Remobilization Of Au Research Articles

Newly discovered Early Cretaceous Au mineralization overprinting Late Jurassic Pb-Zn-Ag mineralization in the Qin-Hang metallogenic belt (South China)

Magmatic and mineralizing events in South China were traditionally thought to be limited between 150 Ma and 130 Ma despite the numerous Mesozoic-aged magmatic-hydrothermal deposits in the region. This viewpoint is being reevaluated in light of new age data. Kangjiawan is a representative Pb-Zn-Ag-Au deposit in the Hengyang Basin in the Qin-Hang metallogenic belt of South China that contains 1.96 Mt of Pb-Zn resource with 4.70% Zn, 6.02% Pb; 2280.5 t of Ag resource with 108.23 g/t of Ag; and 58 t of Au resource with 3.25 g/t of Au. Its mineralization can be divided into four stages: (I) pre-ore quartz−pyrite, (II) quartz−pyrite−galena−sphalerite, (III) quartz−galena−sphalerite, and (IV) calcite−pyrite. Multiple generations of pyrite were identified at Kangjiawan. Generation I pyrite (Py1, formed in Stage I) is coarse-grained, texturally homogeneous, and contains abundant silicate inclusions (e.g., K-feldspar and titanite). Generation II pyrite (Py2, formed in Stage II) is generally fine-grained and exhibits a core-rim texture, with the porous, sphalerite−galena inclusion-bearing core (Py2a) having replaced Py1 and the rim (Py2b) being homogeneous. Generation III pyrite (Py3, formed in Stage IV) is subdivided into Py3a and Py3b, with the homogeneous Py3a often replaced by the porous Py3b. The low Co/Ni ratio of Py1, along with the abundant silicate inclusions, is suggestive of intense fluid-rock interaction during Stage I. Py2a is As-Cu-Pb-Ag-Au−rich, Co-depleted, and has elevated Ag/Co (average 3.24) and As/Co (average 10,217.14) ratios, which are suggestive of extensive fluid boiling during the formation of Py2a. This is also supported by the ubiquitous Py2-cemented hydrothermal breccia. In contrast, Py2b is depleted in As-Cu-Pb-Ag-Au but enriched in Co, which is indicative of non-boiling conditions. Subsequent sealing of fractures by veins may have caused the formation of coarse-grained sphalerite and galena in Stage III. The similar 207Pb/206Pb values among Py1 (average 0.849), Stage II galena (average 0.849), Stage III galena (average 0.849), and the nearby Shuikoushan granodiorite stock (corrected average 0.848) suggests that mineralization in stages I−III may represent distal skarn-type Pb-Zn-Ag mineralization genetically related to the Shuikoushan stock. This interpretation is supported by the similar ages of mineralization Stage I (apatite U-Pb: 159.4 ± 1.0 Ma) and Stage II (fuchsite 40Ar-39Ar: 158.1 ± 0.4 Ma) to the age of Shuikoushan stock (zircon U-Pb age: ca. 158 Ma). As the primary Au-hosting mineral at Kangjiawan, the As- and Au-rich Py3a occurs in sharp contact with porous Co-Ni-Se-Bi-Mo-As−poor Py3b, which commonly contains abundant fine-grained galena and sphalerite inclusions. This suggests that Py3b formed via coupled dissolution−re-precipitation of Py3a. This coupled dissolution−re-precipitation reaction favors the remobilization of Au from within the Py3a structure and its re-enrichment as Au inclusions in Py3b; this is supported by the anomalous Au peaks in time-resolved signal profiles from laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and the abundant outliers for Au concentrations in Py3b. The markedly distinct trace-element geochemistry of Py3 compared to those of Py1 and Py2, and the U-Pb age of Stage IV calcite (ca. 138 Ma), are indicative of an Early Cretaceous Au mineralization event. Published calcite C-O isotope data, coupled with the narrow range of δ34S values (0.6‰−2.2‰) of Py3, suggest that the Kangjiawan Au mineralization may be associated with a concealed Early Cretaceous pluton. We propose that the Kangjiawan deposit formed via the overprinting of Late Jurassic distal skarn Pb-Zn-Ag mineralization by Early Cretaceous magmatic-hydrothermal Au mineralization, which demonstrates that 150−135 Ma may also be an important period for the formation of the magmatic-hydrothermal mineralization belt.

Gold remobilization of the Sanshandao gold deposit, Jiaodong Peninsula, Eastern China: Perspective from in-situ sulfide trace elements and sulfur isotopes

The Sanshandao is a typical altered-rock-style gold deposit (>260 t @ 3.96 g/t). The deposit is located at the northernmost end of the NE-trending Sanshandao fault in the northwestern Jiaodong Peninsula. The orebodies are mainly hosted by the footwall (minor in hanging-wall) of the fault, which cut Jurassic granites. According to the mineral assemblage and crosscutting relationship, five hydrothermal/mineralization stages were identified: (I) veinlet-disseminated quartz + pyrite ± chalcopyrite, (II) quartz + pyrite + chalcopyrite + native Au veining, (III) quartz + pyrite + galena + sphalerite + native Au + chalcopyrite veining, (IV-a) veinlet-disseminated siderite + pyrite + galena + sphalerite, (IV-b) ore-barren siderite, and (V) ore-barren calcite veining. Stages II and III auriferous pyrite contains visible native gold, and can be divided into four generations (Py2a, Py2b, Py3a, and Py3b). Py2a contains little native gold, whereas Py2b, Py3a, and Py3b contain much coarse-grained native gold. Gold in these pyrite generations correlates positively with the As content. Typical Py2a and Py3a porous/sieve-like texture indicates that Au remobilization has occurred, which may have controlled mainly by coupled dissolution-reprecipitation (CDR). Relative Sb-Te enrichments in Py2b and Py3a imply that a melt with low-melting point chalcophile elements (LMCEs) also facilitated Au remobilization by efficiently scavenging Au from preexisting minerals. Combined with the trace element compositions, we show that CDR may have liberated Au and LMCEs, and the subsequent fluid and LMCEs melt have altogether caused Au remobilization. The narrow δ34SV-CDT range of the pyrite from stages II-III (11.75–12.22‰ (Py2a), 10.83–12.39‰ (Py2b), 11.63–12.88‰ (Py3a), 11.91–12.67‰ (Py3b)), together with the compiled S-isotope dataset (from literature data) (δ34SV-CDT = mostly 7–10‰), indicate that reduction of marine sulfates represents the main sulfur source.

