Archean Gold Deposits Research Articles

Biotite chemistry as an indicator of hydrothermal deposit types and fluid sources: Insights from big data compilation, multivariate statistical analysis, and machine learning

Identification of a deposit type in an early stage of exploration can save money and time. However, despite several studies, there is a lack of simple deposit classifiers based exclusively on the major and trace element chemistry of minerals, which is relatively easier to measure. Furthermore, the identification of fluid source is crucial for any ore-genetic study, as it exerts a first order control on physicochemical characteristics of fluid, which ultimately control the alteration mineralogy, metal solubility and mobilization, and metal association in a deposit. The stable isotope composition (e.g., δ34S, δ13C, δ18O, δ11B, etc.) of rocks and minerals is conventionally used to constrain the source of ore-forming fluids, however, the potential of mineral chemistry for constraining fluid source is not well-explored. In this study, we compile the major element composition (wt% of oxides) of a large number (n = 1932) of hydrothermal biotite from various ore settings and execute multivariate statistical analysis and machine learning-based predictive modeling to explore the potential of using biotite chemistry as an indicator of deposit type and fluid sources. The atoms per formula units (i.e., apfu values), and calculated parameters (e.g., Xphl, chlorine intercept value, log(fH2O/fHCl)fluid) is used to perform principle component analysis (PCA) and partial least square discriminant analysis (PLS-DA). The results show that among the different ore settings, iron-oxide copper‑gold, Archean gold, porphyry, and magmatic-hydrothermal rare earth metals deposits and among the fluid sources, (meta-)evaporitic, magmatic-hydrothermal, and metamorphic fluid sources can be reliably discriminated using biotite chemistry. We also show that skarn, hydrothermal platinum group elements, and carbonatite-related rare earth elements deposits may not be discriminated using biotite chemistry. Similarly, mafic rock-derived, carbonatite-derived, and meteoric fluid sources may not be discriminated by biotite chemistry. We propose multiple binary and ternary classifier diagrams, which can be used as discriminator of deposit types and fluid sources. Furthermore, we have developed four different PLS-DA predictive modeling, two for deposit types and two for fluid sources, which can predict deposit types and fluid sources with good accuracy. We provide a Windows© based program that can be used to discriminate deposit types using our PLS-DA predictive modeling. The binary and ternary classifiers and the Windows© programs are tested with recently published biotite data, and it has been shown that these classifiers and programs can efficiently separate deposit types and fluid sources. We also discuss about the limitations of the classifiers.

Evaluating Geochemical Discriminants in Archean Gold Deposits: A Superior Province Perspective with an Emphasis on the Abitibi Greenstone Belt

Abstract Discriminating Archean Au deposit types and related ore-forming processes is challenging but paramount for increasing Au exploration success. This study tests the validity of applying geochemical data generated from conventional bulk versus modern in situ methods as discriminants for classifying Au deposits in the Archean Swayze greenstone belt with further comparison to other deposits in the contiguous Abitibi greenstone belt and Red Lake area (Superior Province, Canada). The study used five well-characterized Au settings, based on new mapping, as a basis for evaluating in situ (δ18Oquartz, δ33, 34Ssulfide, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite, electron microprobe analysis of gold), and whole-rock geochemical datasets to resolve whether intrusion-related Au deposits can be discriminated from orogenic-type Au deposits. Results show that the in situ methods provide insight into processes related to Au mineralization, both primary and subsequent remobilization and upgrading, and define elemental and isotopic correlations that cannot be resolved using conventional bulk methods. For example, when comparing the whole-rock to laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace metal data, only Ag, Te, and Hg show a consistent positive correlation with Au across deposit types in both data sets. Furthermore, the wholerock datasets combined with in situ isotopic analysis suggest the Archean sanukitoid-associated Au deposits represent a distinct group of intrusion-related deposits with mineralization characterized by low δ34Spyrite (<–5 to –25‰), inferred high fO2, an Hg-Te signature, and hosted in intrusions of <2690 Ma that predate shearing. The data and interpretations presented herein provide a baseline that can be widely utilized in future studies of Au deposits.

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.

