Alteration Styles Research Articles

New stratigraphic and genetic model for the dolomitic Cretaceous Pinda reservoirs in Angola. Part II - Compelling arguments against early dolomitization and early leaching

This is the second contribution to the stratigraphic and genetic model of the Pinda Group. The mid-Cretaceous Pinda Group of the Congo basin in the northern Angolan offshore is a transgressive supersequence. Its lower part is made of Bufalo and Pacassa facies that correspond respectively to mixed and calcareous-dolomitic grain-dominated fabrics. Both facies were strongly diagenetically altered, mostly by dolomitic replacement and leaching. Contrary to opinions expressed by other authors that both styles of alteration are early diagenetic phenomena, there is substantial evidence for a burial origin with dolomitic replacement and leaching of calcite having started at depths not less than 150 m and 400 m, respectively.

New stratigraphic and genetic model for the dolomitic Cretaceous Pinda reservoirs in Angola Part II – Compelling arguments against early dolomitization and early leaching

This is the second contribution to the stratigraphic and genetic model of the Pinda Group. The mid-Cretaceous Pinda Group of the Congo basin in the northern Angolan offshore is a transgressive supersequence. Its lower part is made of Bufalo and Pacassa facies that correspond respectively to mixed and calcareous-dolomitic grain-dominated fabrics. Both facies were strongly diagenetically altered, mostly by dolomitic replacement and leaching. Contrary to opinions expressed by other authors that both styles of alteration are early diagenetic phenomena, there is substantial evidence for a burial origin with dolomitic replacement and leaching of calcite having started at depths not less than 150 m and 400 m, respectively.

Evaluating the fidelity of the cerium paleoredox tracer during variable carbonate diagenesis on the Great Bahamas Bank

Abstract Inferring redox conditions for ancient marine environments is critical to our understanding of biogeochemical cycles over Earth history. Because of the redox sensitivity of cerium (Ce) relative to other rare earth elements (REEs) and its uptake in marine carbonates, the Ce anomaly (Ce/Ce*) is widely applied to ancient carbonates as a proxy for local redox conditions in the water column. However, carbonate sediments and rocks are particularly vulnerable to multiple stages and styles of post-depositional diagenetic alteration where the diagenetic redox conditions and fluid compositions can vary widely from overlying seawater. Evaluations of the effects of this post-depositional alteration for the Ce anomaly have mostly been limited to ancient carbonate rocks rather than recent, well-characterized analog facies. Here, we report on analyses of REE plus yttrium concentrations (REY) and Ce anomalies in bulk carbonate samples from drill cores collected in the Bahamas (Clino and Unda) that allow us to track loss or retention of primary signals of initial oxic deposition through a range of subsequent alteration scenarios mostly under anoxic conditions. Specifically, these materials have experienced well-constrained overprints linked to meteoric processes and marine burial diagenesis, including dolomitization. Our results show that, regardless of mineralogy, diagenetic fluid composition, and redox state, the REY patterns in these carbonates, including the Ce anomaly, are similar to those of modern oxic seawater, indicating that they likely record the seawater signatures of primary deposition. As such, the Ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even when subject to multiple stages and styles of diagenetic alteration, confirming its utility in studies of ancient marine redox.

Intense aqueous alteration on C-type asteroids: Perspectives from giant fine-grained micrometeorites

This study explores the petrology of five giant (>400 μm) hydrated fine-grained micrometeorites from the Transantarctic Mountain (TAM) micrometeorite collection. For the first time, the extent and mechanisms of aqueous alteration in unmelted cosmic dust are evaluated and quantified. We use a range of criteria, previously defined for use on hydrated chondrites, including phyllosilicate fraction, matrix geochemistry and micro textures. Collectively, these micrometeorites represent ∼2.22 mm2 of intensely altered hydrated chondritic matrix (with petrologic subtypes of <1.2 in the scheme of Howard et al. (2015)) and reveal a range of alteration styles. Two particles are found to contain pseudomorphic chondrules with thick fine-grained rims, while another micrometeorite contains several aqueously altered CAIs. Their outlines range from well-defined to indistinct, demonstrating that the advanced stages of aqueous alteration progressively remove evidence of coarse-grained components. The remaining two micrometeorites entirely lack coarse-grained components but are similarly altered. Thus, the combined chondrule-to-matrix ratio among these giant micrometeorites is extremely low (6.45 area%), and significantly below the average ratio found in typical CM or CR chondrites (∼20%, Weisberg et al., 2006). Our findings are consistent with previous analyses from smaller Antarctic micrometeorites, which suggest that chondrules (and CAIs) derived from hydrated carbonaceous chondrite parent bodies are underrepresented among the micrometeorite flux, even when considering contributions from coarse-grained micrometeorites. Therefore, to explain the relative paucity of anhydrous material, we propose that the flux of fine-grained micrometeorites is primarily derived from intensely aqueously altered, primitive C-type asteroids, which have lost the majority of their refractory coarse-grained components by replacement with secondary phyllosilicate minerals.

