Archean Orogenic Gold Deposits Research Articles

Deformation and paleopiezometry of auriferous quartz veins in Archean orogenic gold deposits of the Abitibi greenstone belt

Most Archean orogenic gold deposits are associated with major faults such as the Larder Lake-Cadillac fault zone (LLCFZ) in the Superior craton and are hosted in or adjacent to second-order structures as quartz vein systems. Microstructures induced by quartz deformation in these faults or shear zones, such as typical textures of recovery and dynamic recrystallization, give indication of the P-T conditions during deformation. Electron backscatter diffraction (EBSD) analysis of grain lattice orientation and grain size determination, for flow stress calculation, are commonly performed on mylonite or quartzite samples from shear zones. This study tests the validity of such methods on hydrothermal auriferous quartz veins to better constrain the conditions of quartz vein deformation in the vicinity of the LLCFZ and evaluate the effect of depth and local variations on the microstructures and flow stress. Samples were collected in 24 gold deposits and showings along the LLCFZ. The microstructural observations and EBSD analysis allowed the distinction of six microstructural types within the studied samples. Samples analyzed with EBSD display a log-normal distribution of recrystallized grain sizes that is consistent with data for natural samples. The calculated flow stress is highly variable between samples and ranges from 38.2 + 5.1/-6.1 to 74.5 + 2.2/-3.0 MPa in surface samples only. Quartz recrystallization overprints primary textures in all but one samples, which indicates that recrystallization and its associated deformation occurred after the complete crystallization of the veins. Samples with bimodal distribution of recrystallized quartz grain sizes highlight the potential of quartz veins to record several deformation events. Samples are recrystallized at higher temperature and lower stress in the east than in the west of the LLCFZ, which could indicate the exposure of a deeper crust in the east. These data underline the potential of quartz vein microstructures to contribute to the evaluation and evolution of deformation conditions of auriferous quartz vein in an orogenic context.

Nanoscale Distribution of Elements in Gold: Examples from Contrasting Deposit Types

Abstract Quantifying the distribution and nature of trace elements in native gold is a novel approach to understanding the genesis of gold deposits and has implications for metallogenic processes, mineral exploration, and metallurgical processing. Most previous research on the formation of gold deposits has utilized proxies for gold, rather than gold itself, to infer its source, transport, and depositional mechanisms, despite the difficulty in establishing unequivocal relationships among the proxies and gold. This approach has left many questions regarding the formation of high-grade coarse gold. This contribution presents results on the composition and distribution of trace elements in native gold using electron probe microanalysis and, for the first time, atom probe tomography. Gold grains selected are from a deformed and metamorphosed Archean orogenic gold deposit (Preston Mine) and a young, undisturbed epithermal gold deposit (McLaughlin mine) which are endmembers in terms of deposit type and geologic time. Results show that Cu, Ag, and Hg are homogenously distributed in gold while Sb is not, which raises questions regarding the substitution mechanisms of large radii metallic elements in gold. These data provide new insights into the incorporation of trace elements in gold, as well as primary and secondary processes related to the formation of high-grade gold deposits through time.

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt‐hosted gold mineralization at the Tau deposit of the Mupane gold mine, Botswana

AbstractThe Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

Ediacaran - Cambrian fluid flow in Archean orogenic gold deposits: Evidence from U–Pb SHRIMP hydrothermal monazite ages of the metaturbidite-hosted Córrego do Sítio and Pilar deposits, Quadrilátero Ferrífero, Brazil