Evidence of gold related to Neoarchean alkaline magmatism in the Abitibi greenstone belt (Canada) from mineral parageneses and microscale trace element geochemistry on pyrite

Most of Archean gold deposits are classified as orogenic gold systems (OGS), precipitated by metamorphic fluids during compressional deformation at convergent plate margins. However, Neoarchean gold deposits are also found spatially related to post-collisional alkaline intrusions such as syenites, suggesting a genetic link between gold and magmas. To bring novel evidence on this relation, ore parageneses related to Bachelor/O’Brien, Beattie, Douay and Golden Arrow gold deposits were studied together with trace elements distribution at micron-scale in Au-bearing pyrites. Further, the δ34Spyrite signature in the Beattie deposit were compared to epithermal Phanerozoic analogues in post-collisional tectonic settings. Except for Golden Arrow, results show a paragenetic assemblage of K-feldspar-albite-anhydrite-barite-celestite-apatite-rutile-calcite-dolomite which is also common in Phanerozoic magmatic-hydrothermal systems. Gold-bearing pyrites are characterized by a higher abundance of most trace elements compared to OGS. Positive Au-Ag-Bi-Te-Tl-W-anomalies – which are not observed in OGS – suggest deposition from highly oxidized fluids, while Au precipitation in OGS is mainly controlled by reduced fluids. The presence of highly oxidized environments is also confirmed by very low δ34Spyrite values ranging from −17.3‰ to −13.9‰ VCDT found at Beattie, in the range of those found in Phanerozoic deposits of Tuvatu and Emperor in Fiji. It is proposed that Archean gold related with late alkaline continental magmatism is deposited both at the top of porphyritic system and in epithermal environment, before the acme of deformation and metamorphism.

Albite ± Actinolite-Altered Porphyry Dykes in Archean Gold Deposits of the Boulder Lefroy-Golden Mile Fault System, Yilgarn Craton, Western Australia: Petrography, Chronology, and Comparison to Canadian Albitites

The Boulder Lefroy-Golden Mile fault system in the Archean Yigarn Craton is the most productive gold-mineralized structure in Australia (>2300 t Au). The New Celebration deposit (51 t Au) is part of a group of hematite- and anhydrite-bearing mesothermal deposits and Fe-Cu-Au skarns associated with monzodiorite-tonalite intrusions in the strike-slip fault system. Ore-grade biotite-carbonate and late sericite-carbonate-alkali feldspar replacement is bound to the contacts of a felsic (low Cr, Ni, V) quartz-plagioclase porphyry dyke dated at 2676 ± 7 Ma. The sodic-potassic alteration of the felsic boudinaged dyke contrasts with the albite-actinolite alteration in the adjacent mafic (high Cr, Ni, V) plagioclase porphyry dated at 2662 ± 4 Ma, although both share the same sulfide-oxide assemblage: pyrite ± chalcopyrite, magnetite ± hematite. The younger porphyry locally crosscuts foliation and is bordered by post-kinematic actinolite-pyrite selvages overprinting talc-chlorite-phlogopite-dolomite schist. It contains auriferous pyrite (70 ppb Au; 610 ppb Ag) where sampled for zircon U-Pb chronology at +224 m elevation. Above the sample site, the dyke was mined as gold ore (1–6 g/t Au) at +300–350 m. Temperature estimates based on actinolite-albite pairs (300–350 °C) agree with the fluid inclusion trapping temperature of main-stage auriferous veins (330 ± 20 °C). These relationships are interpreted to indicate syn-mineralization emplacement. Gold-related albite-altered porphyry dykes (albitites) also occur in the world-class Hollinger-McIntyre (986 t Au) and Kerr Addison-Chesterville deposits (336 t Au), Abitibi greenstone belt, Canada.

