Elevated Au Concentrations Research Articles

Magnetite geochemistry of giant alkalic-type epithermal gold deposits: Insights into the magmatic evolution of mineralized alkaline systems

Magnetite, a common mineral in alkaline magmatic complexes hosting giant epithermal Au deposits, is examined to obtain insights into the magmatic evolution and ore fertility of alkaline systems. Textural and compositional analyses of magmatic magnetite from Cripple Creek, USA, and Lihir Island, Papua New Guinea, reveal distinct magma sources and evolution paths. High HFSE concentrations and high La/Yb ratios in magnetite from Cripple Creek suggest relatively low-degree partial melting of lithospheric and asthenospheric mantle sources, whereas magnetite from Lihir Island indicates higher degree partial melting from subduction-modified mantle. Chromium concentrations track magma evolution, while V systematics record decreasing oxygen fugacity with magma differentiation. Sulfide saturation is observed at Cripple Creek, potentially affecting ore fertility. Magnetite itself bears the capability to scavenge Au from melts as indicated by Au concentrations of up to 40 ng/g. This might point towards elevated Au concentrations in the parental magmas, although experimental data on Au partitioning is lacking. Magnetite compositions overlap across different mineralization styles, challenging its reliability as an exploration tool but analysis of noble metals in magnetite and its inclusion cargo could offer promising alternative approaches. Overall, magnetite serves as a valuable tool for investigating ore deposit petrogenesis and holds potential for exploration.

A modified paleoplacer model for the metaconglomerate-hosted gold deposits at Jacobina, Brazil

Evidence is presented that the metaconglomerate-hosted Jacobina gold deposits in Brazil represent paleoplacers that became partly remobilized during later metamorphic overprint analogous to Witwatersrand-type deposits elsewhere, notably in South Africa. This includes strong lithological and sedimentological control on the gold, presence of detrital minerals with gold inclusions, and detrital gold particles. Detrital, synsedimentary, and post-depositional pyrite types can be differentiated. Whereas the first two types can be linked to gold accumulation, the latter was associated with gold dispersion. Synsedimentary pyrite has the highest Au content, from which elevated Au concentrations in Archean rivers can be inferred. The nature and extent of post-depositional alteration, mainly in the course of the Paleoproterozoic Orogeny, distinguishes the Jacobina deposits from other Witwatersrand-type gold deposits. Phase equilibria and Zr-in-rutile thermometry indicate peak metamorphic temperatures of ca. 600 °C. Both Mg-chlorite and Fe-chlorite formed in disequilibrium at approximately 280–340 °C during retrograde metamorphism. An igneous signature in the chemistry of some of the tourmaline and the remobilization of gold associated with Fe-oxides, near intrusive rocks, point at a local magmatic influence on the post-depositional mineralization stage. Whether magmatic hydrothermal fluids added Au to the system at that stage remains to be determined. Remobilization during regional metamorphism was insufficient to form substantial ore bodies but led to purification of the initially detrital gold particles that now contain relatively little Ag and Cu and lack Hg.

Initial exploration results of the Collins epithermal Au‐base metal prospect, Aceh, Indonesia

AbstractInterpretation of various exploration data, in particular geochemical prospecting, offers a powerful and rapid assessment of grass‐root projects in a green‐field terrain. Here, we present an example of the Collins epithermal prospect in Aceh Province, Indonesia. In this area, the Au+ base‐metal‐bearing sheeted quartz veins (individually mostly 2–4 cm wide), which are controlled by a 250 m wide by 800 m long NNE‐trending structural corridor within Paleogene sandstone and volcanic rocks, are the product of two main stages of deposition. Stage I formed veins with a sliver of cryptocrystalline quartz wall zone followed by an inner zone of comb quartz with interstitial rhombic adularia that terminates in open space. Stage I or main‐stage sulfide mineralization consisting of early galena + sphalerite and later chalcopyrite occurs with the quartz + adularia. Small amounts of galena also occur in the wall zone. Stage II mineralization brecciated Stage I veins and overprinted them with silicification characterized by vuggy texture. Mineralization associated with this episode consists of earlier chalcopyrite + sphalerite + tennantite–tetrahedrite and later, vug‐filling Au–Ag alloy (Ag0.37–0.41Au0.62–0.59). The above mineralized veins are successively flanked by silicic selvages, an illite + chlorite + pyrite ± kaolinite zone and a chlorite + epidote + carbonate + pyrite zone. Local supergene alteration induced replacement of galena by plumbogummite and anglesite and chalcopyrite by covellite. Data from fluid inclusion microthermometry in quartz indicated that the inner zone of Stage I veins formed from fluids with a 2.3 wt% salinity (0.5–3.3 wt% NaCl equivalent), at 174°C (155–211°C). Combining these physico‐chemical parameters with the mineral assemblage, the mineralization occurred under a reduced environment. Rock and soil assays indicate that elevated Au concentrations (up to 16.5 ppm over 1 m) occur along northeast‐trending zones and show a strong correlation with Pb, while Cu (up to 2.58% over 1 m), Zn, As, Sb, and Mo anomalies lie mostly at the periphery. The high‐grade mineralized veins correlate with moderate to high resistivity and chargeability zones, and the pseudosections of such geophysical signals are interpreted as reflecting coalesced or enlarged veins at depth, or inclined veins in other localities. The intermediate sulfidation affinity for Collins points to potential mineralization at depth as well as preservation of Au‐rich and sulfide‐poor zones in the less eroded areas.

