying Mo was rich in pyrite, CO2, and CH4; and (vii) the SR–XRF mapping shows that the concentration of Cu in the mineralizing fluid was high and that of Mo was low when Cu was deposited. Conversely, the concentration of Cu was relatively low and the concentration of Mo was relatively high during deposition of the Mo. This study also shows that the temporal and spatial separation of Cu and Mo in the porphyry copper–polymetallic deposit at Jiama was associated with the emplacement of the Jiama Porphyry. Transportation of mineralized fluid was controlled by the permeability of the wall rocks, and deposition of the metals related to changes along a redox front and pressure releases during hydrothermal brecciation at the roof of the Jiama Porphyry.

Role of Basement Structural Inheritance and Strike-Slip Fault Dynamics in the Formation of the Pataz Gold Vein System, Eastern Andean Cordillera, Northern Peru

Abstract New regional- to vein-scale geologic mapping and structural analysis of the Carboniferous Pataz gold vein system (~10 Moz Au) reveal critical insights into the structural control on gold mineralization along the Eastern Andean Cordillera of northern Peru. The Pataz basement comprises continental volcanic arc and marginal to marine sedimentary rocks, which experienced intensive D2 deformation associated with Late Famatinian northeast to southwest compressive fold-and-thrust belt development. The D2 event produced an E-NE–dipping structural grain, including (1) tilted and F2 folded S1 foliations, (2) local F2 axial planar S2 foliations, and (3) subparallel D2 thrust faults. Intrusions, constituting the ca. 342 to 332 Ma (Mississippian) Pataz batholith, were emplaced along strike of the prominent Río Marañón fault and inherited the D2 basement structures, as evident in the orientation of suprasolidus magmatic flow zones and intrusive contacts within the batholith. Progressive horst-and-graben development affecting the volcanic carapace of the Pataz batholith records late syn- to postmagmatic uplift and transition into a NW-SE–extensional regime. We show that the E-NE–dipping, batholith-hosted gold vein system formed through synchronous activation of two geometric fault-fill vein types, following (1) the moderately E-NE–dipping D2 basement-inherited competency contrasts within the batholith and (2) shallow NE-dipping Andersonian footwall thrusts, during NE-directed shortening (D3a). Both geometric vein types display an early paragenetic stage (I) of quartz-pyrite, progressing texturally from hydraulic breccia into crack-seal laminated shear veins. A second (II), undeformed quartz-pyrite-sphalerite-galena paragenetic stage is observed to fill previously established dilational sites adjacent to newly formed D3b normal faults, which likely formed during regional NW-SE–extensional horst-graben development. Kinematics and relative timing indicate that, upon batholith solidification, D3a transpressional dextral strike-slip ruptures along the Río Marañón fault superimposed a lower-order Riedel-type fault system. Fluid-assisted fault activation preferentially impinged on the D2 basement-inherited competency contrasts within the batholith. Subsequent transition into a transtensional regime led to the D3b normal faulting, providing a feeder system for stage II fluid influx. The tectonic switch may be explained either by increasing tensile strain accommodation upon progressive strike-slip movement within a regional dilational jog or by larger-scale crustal relaxation of the late Gondwana margin upon final Pangea assembly. Our new structural model for the Pataz vein system evolution highlights the importance of basement structural inheritance in controlling the localization of gold mineralization along polycyclic supercontinent margins. We provide valuable insights for exploration targeting of complex vein arrays within rheologically heterogeneous host rocks.

The large Bakyrchik orogenic gold deposit, eastern Kazakhstan: Geology, mineralization, fluid inclusion, and stable isotope characteristics