Differentiated Archean Dolerites: Igneous and Emplacement Processes that Enhance Prospectivity for Orogenic Gold

Abstract Magnetite-bearing granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits because they are chemical traps for orogenic fluids. Despite their economic importance, there is a poor understanding of how melt composition, crystal fractionation, sill geometry, and depth of emplacement increase the volume of host rock that is most favorable for gold precipitation during orogenesis. We use drill core logging, whole-rock geochemistry, magnetic susceptibility, gold assay, and thermodynamic modeling data from 11 mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields superterrane (Yilgarn craton, Western Australia) to better understand the influence of igneous and emplacement processes on gold prospectivity. Orogenic gold favors differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of magnetite-bearing quartz dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick and broadly coeval sedimentary sequences. Sill thickness is an important predictor for gold prospectivity, as it largely controls cooling rate and hence fractionation. The parental melts of gold-mineralized sills fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) at depth before emplacement in the shallow crust. A second fractionation event at shallow levels (<3 km) operated both vertically and laterally, resulting in an antithetic relationship between quartz (magnetite) dolerite and cumulates (pyroxenites and peridotites). By comparison with younger mafic sills emplaced in synsedimentary basins, we argue that the geometry of these high-level sills was more irregular than the often-assumed tabular form. Any irregularities in the lower sill margin act as traps for early formed (dense) ferromagnesian minerals, now represented by pyroxene and peridotite cumulates. In contrast, irregularities in the upper sill margin trap the buoyant fractionated liquids when the sill is more crystalline, through magma flow on the scale of <1 km. Sills derived from iron-poor melts are rarely mineralized and, all else being equal, probably have to be thicker than Fe-rich sills to be similarly prospective for orogenic gold. Finally, we provide a list of quantifiable parameters that can be incorporated into an exploration program targeting differentiated dolerites that host orogenic gold.

GENETIC CLASSIFICATION OF ARCHEAN LODE GOLD DEPOSITS

Main features of the Archean gold deposits spatial distribution are outlined. Geology and ore composition of two deposits within the Canadian shield distinguished by tectonic position and host rock lithology are described. Delineation of two separate genetic types of the Archean gold deposits, orogenic and granitoid-related, similar to Phanerozoic accretion-collision belts is validated on that basis.

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.

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.

Quadruple sulfur isotopic fractionation during pyrite desulfidation to pyrrhotite

The chemical and mineralogical conversion of pyrite to pyrrhotite has been studied in many aspects, but the associated quadruple sulfur isotopic fractionation between these phases has not been investigated previously. In this study, Ruttan pyrite and pyrite from a Neoarchean carbonaceous phyllite drill core were heated to 675 °C, and the released sulfur was reacted with iron metal, forming compact sulfide rims. The products of both desulfidized pyrite and sulfidized iron metal are pyrrhotite, following the equations n(1-x)FeS2 = nFe1-xS + (1-2x)Sn and Sn + n(1-x)Fe = nFe1-xS, respectively. The mean δ34S of the Ruttan pyrite experiment products has shifted slightly with the residue becoming slightly heavier and the sulfidized iron becoming slightly lighter than the starting Ruttan pyrite. However, the shift is small (0.5‰ and 1.2‰, respectively), yet the range of δ34S increases in the pyrrhotite products, suggesting that the evaporation of sulfur during pyrite breakdown is not occurring uniformly. A shift in δ34S between the initial Archean pyrite and resultant pyrrhotite products is likely a real difference in composition of the analyzed pyrite and the pyrite extracted for the experiment. The compositions of the pyrrhotite run products are quite close with the difference in Δ33S and Δ36S being within ca. 0.1‰. They keep constant with the starting Ruttan pyrite while differ slightly from the initial Archean pyrite, which is likely due to heterogeneity in the original pyrite. The experiments indicate that the fractionation in Δ33S and Δ36S during pyrite desulfidation, sulfur evaporation transfer and condensation and iron metal sulfidation is negligible. Therefore the preservation of Δ33S and Δ36S between initial pyrite and released sulfur during pyrite desulfidation is demonstrated, establishing Δ33S and Δ36S as effective tools of tracing sulfur sources of Archean gold deposits and opening up pyrrhotite as an appropriate phase for studying Archean S-MIF.