Bacillus cereus, a geobiological marker for gold prospecting isolated from soil from the Jiaodong Gold Mine

An efficient method for new gold deposits exploration is of great interest as the existing deposits depleted. In this paper, we investigated the possibility of using B. cereus in the soil from Jiaodong Gold Mine as a biogeochemical indicator of gold. Cell counting, growth curve assay, SEM/TEM-EDS and X-ray Absorption Near Edge Spectroscopy (XANES) techniques were used to investigate the sporulation characteristics, reducing capability and detoxification mechanisms of the B. cereus strain in the environment containing gold. The results showed that: (1) The frequencies of B. cereus spores were higher in areas where elevated concentrations of Au were detected, which was not observed for other elements, such as As, Cu, Zn, Cd and Pb. (2) B.cereus has the ability to facilitated the solubilization of Au. The dissolved gold subsequently stimulated B.cereus spores formation, and mother cells release the contents of their cytoplasm to ameliorate gold toxicity. (3) Toxic Au (III)-complexe was reduced to nontoxic Au (0) nanoparticles by B. cereus. Before reduced to Au (0), cellular Au was coupled with the formation of AuS intermediate species. The results showed that B.cereus spores were confirmed to exist in the gold mine soil, and the reduction and detoxification of gold were mediated by the cytoplasm released by the mother cells, which led to the formation of gold nanoparticles.

Metallogenic model of the Jinchang Au-Ni deposit in the Ailaoshan belt, SW China, determined on the basis of pyrite trace element contents, in-situ sulfur isotope composition and PGE geochemistry

The Ailaoshan metallogenic belt in southwestern China contains numerous shear zone-related Au deposits with Au reserves greater than 200 tonnes. However, the source of ore-forming fluids and metals is controversial, hampering the construction of genetic models. In this study, we selected the Jinchang Au-Ni deposit in the northern segment of the Ailaoshan belt for detailed investigation of pyrite chemistry and in-situ sulfur isotope compositions using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The petrographic and pyrite chemistry analyses revealed four types of hydrothermal pyrite. Py1 pyrite is euhedral to subhedral and has in-situ sulfur isotope (δ34S) values of −5.30 to 6.26‰ (median: −2.93‰) and low concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. Py1 is interpreted to have formed during the early hydrothermal stage. Py2 pyrite is subhedral to anhedral and porous with variable size. Although Py2 and Py3 show different textural relationships, we conclude that both of them formed during the intermediate hydrothermal stage as evidenced by similar sulfur isotope compositions and trace element contents, and common intergrowth textures. Py2 and Py3 pyrite have in-situ δ34S values ranging from −11.6 to 1.29‰ (median: −4.45‰) and relatively elevated concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. In Py2 and Py3 pyrite, Ni concentrations correlate with Co concentration, whereas Au exhibits moderate positive correlations with Ni, Co, and As, indicating that Au, Ni and other metals were predominantly sourced from the Jinchang ultramafic rocks. Py4 pyrite is euhedral in shape and has relatively low in-situ δ34S values of −23.5 to −4.34‰ (median: −8.33‰). The metal concentrations in Py4 pyrite are lower than those in Py2 and Py3 pyrite. In general, the metal concentrations in the different pyrite populations display systematic changes, with low levels in Py1, higher levels in Py2 andPy3, and low levels again in Py4.The average total platinum-group element (PGE) concentration of the Jinchang Ni-bearing ultramafic rocks is 26.3 ppb, similar to the average value of the primitive mantle of 22.0 ppb. The Au-Ni ores from the Jinchang deposit have an average Pd/Pt ratio of 5.98, similar to that of hydrothermal Ni ores but higher than that of typical magmatic Ni ores. In contrast, the Jinchang ultramafic rocks have low Pd/Pt ratios between 0.57 and 1.13, suggesting that the PGEs in the Jinchang intrusion are of magmatic origin. Similarly, the Rh/Ir ratios of Au-Ni ores are consistent with those of hydrothermal Ni ores but differ from those of magmatic Ni ores.The pyrite chemistry, in-situ sulfur isotope compositions, and PGE geochemistry suggest that the ore-forming fluids and metals associated with the Jinchang Au-Ni deposit were dominantly derived from the Jinchang ultramafic intrusion. Mesozoic felsic magmatism or Cenozoic regional strike-slip activity may have triggered migration of the ore-forming fluids, leaching of metals from the Jinchang intrusion and precipitation of these metals at the boundary between the intrusions and carbonaceous rocks.