The fluid evolution of the Nimbus Ag-Zn-(Au) deposit: An interplay between mantle plume and microbial activity

The Nimbus Ag-Zn-(Au) volcanic-hosted massive sulfide (VHMS) deposit represents an exceptional orebody in the Yilgarn Craton of Western Australia. It is located in a zone of juvenile crust running N-S through the Eastern Goldfields Superterrane. However, unlike most other VHMS occurrences associated with the 2690–2680 Ma rift sequence (Teutonic Bore, Jaguar, Bentley, Erayinia), it is hosted by the ca. 2705 Ma plume-related stratigraphy, which is more typically associated with komatiite-hosted nickel-sulfide mineralization. The Nimbus deposit displays hybrid VHMS-epithermal characteristics resulting from low temperature and shallow water conditions, which developed a quartz-carbonate-sericite dominated alteration assemblage in the dacite host rocks. Sulfide mineralization comprises pyrite, sphalerite, galena, arsenopyrite, Ag-Sb-Pb-Bi sulfosalts, and rare chalcopyrite. In this study, we applied a combination of in-situ analytical techniques to monitor the evolving sulfur isotope signature of the mineralizing fluids, and propose that the Nimbus VHMS deposit developed as a result of a bimodal fluid history. In the first stage, the hydrothermal system – powered by plume-related magmatism – deposited a series of barren pyrite lenses with colloform textures. Their variable but consistently negative mass-independent sulfur isotope fractionation signature (Δ33S = −0.81%) reflects the interaction between mantle-derived magmatic fluids and Archean seawater. The ubiquitous presence of carbon-rich porous textures and the remnants of carbonaceous “nests” in colloform pyrite also indicate a significant contribution of microbial sulfate reducers during the incipient stages of sulfide precipitation. The gradual formation of colloform lenses eventually sealed the hydrothermal system establishing a physical barrier that limited the interaction with seawater. This process promoted the onset of higher temperature and pressure conditions required to form the high-grade Zn-Ag mineralization, sourcing sulfur and fluids largely from a magmatic reservoir (Δ33S = +0.09%). Whereas the identification of a predominantly magmatic sulfur source is supported by sulfur isotope signatures, the recognition of a magmatic origin of the fluid may only be inferred indirectly. It is consistent with the observed quartz-sericite alteration style and the proposed mechanism of sulfide deposition via decompression, which does not require any interaction with seawater. The trace element distribution of the ore-related sulfides also supports a closed hydrothermal system developed through multiple fluid pulses. Indeed, this ore-forming scenario is commonly favored by high confining pressures, which at Nimbus were established following the deposition of the colloform pyrite lenses that progressively sealed the hydrothermal system creating the favorable conditions for mineralization.

Influence of weathering and hydrothermal alteration on the REE and δ56Fe composition of iron formation, Cauê Formation, Iron Quadrangle, Brazil