Origin of biotite-rich xenoliths in the Eocene Beiya porphyry: Implications for upper-crustal Au remobilization and formation of giant porphyry Au systems in a collisional setting

Origin of biotite-rich xenoliths in the Eocene Beiya porphyry: Implications for upper-crustal Au remobilization and formation of giant porphyry Au systems in a collisional setting

Deciphering multiple ore-forming processes of the Shuangqishan orogenic gold deposit, Southeast China by in situ analysis of pyrite

Gold deposits are characteristically formed via multiple mineralization and remobilization events. Inferences on the complex formation history of such deposits from ore textures can be ambiguous in the lack of chemical and isotopic evidence as exemplified by the Shuangqishan orogenic gold deposit in Southeast China. We, therefore, conducted LA-ICP-MS spot analysis coupled with LA-MC-ICP-MS sulfur and lead isotope analysis on texturally complex gold-bearing pyrite from the Shuangqishan deposit. Gold distribution in different syn- to late-ore pyrite generations indicate initial evenly deposition of nano- to micro-inclusions of Au-Ag-Pb-Bi tellurides within pyrite grains and subsequent Au remobilization following brittle deformation and fluid-mediated coupled dissolution-reprecipitation (CDR) reactions. Gold dissolved from the pyrite during the reworking event became re-concentrated and re-precipitated as visible gold either as inclusions in pyrite or larger free gold grains within microfractures of pyrite. The distribution of Bi-Te-Pb-Ag assemblage is comparable to that of Au, illustrating that these trace metals became redistributed in a similar fashion. Early, undeformed pyrite has the least radiogenic Pb isotopic compositions. Late, porous and inclusion-rich pyrite, brecciated pyrite, and recrystallized Te-Bi minerals have variable but in general more radiogenic Pb isotope ratios (higher 206Pb/204Pb and 207Pb/204Pb ratios), which are attributed to modification and precipitation from a more evolved crustal fluid during the re-precipitation of native gold. The Pb isotopic signature of the younger pyrite generations was likely inherited from Pb released during modification of earlier proto-pyrite and a second, crustal lead source that was involved in the hydrothermal system during later remobilization. The δ34S values for the early proto-pyrite and the reworked pyrite generations within the individual vein stage are broadly similar. The variations in sulfur isotope compositions reflect the change in physicochemical conditions (temperature and oxygen fugacity) during proto-ore formation. Our study highlights the importance of metal recycling from early proto-auriferous pyrite during later reworking and gold remobilization, which led to visible gold and high-grade ore shoot formation. The extensive variations of trace element concentrations and S-Pb isotope compositions of proto and reworked pyrites record the complicated Au mobilization and deposition processes and help to understand how gold ores evolved.

In situ trace elements and sulfur isotopes of sulfides in the Dabaiyang Te-Au deposit, Hebei Province, China: Implications for Au remobilization from pyrite

Coupled dissolution–reprecipitation (CDR) reactions and low-melting point chalcophile elements (LMCE) melt scavenging are effective mechanisms for gold remobilization and are important for the formation of Te-Au deposits. The Dabaiyang deposit is a typical Te-Au deposit in the Zhangjiakou district and contains tellurides, including altaite, hessite, petzite, tetradymite and tellurobismuthite. Two generations of pyrite have been recognized in the deposit: early pyrite without fractures or alteration (Py1) and later pyrite with more porosity, inclusions and fractures (Py2), and Py2 could be separated into Py2a that containing many fractures without mineral inclusions and Py2b that containing microfractures, micropores and mineral inclusions. Pyrite containing large amounts of mineral inclusions and free of microfractures (Py-MI) has also been recognized, but timing relationships between Py-MI and Py1 (or Py2) is hard to establish. The Pyrite textures indicate that Py2a underwent brittle deformation and that Py2b underwent CDR reactions and Te-melt scavenging. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that Py2a contains trace elements similar to Py1, while Py2b contains higher Au, Ag, Te and other trace elements, indicating that brittle fracturing has no effect on element mobilization and reprecipitation, and porous pyrite has Au-Ag-Te concentrations. High Co/Ni ratios and Te contents and low Se contents show that the Dabaiyang deposit has a genetic relationship with magmatic activity. In situ δ34S values of sulfides range from −15.77‰ to −6.09‰ (mostly from −13.89 to −10.06‰), which is consistent with bulk sulfide δ34S values. The negative δ34S values more likely resulted from sulfur isotope fractionation in high fO2 hydrothermal fluids. CDR reactions liberated Au, Te, Ag and other trace elements from pyrite and formed porous textures. Then, Te-rich melts scavenged Au from fluid and wallrocks during migration. Finally, the melt precipitated as mineral inclusions in pores or fractures in pyrite. CDR reactions and LMCE melt scavenging played an important role in upgrading the ores and the occurrence of visible native gold in the Dabaiyang gold deposit, as well as other Te-Au deposits in the Zhangjiakou district.