Geochemical and geochronological characteristics of the Um Rus granite intrusion and associated gold deposit, Eastern Desert, Egypt

The Um Rus tonalite-granodiorite intrusion (∼6 km2) occurs at the eastern end of the Neoproterozoic, ENE-trending Wadi Mubarak shear belt in the Central Eastern Desert of Egypt. Gold-bearing quartz veins hosted by the Um Rus intrusion were mined intermittently, and initially by the ancient Egyptians and until the early 1900s. The relationship between the gold mineralization, host intrusion, and regional structures has always been unclear. We present new geochemical and geochronological data that help to define the tectonic environment and age of the Um Rus intrusion. In addition, field studies are integrated with EPMA and LA-ICP-MS data for gold-associated sulfides to better understand the formation and distribution of gold mineralization.The bulk-rock geochemical data of fresh host rocks indicate a calc-alkaline, metaluminous to mildly peraluminous, I-type granite signature. Their trace element composition reflects a tectonic setting intermediate between subduction-related and within-plate environments, presumably transitional between syn- and post-collisional stages. The crystallization age of the Um Rus intrusion was determined by in situ SHRIMP 206Pb/238U and 207Pb/235U measurements on accessory monazite grains. The resultant monazite U–Pb weighted mean age (643 ± 9 Ma; MSWD = 1.8) roughly overlaps existing geochronological data for similar granitic intrusions that are confined to major shear systems and are locally associated with gold mineralization in the Central Eastern Desert (e.g., Fawakhir and Hangaliya). This age is also consistent with magmatism recognized as concomitant to transpressional tectonics (D2: ∼650 Ma) during the evolution of the Wadi Mubark belt. Formation of the gold-bearing quartz veins in NNE-SSW and N–S striking fault segments was likely linked to the change from transpressional to transtensional tectonics and terrane exhumation (D3: 620–580 Ma). The development of N–S through-going fault arrays and dike swarms (∼595 Ma) led to heterogeneous deformation and recrystallization of the mineralized quartz veins.Ore minerals in the auriferous quartz veins include ubiquitous pyrite and arsenopyrite, with less abundant pyrrhotite, chalcopyrite, sphalerite, and galena. Uncommon pentlandite, gersdorffite, and cobaltite inclusions hosted in quartz veins with meladiorite slivers are interpreted as pre-ore sulfide phases. The gold-sulfide paragenesis encompasses an early pyrite-arsenopyrite ± loellingite assemblage, a transitional pyrite-arsenopyrite assemblage, and a late pyrrhotite-chalcopyrite-sphalerite ± galena assemblage. Free-milling gold/electrum grains (10s μm-long) are scattered in extensively deformed vein quartz and in and adjacent to sulfide grains. Marcasite, malachite, and nodular goethite are authigenic alteration phases after pyrrhotite, chalcopyrite, and pyrite and arsenopyrite, respectively.A combined ore petrography, EPMA, and LA-ICP-MS study distinguishes morphological and compositional differences in the early and transitional pyrites (Py I, Py II) and arsenopyrite (Apy I, Apy II). Py I forms uncommon small euhedral inclusions in later Py II and Apy II. Py II forms large subhedral crystals with porous inner zones and massive outer zones, separated by narrow As-rich irregular mantles. The Fe and As contents in Py II are variable, and the LA-ICP-MS analysis shows erratic concentrations of Au (<1 to 177 ppm) and other trace elements (e.g., Ag, Te, and Sb) in the porous inner zones, most likely related to discrete sub-microscopic sulfide inclusions. The outer massive zones have a rather homogenous composition, with consistently lower abundances of base metals and Au (mean 1.28 ppm). The early arsenopyrite (Apy I) forms fine-grained euhedral crystals enriched in Au (mean 17.7 ppm) and many other trace elements (i.e., Ni, Co, Se, Ag, Sb, Te, Hg, and Bi). On the other hand, Apy II occurs as coarse-grained subhedral crystals with lower and less variable concentrations of Au (mean 4 ppm). Elevated concentrations of Au (max. 327 ppm) and other trace elements are measured in fragmented and aggregated pyrite and arsenopyrite grains, whereas the undeformed intact zones of the same grains are poor in all trace elements. The occurrence of gold/electrum as secondary inclusions in deformed pyrite and arsenopyrite crystals indicates that gold introduction was relatively late in the paragenesis. The LA-ICP-MS results are consistent with gold redistribution by the N–S though-going faults/dikes overprinted the earlier NNW-SSE quartz veins in the southeastern part of the intrusion, where the underground mining is concentrated. Formation of the Um Rus intrusion and gold-bearing quartz veins can be related to the evolution of the Wadi Mubarak shear belt, where the granitic intrusion formed during or just subsequent to D2 and provided dilatation spaces for gold-quartz vein deposition when deformed by D3 structures.

Vorontsovka Carlin-style gold deposit in the North Urals: Mineralogy, fluid inclusion and isotope data for genetic model