Continental margin (Superior-type) iron formation of the Cauê Formation, Iron Quadrangle, Brazil, is composed of a laminated, magnetite-rich chemical meta-mudstone that was altered during hydrothermal circulation of basinal fluids, greenschist to amphibolite facies metamorphism, and subsequent supergene enrichment of Fe. The absence of traction current deposits and storm-generated structures suggests that deposition occurred on the distal shelf, possibly as deep as the continental slope. Paragenesis-based lithogeochemical analyses of two drill cores with contrasting alteration styles highlight the importance of hydrothermal, metamorphic, and supergene processes on the rare-earth element plus yttrium (REE + Y) and δ56Fe compositions of iron formation. The first drill core penetrates to a depth of 540 m in the Ouro Fino Syncline (OFS) where lithofacies have experienced intense hydrothermal and supergene alteration. The second core is from the Gandarela Syncline (GS) and contains 450 m of metamorphosed iron formation that has undergone later supergene enrichment of Fe.Petrographically, euhedral to subhedral magnetite in the OFS and GS is fabric destructive, and depending on the style and degree of alteration, can co-occur with dolomite, siderite, goethite, and/or martite. Synsedimentary magnetite and other primary Fe-oxide phases are absent. The effect of supergene enrichment and weathering is best captured along the depth profile of each core by the inverse relationship between SiO2 and Fe2O3; SiO2 values are lowest and Fe2O3 values highest near the surface. The presence of goethite to 500 m records the maximum depth of these processes in the subsurface. The REE + Y, and Y anomalies from the deepest and least altered lithofacies in both cores are typical of deposition in a redox-stratified water column. The weathered lithofacies show several changes in the REE + Y patterns, Ce anomalies and absence of Y anomalies, caused mainly by fluid percolation.The change in Fe isotopic composition with depth is more complex. In drill core from the OFS, negative δ56Fe values shift from −0.91‰ at depth to less negative values of −0.6‰ near the surface. The most altered samples do not lie on this trend because they are characterized by positive values of 0.30‰ and 1.33‰. The δ56Fe values from the GS increase linearly from −0.11‰ to 1.23‰ towards the surface with the most negative values −0.7‰ and −1.16‰ near the top. The most negative values from the OFS could reflect synsedimentary microbial dissimilatory Fe reduction, and the positive values from the GS may be related to partial Fe2+ oxidation between the hydrothermal Fe source and Fe-(oxyhydr)oxides. However, when interpreted together with mineralogical and REE data, these data likely record the various alteration processes that affected the Cauê Iron Formation. Such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and Fe ore-forming processes.

Controls on the formation of the Changgou gold deposit and implications for exploration within the North Hanyin area, South Qinling Mountains, China

This paper focuses on the area around the Changgou gold mine within the North Hanyin area of the South Qinling Mountains, China. We combine new large‐scale field geological, tectonic, and lithofacies data with the results of previous research and present new information on the geological characteristics of mineralization within this region. These data indicate that the Changgou gold mine is a brittle–ductile shear zone associated gold deposit and that the key controls on the formation of the deposit can be used to identify areas in this region that are favourable for future exploration. The key control on the spatial distribution of mineralization within the Changgou gold mine area is the location of the DSZ3 brittle–ductile shear zone, with secondary faults and fractures controlling the specific locations of gold orebodies. The structural deformation is dominated by the development of an S2 foliation that strikes 290–350° (predominantly inclination 230° and dip 30°). Gold‐bearing pyrite is preferentially aligned along the S2 foliation, and individual orebodies are preferentially located along the S2 foliation. Therefore, this foliation controlled the location of mineralization at both micro‐ and macroscopic scales. Mineralization also occurs in S2‐phase quartz veins that contain visible granular gold, whereas the S3‐phase quartz veins are generally barren and are not related to the location of gold mineralization. Fluid inclusion microthermometry indicates that gold‐bearing quartz veins homogenized at 190–260°C and have salinities (in NaCl equivalent terms) of 6–12%. All of these fluid inclusions contain gas and liquid phases dominated by CO2 and H2O, respectively, indicating that they are CO2–H2O type. The hydrothermal fluids that formed the Changgou deposit are similar to those that generate epithermal deposits (i.e., moderate‐ to low‐temperature, moderate‐ to low‐salinity, and low‐density CO2–H2O fluids). Our research into the geological characteristics of the quartz veins that formed at different times in the study area and their associations with different styles of mineralization and alteration indicates that the hydrothermal fluids associated with the S2 period of deformation were the main ore‐forming fluids associated with the Changgou gold deposit.

Trace Element Analysis of Pyrite from the Zhengchong Gold Deposit, Northeast Hunan Province, China: Implications for the Ore-Forming Process

The Zhengchong gold deposit is located in the central segment of the Jiangnan Orogen in northeastern Hunan Province, South China. The host rocks of this deposit are the Neoproterozoic slates of the Lengjiaxi Group and granodiorite. The structures in the Zhengchong gold deposit are dominated by NE-trending reverse faults, which control the gold-bearing veins. The orebody consists of NE-trending laminated quartz veins and NW-trending quartz veins. The alteration styles include silicification, carbonatization, sulfidation, sericitization and chloritization. The Zhengchong gold mineralization can be divided into four stages: Quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV). Three generations of hydrothermal pyrite were identified: Disseminated euhedral to subhedral cubes in altered wall-rock (PyI), euhedral to subhedral cubes inter-grown with arsenopyrite and tetrahedrite in quartz veins and wall-rock (PyII), and euhedral cubes with microinclusions (native gold, galena, sphalerite, chalcopyrite, tetrahedrite, and pyrrhotite) or metasomatic textures in sulfide-rich veins or veinlets (PyIII). PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals. PyI records the lowest concentrations of Au; PyII and PyIII record similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurs by direct-ion exchange. Invisible gold is uniformly distributed within the arsenian pyrite, and visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure. There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitution that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (&lt;1) and the decrease in the Se/Te ratio (&lt;10) in stage III, indicating that a more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.