Trace element associations in magnetite and hydrothermal pyrite from the Geita Hill gold deposit, Tanzania

Gold mineralization in the Geita Hill deposit is associated with pyrite formed along microfracture networks and sulfidation fronts together with K-feldspar and biotite. The sulfidation fronts are best developed in magnetite-bearing ironstone. The gold is present mainly as electrum and gold tellurides along grain boundaries, and as inclusions in pyrite, quartz, biotite and K-feldspar. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of pyrite and magnetite grains reflect complex fluid-host rock interactions. Magnetite textures and chemistry change with alteration intensity, indicating the progression of the alteration front into the host rock. Pyrite textures are uniform across all rock types and reflect late-tectonic growth linked to multi-staged infiltration of hydrothermal fluids. Trace element distribution patterns in pyrite are locally complex and influenced by host rock chemistry.Gold distribution patterns in pyrite correlate closely with Te, Ag, Bi and Pb, indicating that gold occurs in micro- and nano-inclusions of telluride minerals. This is especially so for gold in quartz veins, whereas gold in ironstone and diorite also occurs as electrum with an average Au/Ag ratio of 0.41. As, Co and Ni in pyrite are lattice bound and occur in high concentrations in ironstone and diorite where they show characteristic growth zoning patterns. Pyrite in quartz veins has As, Co and Ni concentrations that are low and variable. Cr, Cu, Mo, Mn and Zn are present in all rock types in isolated inclusions in pyrite grains, whilst Pb, Bi and Sb occur in more dispersed patches of fine clustered inclusions. The Se content in pyrite is typical for Archean gold deposits, and reflects an average temperature of ~340 °C for the mineralizing fluid. The Co/Ni ratio of pyrite grains varies between 0 and 5.2 in ironstone and diorite, and most likely reflects the equilibration Co/Ni ratio of the host rock. The Co/Ni ratio of pyrite grains in quartz veins varies between 1 and 12, and is consistent with a magmatic-hydrothermal origin for the ore fluid. Trace element distribution patterns in magnetite and pyrite indicate that As, Ni, Co, Cr, Mn and Cu were mostly locally derived, and remobilised into the pyrite during sulfidation of the host rock. The concentrations of these elements are strongly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Bi and Pb were externally derived, and closely correlate in all varieties of pyrite as well as strongly altered magnetite. The alteration footprint of the Geita Hill deposit is limited in extent, and does not involve As and Sb that are typically enriched in Archaean lode-gold systems. Instead, Te and Bi are most characteristic for the deposit and could be of use as path finder elements together with altered magnetite grains.

Textural evolution and trace element chemistry of hydrothermal calcites from Archean gold deposits in the Hattu schist belt, eastern Finland: Indicators of the ore-forming environment

Orogenic style gold mineralizations in the Archean Hattu schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary rocks, volcanic rocks, and felsic intrusives. The gold deposits are related to extensive quartz-dominated vein networks and the gold mineralization occurs as dissemination in altered wall rocks and within hydrothermal quartz veins. The quartz-dominated veins frequently contain calcite, which is also present within the country rocks. We analyze and compare the major and trace element chemistry of these calcites and test whether they carry the signatures of the origin of hydrothermal fluids associated with the gold deposits or record fluid interaction with the local wall rocks. The calcites show textural growth and chemical zoning, considerable chemical variations within samples and the chemical data do show some overlap between the samples. All calcites contain dominantly Ca with other elements being typically below 2 wt%, and they are accordingly classified as rather pure calcites. Amongst minor elements, both Mg and Fe show larger variations than Mn. The concentrations of most trace elements analyzed are relatively low, except for Sr and Y, which can attain values up to 1 wt% and 200 ppm, respectively. Some of the trace elements or trace element ratios in hydrothermal calcite, including Sr, Y, Mn, (La/Lu)N and ∑REE, show correlations with the respective host rock lithologies. This indicates that interaction between hydrothermal fluids and adjacent and diverse host rocks has strongly affected the trace element chemistry of calcite. Chemically variable growth zones in large calcite crystals and distinct grain populations within individual samples cannot be readily explained by host rock control, but instead they are interpreted to reflect changes in fluid composition over time. The hydrothermal calcites have chondrite-normalized REE patterns that are mostly HREE enriched relative to LREE, with very few exceptions that show unfractionated flat or LREE enriched (relative to the HREE) patterns. The REE patterns of the calcites are very different from those of their immediate host rocks, suggesting that they are inherited from the hydrothermal fluid(s) from which they precipitated. The HREE enrichment relative to the LREE visible in the patterns resembles the REE patterns of calcites from hydrothermal vein type deposits of metamorphic origin elsewhere and contrasts with the REE patterns of calcites from magmatic-hydrothermal environments. The REE patterns of the hydrothermal calcites in the Hattu schist belt deposits would therefore be compatible with formation from a fluid system that is essentially derived from metamorphic sources, in agreement with conclusions drawn from the fluid inclusion chemistry.