The Vorontsovka gold deposit in the Tagil zone, North Urals, hosts 101 t Au, averaging 7 g/t Au and including 30 t Au in the regolith, and 120 t Ag, averaging 8 g/t Ag. Early skarn-magnetite mineralisation related to a dioritic intrusion of the shoshonitic series was formed in the Late Emsian. Water and volatiles (mainly CO2) released from host metapelites and limestones were mixed with the metalliferous magmatic fluid during catagenesis to contact metamorphism. The modified fluid extracted ore components from host rocks and syn-volcanic stratiform mineralisation and became enriched in Fe, Mn, Ba, Zn, Pb, Au, Ag and S to deposit these components as sulphide dissemination in the footwall of siliciclastic unit and underlying limestone both brecciated along the trust. At the final Early Eifelian stage, there was an additional inflow of deep mantle/magmatic fluid, enriched in Au, Hg, As, Sb and Tl. The mixing of this fluid with metamorphic water resulted in Carlin-style gold-realgar-stibnite (+Tl minerals) mineralisation mainly in carbonate breccia. The main gold endowment associates with gold‑arsenic ores located in brecciated silicified limestone (jasperoid) and argillised tuff-siltstone. Early ore assemblages were formed at 510–240 °C (including magnetite skarn), whereas overprinting Carlin-style gold-(Fe, As, Sb, Hg, Tl) sulphide mineralisation was deposited at decreasing temperatures mainly from 260 to 110 °C. Fluid inclusion data revealed aqueous‑carbonic, Ca-, Na-, Mg- and K-chloride compositions of ore-bearing fluid of 4.8–9.3 wt% eq. NaCl. Changing fluid pressure corresponded to near lithostatic (0.5–0.6 kb) to hydrostatic (0.15–0.2 kb) conditions. Based on the δ34S ranges of sulphides determined for siliciclastic rocks (−2.3 … +1.8‰) and skarns (−2 … +1.6‰) it is assumed that most of the sulphur derived from magmatic reservoir associated with the mantle. The lead isotopic compositions of sulphides show a relatively narrow range and continue the linear trend of the Early Silurian VMS deposits corresponding to the mixing line of mantle and crustal lead sources. Model PbPb ages for the Vorontsovka sulphides are grouped into two clusters: 425–416 and 398–388 Ma. The second period is close to the UPb isotope age of the final stage of the Auerbakh pluton as well as in a good agreement with the ArAr isotope age of 391.1 ± 4.9 Ma of hydromica from the late gold-arsenopyrite assemblage. The specific features of the Vorontsovka deposit indicative of the Carlin-style mineralisation are as follows: the paleo-continental margin and trust-related position; causing high-K/shoshonitic igneous activity; the fine-disseminated sulphide mineralisation in the highly brecciated carbonate-clastic sequence; jasperoid and argillic alteration accompanying gold orebodies with assay boundaries; multiple fluid source; Au-As-Hg-Tl geochemical association; the abundance of As- and As-Fe-sulphides and AsSb sulphosalts; the occurrence of Hg-, Tl-sulphides and sulphosalts; the association of gold with arsenian pyrite and arsenopyrite overgrowing previously forming pyrite; and the predominance of submicroscopic gold in the ores. A LA-ICP-MS analysis of pyrite confirms that elevated concentrations of Au regularly correlate with higher contents of As, Ag, Sb or Tl.

Geochemistry of Au-bearing pyrite from the Sepon Mineral District, Laos DPR, Southeast Asia: Implications for ore genesis

The Sepon gold-copper district occurs along the Truong Son Foldbelt in a 10 km × 40 km east-trending corridor in southern Laos. This study has investigated the nature of gold in pyrite and ore-forming fluid characteristics using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Proton Induced X-ray Emission (PIXE) techniques. Gold is present as invisible gold (Au+1) and as nanoparticles of gold (Au0) in both sedimentary-rock hosted gold and skarn Cu-Au deposits. Gold is present in an early syngenetic pyrite generation (pyrite 2B) with concentrations to 0.5 ppm. Post-sedimentation processes included formation of quartz veins which host pyrite 3B with concentrations to 1.8 ppm Au. The highest concentrations of gold were found in the rims of hydrothermal vein-hosted pyrite 4A and in skarn related pyrite, suggesting gold concentration at a later stage during the ore-forming processes and not during sedimentation.As rhyodacite-porphyry is proximal to calcareous shale and dolomite that host ores, it is plausible that the emplacement of rhyodacite-porphyry introduced significant amount of hydrothermal fluids that circulated through fractures and favoured high grade (to 293 ppm Au) in pyrite 4A and pyrite SKN2. Gold is preferentially concentrated in As-rich zones and in areas that underwent brittle deformation, as evidenced by fracture-fill coarse-grained pyrite 4A, indicating a structural control to ore formation. The predominance of Se, Bi and Te in pyrite SKN1 and the elevated concentrations of Bi, Cu, Se, Sn and Mo in pyrite SKN2 raise the possibility of derivation from a magmatic source. The presence of primary hematite associated with the skarn suggests prevailing oxidising fluid conditions during Au mineralisation in pyrite SKN2.There is a striking chemical similarity between pyrite 4A core and py-SKN1 and rimmed zones of pyrite 4A and pyrite SKN2, which indicates possibility of two types of hydrothermal fluids. The As-rich fluids with elevated concentrations of Au, Se, Sb, Tl, and Cu recorded in pyrite 4A rims and pyrite SKN2. The As-poor fluids with elevated concentrations of Ni and Pb yielded in pyrite 4A cores and pyrite SKN1. The chemistry of pyrite has also shown that both ore-forming fluids are undersaturated in Au similar to Au-bearing pyrite in the Carlin-type deposits, Nevada, USA.