The Shah-Ali-Beiglou Zn-Pb-Cu (-Ag) Deposit, Iran: An Example of Intermediate Sulfidation Epithermal Type Mineralization

The Shah-Ali-Beiglou epithermal base metal-silver deposit is located in the Tarom-Hashjin metallogenic province (THMP) in northwestern Iran. This deposit is hosted by quartz monzonite dikes of Oligocene age and surrounded by andesite to trachyandesite volcanic and volcaniclastic rocks of Eocene age. The subvolcanic rocks in the study area vary in composition from quartz-monzonite to monzonite and have metaluminous, calc-alkaline to shoshonitic affinity. These rocks have I-type geochemical characteristic and are related to post-collisional tectonic setting. The mineralization occurs as NE-SW and E-W-trending brecciated veins controlled by strike-slip and normal faults, which are associated to the Late Oligocene compressional regime. The mineral paragenesis of the vein mineralization is subdivided into pre-ore stage, ore stage, post-ore stage, and supergene stage. Pre-ore stage is dominated by quartz, sericite, and subhedral to anhedral pyrite as disseminated form. Ore-stage is represented by quartz, sphalerite (from 0.1 mol % to 4 mol % FeS), galena, chalcopyrite, tetrahedrite-tennantite, minor seligmannite and enargite, as vein-veinlet, cement and clast breccias. Post-ore stage is defined by deposition of quartz and carbonate along with minor barite, and supergene stage is characterized by bornite, chalcocite, covellite, hematite, goethite, and jarosite. The ore mineralization is associated with the silicic alteration. The styles of alteration are silicic, carbonate, sericitic, chloritic, and propylitic. Fluid inclusions in sphalerite have a wide range of salinities between 0.35 wt % and 21.4 wt % NaCl equivalent and homogenization temperatures range from 123 to 320 °C. The isotopic values of sulfides vary from 2.8‰ to 6.7‰ suggesting a magmatic source for the sulfur. In the present study, based on geological setting, alteration style of the host and wall rocks, main textures, mineral assemblages, composition of ore minerals, and structural features, it is suggested that the mineralization in the Shah-Ali-Beiglou is similar to intermediate-sulfidation style of epithermal deposits.

The role of hydrothermal alteration in tungsten mineralization at the Dahutang tungsten deposit, South China