Occurrence and remobilization of gold in the Dayingezhuang deposit in Jiaodong, North China Craton: Evidence from textural and geochemical features of pyrite

The Dayingezhuang gold deposit in the Jiaodong district, eastern margin of the North China Craton is hosted in Mesozoic granitic rocks and consists of quartz-sulfide veins/veinlets and sulfide disseminations in alteration envelopes. Previous studies mainly focused on the geochronology, sources of ore-forming fluids and metals to investigate the ore genesis. However, enrichment mechanism of Au and other associated trace metals remain unclear. In this study, we present detailed textures and in-situ LA-ICP-MS trace-element compositions of different generations of pyrite, as well as EMP analysis of Au-bearing minerals to discuss the occurrence and enrichment mechanism of Au at this deposit.Three generations of pyrite (Py1, Py2 and Py3) formed during three hydrothermal ore stages (I, II, and III) at Dayingezhuang. Py1 occurs as disseminations in sericitic alteration assemblages and is characterized by low Au (mean 0.15 ppm), Ag, As and Te contents. The time-resolved depth-concentration profiles indicate that Au in Py1 mainly occurs as nanoparticles and/or micron-sized inclusions. Py2 can be further divided into the early undeformed Py2a and later Py2b, which is the product of deformed Py2a with different degrees of brittle to plastic deformation and recrystallization. Py2a in pyrite-siderite-quartz veins is relatively enriched in invisible Au (mean 0.41 ppm), Ag, As, and Te compared to Py1, and contains numerous micron-sized Au inclusions. In contrast, Py2b contains lesser invisible Au (0.21 ppm) and host abundant gold minerals along the grain boundaries and microfractures. Py3 in polymetallic sulfide veins has little Au. As a whole, Au in pyrite is positively correlated with Ag and Te, which is consistent with the results of EMP analysis showing the occurrence of Au as electrum, native gold and minor petzite in pyrite. Such evidences show that the deformation and recrystallization of auriferous Py2a potentially caused local remobilization of Au (mainly as micron-sized inclusion Au) via solid-state ductile flow and subsequent reconcentration of Au in microfractures of Py2b. The pyrite deformation and Au remobilization events were suggested to be related to the continuous reactivation of the regional Zhaoping Fault contemporaneous with gold mineralization. Our study highlights the importance of remobilization and reconcentration of Au triggered by syn-ore tectonic activities at Dayingezhuang and possibly other Au deposits in the Jiaodong district.

Gold Remobilization: Insights from Gold Deposits in the Archean Swayze Greenstone Belt, Abitibi Subprovince, Canada

Abstract Recognizing if and how Au is remobilized, in solid, melt, or fluid state, is critical for understanding the origin of high-grade ore zones in Au deposits. When evidence for Au remobilization can be demonstrated, then primary versus secondary processes can be distinguished, resulting in a more complete understanding of Au deposit formation. To address this, samples from two Au deposits, Jerome and Kenty, in the Archean Swayze greenstone belt of northern Ontario, Canada, together with archived samples from 39 high-grade Au deposits from the Abitibi greenstone belt across Ontario and Quebec, were geochemically characterized using integrated scanning electron microscopy-energy dispersive spectroscopy and electron microprobe imaging and analyses in addition to laser ablation-inductively coupled plasma-mass spectrometry elemental mapping. These data provided the basis to develop a model for Au remobilization and upgrading of Au that is widely applicable to orogenic gold settings. Data for the Jerome deposit indicate that Au uptake into early pyrite was not due to pulsing of different fluids, but instead was predominantly controlled by S availability, whereby the oscillatory/sector zoning in pyrite resulted from the substitution of As into S sites during rapid growth due to local chemical disequilibrium. In addition, Au-bearing pyrite from both the Jerome and Kenty deposits records textures, such as porosity development coincident with the presence of native gold and accessory sulfide phases, that are strongly suggestive of coupled dissolution-reprecipitation (CDR) reactions that liberated Au and associated elements from earlier auriferous (100–5,000 ppm Au) pyrite. During the remobilization process, Au and Ag were decoupled, which resulted in (1) a change in Au/Ag ratios of 0.5 to 5 in early pyrite to ≈9 in the new native gold (900 Au fineness) and (2) incorporation of Ag into cogenetic secondary mineral phases (e.g., chalcopyrite, tetrahedrite, and galena). Evidence for an association of low-melting point chalcophile elements (LMCE; Hg, Te, Sb, and Bi) with Au at the Jerome, Kenty, and many (>50%) of the 39 historic deposits sampled, along with native gold filling structurally favorable sites in vein quartz in all samples, indicates a fluid might not have been the only factor contributing to remobilization. This systematic Au-LMCE association strongly supports a model whereby Au is released by CDR reactions and is then remobilized by fluid-mediated, LMCE-rich melts that began to form at 335°C and/or by local, nanoparticle (nanomelt?) transport during deformation and metamorphism. Conclusions drawn from this study have implications for Au deposits globally and can account for the common presence of coarse-grained, commonly crystalline, native gold filling fractures in quartz and the paragenetically late-stage origin of gold in veins. They can also better explain the inability of Au in solution remobilization models to account for locally high gold grades, given the relatively low solubility of Au in hydrothermal fluids.