Biogeochemical cycling of gold: Transforming gold particles from arctic Finland

(Bio)geochemical cycling of gold (Au) has been demonstrated in present-day (semi)-arid, (sub)-tropical and temperate environment. Hereby biofilms on Au-bearing mineral- and Au-particle surfaces drive Au dispersion and reconcentration, thereby (trans)forming the particles. However, it is unknown if biogeochemical cycling of Au occurs in polar environments, where air temperatures can reach −40 °C and soils remain frozen for much of the year. Therefore, placer Au-particles, soils and waters were collected at two placer mining districts in arctic Finland, i.e., the Ivalojoki and Lemmenjoki goldfields. Sites were chosen based on contrasting settings ((glacio)-fluvial vs. glacial-till deposits) and depths (surface to 5 m below current surface). Gold particles were studied using a combination of tagged 16S rRNA gene next generation sequencing and electron microscopic/micro-analytical techniques. Across all sites a range of Au-particle morphologies were observed, including morphotypes indicative of Au dissolution and aggregation. Elevated Au concentrations indicative of Au mobility were detected in placer particle bearing soils at both districts. Typically Au-particles were coated by polymorphic biofilm layers composed of living and dead cells embedded in extracellular polymeric substances. Intermixed were biominerals, clays and iron-sulfides/oxides and abundant secondary Au morphotypes, i.e., nano-particles, micro-crystals, sheet-like Au, branched Au networks and overgrowths and secondary rims. Biofilms communities were composed of Acidobacteria (18.3%), Bacteroidetes (15.1%) and Proteobacteria (47.1%), with β-Proteobacteria (19.5%) being the most abundant proteobacterial group. Functionally, biofilms were composed of taxa contributing to biofilm establishment, exopolymer production and nutrient cycling, abundant taxa capable of Au mobilization, detoxification and biomineralization, among them Cupriavidus metallidurans, Acinetobacter spp. and Pseudomonas spp., were detected. In conclusion, these results demonstrate that placer Au-particle transformation and Au dispersion occur in cold, arctic environments. This corroborates the existence of biogeochemical Au cycling in present-day cold environments.

Anomalies in insect nest structures at the Garden Well gold deposit: Investigation of mound-forming termites, subterranean termites and ants

Previous research has demonstrated that small mounds of the termite Tumulitermes tumuli (Froggatt) can successfully be used as a geochemical sample medium for detecting Au mineralization concealed beneath transported cover. This finding is in contrast to the use of large mound forming species typically targeted for exploration in northern Australia and Africa. Large areas of Australia not only have small mound forming termites present but also have subterranean termites and ants. Subterranean termites can have large nests hidden sub-surface. When foraging they sheath their food sources with soil in order to create a microclimate. Here we investigate the use of this material along with nest material from an ant species as sampling media for geochemical exploration in the Yilgarn Craton, Western Australia.Samples were collected along a transect at the Garden Well Au deposit in the Duketon Greenstone Belt of the Yilgarn Craton in Western Australia. Elevated Au concentrations (above a background level ~2ppb) were found in all 3 nest structure types over the mineralization. The mound forming termite (T. tumuli), which forms structures to 60cm in height, had up to 7.4ppb Au. The subterranean termite (Schedorhinotermes actuosus) had up to 8.4ppb, in the sheath material, and an ant species (Rhytidoponera mayri) recorded Au up to 24.4ppb, in the nest material. Vertical movement of Au, Cu and Zn from at least 1.4m deep is supported by evidence in the form of XRD studies and mineral grain counts of nests. These show vertical movement of calcite-rich fragments associated with elevated Au.This is the first investigation of the suitability of subterranean termites for use in geochemical exploration and results suggest that they and ants in the same area represent a potentially useful sample media.

Uranium and gold deposits in the Pine Creek Orogen (North Australian Craton): A link at 1.8 Ga?