The giant Dahutang tungsten deposit has total reserves of more than 1.31 Mt of WO3 with a scheelite/wolframite ratio of ∼1 and is mainly hosted by the Neoproterozoic Jiuling granodiorite batholith (∼820 Ma). The deposit is characterized by four types of alteration, including biotite alteration, phyllic alteration, greisenization and silicification. Whole-rock geochemical analyses showed that the elements Ti, Ni, V, Sc, and Lu exhibited immobility during the four alteration processes. The mobile element geochemistry effectively differentiated among four distinct hydrothermal alteration styles. During the biotite mineralization, there were mass gains in Al2O3, Fe2O3, MnO, MgO, K2O, P2O5, and W and depletions in SiO2, CaO, and Na2O. The phyllic alteration exhibited mass gains in SiO2, Fe2O3, MgO, and W and depletions in CaO, Na2O, and K2O, but Al2O3, MnO, and P2O5 were immobile. The weak greisenization exhibited mass gains in SiO2, Fe2O3, K2O, P2O5 and W and depletions in Na2O, MgO, and CaO, whereas Al2O3 and MnO remained immobile. The silicification exhibited mass gains in SiO2 and W and depletions in Al2O3, Fe2O3, MgO, CaO, Na2O, and K2O, but MnO exhibited immobility. These alterations were related to at least three major hydrothermal fluid systems. Firstly, a hydrothermal fluid caused biotitization zones and Fe + Mn ± W mineralization (mostly biotite) at temperatures ranging from 560 to 450 °C, and the magmatic hydrothermal fluids were derived from the Jurassic porphyritic biotite granite (∼150 Ma) and characterized by alkaline, oxidized, moderate-pressure, and low-salinity features. Secondly, a hydrothermal fluid was formed by the mixing of magmatic fluid with meteoric water and was responsible for the phyllic/weak greisenization alteration zones at temperatures ranging from 440 to 450 °C and characterized by alkaline, weakly oxidized, moderate-pressure, and low-salinity features. Thirdly, a hydrothermal fluid caused greisenization and silicification zones at temperatures ranging from 440 to 160 °C, which were characterized by acidic, reduced, high-pressure, and moderate- to low-salinity features, derived from the Cretaceous fine-grained biotite granite (∼144 Ma), and associated with the main W mineralization at Dahutang.The characteristics of Fe-enriched biotite in the biotitized and greisenized rock – decreasing temperature, high pressure, and high log(fH2O/fHCl) – could facilitate the formation of scheelite deposits, but the high F suppressed it. The decreasing temperature and f(O2) and the Fe-Mn released from biotite contributed to the formation and precipitation of wolframite. During alkaline alteration, biotite and apatite acted as storage places for Fe-Mn and Ca, which were subsequently released by acidic alteration to form scheelite and wolframite. There was no enough Ca was released from biotitized rock for the acidic alteration to provide the WO42− to form scheelite. Meanwhile, Fe was added, and Mn was released to form the scheelite + wolframite deposit. Based on all the results, we have developed a genetic model for tungsten mineralization including superimposed alteration processes (alkaline overprinted by acidic alteration), which led to the gain and loss of elements, and corresponding to two magmatic events, the Jurassic porphyritic biotite granite and the Cretaceous fine-grained biotite granite. The superposition of alterations played an important role in the mineralization of the Dahutang giant tungsten deposit.

The early Cretaceous Mansourabad shale-carbonate hosted Zn–Pb (-Ag) deposit, central Iran: An example of vent-proximal sub-seafloor replacement SEDEX mineralization

The Mansourabad stratiform and stratabound Zn-Pb (-Ag) deposit is hosted by an Early Cretaceous siliciclastic-carbonate sequence deposited in southern Yazd basin, Iran. Mineralization is mainly hosted in silty limestone and dolomite. Based on petrographic studies three sulfide ore facies can be distinguished: feeder zone, bedded and massive-replacement ore facies. Textures include replacement, breccia, massive, framboidal, laminated and vein-veinlet. Sulfide minerals are predominantly sphalerite, galena, and pyrite. Dolomite and quartz are the major non-sulfide gangue minerals. Based on microscopic studies, sulfide mineralization in the Mansourabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as lamina and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet’s textures, and are considered to form by replacement during sub-seafloor fluid flow. Dolomitization and silicification are the major hydrothermal alteration styles in the Mansourabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids.Calcite minerals from the feeder zone and massive-replacement ore facies have 115–250 °C trapping temperature with salinities ranging from 3.55 to 19.82 eq. wt.% NaCl eq. δ34S values of pyrite, sphalerite and galena range from −18.8 to +3.2‰. The highest δ34S values correspond to the feeder zone (+3.2 to +2‰), and the lowest to the bedded ore facies (−18.8 to −9.9‰) and the massive ore (+2.6 to − 4.7‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulfides formed from the reduction of seawater sulfate by bacteriogenic sulfate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulfate reduction likely played an important role in the feeder zone. The δ13CPDB and δ18OSMOW values of host limestone’s and hydrothermal dolomites range from +2.7‰ to +2.0‰, +8.6‰ to +9.2‰, +1.5‰ to +2.8‰ and +10.6‰ to +13.2‰, respectively, and plot the marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks.Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia’s and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor and on the seafloor processes. S, C and O isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal sub-seafloor replacement SEDEX ore deposit.

Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks

The sequence of replacement in groundmass perovskite and spinel from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India) has been established. Two types of perovskite occur in the studied Indian kimberlites. Type 1 perovskite is found in the groundmass, crystallized directly from the kimberlite magma, it is light rare-earth elements (LREE)-rich and Fe-poor and its ΔNNO calculated value is from −3.82 to −0.73. The second generation of perovskite (type 2 perovskite) is found replacing groundmass atoll spinel, it was formed from hydrothermal fluids, it is LREE-free and Fe-rich and has very high ΔNNO value (from 1.03 to 10.52). Type 1 groundmass perovskite may be either replaced by anatase or kassite along with aeschynite-(Ce). These differences in the alteration are related to different f(CO2) and f(H2O) conditions. Furthermore, primary perovskite may be strongly altered to secondary minerals, resulting in redistribution of rare-earth elements (REE) and, potentially, U, Pb and Th. Therefore, accurate petrographic and chemical analyses are necessary in order to demonstrate that perovskite is magmatic before proceeding to sort geochronological data by using perovskite. Ti-rich hydrogarnets (12.9 wt %–26.3 wt % TiO2) were produced during spinel replacement by late hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnets in the mineral sequence and their water content before using them to classify the rock based on their occurrence.