Supergene remobilization of Au in Au-bearing regolith related to orogenic deposits: A case study from Kazakhstan

Our study of the primary and supergene gold from the Suzdal and Raigorodok deposits in Kazakhstan confirms that during the formation of the regolith in both deposits, invisible and fine gold is released from the host sulfide minerals during their oxidation and destruction; and transported in the form of soluble thiosulfate complexes by near-surface solutions. Upon contact with a geochemical barrier, gold can be deposited as crystalline aggregates. In the deposition of this gold the role of geochemical barriers, such as variations of redox, pH and sorption are very important.Evidence of the past processes of dissolution, redistribution and aggregation of gold in the regolith in these deposits is provided by the following features: 1) The presence of dissolution and growth structures on the surface of newly formed gold; 2) The size of the gold grains increases and newly formed gold microcrystals appear on the surface of the grains; 3) Changes in the composition of native gold (e.g. decrease of silver content); 4) The accumulation of gold in subhorizontal localized layers within the regolith; 5) Gold association with supergene minerals (limonite, jarosite, goethite, Mn-hydroxides, opal, gypsum, calcite, clay minerals, etc.).This detailed study of the specific compositional and structural features of supergene gold and the physicochemical conditions of regolith environment allowed the recognition of the releasing, dissolution, transfer and aggregation processes of gold in the regolith.

Supergene gold in silcrete as a vector to the Scuddles volcanic massive sulfides, Western Australia

This study reports data on the first occurrence of economic supergene Au in a near-surface silcrete horizon over the Scuddles volcanic massive sulfide (VMS) deposit in the Golden Grove district, Western Australia. A deep weathering profile was developed on dacite, rhyodacite, siltstone, and breccia that host primary Cu, Zn, and Pb sulfides with Au-Ag ore. From the base, the weathering profile at Scuddles is subdivided into five main zones: (1) supergene sulfide enrichment zone; (2) supergene oxide enrichment zone; (3) ferruginous saprolite; (4) leached zone of kaolinitic saprolite and silcrete; and (5) lateritic zone of mottled clays, ferruginous duricrust, and gravels. Silcrete at Scuddles hosts supergene Au deposit that formed in two generations: the first is intimately associated with Ag halides during supergene enrichment of the primary VMS, and the second is associated with kaolinite in dissolution cavities inside Ag halides during lateritic weathering. These two Au generations imply more than one mechanism of Au remobilization and formation, multiple fluid pathways, and superimposed episodes of weathering under variable timing and climatic conditions. Gold grains are pure, nanocrystalline (up to 10 nm) and clustered together forming microcrystalline aggregates. A few Au grains are residual in silcrete with Ag-Sb-rich cores and Ag-poor rims possibly formed during dealloying of Ag and Sb. Chemically, Au in silcrete is associated with a multi-element concentration of Ag, I, Br, Cl, Sb, Sn, Bi, Hg, Mo, W, Te, and Ge. Gold and Ag in the supergene weathering profile were mobilized to silcrete as a halide complex under acidic and saline conditions generated during the oxidation of massive sulfides at depth. The precipitation of Au-Ag halides in the silcrete may have taken place in response to a rise in pH. Gold was likely remobilized with kaolinite from the surface lateritic zone, facilitated by decays of plant roots and bioturbation. The clustered spongy, cube-octahedral, platy (six-sided), dendritic-, and reniform-like morphologies of Au in cavities inside Ag halides may indicate biogenic-related processes in its precipitation. Recognizing Au-Ag-rich silcrete over the buried VMS at Scuddles highlights the significance of the silcrete in finding buried VMS, particularly if the gossan is absent.

Pyrite deformation and connections to gold mobility: Insight from micro-structural analysis and trace element mapping

The metamorphic transition of pyrite to pyrrhotite results in the liberation of lattice-bound and nano-particulate metals initially hosted within early sulphide minerals. This process forms the basis for the metamorphic-driven Au-upgrading model applied to many orogenic Au deposits, however the role of syn-metamorphic pyrite deformation in controlling the retention and release of Au and related pathfinder elements is poorly understood. The lower amphibolite facies metamorphic mineral assemblage (Act-Bt-Pl-Ep-Alm ± Cal ± Qz ± Ilm; 550 °C) of Canada's giant Detour Lake deposit falls within the range of pressure-temperature conditions (450 °C) for crystal plastic deformation of pyrite. We have applied a complementary approach of electron backscatter diffraction (EBSD) mapping and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) 2D element mapping on pyrite from the Detour Lake deposit. Chemical element maps document an early generation of Au-rich sieve textured pyrite domains and a later stage of syn-metamorphic oscillatory-zoned Au-poor pyrite. Both pyrite types are cut by Au-rich fractures as a consequence of remobilization of Au with trace element enrichment of first-row transition elements, post-transition metals, chalcogens and metalloids during a late brittle deformation stage. However, similar enrichment in trace elements and Au can be observed along low-angle grain boundaries within otherwise Au-poor pyrite, indicating that heterogeneous microstructural misorientation patterns and higher strain domains are also relatively Au-rich. We therefore propose that the close spatial relationship between pyrite and Au at the microscale, features typical of orogenic Au deposits, reflects the entrapment of Au within deformation-induced microstructures in pyrite rather than the release of Au during the metamorphic transition from pyrite to pyrrhotite. Moreover, mass balance calculations at the deposit scale suggest that only a small percentage of Au could have been sourced from pyrite and instead point to the role of substructures in pyrite as depositional traps for Au during syn-metamorphic deformation- and fluid-assisted diffusion Au-upgrading.

Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

The Las Cruces deposit is in the eastern end of the Iberian Pyrite Belt (SW Spain). It is currently being mined by Cobre Las Cruces S.A. The main operation is focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). An Au-Ag-Pb–rich gossan resource (3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag) occurs in the upper part of the deposit. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au mineralization in the upper part of the gossan linked to Fe-oxides lithofacies, (2) Au concentration in the lower part of the gossan associated with leached black shales, and (3) Au ore in the cementation zone related to subvertical fractures. A hydroseparation device has been used to obtain heavy mineral concentrates from selected samples of different ore types. Reflected-light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA) were used to study the separated Au particles. Significant differences between the defined ore types include the Au-bearing lithologies, mineral associations, textural features, particle sizes, morphologies, and fineness. Au-rich minerals include native Au, Au-Ag electrum, and Au-Ag-Hg amalgams. Gold-bearing mineral associations include Pb-oxihalides, Fe-oxides, galena, pyrite, cinnabar, and Ag-sulfosalts. The Au enrichment mechanism in the supergene profile involves (1) dissolution of Au from the primary sulfides as chloride-rich ionic complexes during the weathering of the deposit under subaerial exposure; dissolved Au is transported downward through the supergene profile under acidic and oxidized conditions; (2) destabilization of the Au complexes by Fe-controlled redox reactions; as a consequence, coarse-grained, high-fineness Au particles precipitated in association with Fe-oxyhydroxides. This resulted in secondary concentration in the upper gossan; and (3) after deposition of cover sediments took place a progressive change in the system conditions resulting in a later Au remobilization as hydroxidehalide, hydroxide, thiosulfate, and bisulfide complexes in the lowermost gossan and cementation zone. The main pathways for migration of enriched fluids to the cementation zone are secondary permeability zones linked to Alpine reactivated faults. Deposition of Au seems to be related to fluid interaction with reductant lithologies, including black shales and the primary sulfides.

The Skaergaard PGE and Gold Deposit: the Result ofin situFractionation, Sulphide Saturation, and Magma Chamber-scale Precious Metal Redistribution by Immiscible Fe-rich Melt

The Skaergaard intrusion, Greenland, is the type locality for Skaergaard-type mineralizations. Mineralization levels are perfectly concordant with igneous layering, up to 5 m thick, internally fractionated, and contain crystallized sulphide droplets and precious metal alloys, sulphides, arsenides and telluride. Immiscible Cu-rich sulphide droplets, formed in a mush zone below the roof, scavenged precious metals. They were subsequently dissolved and transported to the floor in late-formed, immiscible, Fe-rich mush melts. Mineralized stratigraphic intervals of floor gabbro formed in ‘proto-macrolayers‘, owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene. The floor mineralization is divided into four stratigraphic sections. Formation of the Lower Platinum Group Element Mineralization (LPGEM) involved: (1) crystallization of the bulk liquid liquidus paragenesis and in situ fractionation; (2) sulphide saturation and formation of sulphide droplets in melt in the upper part of ‘proto-macrolayers‘. After further in situ fractionation, the following steps occurred: (3) the onset of silicate–silicate immiscibility and the consequent loss of buoyant and immiscible Si-rich melt; (4) dissolution of unprotected droplets of sulphide melt present in the Fe-rich mush melt; (5) compaction-driven upwards loss of residual mush melt enriched in, for example, Au. The LPGEM preserves upward increasing bulk Pd/Pt (∼6–13) owing to a continued supply of PGE and Au, with high Pd/Pt. The further development of the LPGEM ceased as the supply of precious metals to the floor waned. The Upper PGE Mineralization (UPGEM) subsequently formed from precious metals recycled in the floor. The UPGEM is characterized by increasing Au substitution in PGE phases, and a decrease in total PGE and Pd/Pt owing to upward fractionation in migrating mush melts and exhaustion of Pd and Pt. An upper Au-rich mineralization level (UAuM) was caused by late remobilization of Au and deposition on grain boundaries in fully crystallized gabbro. Cu concentrations (∼150 ppm) are not correlated with PGE and Au. Repeated Cu mineralization levels (CuM), attaining >1000 ppm, occur above the Au levels, caused by local mush layer sulphide saturation. PGE, Au and Cu distributions in the floor mineralization reflect sub-liquidus, but supra-solidus, processes and reactions in mushes at the roof, wall and floor. Constraints provided by a new model for the mineralization provide the basis for re-evaluation of the solidification processes in the Skaergaard intrusion. We have identified the importance of extensive in situ fractionation and intrusion-wide elemental redistributions in immiscible Fe- and Si-rich silicate melts. Our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards-increasing proportion crystallized from Fe-rich, immiscible mush melt. The roof-rocks are complementary to the floor, with downwards increasing proportions crystallized from the conjugate Si-rich melt. Petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones. Bulk melt remaining in the magma chamber evolved not, as generally assumed, as a result of loss of crystals grown from the bulk melt, but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction. Redistribution of Fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates.