Archean orogenic gold deposits are hosted in the Rio das Velhas greenstone belt in the eastern sector of the Quadrilátero Ferrífero, Southern São Francisco Craton region, southeastern Brazil. These include the gold deposits along the NE-trending Córrego do Sítio lineament, stretching for some 15 km, and the Pilar deposit, object of the present study. These deposits occur in an area close to the boundary of the São Francisco craton, which is commonly interpreted to have preserved the Archean and Paleoproterozoic crust from the effects of Ediacaran-Cambrian Araçuaí orogenic front. The gold deposits were formerly interpreted as exclusively Archean (ca. 2.7 Ga). However, recent geochronological data suggest the imprint of the late Ediacaran-Cambrian Brasiliano Orogeny in host rocks of these deposits, as structural modification or hydrothermal alteration assemblages that postdates Archean mineralization. To elucidate those issues, contextual (thin section) in-situ U–Pb SHRIMP dating was conducted on hydrothermal monazite crystals from the Carvoaria and Cachorro Bravo deposits of the Córrego do Sítio lineament and from the Pilar gold deposit. Hydrothermal monazite in mineralized metapelites from Carvoaria yielded a U–Pb Discordia with intercepts at 2,514 ± 22 Ma and 555 ± 19 Ma. Three younger, age-equivalent crystals are concordant and yielded a U–Pb Concordia age of 539 ± 9 Ma, identical within uncertainties to the lower intercept age. Monazite from the Cachorro Bravo deposit yielded U–Pb Concordia ages of 551 ± 10 Ma and 510 ± 11 Ma. Monazite from the veined (silicified) sulfide-schist from the Pilar deposit is depleted in U, precluding the calculation of robust U–Pb ages, and disclosed a mean 208Pb/232Th average age of 508.2 ± 6.4 Ma. These results reinforce the proposal of a strong Ediacaran-Cambrian imprint related to the final stages of the Brasiliano orogenic event, affecting Archean gold deposits throughout the Quadrilátero Ferrífero. Consequently, the results point to the importance of mapping Brasiliano-related structures that control the spatial arrangement of the gold deposits, such as the Córrego do Sítio lineament, consisting of an important exploration target. This major NE-SW trending strike-slip shear zone hosts several gold deposits and might represent an Archean structure reactivated during the Brasiliano Orogeny that possibly led the large volume of Ediacaran-Cambrian post-collisional hydrothermal fluids among the east sector of the Quadrilátero Ferrífero.

Integrated geophysical signatures and structural geometry of the Kabinakagami Lake greenstone belt, Superior Province, Ontario, Canada: Exploration implications for concealed Archean orogenic gold deposits

Geophysical exploration for concealed orogenic gold deposits is challenging because petrophysical contrasts between the gold lodes and their host rocks are normally subtle. Most greenstone-hosted lode‑gold deposits do not contain strong conductors as they normally contain <5 vol% of poorly conductive disseminated arsenopyrite and pyrite. Where there is no outcrop, reliance must be placed on lithological and structural interpretations from aeromagnetic data.Airborne Induced Polarization (AIP) Cole-Cole parameters from helicopter time-domain electromagnetic (HTEM) surveys are demonstrated to be effective in geophysical exploration for concealed gold deposits in the Kabinakagami Lake greenstone belt in Ontario, Canada. Interpretation reveals that most AIP positive responses are associated with clays and all known gold deposits in the study area are in resistive rock formations. Hence, resistivity-scaled chargeability (RSC) can be used to detect weakly chargeable zones within these resistive units.Deep Neural Network (DNN) predictive targeting analysis is utilized to integrate the AIP apparent resistivity, resistivity-scaled chargeability (RSC), EM induction time-constant, and magnetic data. The DNN results imply that the known gold occurrences coincide with high DNN probabilities, and hence exploration targets can be identified based on DNN analysis. From a geological perspective, gold prospects also occur at triple-point junctions between granite intrusions and at probable intersections between potentially hosting shear zones and oblique cross faults or dykes, both common structural geometries for orogenic gold deposits. Consequently, DNN predictive targeting results based both on HTEM and magnetic data, and consistent with expected structural geometries, are proposed as a vectoring tool in the generation of drill targets during exploration for concealed orogenic gold deposits in greenstone belts such as those of the Superior Province, Canada.

The Perron Gold Deposit, Archean Abitibi Belt, Canada: Exceptionally High-Grade Mineralization Related to Higher Gold-Carrying Capacity of Hydrocarbon-Rich Fluids

The Perron deposit, an Archean orogenic gold deposit located in the Abitibi belt, hosts a quartz vein-type gold-bearing zone, known as the high-grade zone (HGZ). The HGZ is vertically continuous along &gt;1.2 km, and is exceptionally rich in visible gold throughout its vertical extent, with grades ranging from 30 to 500 ppm. Various hypotheses were tested to account for that, such as: (1) efficient precipitating mechanisms; (2) gold remobilization; (3) particular fluids; (4) specific gold sources for saturating the fluids; and (5) a different mineralizing temperature. Host rocks recorded peak metamorphism at ~600 °C based on an amphibole geothermometer. Visible gold is associated with sphalerite (&lt;5%) which precipitated at 370 °C, based on the sphalerite GGIMFis geothermometer, during late exhumation of verticalized host rocks. Pyrite chemistry analyzed by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) is comparable to classical orogenic gold deposits of the Abitibi belt, without indication of a possible magmatic fluid and gold contribution. Comparison of pyrite trace element signatures for identifying a potential gold source was inconclusive to demonstrate that primary base-metal rich volcanogenic gold mineralization, dispersed in the host rhyolitic dome, could be the source for the later formation of the HGZ. Rather, nodular pyrites in graphitic shales, sharing similar trace element signatures with pyrite of the HGZ, are considered a potential source. The most striking outcome is the lack of water in the mineralizing fluids, implying that gold was not transported under aqueous complexes, even if fugacity of sulfur (−6) and oxygen (−28), and pH (~7) are providing the best conditions at a temperature of 350 °C for solubilizing gold in water. Fluid inclusions, analyzed by solid-probe mass spectrometry, are rather comparable to fossil gas composed mostly of hydrocarbons (methane and ethane and possibly butane and propane and other unidentified organic compounds), rich in CO2, with N2 and trace of Ar, H2S, and He. It is interpreted that gold and zinc were transported as hydrocarbon-metal complexes or as colloidal gold nanoparticles. The exceptional high content of gold and zinc in the HGZ is thus explained by the higher transporting capacity of these unique mineralizing fluids.