Geology and Geochronology of the Golpu Porphyry and Wafi Epithermal Deposit, Morobe Province, Papua New Guinea

The Wafi-Golpu district of Papua New Guinea contains contrasting high- and intermediate-sulfidation styles of epithermal veins and alteration that have overprinted and partially obscured the giant, high-grade Golpu porphyry Cu-Au deposit. The initial stages of magmatic-hydrothermal activity produced high-grade porphyrystyle assemblages that are zoned about the Golpu diorite intrusions, with most mineralization formed between 8.76 ± 0.02 and 8.73 ± 0.01 Ma. Porphyry mineralization was followed by the eruption of the Wafi diatreme, after which upward-widening zones of epithermal alteration and mineralization have overprinted both the mineralized Golpu diorites and the Wafi diatreme. Above the top of the Golpu porphyry, vein and alteration assemblages include quartz (locally vuggy), alunite, pyrophyllite, kaolinite, dickite, and diaspore, with high-sulfidation mineralization consisting of pyrite-covellite-chalcocite-tennantite ± enargite ± bornite that transitions outward to a distal assemblage of disseminated pyrite-sphalerite ± tennantite. A zone of intermediate-sulfidation epithermal carbonate-sulfide ± quartz ± adularia veins and montmorillonite-chlorite ± muscovite ± illite alteration occurs along the margins of the high-sulfidation domain. Most of the epithermal Au occurs near the transition from the high- to intermediate-sulfidation domains. New geochronological results indicate that the main stage of Golpu porphyry mineralization to the last stage of Wafi epithermal veins lasted between 120 and 220 k.y.

Geology and Isotope Geochemistry of the Wainaulo Cu-Au Porphyry Deposit, Namosi District, Fiji

The late Miocene, calc-alkalic, Wainaulo Cu-Au porphyry deposit of the Namosi district, Fiji, hosts distinct styles of alteration and mineralization that overlapped to produce a substantial porphyry Cu-Au resource. The early stages produced medium-grade Cu, low-grade Au and concentric calc-potassic to propylitic alteration that is zoned around the early-stage diorite intrusions. Discrete zones of high-grade Cu and Au and calc-sodic alteration were then superimposed during the intrusion of the main-stage quartz diorites and the formation of quartz-sulfide and epidote-sulfide veins. As the magmatic-hydrothermal system waned, lower Cu and Au grades, with a weaker intensity of calc-sodic alteration and lower density of veins, were produced coincident with emplacement of subsequent quartz diorite intrusions. Late-stage anhydrite-pyrite veins and chlorite-illite alteration overprinted the quartz diorite intrusive complex. The final hydrothermal event consisted of argillic alteration that was concentrated in and around steeply dipping, ENE-trending shears. These structures appear to have controlled the emplacement of the quartz diorite complex and distribution of high-grade Cu-Au mineralization, suggesting they were active during the pre- and synmineralization stages. Stable and radiogenic isotopic data provide evidence for direct seawater contributions to the magmatichydrothermal system. Measured δ34Ssulfide (–5.0 to 3.8‰) and δ34Ssulfate (9.0–16.8‰) values are consistent with a predominantly magmatic source, whereas an elevated bulk sulfur composition (6.7‰) suggests mixing with an isotopically heavy fluid (e.g., seawater). Estimates of δDfluid derived from epidote (–9.1 to 11.3‰) and δ18Ofluid from epidote and anhydrite (–0.2 to 4.7‰) approach that of Vienna standard mean ocean water, and the initial Sr isotope ratios of epidote (0.70364–0.70378) suggest a component of seawater Sr ranging from 3.2 to 5.8%. These results are consistent with the inferred submarine paleogeographic setting and may explain the abundance of albite- and epidote-rich alteration assemblages at Wainaulo.

Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines

The Boyongan and Bayugo porphyry Cu-Au mineral deposits, discovered under postmineralization cover during the previous decade, are part of an emerging belt of intrusion-centered Au-rich Cu mineral deposits and prospects in the Surigao district of northeast Mindanao, Philippines. Since their formation in the early Pleistocene, exhumation and weathering of these deposits have led to the development of a 600-m-thick oxidation profile at Boyongan and a modest (30–70 m) oxidation profile at Bayugo. Debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the weathered deposits, preserving their supergene profiles. The mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages. Three premineralization diorite porphyry stocks and a silt-sand matrix breccia complex represent early stages of magmatism and brecciation. Significant Cu and Au introduction followed these events and occurred in association with small early-mineralization diorite porphyry stocks at Boyongan and Bayugo. Within the diorite complex, the two mineral deposits are spatially distinct, separated by approximately 1 km of premineralization diorites. Inter- and late-mineralization intrusions were emplaced as the magmatic-hydrothermal system waned. A characteristic progression of vein and K silicate alteration styles affected each of the synmineralization intrusions. Rare comb quartz unidirectional solidification textures (stage 0) mark the transition from magmatic to hydrothermal conditions. Quartz-poor wispy magnetite-biotite-K-feldspar veinlets characterize stage 1. Stage 2 veins consist of quartz with selvage and/or disseminated magnetite or biotite and K-feldspar halos. Stage 3 quartz veins have K-feldspar halos but generally lack magnetite and biotite. Stage 4 veins consist of massive bornite-chalcopyrite and chalcopyrite-pyrite with K-feldspar halos. Stage 3 quartz veins and stage 4 sulfide veins host Cu-Au mineralization of the greatest volumetric significance, reflecting the general paucity of sulfide in the earlier vein stages. Despite the simplicity of this sequence, detailed paragenetic reconstructions reveal that this characteristic progression of veining and K silicate alteration was repeated with the emplacement of each synmineralization intrusive event, revealing multiple magmatic-hydrothermal cycles of alteration and mineralization. At Boyongan and Bayugo, intense and pervasive illite alteration, in association with pyrite, chalcopyrite, and tetrahedrite-tennantite, developed in narrow structures crosscutting quartz-K-feldspar veins. Debris flows in the burial sequence above and adjacent to the Boyongan/Bayugo complex also contain abundant clasts with intense, pervasive illite and alunite-pyrophyllite-dickite-kaolinite alteration assemblages that have overprinted K silicate-style quartz veins. In both mineral deposits, Cu and Au are associated with intense quartz-vein stockworks composed primarily of K silicate stage 3 veins. Despite this association, not all of these quartz-vein stockworks contain Cu and Au to the same tenor. Quartz-vein stockworks in the eastern high grade of Boyongan have been intersected over a vertical interval of 800 m, having affected much of the early-mineralization stock. However, hypogene Cu grades exceed 0.5% by weight only in the upper 300 m of the stockwork (in the cupola of the early-mineralization stock). Superior grade development in high-grade zones at Boyongan, locally exceeding 2% Cu and 3 g/t Au, developed where fertile vein stages from two or more magmatic-hydrothermal cycles affected the same wall rock. The documented paragenetic relationships demonstrate multiple discrete cycles of K silicate-stage veining and alteration associated with each synmineralization intrusive event. All such events predate formation of feldspar-destructive illite-smectite-chlorite, illite-pyrite, and quartz-alunite-clay assemblages. Existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly, in a period of less than 200,000 years (2.3–2.1 Ma). Geologic and geochronological constraints on the life cycle of Boyongan and Bayugo describe an extremely dynamic history of emplacement, exhumation, weathering, and burial over a period of 2.3 m.y. The study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting.

Petrography and geochemistry of granitoids from the Samphire Pluton, South Australia: Implications for uranium mineralisation in overlying sediments