Using hyperspectral imaging to vector towards mineralization at the Canadian Malartic gold deposit, Québec, Canada

Canadian Malartic is a large-tonnage, low-grade Archean gold deposit (16.3 Moz, 1.08 g/t Au) located in the Abitibi region of Québec, Canada. A large part of the mineralization is hosted in the Pontiac Group metasedimentary rocks, which consist of mudstones to greywackes at upper greenschist to amphibolite facies. In exploration and production environments, these lithologies are challenging to characterize by conventional core logging methods, while in a research setting thin section-sized samples (2 cm × 4 cm) do not capture the full extent of mineralogical variability, which can extend from centimeters to meters away from mineralized zones. Here, high-resolution hyperspectral imagery (0.2–1.0 mm/pixel) in both shortwave infrared (SWIR, 1000–2500 nm) and longwave infrared (LWIR, 8000–12000 nm) is acquired for over two thousand meters of drill core, and is used to visualize changes in mineralogy and mineral chemistry related to metamorphism and hydrothermal alteration. Unaltered metasedimentary rocks contain metamorphic white mica with AlVI contents varying between 1.90 and 1.75 apfu (2195 to 2203 nm), depending on metamorphic grade. Hydrothermal alteration is characterized by white mica which becomes progressively more phengitic with increasing alteration intensity, with AlVI contents ranging from 1.70 to 1.50 apfu (2204 to 2212 nm). Phengitic white mica extends from meters to tens of meters away from major mineralized zones, and can be used as a vector towards mineralization in an exploration setting. White mica composition is correlated to Au content, and can be used to discriminate between unmineralized (<0.3 g/t Au, 2195–2204 nm), weakly mineralized (>0.3 g/t Au, 2205–2208 nm), and highly mineralized (>1.0 g/t Au, >2208 nm) samples, which is a simple metric that can directly be applied to sort ore in a production environment. The Mg# (molar Mg/[Mg + Fe]) of biotite, on the other hand, is unaffected by metamorphic grade, and consistently is Mg# 55–60 (2251–2250 nm) in unaltered samples. In mineralized samples, biotite is Mg-rich (Mg# >65, <2249 nm), but rapidly grades into background compositions outside of the ore zones. LWIR hyperspectral data is used in drill core to estimate silicification abundance from the strength of the 9200 nm quartz reflectance peak. Peak strength was generally in good agreement with the silicification abundance estimated by core logging, and is therefore a metric that can directly be used for characterization of this parameter in exploration and production environments. Over 1500 SWIR point measurements were also acquired in a 50 km by 15 km area around the deposit, and were used to delineate metamorphic isograds on regional scales, and a large 12 km by 3 km hydrothermal alteration halo around the Canadian Malartic deposit. Similar measurements, if performed along the over 200 km long Cadillac-Larder Lake Deformation zone, could be used to rapidly delineate other potentially mineralized areas.

Multistage mineralization at the hypozonal São Sebastião gold deposit, Pitangui greenstone belt, Minas Gerais, Brazil