A metasedimentary source of gold in Archean orogenic gold deposits

AbstractThe sources of metals enriched in Archean orogenic gold deposits have long been debated. Metasedimentary rocks, which are generally accepted as the main metal source in Phanerozoic deposits, are less abundant in Archean greenstone belts and commonly discounted as a viable metal source for Archean deposits. We report ultralow-detection-limit gold and trace-element concentrations from a suite of metamorphosed sedimentary rocks from the Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic decreases in the Au content with increasing metamorphic grade indicate that Au was mobilized during prograde metamorphism. Mass balance calculations show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed to the sillimanite metamorphic zone. The total gold resource in orogenic gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au mobilized from the estimated total volume of high-metamorphic-grade Pontiac sedimentary rock in the region (25,000 km3), indicating that sedimentary rocks are a major contributor of metals to the orogenic gold deposits in the southern Abitibi belt.

Integration of multiple sulfur isotopes with structural analysis unveils the evolution of ore fluids and source of sulfur at the Kanowna Belle Archean orogenic gold deposit, Yilgarn Craton, Western Australia

The Kanowna Belle deposit is a world-class Archean orogenic gold system that witnessed multiple fluid episodes over a protracted deformation history. The hydrothermal fluid circulation episodes at the Kanowna Belle deposit initiated with the precipitation of early gold-bearing carbonate-famatinite-pyrite-telluride-electrum veins (V1a). These early veins were subsequently folded during a NE-SW shortening event (D1bKB) that led to the development of sericite-chlorite-pyrite stringers (V1b) and foliation (S1KB) dated at c. 2658 ± 9 Ma (U-Pb, xenotime). D1bKB structures are overprinted by quartz-carbonate-sericite-pyrite-gold veins (V2) controlled by the reverse faulting formed as a result of N-S shortening during D2KB. A subsequent deformation event (D3aKB) is related to sinistral shearing produced under ENE-WSW shortening and associated with the development of the Troy lodes and deposition of quartz-pyrite-sericite-gold veins (V3a) dated at c. 2628 ± 9 Ma (U-Pb, xenotime). The application of multiple sulfur isotope analyses of sulfides related to the different mineralization events resolves the hydrothermal fluid isotopic evolution through time. Despite the ore mineralogy differences of the V1, V2, and V3 vein sets, their associated sulfides yield small positive ∆33S (+ 0.1 to + 0.4‰; n = 231) values with two outliers (∆33S = + 0.5‰ and + 0.6‰) across all lithology types. The constant value of MIF-S through the three temporally different gold mineralization episodes implies that sulfur was derived from a single homogenized source of sulfur distal from the deposition site, irrespective of the Au endowment. The consistent small positive ∆33S sulfur isotope signature may support that the Archean orogenic gold system sourced sulfur and possibly hydrothermal fluids from a mantle/magmatic dominated source that homogenized with crustal sulfur at depth prior to gold deposition.

Geophysical implications for the exploration of concealed orogenic gold deposits: A case study in the Sandy Lake and Favourable Lake Archean greenstone belts, Superior Province, Ontario, Canada