The Pine Creek Orogen (PCO) in the North Australian Craton (NAC) hosts world-class unconformity-related U deposits and numerous orogenic or intrusion-related Au deposits, whose timing and genesis are the subject of ongoing controversy. This study reports new geochronology results for an unconformity-related U deposit, Ranger 1 Number 3 orebody, to constrain the timing of processes leading to alteration development and U mineralisation within a regional geological and metallogenic context. It also examines why some U deposits in the eastern PCO contain elevated Au concentrations, unlike their unconformity-related U counterparts in Canada and elsewhere. Key results are as follows. (i) The Ranger 1 deposit and the PCO Au deposits share several characteristics of hydrothermal mineralogy, including chlorite—white-mica—monazite—(±xenotime)-bearing alteration assemblages, some sulfides (pyrite, chalcopyrite), silica (quartz veining or silicification), and Au. (ii) The primary host structure at the Ranger 1 deposit is a shear zone which was reactivated at low-grade metamorphic conditions, as during formation of many of the syn-tectonic Au deposits in the PCO. (iii) Ion probe U–Pb dating of low-Th monazite in an assemblage of hydrothermal white-mica and Fe-rich chlorite from an outer alteration zone at the Ranger 1 No. 3 orebody yielded an age of 1800±9Ma. This age is ∼60 million years earlier than the oldest previously reported age of unconformity-related U mineralisation in the PCO and is within error of several U–Pb ages reported for monazite and xenotime associated with Au mineralisation in the PCO. (iv) Iron-rich chlorite intimately associated with the dated monazite is similar compositionally and texturally to chlorites in alteration zones at other U deposits in the PCO.Two scenarios for the formation of the Ranger 1 deposit are consistent with the available evidence. We prefer a two-stage model where an Fe-chlorite—white-mica—monazite hydrothermal alteration assemblage, with probable gold was introduced replacing medium-grade metamorphic assemblages within a reactivated shear zone at ca. 1800Ma, contemporaneous with the formation of some Au deposits within the PCO. Subsequently, U ore was formed within this environment at ∼1740Ma and/or later, via the reaction of oxidised U- and Mg-bearing basin-derived fluids with chemical reductants in the basement.The new U–Pb geochronology result for monazite points to an important hydrothermal event at the site of the Ranger 1 deposit during regional tectonothermal activity of the Shoobridge Event at ∼1.8Ga in the PCO, and suggests that this pre-ore alteration and tectonism was a key factor in localising later unconformity-related U mineralisation. The pre-ore alteration at ∼1800Ma at the Ranger 1 deposit therefore provides a link between the genesis of the unconformity-related U deposits and the orogenic or intrusion-related Au deposits that formed in the PCO during the Shoobridge Event. At the broader scale, the newly identified ∼1800Ma hydrothermal activity at the Ranger 1 deposit may be part of a craton-wide metallogenic and tectonothermal event at ∼1.8Ga that includes Au mineralisation in the PCO and in The Granites-Tanami Orogen, as proposed in other studies.

Behavior of gold in a magma at sulfide-sulfate transition: Revisited

We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au ( D AuMSS-sil) is about 100–200, whereas the partition coefficient of sulfide liquid/basalt ( D AuSuL-sil) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity ( f O2) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma.

Trace element distribution in uraninite from Mesoarchaean Witwatersrand conglomerates (South Africa) supports placer model and magmatogenic source

The first systematic analyses of the trace and rare earth element (REE) distribution in uraninite from various gold-bearing conglomerates of the Mesoarchaean Central Rand Group in South Africa’s Witwatersrand Basin by in situ laser ablation-inductively coupled plasma-mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof. The chemistry of commonly rounded to sub-rounded uraninite is highly variable from grain to grain but generally marked by elevated Th, W, Bi, Mo, Ta, Y, REE contents and unusually high Au concentrations. Especially, the high Th contents and the chondrite-normalised REE patterns are incompatible with post-sedimentary hydrothermal genetic models for the U mineralisation and point to derivation of the detrital uraninite from a high-temperature, magmatogenic, presumably granitic to pegmatitic source. The elevated Au concentrations (of as much as 67 ppm) in this uraninite are unique to the Witwatersrand and hint at a granitic hinterland that was enriched in both U and Au, thus presenting a potential source domain for some of the detrital gold in the Witwatersrand conglomerates. Minute fracture fills of brannerite in close proximity to the larger, rounded uraninite grains are devoid of detectable Bi, Mo, REE and Au and have only very low concentrations of Th, W, Ta and Y. This is explicable by crystallisation from a low-temperature hydrothermal fluid. Thus, Witwatersrand U phases show, analogous to many other ore constituents, such as pyrite and gold, clear evidence of partial, short-range mobilisation of originally detrital particles by post-sedimentary fluids.

Fine fraction regolith chemistry from the East Wongatha area, Western Australia: tracing bedrock and mineralization through thick cover

The fine fraction (&lt;50 μm; ‘silt – clay’) of regolith from an area dominated by thick (up to 120 m) transported regolith is shown to be a viable sampling medium for detecting bedrock and bedrock-hosted mineralization. A partial digest of the fine fraction of regolith reveals that elements which have migrated vertically from bedrock have become sequestered at a shallow depth in regolith. Nickel, Cr, Li, Mn, V, Co and Rb are useful in tracing the extent of different bedrock types, with Ni, Cr, V and Co particularly useful for identifying more mafic bedrock. Higher concentrations of Au (maximum 29 ppb; median 2 ppb; n = 835) are spatially related to either Archean greenstones or unconformity-related uranium – base metal – gold mineralization. Although some samples with anomalous Au concentrations are carbonate-rich, there is no consistent relationship between elevated Au concentrations and the carbonate content of regolith. Deionized water digestion of the &lt;50-μm fraction of 50 samples correlates with aqua regia data for several elements (particularly Au, Ni), suggesting that some mineralization- or bedrock-related elements in the fine fraction of regolith are either labile or microparticulate.