The large (>13 Moz Au, averaging 5–6 g/t Au) Bakyrchik Au deposit in eastern Kazakhstan is part of a district-scale mineralized zone, situated in an accretionary terrane. The deposit is localized within a shear zone in carbonaceous sedimentary rocks, which crosscuts the stratigraphy and comprises subconcordant dikes of high-K calc-alkaline (to shoshonitic?) rocks (lamprophyre to monzogabbro to monzodiorite). The tectonized/cataclased carbonaceous rocks contain numerous banded (bedded?) pyrite clasts and bands and abundant disseminated auriferous sulfides (pyrite and acicular arsenopyrite); the latter overprint pyritic clasts and occur proximal to (although mostly outside of) the concordant quartz veins and associated quartz-cemented hydrothermal breccias. Late granodiorite-plagiogranite dikes postdate auriferous sulfides and associated quartz veins/hydrothermal breccia but are overprinted by the late quartz veins containing free native gold and some Cu-Zn-Pb sulfides. This mineralization, however, is minor at the deposit.The most abundant stylolitic quartz veins accompanied by auriferous sulfide mineralization were formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >315–300 to 265–240 °C and pressures of 0.8 to 1.2 kbar. Boiling was a dominant process during the formation of hydrothermal breccias accompanied by most intense auriferous sulfide (pyrite, arsenopyrite) mineralization at the temperatures of 250–230 °C and pressures of ~1.1 kbar. The late quartz veins were also formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >350–315 to 235–225 °C and pressures of 1.1 to 1.3 kbar. The stable isotope data support a variable – from sedimentary to magma-derived - source of sulfur (δ34S = −20 to ~0‰), with a dominantly sedimentary to metamorphic source of carbon (δ13C = −27.4 to −22.5‰).The deposit is polygenetic and exhibits combined synsedimentary-exhalative, metamorphic to intrusion-related, magmatic-hydrothermal signatures, with strong structural control and close association with various dikes. Most of auriferous sulfide mineralization occurred after or nearly coeval to the intrusion of mafic to intermediate dikes, with a strong positive correlation of Au with As, Ssulf, Ag, Sb, and Hg. Homogenized δ34S values of auriferous pyrite and arsenopyrite (from −2.4 to −0.5‰), close to the meteorite standard, indicate dominantly magma-derived sulfur. Late quartz veins with free native gold and minor Cu-Zn-Pb sulfides are more distinctly related to granodiorite-plagiogranite intrusions.

Modelling gold potential in the Granites-Tanami Orogen, NT, Australia: A comparative study using continuous and data-driven techniques

The Granites-Tanami Orogen (GTO), a poorly exposed component of the North Australian Craton, is host to significant Paleoproterozoic orogenic gold mineralization amounting to a total endowment of greater 20 Moz Au. This study adopted a multi-technique approach to mineral potential modelling (MPM) of orogenic gold deposits in the GTO, using four different techniques: Continuous fuzzy gamma, geometric average and data-driven index overlay and data-driven random forest methods. The predictive models generated in this study delivered the first public-domain gold prospectivity maps for the Northern Territory (NT)-portion of the GTO. The predictor maps at the core of these models represent mappable proxies of mineralizing processes believed to have been critical in the formation of the orogenic gold deposits, and were generated in the framework of a mineral systems approach. Normalized density, prediction-area and area under the receiver operating characteristic curve methods were used to test the robustness of these predictors and determine the most effective. The performance of the gold potential models generated in this study was assessed using the improved prediction-area plot procedure, with the data-driven random forest approach having delivered the best performing model. The multi-technique approach taken in this study, in which multiple mineral potential models are generated for the same study area and using the same predictor maps, is a relatively new and, as of yet, underutilized approach to MPM that, in contrast to the standard procedure (i.e., generation of a single data- or knowledge-driven model) ensures optimal utilization of the available conceptual and empirical information, helps to better understand and minimize stochastic and systemic uncertainties, facilitates cross-validation of the resulting prospectivity models, and allows the resulting models to be compared and contrasted. From a practical, exploration point of view, the models served as decision-support tools that helped to improve confidence in targeting underexplored, gold-prospective areas in the GTO more effectively and identify new search areas previously not recognized as areas of high gold potential.

A trace metal, stable isotope (H, O, S), and geochronological (U-Pb titanite) characterization of hybridized gold orebodies in the Missanabie-Renabie district, Wawa subprovince (Canada)

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.