The Chapada Cu–Au deposit, Mara Rosa magmatic arc, Central Brazil: Constraints on the metallogenesis of a Neoproterozoic large porphyry-type deposit

The large Chapada Cu–Au deposit is situated in Goiás Magmatic Arc, Central Brazil, which is part of an accretionary orogen with tectonic evolution between ca. 900 and 600Ma during the Brasiliano Orogeny. With a current reserve (proven and probable) of 420.94 Mt Cu (0.29%) and 479.84 Mt Au (0.25g/t) or 3.83 Moz Au, the Chapada Cu–Au deposit is an important example of a large-tonnage, low-grade porphyry copper–gold deposit of Precambrian age, in which the mineralization is disseminated and hosted mainly by biotite-rich schists and gneisses.In this work, we propose a two-stage genetic model for the Chapada deposit that includes an early porphyry-type Cu–Au event overprinted by a shear zone-hosted Cu–Au event. The first mineralization event represents a rare case of metamorphosed Neoproterozoic porphyry-type deposit which preserves distinct hydrothermal alteration–mineralization zoning developed at the interface between equigranular (867±8Ma) to porphyritic (884±15Ma) medium- to high-Kcalc-alkaline dioritic rocks and mafic to intermediate metavolcanic and metapyroclastic rocks. The εNd values are positive (from +2.17 to +5.39), thus indicating the juvenile nature of the original rocks for both metavolcano-sedimentary and plutonic rocks. In this context, kyanite-rich rocks are interpreted as products of metamorphosed argillic hydrothermal alteration halo and occur as linear alignments oriented parallel to the Mara Rosa magmatic arc trend. The potassic hydrothermal alteration halo, which includes the main host rocks of the Chapada deposit, is characterized by biotite schist, muscovite–biotite schist, amphibole–biotite schist and biotite gneiss. The ore is represented predominantly by platy deformed chalcopyrite and pyrite disseminations along foliation plans (or axial surfaces of folds) and, to a lesser extent, by small massive concentrations in fold hinges. The ore shows a systematic oxide/sulfide zonation, including the peripheral pyritic shell and the central magnetite-rich zone. In general, the geological context of the deposit lacks preserved stocks, dykes and breccias, while deformed A-type quartz veins/veinlets are abundant.The second mineralization event represents an epigenetic shear zone-hosted Cu–Au mineralization associated with Dn+1 and occurred between 630 and 580Ma during the late orogenic to post-orogenic stage of the Mara Rosa magmatic arc, which may be interpreted as i)a shear zone-hosted Cu–Au remobilization; ii) an orogenic Cu–Au mineralization, or iii) an intrusion-related Cu–Au mineralization. In the latter case, the mineralization could be related to the ilmenite-bearing younger metadiorite (634.8±6.8Ma) with disseminations of chalcopyrite, pyrite and pyrrhotite.The narrow range of pyrite and chalcopyrite δ34S values close to 0‰ (between −2.90 and +0.76‰) indicates that both the porphyry copper–gold mineralization and the epigenetic mineralization were generated by sulfur of magmatic origin.

Geology, geochemistry and metallogenesis of the Selinsing gold deposit, central Malaysia

Malaysia is endowed with a number of major sediment-hosted, orogenic gold deposits that lie parallel to but east of the Bentong-Raub Suture Zone, which runs approximately N–S in central Malaysia. The Selinsing gold deposit is one of the major economic deposits in central Malaysia and is hosted by (meta)-sedimentary units. The deposit is affected by later deformation and metamorphism during the Triassic due to the collision of the Sibumasu Terrane with the Sukhothai Arc. Laser ablation ICP-MS U–Pb zircon dating of the host rocks indicates a Carboniferous (ca. 331–300Ma) maximum depositional age. Ore minerals include pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, pyrrhotite and gold. Detailed textural and geochemical studies revealed five distinctive pyrite types, sub-rounded framboidal pyrite (Pyrite 1), rounded recrystallized pyrite (Pyrite 2), anhedral inclusion-rich pyrite (Pyrite 3), anhedral to subhedral clean pyrite (Pyrite 4) and coarse euhedral clean pyrite (Pyrite 5). Laser ablation ICP-MS trace element analysis of the framboidal pyrite indicates enrichment in V, As, Mo, Ni, Se, Te, Ag, and Zn relative to the other pyrite types. Pyrite sulfur isotope composition (1.2–9.2‰) suggests a likely mixture of magmatic and local sedimentary source for the sulfur. Pyrite Pb isotope results (Pb206/Pb204: 18.94–19.10; Pb207/Pb204: 15.72–15.73) suggest a high-U source rock for the lead. Fluid inclusions are CO2-rich (95–100mol%) and yielded homogenization temperatures from 194 to 348°C and salinities between 1.23 and 9.98wt.% NaCl equiv. suggesting involvement of metamorphic fluids during ore formation. Based on these results, we suggest a two-stage model for the formation of the gold deposit at Selinsing. Stage 1 consists of early enrichment of the elements Au, V, As, Mo, Se, Te, Ni, Ag, and Zn in framboidal pyrite in the sediments, whereas Stage 2 is characterized by multiple post-diagenetic deformation and metamorphic processes, favoring the development of fluid pathways and Au remobilization into later pyrites. Regionally, the Selinsing gold deposit shares geological similarities (in term of host rocks, ore mineralogy and geochemistry) with many other sediment-hosted, orogenic gold deposits in mainland SE Asia such as the Sepon gold deposit (Laos), Langu gold deposit (Thailand), Modi Taung and Meyon gold deposits (Myanmar), and Phuoc Son gold deposit (central Vietnam). We suggest that our two-stage model may have important implications to mineral exploration for future research on sediment-hosted/orogenic gold deposits in SE Asia.