São Sebastião is a recently discovered Archean gold deposit at the northwesternmost part of the Quadrilátero Ferrífero (QF), southern São Francisco craton, Brazil. The gold ore is strata-confined in two levels of banded iron formation (BIF) of the basal stratigraphic unit in the Pitangui greenstone belt (2.8 Ga). Pitangui is correlated to the widely known Rio das Velhas greenstone belt, which hosts several orogenic gold deposits such as the world-class Cuiabá. The gold mineralization at São Sebastião is associated to replacement-style sulfidation over magnetite, where the main developed phases are: pyrrhotite, chalcopyrite, arsenopyrite, pyrite and arsenian-pyrite. Native Au occurs as inclusions in pyrrhotite formed at dilatation domains (e.g., breccias, fold hinges) during compressional events (Rio das Velhas Orogeny at 2.8–2.75 Ga), but mainly as inclusions in late- to post-kinematic pyrite and arsenopyrite, as fracture infills in arsenopyrite and in contact with gangue minerals. Electron-microprobe analyses in arsenopyrite reveal a 465–560 °C precipitation temperature interpreted from As atomic percentage and indicate that high-temperature fluids were active after the deformational event. The temperature increase caused the incorporation of LA-ICP-MS-detected lattice-bound Ni and Co in arsenian-pyrite and arsenopyrite, related to the mafic-ultramafic signature of boundary rocks. High temperature conditions are supported by pyrite and ISS (intermediate-solid solution in the Cu-Fe-S system) formation at 600 °C and melting-derived textures resulting from the precipitation of Bi- and As-melts, typical of amphibolite-facies environments with high fS2. Whole-rock geochemical assays of drill core samples display two clear trends on Ag, Bi, Cu vs. Au binary plots and on principal component analysis results. The trends are a product of the heterogeneous spatial distribution of melts and high-temperature fluids. Melting features and superposition of mineralization styles are common in hypozonal gold deposits experiencing higher heat influx. At São Sebastião, the thermal effect is likely related to the 2.7 Ga granitic intrusions that surround the Pitangui belt. São Sebastião shows distinct characteristics than those described for the mesozonal gold deposits in the Rio das Velhas greenstone belt and may depict the deeper roots of the QF gold system.

Predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the Canadian Malartic ore and host rocks, Pontiac Subprovince, Québec, Canada

The Canadian Malartic deposit is a world class intrusion-related Archean gold deposit hosted in the Pontiac Subprovince, Superior Province, in Québec, Canada. Laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks. The various rock types present in the Malartic District, mainly meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities. Using support vector machines, it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89%. Within the meta-sedimentary rocks class, variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration. These are caused by the destruction of iron-bearing silicate minerals and magnetite in unaltered rocks (10−4 to 10−3 SI) to form pyrite, carbonates, K-feldspar and Fe-depleted hydrothermal biotite in altered rocks (10−5 to 10−4 SI). Within the felsic-intermediate intrusive rocks, grain densities below 2.7 g/cm3 and magnetic susceptibilities in the 10−6 to 10−4 SI range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration. However, magnetic susceptibility and grain density of these intrusive rocks are also dependent on their Fe2O3/Al2O3 and TiO2/Al2O3 ratios, which are not related to the hydrothermal alteration footprint, but are rather due to distinct protolith compositions. Within the mafic dykes, grain density is the best indicator of hydrothermal alteration. Unaltered mafic dykes (2.95 to 3.10 g/cm3) are mostly composed of amphibole, whereas altered mafic dykes (2.70 to 2.95 g/cm3) have reduced amphibole contents and higher abundances of carbonates, pyrite, quartz, and biotite alteration. The support vector machine classifier is extended to predict if meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average F1 scores of 73%, 69% and 93%, respectively. In altered meta-sedimentary rocks, the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples. The classifier is further extended to predict if the gold content of meta-sedimentary rocks is above or below threshold values of 0.01, 0.1 and 1 ppm with average F1 scores of 83%, 80%, and 76%, respectively. Using conceptual models of the rock physical properties at the Malartic District scale, it is shown that ground magnetic surveys are the most promising geophysical tool for early-stage greenfield exploration of this type of deposit. However, depending on the scale at which the surveys are conducted, magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks. This in part explains why past airborne geophysical exploration campaigns for this type of deposit in the Malartic District were inconclusive. Finally, the machine learning process used in this case study can be applied in advanced exploration stages, during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models.