In-situ sulfur isotopes analysis as an exploration tool for orogenic gold mineralization in the Granites-Tanami Gold Province, Australia: Learnings from the Callie deposit

Orogenic gold deposits are commonly associated with a small alteration and geochemical footprint that often hinders the exploration process. In the Granites-Tanami Gold Province; however, we demonstrate that sulfur isotope analyses represent a powerful exploration tool for orogenic gold mineralization. The Granites-Tanami Gold Province, situated in the Northern Territory (Australia), hosts numerous gold occurrences including the world-class Callie deposit (c. 14.2 Moz Au). In this paper, we use sulfur isotopes to: 1) characterize the δ34S signature of the sulfides associated with gold mineralization in order to assist with exploration in the Granites-Tanami Gold Province, and 2) monitor the temporal fluid evolution of the Callie deposit. We present in-situ sulfur isotope analyses of sulfides associated to gold mineralization, diagenetic sulfides, and sulfides associated to a barren, post-mineralization hydrothermal event. Our results show that the sulfides associated with mineralization yield δ34S values that range from +4‰ to +11.5‰ (n = 114), the diagenetic sulfides yield δ34S values that range from +0.0‰ to +1.7‰ (n = 41) and the post-mineralization sulfides yield δ34S values that range from +2.3‰ to +3.3‰ (n = 20). The range of δ34S values obtained from mineralized samples at the Callie deposit coincides with δ34S values reported in sulfides associated to other orogenic gold deposits of the Granites-Tanami Gold Province. The mineralization-related δ34S signature in the Granites-Tanami Gold Province is distinct from both diagenetic and post-mineralization sulfides providing a unique isotopic signature for the sulfides associated to gold mineralization. Therefore, we propose that the application of routine sulfur isotope analysis may represent a valuable tool for mineral exploration in the area. We also report a temporal shift toward lighter δ34S values from various sulfides associated to the stratabound mineralization at the Callie deposit. We interpret the shift in δ34S values as the expression of a progressive increase in the oxygen fugacity of the ore fluid resulting from progressive alteration of the host-rock.

Petrogenetic and tectonic controls on magma fertility and the formation of post-subduction porphyry and epithermal mineralization along the late Cenozoic Anatolian Metallogenic Trend, Turkey

The late Cenozoic Anatolian Metallogenic Trend in Turkey is a manifestation of westerly Aegean subduction and easterly Arabian continental collision along the Western Tethyan Orogenic Belt. The 1500 × 200 km late Cenozoic Anatolian magmatism hosts many gold-rich porphyry and epithermal deposits and prospects (~ 27 Moz Au) that formed during the tearing of the Aegean slab and Arabian slab break-off events that initiated at 15 Ma in western Anatolia and 25 Ma in eastern Anatolia. Although the temporal and spatial correlations between slab segmentation, late Cenozoic magmatism, and associated gold-rich porphyry and epithermal mineralization has been intuitively proposed, the genetic relationships between asthenospheric upwelling, subcontinental lithospheric mantle instability, and magma fertility have not been well established. Herein, new geochemical and radiogenic Sr, Nd, and Pb isotope data from Miocene to Pliocene magmatic rocks that are genetically related to porphyry, epithermal, and skarn systems from western, central, and eastern Anatolia are presented and interpreted with previously published geochemical data from late Cenozoic intrusion-related mineral systems throughout Anatolia. The magma fertility of the late Cenozoic post-subduction igneous suites along the late Cenozoic Anatolian Metallogenic Trend is assessed and is best indicated by the SiO2 (61.8 ± 5.5 wt%) content, Sr/Y (> 20) and Ba/Ta ratios (> 428), and (eNd)i values (< 0). Fertile magmatism dominantly resulted from the melting of the metasomatized Anatolian SCLM and was accompanied by amphibole ± clinopyroxene and plagioclase fractionation in the absence of garnet residue as indicated by low (Dy/Yb)N and Eun/Eu* ratios. Magma fertility indicators evolved through time along the late Cenozoic Anatolian Metallogenic Trend. In western Anatolia, fertile magmatism peaked in the middle to late Miocene (15–9 Ma) and was preceded by less-fertile early Miocene magmatism (21–18 Ma) and followed by barren Pliocene to Quaternary, alkaline lavas. The geochemical signatures of mineralized igneous units in central and eastern Anatolia indicate that fertile magmas formed early as regional magmatism evolved towards less hydrous, less oxidized, shallower, and mantle-dominant signature. The reduction of magmatic fertility since the late Miocene throughout Anatolia was caused by the upwelling of hot, dry asthenosphere through the Aegean slab tear and Arabian slab break-off, SCLM instability, and subsequent removal in absence of an over-thickened crust. The spatial migration of slab ruptures and slab gap opening modified mantle fluxes, reduced the SCLM thickness and volume of fusible material, and caused the migration of melting source. Thus, the segmentation of the Southern Neotethyan oceanic slab triggered the metallogenic events along the Anatolian Trend but subsequently contributed to the cessation of production of fertile magma.