Supergene Alteration of Primary Ore Assemblages from Low-Sulfidation Au-Ag Epithermal Deposits at Pongkor, Indonesia, and Nazareño, Perú

The textures and mineralogical assemblages of high-grade ore samples from the low-sulfidation Au-Ag epithermal deposits of Pongkor (2 Ma; West Java, Indonesia) and Nazareno (10 Ma; Cordillera Shila, southern Peru) were studied to investigate processes related to post-depositional alteration of primary mineral parageneses. Both deposits are characterized by a late primary high-grade base and precious metal stage composed of chalcopyrite-sphalerite-galena-pyrite–polybasite-pearceite–tennantite-tetrahedrite–electrum. Minerals of the polybasite-pearceite series, the main Ag carriers, were found to be transformed into acanthite. Acanthite was also found locally in association with native silver, as a replacement product of base metal sulfides, as veinlets, alternating rhythmically with supergene iron oxyhydroxides replacing pyrite, as well as associated with barite, iron oxyhydroxides and covellite as boxwork fillings. Primary electrum grains with a composition of about Au0.6Ag0.4 are partially replaced by Au-Ag sulfide, locally identified as uytenbogaardtite (Ag3AuS2), and/or by acanthite, with both of the replacing minerals commonly associated with vermicular electrum of a greater fineness (up to Au0.8Ag0.2). The occurrence of the replacement textures near the present-day surface and the associated secondary minerals, strongly suggests a supergene origin for the Au and Ag sulfide phases. The comparison of Pongkor and Nazareno also suggests that supergene remobilization of Au and Ag is geochemically significant and can occur in widely differing morphological and climatic contexts.

Hydrogeochemistry in the Yilgarn Craton

The hydrogeochemistry of the Yilgarn Craton and its margins has been extensively investigated, with particular emphasis on the chemistry of Au. Four groundwater regions have been delineated based on variations in salinity, acidity and oxidation potential: (1) Northern (N Yilgarn and margins) – Fresh and neutral, trending more saline in the valley axes; (2) Central – Neutral and brackish (commonly <1% TDS) to saline (about 3% TDS), trending to hypersaline (10–30% TDS) at the salt lakes, with common increases in salinity with depth; (3) Kalgoorlie – Commonly acid (pH 3–5), except where buffered by extremely alkaline materials (e.g. ultramafic rocks), and saline within the top part of the groundwater mass, trending more neutral (pH 5–7) and hypersaline at depth and within a few kilometres of salt lakes; and (4) Eastern (E Yilgarn and Officer Basin) – Saline to hypersaline, neutral to acid and reducing. Dissolved concentrations of many ions are low, due to the presence of lignites in the channel sediments. These regional variations have major effects on the concentrations of many elements. Aluminium, Li, Y, REE and U are dominantly controlled by pH and thus have higher concentrations in acid groundwaters, such as those in the Kalgoorlie region. Dissolved concentrations of Mn, Co, Ni, Cu and Zn are less closely correlated with acidity, and show scope for lithological discrimination, but there is no apparent relationship with Au mineralization. Dissolved Cr shows an absolute correlation with ultramafic rocks, apparently irrespective of pH, possibly due to its presence as chromate (i.e. Cr 6+ as CrO 4 2− ). Concentrations of As, Sb Mo, W and Bi are low in acid groundwaters, but are higher above pH 6.5, particularly in the Central region. Therefore, acid groundwaters (particularly in the Kalgoorlie district) will be poor media for the use of these elements as exploration pathfinders. Molybdenum differs from the other elements in this group in having significant concentrations in acid groundwaters, although lower than in neutral and alkaline groundwaters. Dissolved Au is commonly the best pathfinder for Au mineralization. It occurs dominantly as halides (chloride and/or iodide) and has enhanced concentrations (to >1 ppb) under the acid/saline/oxidizing conditions common in the Kalgoorlie region, whereas concentrations in the northern Yilgarn are two orders of magnitude less. This implies that supergene Au remobilization should be considerably less in the northern Yilgarn than in the Kalgoorlie region. Additionally, the threshold dissolved Au concentration as used for Au exploration differs significantly between regions.

New Evidence for Multiple Periods of Gold Emplacement in the Porcupine Mining District, Timmins Area, Ontario, Canada