Since 2000, gold discovery rates have not reflected the record levels of exploration expenditure, with those discoveries that are made being predominantly in areas of post-mineral cover. Hence, gold exploration has been increasingly focused on concealed exploration targets where geophysics takes on an increasingly important role in the exploration process.Archean orogenic gold deposits are attractive exploration targets that represent a major proportion of global gold resources. Orogenic gold deposits consistently form late during the tectonic and structural evolution of their host terranes, normally during a transition from compressional to transpressional tectonic stress regimes. Thus, for target generation, the late timing of gold mineralization within the structural history of a prospective orogenic belt is crucial, as it means that current geological maps and sections reflect the geometry of the gold deposits at the time of formation unless there has been subsequent orogeny. Lithological contacts between granitic intrusions and volcano-sedimentary greenstone sequences, as well as between volcanic and sedimentary sequences, are important rheological and geochemical trap sites. Lithosphere-scale faults and dilational jogs in second-order faults act as ore-fluid conduits with anticlinal folds as structural traps for ore-fluid infiltration and gold deposition given suitable host rocks and fluid seals.Geophysical exploration for concealed orogenic gold deposits is challenging due to normally subtle petrophysical contrasts between the gold lodes and their host rocks. Most greenstone-hosted lode-gold deposits do not contain strong conductors as they normally contain <5 vol% of disseminated arsenopyrite and pyrite, which are poor conductors, and rarely contain significant quantities of strong conductors such as pyrrhotite. However, pyrrhotite may be a component of rock sequences such as sulfidic shales or cherts adjacent to the orogenic gold lodes. It is proposed that the combination of HTEM and magnetic surveys be used to explore for concealed orogenic gold deposits, using the Favourable Lake and Sandy Lake greenstone belts in the Superior Province, Canada, as a case study. AIP Cole-Cole time-constants, combined with magnetic and EM data, can efficiently detect disseminated pyrite/magnetite at shallow depths, indicating their potential use as vectoring tools for concealed Archean orogenic gold deposits.

Timing and duration of Archean orogenic gold deposits in the Bourlamaque pluton, Val d’Or mining camp, Abitibi, Canada

In the Val-d’Or mining district of the Archean Superior Province, the relationship between regional metamorphism/deformation and the timing of orogenic vein-type gold mineralization is debated; some authors arguing that gold mineralization was coeval with the regional D2 deformation event and broadly synchronous with metamorphism, and others suggesting that it is 60 to 100 Ma younger than peak metamorphism. Using the 40Ar/39Ar method and the compilation of U-Pb or Pb-Pb ages of ore-related minerals, this contribution presents a geochronological study on the timing and the inferred duration of the formation of vein-type gold deposits hosted by the Bourlamaque pluton in the Val d’Or mining district. We present an extensive set (49 samples) of 40Ar/39Ar CO2 laser probe data acquired on amphibole and white mica single grains from three major gold deposits, the Lac-Herbin, Beaufor and Beacon-2 mines. In each deposit, gold mineralization is host by shear and extensional quartz veins associated to steeply south-dipping networks of ductile shear zones. Barren north-dipping faults also occur at each deposit and are attributed to post-mineralization structural event(s). Amphiboles 40Ar/39Ar analyses yield magmatic crystallization ages clustering around 2704 Ma and preserve evidences for at least two subsequent disturbances linked to probable hydrothermal circulation events at c. 2667 Ma and c. 2652 Ma. White micas 40Ar/39Ar age spectra from auriferous quartz veins and hosting mylonitic shear zones are typical of long-lasting and sequential dynamic recrystallization and suggest that Au-bearing hydrothermal circulation was establish, at least, around c. 2650 Ma, and followed by a succession of hydrothermal pulses at c. 2597, 2575, 2551, 2500 and 2452 Ma. 40Ar/39Ar ages distribution among the different deposits as well as between the auriferous quartz veins and hosting shear zones are consistent with fluids production and circulation along an active network of seismically active fault zones.

Assessing the variability of expert estimates in the USGS Three-part Undiscovered Mineral Resource Assessment methodology: A call for increased skill diversity and scenario-based training

Abstract Future mineral exploration will necessarily be conducted in increasingly challenging and uncertain search spaces as near-surface, high-quality ore deposits are progressively depleted. Faced with this increase in task complexity, an important consideration from an exploration management perspective is the behavioural aspect of information interpretation and decision-making. One such challenging search space is the Sandstone Greenstone Belt, Western Australia, covering an area of approximately 920 km2, that is deemed prospective for the discovery of archean orogenic gold deposits, with mined (historic production), inferred and indicated resources (JORC 2004 and 2012) totalling 54 t Au. Gold endowment estimates made by geoscience experts, during an exploration project evaluation workshop, were compared with estimates from a group of non-geoscientists, made during a separate but identical workshop. Significant differences were identified between the estimates of the expert geoscientists and the non-geoscience expert group, with the latter proving more conservative. However, a portion of the geoscience experts (N = 11) group produced conservative estimates, comparable to a non-geoscience expert group (N = 10), with both suggesting the existence of additional gold deposits of similar size and quality to known resources (with group estimates for median total endowment of 99 t and 120 t Au, respectively). The remaining geoscience experts (N = 11) presented significantly more optimistic, albeit inconsistent, estimates for the gold endowment of the project area, predicting the existence of undiscovered deposits significantly larger than those already defined in the belt (with a group estimate for median total endowment of 350 t Au). Although the true undiscovered gold endowment within the project area remains uncertain, several possible factors can explain the variations in estimates. These include the application of contrasting strategies, with participants opting to apply more empirical or conceptual methods, and to differences in background experience, resulting in distinct skillsets and varying ability to estimate uncertainty. To improve the quality of expert estimates, it is suggested that individual expertise and appropriate assessment strategies can be developed through scenario-based training courses, and that greater skill and experience diversity within exploration teams is desirable, leading to more balanced aggregate estimates. Further research is warranted to determine which, if any, of the proposed factors account for these disparities. This research could be used to adapt the composition of exploration teams and develop training programs to promote the development of expertise in predictive exploration targeting, in order to promote discovery of future mineral resources.