Calcrete and plant inter-relationships for the expression of concealed mineralization at the Tunkillia gold prospect, central Gawler Craton, Australia

Transported and typically weathered material that extends over much of Australia presents a challenge for the successful application of surficial geochemical exploration methods. The Tunkillia Au-prospect in the Gawler Craton, South Australia provides a case study where unconsolidated and potentially laterally transported aeolian and alluvial sediments overlie a deeply weathered profile with variable supergene and primary Au mineralization at depths typically exceeding 40 m. The Tunkillia Au prospect was discovered through a regional calcrete sampling program, but elevated Au concentrations in this material do not always express underlying mineralization, such is the case at Tomahawk, an area of elevated Au in calcrete results overlying limited, low grade mineralization. The distribution and sampling accessibility of calcrete is also not continuous across the landscape, where in this region young siliciclastic linear dune ridges bury older calcrete that are otherwise exposed or only at shallow depths in dune swales or across sheetwash plains. Vegetation cover is more continuous across this landscape and is dominated by several main species. Sampling of dominant plant species: Casuarina pauper , and Eucalyptus concinna , has been conducted across Tomahawk where some sites of elevated Au in calcrete have been drilled. Results show a limited spatial association between calcrete geochemistry and plant biogeochemistry. This demonstrates that plant sampling is able to express low grade, discrete areas of buried Au mineralization typically on the scale of tens of metres, whereas the expression provided from calcrete is more levelled, and typically representative of larger areas, especially when they have formed within transported detritus host materials. It is recommended that although plant biogeochemistry and calcrete geochemistry may have their strengths and weaknesses, such as scale constraints, in regional geochemical exploration programs when these methods are integrated they potentially provide a more comprehensive and widely available geochemical exploration assessment across a wider range of regolith-landform settings.

The source of pedogenic carbonate associated with gold-calcrete anomalies in the western Gawler Craton, South Australia

Elevated Au concentrations above bedrock mineralization are commonly closely associated with calcrete in arid residual and semi-residual soils in Australia. The origin of Australian calcrete has been argued for many years but there have been very few published studies. Calcrete from Au deposits and prospects, and elsewhere in the Gawler Craton (South Australia) were studied for their geochemical composition, and Sr and C isotope ratios. By comparing carbonate 87Sr/ 86Sr ratios with underlying weathered rock and bedrock, it is demonstrated that many samples have overwhelmingly atmospheric Sr (> 94%) and, by inference, Ca (> 98%) rather than a local bedrock or soil mineral origin. The isotopic composition of calcretes at the Challenger Gold Deposit lie on a mixing trend of decreasingly marine Sr from the Nullarbor Plain to Tarcoola, 300 km to the east. The δ 13C PDB values (− 7.5‰ to + 0.1‰) suggest that the carbonate C in calcrete has a mixed origin derived from C3 and C4 plants. A model is presented to explain how the Au–Ca association may form in calcrete at Challenger based on these new Sr and C isotope data and in which the role of plants and atmospherically deposited Sr and Ca combine with pre-existing Au in the soil. The implication for mineral exploration is that, since most of the Ca is derived from marine sources, it is not a pre-requisite that autochthonous soil minerals (derived from underlying weathered bedrock) supply Ca for calcrete to form and for the Au–Ca association to occur. Thus, the independence of the Ca with respect to underlying lithology reaffirms the usefulness of calcrete as a sampling medium for Au exploration.

Gold Concentrations in Springs at Waiotapu, New Zealand: Implications for Precious Metal Deposition in Geothermal Systems

Gold and silver from a selection of springs at Waiotapu, New Zealand, was preconcentrated onto activated charcoal and analyzed by ICP-MS. The measured gold concentrations were combined with field and laboratory analyses of other components, thermodynamic data and geochemical modeling software to calculate the gold saturation index in each spring, assuming that reduced sulfur ligands largely control gold solubility. The springs that were selected for analysis have elevated concentrations of components that are residually enriched by boiling, so their composition is strongly related to that of the geothermal reservoir. Therefore, these analyses provide information about geochemical processes that operate beneath the springs as well as those at work within the springs. Previous geochemical investigations at Waiotapu indicated that dissolved gold concentrations are elevated within Champagne Pool and in precipitates surrounding this spring. Therefore, it is likely that the reservoir fluid that feeds springs at Waiotapu also contains dissolved gold. Champagne Pool has the highest gold concentration measured in this study, 109 ngL–1 dissolved and 362 ngL–1 total. This spring is slightly undersaturated with gold in solution, but the total concentration is higher than the solubility of Au(s). Undersaturation with respect to Au(s) is consistent with deposition of gold by adsorption and concentration within an As- and Sb-sulfide precipitate that forms around Champagne Pool and is similar in magnitude to that expected from modeling of gold adsorption onto As- and Sb-sulfide precipitates. Elevated Au concentrations of gold (40–90 ngL–1) at two sites downstream from Champagne Pool indicate that deposition of gold within Champagne Pool is inefficient. The two downstream sites are substantially oversaturated with respect to reduced S-Au species; however, assumptions used in solubility calculations are unlikely to be valid at these sites because the oxidation state of the fluids after discharge was not determined. Despite uncertainty in the saturation index calculated at these sites, the elevated concentrations of gold and apparent oversaturation indicate that gold is transported by other ligands, such as polysulfides and thiosulfates or as colloidal particles downstream from Champagne Pool. Several springs at Waiotapu with moderate concentrations of dissolved reduced sulfur (1–6 mgL–1) have very low dissolved concentrations of gold and are substantially undersaturated with respect to Au(s). Deposition of gold by precipitation requires loss of reduced sulfur ligands through processes such as boiling, oxidation, acidification or sulfide precipitation. Therefore, the low concentrations of gold in springs that contain substantial reduced sulfur indicates that gold is deposited by processes that can cause substantial undersaturation of Au(s), such as adsorption, coprecipitation, or both. Deposition of gold by adsorption or coprecipitation is consistent with the occurrence of gold as impurities in epithermal sulfide minerals. These results do not rule out direct precipitation of Au(s) at Waiotapu but indicate that other depositional mechanisms could also be important in the geothermal system beneath the springs.