The Porcupine mining district is the largest lode gold-producing district in North America, with production in excess of 62,000,000 troy oz. Nearly all of the Au has come from quartz-carbonate lode systems hosted by metamorphic rocks of greenschist facies. Field relationships and radioisotope ages of intrusions indicate that gold emplacement occurred over a span of at least 7 m.y. Depositional events that predated formation of the Timiskaming unconformity surface are manifested by auriferous clasts of detrital pyrite, banded quartz sulfide, and quartz sulfide in basal Timiskaming conglomerate. Pyritic clasts contain on average 2.54 ppm Au and are conclusive evidence for the existence of pre-Timiskaming-aged auriferous pyritic deposits. These clasts may have been derived from pyritic bodies in carbonaceous argillites of the Tisdale Group, from sulfide facies iron-formation of the Deloro Group, or from pyritic deposits of unknown genesis. Auriferous banded sulfidic quartz clasts were probably derived from iron-formation of the Deloro Group and auriferous sulfidic quartz clasts possibly from epigenetic quartz sulfide veins. Intrusion-related and post-Timiskaming gold deposition is evidenced by Cu-Au-(Mo) mineralized porphyry and younger quartz-carbonate gold veins. The magmatism that emplaced feldspar and quartz-feldspar porphyries was accompanied by Cu-Au-(Mo) mineralization at the Dome mine, where a conformable quartz-feldspar porphyry intrusion and its host greenstones are pervasively and atypically copper and gold mineralized, and the porphyry is anomalously molybdenum enriched. A new U-Pb zircon date on this mineralized porphyry fixes its emplacement and the maximum age of Cu-Au-(Mo) deposition at 2688 ± 2 Ma. This mineralization was overprinted by later gold mineralization in the form of planar, sheeted, and en echelon auriferous quartz veins and by mineralized shear and/or fault zones. Field relationships indicate that these veins significantly postdate emplacement of the porphyries and Timiskaming sedimentation. The latest vein systems form the bulk of the ore at the Dome mine and are similar to the most economically significant veins of the district. The largest gold deposits of the district are spatially associated with, but not in, porphyries similar to those exposed at the Dome mine. This association has led to considerable speculation regarding the genetic relationship of felsic porphyry emplacement to ore formation. The results of this study show that although some Au was deposited during a Cu-Au-Mo event temporally linked to porphyry emplacement, most of the Au at the Dome mine was emplaced significantly later than the porphyries. The documentation of at least three broad periods of Au emplacement, at least two of which are premetamorphic, is permissive and supportive of genetic hypotheses that invoke metamorphogenic remobilization of Au from earlier formed deposits. Alternatively, the coincidence of early gold enrichments and later economic concentrations may reflect a single, unidentified gold source from which successive but unrelated hydrothermal activity mobilized, transported, and deposited Au, with no direct genetic link between early and late episodes of gold deposition.

Geochemistry and exploration significance of ironstones and barite-rich rocks in the Proterozoic Willyama Supergroup, Olary Block, South Australia

The Proterozoic Willyama Supergroup in the Olary Block, South Australia, contains a number of occurrences of ironstones and barite-rich rocks. The barite-rich rocks are present as lenticular horizons, which grade laterally and vertically into quartz-magnetite ± hematite and magnetite-hematite rich iron-formations with enclosing quartz-albite rocks. The stacking of these iron-formations and barite-rich rocks are interpreted as facies changes in the original sedimentary environment, competing volcanogenic and chemical sedimentation and multiple hydrothermal fluid events resulting in the deposition of different hydrothermal products. Epigenetic ironstones may have formed by replacement of iron-formations and associated rocks due to oxidizing, saline hydrothermal fluids causing Fe 3+, Cu and Au remobilization during and/or after prograde metamorphism. Barite-rich rocks, ironstones and iron-formations are characterized by exceptionally low Pb and Zn values, however, scattered elevated Cu and Au concentrations point to a potential for stratabound Cu-Au mineralization. Localized manganiferous iron-formations are characterized by the presence of Fe-Mn garnet, F-apatite, Mn-grunerite and Mn-fayalite and elevated concentrations of As, Au, Mn, P, U and Zn. They represent pure chemical precipitates from high-temperature hydrothermal fluids and thus have mineralogical and geochemical characteristics similar to those of banded iron-formations associated with stratiform Pb-Zn-Ag sulfides in the adjacent Broken Hill Block.

Mineralogy, morphology and chemistry of gold in the stone line lateritic profile of the Posse deposit, Central Brazil

Primary Au mineralization at Posse is hosted within felsic meta-volcanic rocks of the Proterozoic Mara Rosa volcano-sedimentary sequence. The weathered zone above ore is characterized by a degraded laterite profile dominated by a saprolitic zone above which is a stone line rich in quartz and pisoliths. More than 80% of the Au in the primary ore is present as native Au intermixed with telluride minerals and the distribution of Ag within the Au grains is heterogeneous. During weathering telluride minerals are readily oxidized leaving a Fe-oxide material intermixed with unaltered native Au. Both mechanical and chemical processes have contributed to the redistribution of Au. The role of organic and hydroxy complexes are thought to be important in the chemical remobilization of Au near the surface. Reprecipitation of Au takes place in the saprolite resulting in Au grains with well-developed crystalline morphology. The fineness of these grains is similar to that of primary Au but the distribution of Ag is typically homogeneous. Gold has also been reprecipitated, especially in the stone line, by adsorption onto iron-oxides, causing Au enrichment in the coarse fractions. The stone line and topsoil are dominated by relatively large primary and reprecipitated Au grains, due to the preferential mechanical removal of the fine-grained Au by surficial erosion.

Geology, geochemistry, and origin of high sulfidation Cu-Au mineralization in the Nansatsu District, Japan

The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of 95 wt percent SiO 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO 2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite+ or -pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines. Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization. Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (delta 18 O = 7+ or -2 per thousand, delta D = -25+ or -5 per thousand, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a delta 18 O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the delta 18 O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCl equiv (some inclusions contain daughter minerals of halite and sulfur). The T h values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees C at Akeshi to about 200 degrees C at Iwato and 230 degrees C at Kasuga. The presence of vapor-rich inclusions, some with T hv similar to T hl , indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200 degrees to 300 degrees C, AuCl 2 (super -) complexing may dominate over HAu(SH) 2 at salinities above about 2 wt percent NaCl. Several conditions are conducive for high sulfidation mineralization to occur: (1) a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid, (2) the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich, (3) the gas-rich (HCl and SO 2 +H 2 S) vapor ascends to the surface with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and (4) the dense, metal-bearing fluid also ascends into this leached zoned and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water. If the saline liquid is not released from its source adjacent to the magma, due to lack of fracturing, or if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached barren rock which is characteristic of many eroded volcanic terranes.