Mica composition as a vector to gold mineralization: Deciphering hydrothermal and metamorphic effects in the Malartic district, Quebec

Canadian Malartic, with a total endowment of 16.3 Moz Au, is an important example of a large-tonnage, low-grade Archean gold deposit (current reserves of 204 Mt @ 1.08 g/t Au). It is located in the southern Superior Province in contact with, and immediately south of the east-west trending Cadillac-Larder Lake fault zone, which delineates the boundary between the Pontiac and Abitibi subprovinces. The deposit is hosted by Pontiac Group metaturbidites, Piché Group mafic-ultramafic metavolcanics and by porphyritic quartz monzodiorite to granodiorite that intruded these lithologies. The metamorphic grade increases southward from upper greenschist facies, which characterizes the immediate metamorphic environment of the Canadian Malartic deposit, to mid-amphibolite facies; the garnet isograd is crossed 1.5 km south of the deposit, and the staurolite isograd about one kilometer further south. Textural, structural and geochronological observations are consistent with a syn-kinematic (D2), early- to syn-peak metamorphic (M2) timing for the main ore-forming event.Mineralization in the metasedimentary rocks and porphyritic intrusions consists of disseminated native gold and minor gold-tellurides within stockworks of quartz-biotite-microcline-carbonate±pyrite veinlets (v2) and associated pervasive microcline-albite-biotite±(white mica)-carbonate-pyrite alteration. A zonal distribution of alteration is centered on structures that acted as preferential pathways for the hydrothermal fluids (e.g., Sladen Fault, NW-SE deformation zones). In the metasedimentary rocks, the proximal potassic alteration zone is dominated by microcline-albite±quartz with variable proportions of phlogopite (±white mica), carbonate minerals (calcite-ankerite±Fe-dolomite), quartz, pyrite and rutile. It grades outwards into a distal potassic-sericitic alteration zone characterized by relatively abundant Mg-rich biotite and phengitic white mica, as well as microcline, albite, quartz, calcite, pyrite and rutile. The zonal distribution of alteration features was due in part to a decrease in the total activity of sulfur species (∑aS) and oxygen fugacity (fO2) away from the main hydrothermal corridors, which is manifested by systematic changes in Fe-sulfide and (Fe-Ti)-oxide mineralogy.The effects of sulfidation (∑aS), oxidation (fO2) as well as K+, Fe2+ and H+ activity on biotite and white mica compositions were assessed from silicate-oxide-sulfide equilibria. Increasing ∑aS-fO2 conditions proximal to the hydrothermal fluid pathways caused pyrite to be stabilized over biotite, and iron sequestration in the sulfide phase in turn promoted the stability of magnesian biotite. Increasing sulfur metasomatism towards the hydrothermal centers was associated with systematic increases in biotite Mg# [Mg/(Fetotal+Mg)] and fluorine concentration, in agreement with the Fe-F avoidance principle. Alteration was also associated with a decrease in Al in biotite and white mica, coincident with increases in Si and Fe + Mg concentrations. These compositional trends were controlled by a Tschermak exchange reaction and are consistent with a gradual decrease in aK+ and/or pH upon progressive rock-buffering of a mildly alkaline, potassium-rich ore-forming fluid.Multicomponent phase equilibria (pseudosections), constrained for specified bulk rock compositions typical of non-mineralized Pontiac Group metasedimentary rocks, are used to model the expected composition of mica solid solutions under varying metamorphic P-T conditions. The thermodynamic modelling of biotite and white mica compositions suggests that the district-scale compositional trends documented for biotite and white mica along the metamorphic gradient, including the progression of the Tschermak substitution towards more aluminous compositions southwards, were likely the product of increasing metamorphic P-T conditions, rather than a distal effect of hydrothermal alteration.Mica mineral chemistry is clearly a sensitive indicator of hydrothermal and metamorphic processes at Canadian Malartic. Our results show that biotite and white mica compositions provide valuable tools that can be used to define hydrothermal fluid pathways in and around the deposit. These factors should be particularly useful for mapping the zonation of alteration features that characterize the footprint of major gold deposits in metamorphic terranes where micas typically display a spatial distribution ideal for defining mineral-chemical vectors to ore.