Remobilization and adsorption of colloidal gold on nano-particulate chlorite at the Hardrock Archean orogenic gold deposits: A new tool for gold exploration

At the Hardrock Archean orogenic gold deposit, Geraldton, Canada, a correlation was found in drill core samples between the mass of elements lost on ignition (LOI) and gold concentration, and it was demonstrated that the measured LOI was in fact controlled by chlorite. The reason why LOI and gold were correlated was investigated, analyses were made to verify if chlorite and/or other sheet silicate minerals were hosting gold, and a new geochemical exploration tool was developed based on these findings. Approximately 50 drill core samples were disaggregated by gelifraction and separated into 5 different size fractions, a process which also removed the heavy minerals out-, and preferentially enriched the sheet silicates in the (sub) microscopic fractions. The concentration of gold was measured in these size fractions and their enrichment in gold over the corresponding whole rock samples was calculated. Only for the small fractions, it was discovered that for rocks having greater amount of gold, the small fractions were preferentially depleted in gold compared to their related whole rocks, whereas for rocks having lesser amounts of gold, the small fractions were relatively enriched in gold. This was interpreted to show that gold was hosted in sheet silicates for rocks hosting lesser amounts of gold, by contrast to its usual occurrence in sulfides for gold-rich rocks. Also only for small size fractions, the gold enrichment factor was positively correlated only with chlorite, confirming that chlorite was hosting gold. X-ray diffraction analyses of the different size fractions further showed that although chlorite had resisted to meteoric recrystallization, the identification of muscovite and illite variably interstratified with montmorillonite and vermiculite, as well as palygorskite, sepiolite and halloysite highlighted some of the weathering-related recrystallization. It is thus proposed that at Hardrock, the sheet silicates were affected by groundwaters after the emplacement of these rocks and that the weathering remobilized some gold from primary pyrite/arsenopyrite, to secondary colloidal-size chlorite.The idea that gold could be transported as colloidal particles in hydrothermal solutions appeared early during the 20th century. Experiments were made showing that gold could be transported as colloids at temperatures reaching up to 350 °C, Bonanza gold veins from Nevada were interpreted to have formed from colloidal gold and silica depositing as a gel, placer deposits were interpreted to have been formed from fluids transporting colloidal gold, and colloidal gold was identified in fluids escaping from black smokers from the South Pacific Ocean. At Hardrock, the concentration of gold in colloids and that of ionic gold in the accompanying liquid was determined by dialysis and showed colloidal/ionic gold distribution coefficients ranging from 5 to 3000. This was interpreted to suggest that gold was remobilized out of sulfides and transported downstream from the hydrologic flow mostly as colloids. The gold sols must eventually have adsorbed to nano-particulate chlorite, which could also have been transported further downstream. These results demonstrate that the separation of (sub) microscopic sheet silicates using this new method and the measurement of their gold enrichment factors over the related whole rocks appears to be an efficient tool for the exploration and discovery of gold in Archean settings and could effectively lead to increased resources, although further testing of the method is required over horizontal and vertical distances from known mineralization.