Bacillus cereus, gold and associated elements in soil and other regolith samples from Tomakin Park Gold Mine in southeastern New South Wales, Australia

Efficient exploration for new Au deposits is increasingly important as existing deposits become depleted. This is particularly relevant in Australia, where exploration can be difficult because of a thick regolith cover. New and effective methods of exploration need to be developed, and possibilities lie in geomicrobiological methods. For instance, Bacillus cereus, a common soil bacterium, has been shown to act as a biogeochemical indicator for concealed mineralisations, including vein-type Au deposits. We report the results of the first Australian case study of the association of B. cereus and Au at the Tomakin Park Gold Mine in southeastern New South Wales. Soil samples from the Ah horizon were analysed for B. cereus spores and 56 major and trace elements. The results show enrichment of Au, As, B. cereus spores and, to a lesser extent, Sb, Bi and Pb over the top of the Au deposit. Gold concentrations over the mineralisation range from 100 ppb to 1.1 ppm compared to a background of 2 ppb and As concentrations are enriched to 100 ppm from a background of 5 ppm. B. cereus spore counts were up to 10 times higher in soils with elevated concentrations of Au. Factor analysis indicates four main associations: TiO 2+lanthanides+actinides; CaO+MgO+Cs+Be+Ba(+Ga+Pb+Rb); B. cereus+Au+As+Sb+Bi(+Pb); Fe 2O 3+MnO+Co+Ni+Cu+Mo. Selective sequential leaching was used to study the fractionation of Au and As in soils, other regolith materials and Au-bearing vein quartz to infer their mobility and bioavailability. In unweathered quartz vein material, the majority of the Au was extracted only in the strongest, final step, with aqua regia. However, in soils from the Ah horizon, 50% of the Au was present in the water-, ammonium acetate- and sodium pyrophosphate- and hydroxylamine hydrochloride-extractable fractions. In contrast, As displays little change in fractionation with an increasing degree of weathering, and is predominantly associated with the operationally defined Mn- and Fe-oxides and oxyhydroxides. These results indicate that: (i) Au is mobilised during the weathering of the host rock; (ii) Au is bioavailable in these soils; and (iii) the increase in B. cereus spores is likely to be linked to elevated concentrations of bioavailable Au in these soils. The results indicate also that an effective biogeochemical exploration technique may be developed, where B. cereus spore counts are measured in the field and used as a pre-screening method to target areas useful for further sampling and complete geochemical analysis.

Groundwater flow in playa lake environments: impact on gold and pathfinder element distributions in groundwaters surrounding mesothermal gold deposits, St. Ives area, Eastern Goldfields, Western Australia

Geochemical sampling of groundwater may be an effective tool in exploring for Au deposits in areas of transported overburden. However, to use hydrogeochemistry effectively, we need to understand which elements are useful pathfinders and how their distribution patterns are affected by groundwater flow and geochemistry, especially in areas that have hypersaline groundwater. A hydrogeochemical survey was completed over the strongly Au-endowed St. Ives area of the Eastern Goldfields of Western Australia. Extensive geochemical and hydrogeological data were measured from 80 holes drilled on a kilometre-spaced grid. A numerical model was developed to quantify the groundwater flow regime in the study area. Also, hydrogeological data in combination with the geochemistry of the waters were used to identify a complex flow regime adjacent to Lake Lefroy, a playa lake in the area. Regional brine flows towards the salt lake, evaporation occurs and hypersaline (playa) brines are formed that sink and flow away from Lake Lefroy and beneath the regional brines. Density-driven, convective mixing of these two brines is responsible for forming a zone of brines with transitional compositions located near the edge of Lake Lefroy. Knowledge of the groundwater flow regime was used to interpret the distribution of elements around bedrock Au mineralization. An Au-only groundwater anomaly (&gt;10 ppt, maximum 52 ppt) is located downstream of the Junction Au ore body. No anomaly was identified in deep brines associated with the nearby Argo/Apollo Au ore body; however, shallow regional brines sampled in a previous survey showed elevated Au values overlying and downstream of the mineralization. The lack of elevated Au concentrations in deeper groundwater associated with Argo/Apollo is because the samples collected were hypersaline playa brines sourced from Lake Lefroy that had not flowed over the mineralization. Possible pathfinder elements, such as Sb, Bi and Te, were either present in concentrations too low to detect or showed no spatial pattern related to known mineralization; however, a strong correlation was noted between some of their concentrations and salinity. Evaluation of the groundwater anomalies observed in this study suggests that hydrogeochemical exploration is most effective at a resolution of 0.5–4 km spacing, and does not require fully cased and packed wells, at least in cases where no confined aquifers are intersected in drilling. The results of this study illustrate the importance of understanding groundwater flow regimes in planning and interpreting hydrogeochemical surveys.

Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada

Abstract This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.