The use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the Malartic district, Québec

The world-class, oxidized intrusion-related Canadian Malartic gold deposit, with reserves estimated at 5.56 Moz Au grading 1.10 g/t Au, and a total geological endowment of 16.3 Moz Au, is one of the largest gold deposits in the Archean Superior Province of Canada. The gold mineralization is hosted predominantly by Pontiac Group metasedimentary rocks, Piché Group metavolcanic rocks, and quartz monzodiorite to granodiorite porphyritic intrusions. The ore takes the form of a low-grade envelope of disseminated pyrite (0.35 to 1 g/t Au) grading inwards into higher grade (>1 g/t Au) stockwork and breccia zones. Hydrothermal alteration in the metasedimentary rocks is zonally distributed around the fluid pathways. Proximal alteration is characterized by a microcline±albite-quartz replacement-type assemblage, with lesser phlogopite, calcite±Fe-dolomite, pyrite and rutile. The distal alteration assemblage comprises biotite, microcline±albite, phengite, quartz, calcite, pyrite and rutile.In this study, we assess the magnitude and distribution of fluid-rock interaction in the metasedimentary rocks of the Malartic district. The metaturbidites are separated into four lithotypes based on grain-size to reduce the effects of primary depositional processes on mass change calculations. Despite the variability in protolith composition, the metasedimentary rocks define a geochemically consistent, cogenetic sequence. The results of the mass transfer calculations indicate progressive gains in CO2, S, K2O and LOI, as well as Au, Te, W, Ag, As, Be, Sb, Bi, Mo and Pb, from background, to distal and proximal alteration zones (relative to the least-altered samples). Molar element ratio analysis (alkali/aluminum) indicates an increase in alkali metasomatism (K and Na) adjacent to the main hydrothermal fluid pathways, which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase, biotite and white mica.Ore-associated pathfinder elements delineate broad enrichment patterns around the deposit, and are used to understand hydrothermal fluid circulation in the Malartic district. A statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes. Generally, the alteration forms envelopes that extend along the S2 fabric, with the largest lithogeochemical anomalies (e.g., Au, W, Te and Ag) reaching up to 10 km in length, and 2 km in width. The results of this study demonstrate that whole-rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context.

Appraisal of the USGS Three-Part Mineral Resource Assessment through estimation of the orogenic gold endowment of the Sandstone Greenstone Belt, Yilgarn Craton, Western Australia

Regional-scale ground selection presents a significant risk in mineral exploration targeting. As any exploration search space contains a finite and fixed number of ore bodies, selection of barren or depleted search spaces incurs wasted exploration expenditure. By defining regions of estimated high mineral endowment, via generation of quantitative information as to the value of potential discoveries, mineral exploration effectiveness, project acquisition, and exploration portfolio management can be improved. The Three-Part Mineral Resource Assessment, developed by the United States Geological Survey (USGS), is a methodology designed to assist in this process. Here, two separate assessments are applied to estimate the orogenic gold endowment of the Sandstone Greenstone Belt in the Yilgarn Craton, Western Australia. First, the results of a global regression assessment (median endowment of 33 t/1.16 Moz Au, mean of 130 t/4.59 Moz Au), based on an available, but outdated, global grade-tonnage model for low-sulfide gold-quartz vein deposits, suggest that most outcropping and shallow orebodies in the Sandstone Greenstone Belt have been discovered and fully delineated. In contrast, an expert assessment (median endowment of 210 t/7.41 Moz Au, mean of 220 t/7.76 Moz Au), using expert-derived grade and tonnage distributions specific to the Sandstone Greenstone Belt, predicts that significant gold mineralization remains to be discovered, largely beneath cover. The contrasting assessments imply a step change in exploration focus, from surficial deposits at the time of the USGS global grade-tonnage model to those buried under cover at the present time. The expert assessment, in contrast to the global regression assessment, implies the potential for deeper exploration targets within the Sandstone Greenstone Belt. Based on the predicted total number of deposits, much of the undiscovered mineral endowment is likely to be distal to known ore bodies, beneath cover, and at greater depths within the belt.