Protracted and polyphased gold mineralisation in the Agnew District (Yilgarn Craton, Western Australia)

The Archean orogenic gold deposits exhibits common characteristics including coeval emplacement age often proposed to be associated with a single late tectonic gold mineralisation event. Although valid for a large majority of deposits in the Yilgarn Craton, this model does not account for certain atypical gold deposits. In the world-class Agnew District (Yilgarn Craton, Western Australia) structural and paragenetic relationships, combined with a compilation of geochronology and multiple sulfur isotopes, indicate that mineralisation developed during a two-stage process involving different fluid sources. The initial event dated at c. 2660 Ma is related to the onset of folding during Kalgoorlie Orogeny and presents characteristics compatible with magmatic intrusion related mineralisation. A second mineralisation event dated at c. 2630 Ma presents a mineral and metal inventory typical to Archean orogenic-like gold mineralisation, and developed at a later stage of the orogenesis. Measurements of δ34S and Δ33S from spatially constrained samples from the two temporally distinct mineralising events further ascertain contrasting fluid compositions and/or reservoirs. Combined with geochronological data obtained on the poly-magmatic Lawlers granitic complex, we argue for a protracted ore forming process. The first stage of mineralisation has a magmatic affinity and is followed by a second stage of mineralisation whereby Au-bearing hydrothermal solution incorporated an Archean sedimentary sulfur reservoir. We propose that the change in source reservoir for sulfur in hydrothermal fluids may reflect a regional change in tectonic regime from convergent to transcurrent motion, allowing fluids to be first sourced from mantle-derived magmas followed later by metamorphic fluids.

Major and trace element geochemistry of tourmalines from Archean orogenic gold deposits: Proxies for the origin of gold mineralizing fluids?

Orogenic style gold mineralizations in the Archean Hattu Schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary and volcanogenic rocks, as well as felsic intrusives. The gold mineralizations occur as dissemination in altered wall rocks and within hydrothermal quartz veins. Hydrothermal tourmalines are often associated with the gold mineralizations occurring in the quartz veins, as alteration minerals in contacts of the metasedimentary host rocks and quartz veins, but also within mineral assemblages of the metasedimentary and magmatic rocks. We characterize and compare the major, trace and rare earth element chemistry of these tourmalines in order to evaluate their suitability as a petrogenetic tool for tracing the fluid sources of the hydrothermal system. By comparing the chemistry of tourmalines from gold mineralization with those from metamorphic and magmatic host rocks, we test whether tourmaline composition can be used to identify the source rocks of the gold transporting hydrothermal fluids. All analyzed tourmalines belong to the alkali super group and plot along the schorl-dravite join. The hydrothermal tourmalines have Li, Sr and V concentrations comparable to metamorphic tourmalines and clearly distinct from magmatic tourmalines. Because the concentrations of Ni, Pb, Cr, Mn, Ga, Zn and Sn show a wide overlap between magmatic, metamorphic and hydrothermal tourmalines, these elements do not permit to discriminate between different source rocks. The co-variations of many elements in hydrothermal tourmalines, when plotted against Li, show more similarity to metamorphic tourmalines and the whole-rock compositions of metasedimentary and metavolcanic host rocks than to their magmatic counterparts. This indicates that the major and trace element composition of hydrothermal tourmalines in the Hattu Schist Belt is predominantly controlled by the host rocks and local fluid-rock interactions, and does not reflect the distal fluid sources. The effect of local fluid-rock interaction is also manifested by the REE patterns of tourmalines. The magmatic tourmalines have distinct LREE enriched patterns resembling the whole-rock REE patterns of granitic intrusives, while the metamorphic and hydrothermal tourmalines have flat or weakly fractionated patterns similar to metavolcanic and metasedimentary host rocks. Taken together, the tourmaline data suggest that hydrothermal tourmalines associated to gold mineralizing fluids are most likely genetically related to metamorphic rock sources without important contributions of magmatic fluids, and that local fluid-rock interaction exerted a major control on tourmaline chemistry.

Trace-element geochemistry of pyrite and arsenopyrite: ore genetic implications for late Archean orogenic gold deposits in southern India

AbstractThe distribution of Au and associated trace elements in pyrite and arsenopyrite from late Archean Hutti and Hira-Buddini orogenic gold deposits, eastern Dharwar Craton, southern India was investigated by laser ablation-inductively coupled plasma-mass spectrometry. X-ray element maps acquired by electron probe microanalyser reveal oscillatory zoning of Co and As indicating the crystallization of pyrite and arsenopyrite in an episodic fluid flow regime in which fluid salinity fluctuated due to fault-valve actions. The absence of any relationship between Au and As in pyrite obviate the role of As in the incorporation of Au into pyrite, particularly here and may be generally the case in orogenic gold deposits. On the other hand, positive correlations of Au with Cu, Ag and Te suggest possible influence of these chalcophile elements in the enhanced gold concentrationin sulfides. Pb-Bi-Te-Au-Ag bearing micro-particles (&lt;2 μm) are observed exclusively in micro-fractures and pores in arsenopyrite. The absence of replacement features and element gradient suggests direct precipitation of Pb, Bi, Te, Au and Ag from a fluid that was unreactive towards arsenopyrite. An intermittent fall in fluid pressure caused by the fault-valve action would have resulted in the sporadic precipitation of Au, Pb, Ag, Bi and Te.