Trace element mapping of pyrite from Archean gold deposits – A comparison between PIXE and EPMA

Chemical zoning of pyrites can record the evolution of mineralising fluids at widely varying P–T conditions ranging from diagenesis to medium-grade metamorphism. If preserved, zoning can reveal growth textures, brecciation and veining, resorption and recrystallisation events, thus shedding light on the processes that contributed to ore formation. Chemical zoning of sulfides is invisible in optical microscopy, but can be studied by chemical etching, high-contrast back-scattering electron images, and elemental imaging. In this study we compared micro-PIXE and WDS-EPMA elemental maps on the chemically zoned pyrites in mineralised vein-bearing samples from the Sheba and Fairview gold mines in the Barberton Greenstone Belt, South Africa. Elemental images show complex distribution of trace elements, suggesting multiple events of pyrite crystallisation and gold deposition. EPMA maps show fine-scale variations reflecting growth and recrystallisation textures marked, in particular, by variations of As, Ni, and Co. In PIXE maps, gold occurs both as finely-distributed and discrete inclusions, suggesting incorporation in the pyrite structure as solid solution, and deposition as electrum inclusions, respectively. Micro-PIXE and EPMA provide complementary information, forming together a powerful tool to obtain information on chemical zoning of pyrites in ore deposits.

Constraints on the Genesis of the Archean Oxidized, Intrusion-Related Canadian Malartic Gold Deposit, Quebec, Canada

With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piche Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite ( δ 18Ofluid of 5.2 to 9.8‰, δ Dfluid of −52.0 to −45.0‰) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( ![Formula][1] from −4.5 to +3.3‰, with small, positive Δ33S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km. Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized ![Formula][2] , CO2- and sulfur-rich (∑ a S ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piche Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower ![Formula][3] . Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced ![Formula][4] and caused destabilization of gold bisulfide species, leading to precipitation of native gold. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif

No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits

The source of the auriferous fluids that formed Archean greenstone gold deposits remains controversial; metamorphic fluids, fluids derived from felsic magmas, mantle fluids, and meteoric water have all been suggested. We report the results of ion microprobe (sensitive high-resolution ion microprobe stable isotopes) multiple sulfur isotope analyses ( 32 S, 33 S, 34 S) conducted to constrain the source of sulfur in four Archean gold deposits. The analyses are of pyrite samples that are genetically related to the gold mineralization from the Bellerophon, Victory, and Wallaby deposits in the Eastern Goldfield superterrane of the Yilgarn craton, Western Australia, together with sulfide from black shale from the Golden Mile deposit and St. Ives gold camp. Ore-associated pyrite from the Beattie gold deposit of the Abitibi greenstone belt of the Superior craton in Canada was measured to test whether the results obtained from the Yilgarn craton apply to other Archean gold deposits. Sulfur analyses from sulfide from shale show mass-independent fractionation (MIF) with Δ 33 S as high as +1.8‰, whereas those from the 4 Archean gold deposits are dominated by mass-dependent fractionation, with only a minor contribution of MIF sulfur. In this respect, Archean gold deposits contrast with Archean NiS deposits, which are dominated by MIF sulfur. The absence of MIF in the pyrite from the four Archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source, but inconsistent with the metamorphic and meteoritic fluid hypotheses.

Oxidation state of europium in scheelite: Tracking fluid–rock interaction in gold deposits

We used µ-XANES spectroscopy to measure the oxidation state of europium in-situ at near-µm resolution in hydrothermal scheelite from the giant Archean gold deposits of Kalgoorlie, Western Australia. By combining these measurements with µ-XRF imaging, it is possible to distinguish inhomogeneities in Eu 2+/Eu 3+ ratios that developed during mineral precipitation from the effects of subsequent hydrothermal alteration and weathering. Thermodynamic analysis reveals that under the conditions typical of the formation of many Au deposits, the Eu 2+/Eu 3+ ratio in the hydrothermal fluid is highly sensitive to pH. The range in pH calculated from the Eu 2+/Eu 3+ ratios in the analyzed scheelite corresponds to a maximum in Au solubility under the ore-forming conditions, and suggests buffering of pH by the CO 2(aq)-rich fluid. The primary heterogeneity of the Karlgoorlie scheelite most likely results from pH oscillating between fluid- and rock-buffered conditions, and reflects the dynamics of the hydrothermal system.