Atmospheric sulfur in the orogenic gold deposits of the Archean Yilgarn Craton, Australia

The origin of sulfur and gold in Archean orogenic gold systems should provide significant insights into the dynamics of fluid movement in the crust of the early Earth, but is poorly constrained and highly debated. Our natural laboratory to address this knowledge gap is the metal-endowed Yilgarn Craton (Western Australia), where we measured the multiple sulfur isotope signatures of representative sulfide-bearing auriferous samples from 24 Archean orogenic gold deposits varying in size and geological setting. Utilizing chemically conservative mass-independent fractionated sulfur (MIF-S) isotope signatures, we fingerprinted a major source of sulfur in these deposits. Contrary to previous studies, our data show that they display MIF-S isotope anomalies, with Δ 33 S values ranging from –1.18‰ to +2.04‰; most of the studied deposits show a sulfur signature that is consistent with a crustally derived source. Unlike smaller deposits, which may form with sulfur derived from a single sedimentary sulfur source, therefore providing a coherent Archean atmospheric signal in their Δ 33 S-Δ 36 S slope (~0.9–1.5), the formation of giant deposits may require sourcing from a wider range of sulfur reservoirs and using different processes, as reflected in their apparently random Δ 33 S-Δ 36 S slopes.

Signatures of Multiple Mineralization Processes in the Archean Orogenic Gold Deposit of the Pampalo Mine, Hattu Schist Belt, Eastern Finland

The Hattu schist belt is an emerging gold-producing ore district in the western part of the Archean Karelian Province of the Fennoscandian Shield. The belt consists of 2.76 to 2.70 Ga tonalite, granodiorite, and leucogranite intruded into a mafic-felsic epiclastic-volcanic sequence of an only slightly older age. Complex and successive folding, shearing, and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism (550° ± 50°C; 3–5 kbar) at ca. 2.70 Ga. Orogenic gold deposits are hosted by the highly strained zones that developed during the Archean deformation of the belt. However, previous K-Ar and Rb-Sr geochronological studies indicated that a second tectonothermal overprint affected the Hattu schist belt between 1.7 and 1.8 Ga during the Svecofennian orogeny. Results of our field, mineralogical, textural, and Pb isotope studies suggest that ore deposition at Pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes, at around 2.72 Ga. Albitization and quartz-tourmaline-biotite-muscovite veining characterize this hydrothermal activity. The major stage of gold ore deposition can be confined to the subsequent development of high-strain zones in an intermediate-felsic tuffaceous unit (mafic schist) characterized by biotite-carbonate-pyrite alteration. Ar-Ar studies revealed the complete resetting of the Ar-Ar system in muscovite and biotite due to the Svecofennian orogeny, with closure of the isotopic system at 1.81 Ga. Results of Pb isotope studies of hydrothermal K-feldspar, galena, and altaite by laser ablation-inductively coupled plasma-mass spectrometry indicate that K-feldspar alteration and remobilization of metals also took place during the Svecofennian reactivation of high-strain zones. The petrographic manifestation of this process is the replacement of albite by hydrothermal K-feldspar. Fluid inclusion data from hydrothermal K-feldspar suggest that carbonic-aqueous, low-salinity fluids interacted with the Archean ore at 350° to 400°C and 1.8 to 2.4 kbar during the Svecofennian overprint. Results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar-porphyry units. Therefore, the Svecofennian overprint locally also enhanced the grade of gold mineralization, mostly along competency differences of the mafic schist and feldspar porphyry blocks/dikes. Later percolation of relatively low temperature (<300°C) saline basin fluids in some fractures of the crystalline basement also left their Pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore. Occurrences of hydrothermal proto-ores, as well as fluid flow events along structures reactivated by overprinting orogenic processes, are not exceptional features in Archean orogenic gold provinces. This study shows that the combination of in situ Pb isotope studies of U-poor alteration minerals and Pb-rich ore minerals, together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies, can be very useful in determining the significance and conditions of overprinting processes, as well as their potential implications for genetic and exploration models.

The Archean BIF-hosted Lamego gold deposit, Rio das Velhas greenstone belt, Quadrilátero Ferrífero: Evidence for Cambrian structural modification of an Archean orogenic gold deposit

ABSTRACT The Lamego orogenic gold deposit (1.3 Mt measured resources at 7.26 g/t Au) is located at the south-western end of the 5 km-long Cuiaba–Lamego trend, Quadrilatero Ferrifero region, Brazil. Both Archean orogenic gold deposits are situated in the Rio das Velhas greenstone belt with the lithological succession at Lamego consisting of metamorphosed (greenschist facies) mafic volcanic rock, chert and banded iron formation (BIF), and carbonaceous and micaceous pelites. The Lamego fold, which controls the Lamego deposit, is the most visible structure related to the D1–D2 event, with a perimeter of about 4.8 km and a maximum width of 450 m. The fold is defined by the layering of the mafic unit with minor BIF and large exposures of carbonaceous and micaceous pelites. It is a rootless, reclined, isoclinal, cylindrical fold with an axial trace striking northwest–southeast dipping 20° to 30°. The hinge zone is thickened and the limbs are thinned, with the limbs dipping 20° to 30° to the SE. Orebodies consist of the Lamego BIF, where gold-mineralized zones are related to iron-rich bands, and associated silicification zones. Replacement-style mineralization is associated with sulfide bands; mainly pyrite, As-rich pyrite, and arsenopyrite. Two structural generations, G1 and G2, are recognized and encompass a set of structural elements. The G1 structural generation developed in a progressive deformation event and resulted in structures oriented from NE to SW and dipping to the SE. Structures pertaining to the G2 structural generation are oriented N–S dipping to the W. The four major high-grade gold orebodies are Carruagem, Queimada, Arco da Velha, and Cabeca de Pedra. Their gold grade shows a spheroidal pattern and a distribution that varies along the S1–2 foliation. These lenses represent the hinge zone of F2 reclined folds with the plunge of the orebodies controlled by the F2 fold axes. The lower-grade gold lenses are controlled by pinch and swell, and locally expressed quartz boudins developed during D1–D2. They have two orthogonal directions, one to the NW–SE and the other to the NE–SW, thereby defining chocolate-tablet style boudinage. Hydrothermal monazite grains in a mineralized mafic volcanic rock indicate that mineralization formed at 2730 ± 42 Ma (U–Pb SHRIMP). Younger monazite dated at 2387 ± 46 Ma, and xenotime dated at 518.5 ± 9 Ma suggest Siderian and Cambrian imprint in the Lamego deposit area. The Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed in the Lamego deposit by the S3 crenulation cleavage (trend NS and dip steeply to the E). Importantly, the Cambrian structural modification is responsible for the present geometry of the Lamego orebodies, but is not associated with hydrothermal alteration; it has not introduced any new gold and also has not caused remobilization of the orebodies or ore minerals.

Tourmaline from the Archean G.R.Halli gold deposit, Chitradurga greenstone belt, Dharwar craton (India): Implications for the gold metallogeny

Tourmaline occurs as a minor but important mineral in the alteration zone of the Archean orogenic gold deposit of Guddadarangavanahalli (G.R.Halli) in the Chitradurga greenstone belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli gold deposit as (a) clusters of very fine grained aggregates which form a minor constituent in the matrix of the altered metabasalt (AMB tourmaline) and (b) in quartz-carbonate veins (vein tourmaline). The vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as (i) albite-tourmaline-ankerite-quartz veins (vein-1 tourmaline) and (ii) albite-tourmaline-calcite-quartz veins (vein-2 tourmaline). Both the AMB tourmaline and the vein tourmalines (vein-1 and vein-2) belong to the alkali group and are classified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tourmalines are unzoned. Mineral chemistry and discrimination diagrams reveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the vein tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tourmalines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-à-vis gold mineralization is spatially linked to the juvenile magmatic accretion (2.56–2.50 Ga) east of the studied area in the western part of the eastern Dharwar craton.

Are Iron-Rich Sedimentary Rocks the Key to the Spike in Orogenic Gold Mineralization in the Paleoproterozoic?

Paleoproterozoic rocks with ages from 2100 to 1800 Ma are recognized as belonging to one of the three main periods in Earth history that host orogenic gold deposits. In contrast to older Archean orogenic gold deposits, which are hosted predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Paleoproterozoic. Unusually iron rich 1840 Ma marine mudstones in the Tanami region host one world-class gold deposit and many other large gold deposits. Fluid-rock modelling at 300° and 325°C suggests a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region. These northern Australian Paleoproterozoic iron-rich sedimentary rocks could be linked globally to iron-rich and gold-bearing sedimentary rocks deposited between 2100 to 1800 Ma in Homestake, South Dakota (United States), and in Ghana, and Brazil. Worldwide, from about 2400 to 1800 Ma, the Paleoproterozoic is also marked by the deposition of mainly Superior style BIFs, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in northern Australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic Au deposits hosted in rocks aged from 2100 to 1800 Ma.