The Blackbush uranium deposit (JORC Inferred Resource: 12,580tonnes U), located on the north-eastern Eyre Peninsula, is currently the only sediment-hosted U deposit investigated in detail in the Gawler Craton. Uranium is hosted within Eocene sandstone of the Kanaka Beds, overlying Mesoproterozoic granites of the Samphire pluton, affiliated with the Hiltaba Intrusive Suite (~1.6Ga). These are considered the most probable source rocks for uranium mineralisation. By constraining the petrography and mineralogy of the granites, insights into the post-emplacement evolution can be gained, which may provide an exploration indicator for other sediment-hosted uranium systems. Three geochemically distinct granite types were identified in the Samphire Pluton and correspond to domains interpreted from geophysical data. All granites show complex alteration overprints and textures with increasing intensity closer to the deposit, as well as crosscutting veining. Alkali feldspar has been replaced by porous K-feldspar and albite, and plagioclase is overprinted by an assemblage of porous albite+sericite±calc-silicates (prehnite, pumpellyite and epidote). This style of feldspar alteration is regionally widespread and known from Hiltaba-aged granites associated with iron-oxide copper‑gold mineralisation at Olympic Dam and in the Moonta-Wallaroo region. In two granite types biotite is replaced by calcic garnet. Calc-silicates are indicative of Ca-metasomatism, sourced from the anorthite component of altered plagioclase. Minor clay alteration of feldspars is present in all samples. Mineral assemblages in veins include quartz+hematite, hematite+coffinite, fluorite+quartz, and clay minerals. Minor chlorite and sericite are found in all vein types. All granite types are anomalously rich in U (concentrations between 10 and 81ppm). Highly variable Th/U ratios, as well as hydrothermal U minerals (mostly coffinite) in granites and veins, are clear evidence for U mobility. Uranium may have been preconcentrated in veins in the upper parts of the pluton, and was subsequently leached after deposition of the sediment.

Geochemistry of the barite deposits near Adana-Feke area (Eastern Taurides)

The barite deposits that emerge as veins and lodes within or along the contact zones of recrystallized limestone and dolomite units of Cambrian Degirmentas Formation are of epigenetic character. Depositional styles, mineral paragenesis, host rock alteration styles, and high SrO contents of the barites suggest a hydrothermal origin for their formation. However, considering their rare earth element compositions, barite samples surround the field for seawaters when plotted on CeN/YbN-YbN and CeN/SmN-CeN/YbN diagrams. Fluid inclusion data obtained from the barites indicated a formation from a fluid with low salinity (0.9-1.6 wt.% NaCl eq.) at homogenization temperatures between 78 and 190 °C. Sulfur isotopic compositions of barites vary between +32.2 and +36.3%c, and their Sr/ Sr values range between 0.709885 and 0.749652. Depositional styles, mineral assemblage, trace element compositions, fluid inclusion data as well as δ34S and 87Sr/86Sr isotopic data indicate a model in which hydrothermal fluids were possibly derived from a buried intrusion mixed with meteoric and marine waters, further interacting with the Precambrian-Phanerozoic metaclastic and Cambrian carbonate rocks to form barite deposits.

Isotope geochemistry of the Sarekuobu metavolcanic-hosted gold deposit in the Chinese Altay (NW China): Implications for the fluid and metal sources

The Late Triassic Sarekuobu lode gold (Au) deposit (reserves: 8.00 t Au at 3.68 g/t) is situated in the Kelan volcanic basin of the Chinese Altay Orogen, NW China. Gold mineralization is hosted in the deformed metavolcanic sequences of the Devonian Kangbutiebao Formation. Multi-isotope analyses were conducted to constrain the ore-forming fluid and metal sources of the Sarekuobu deposit. The measured δDV-SMOW values of the fluid inclusions from the mineralized quartz (stages II and III) are of −132.9‰ to −78.5‰. The measured δ18OV-SMOW values of the quartz range from 11‰ to 13.6‰ (the corresponding calculated δ18OH2O values for the ore fluids: 4.2–6.9‰), which are consistent with those of typical orogenic Au deposits worldwide and fall between the metamorphic water and meteoric water fields in the δ18OH2O vs. δDV-SMOW diagram. This implies that the ore-forming fluids were mainly derived from metamorphic devolatilization with some meteoric water input. Calcite in the late mineralization (Stage III) veinlets contains δ13CV-PDB of −5.2‰ to −1.2‰ and δ18OOV-PDB of −20.4‰ to −19.9‰, similar to the carbonates (limestone and marble) of the Kangbutiebao Formation, and plots near the marine/continental carbonate field(s) in the δ13CV-PDB vs. δ18OOV-PDB diagram. This suggests that the carbon in the late-stage hydrothermal fluids was originated from marine/terrestrial carbonates with organic sediment decarboxylation input. The δ34S values for the Au-hosting sulfides (1.8–8.2 ‰) overlap with the volcanic wall rocks of the Kangbutiebao Formation (−4.7–18.7‰), which suggests that the sulfur was most likely wall rock-derived. The Sarekuobu sulfides contain 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.071–18.874, 15.520–15.669, and 37.860–38.961, respectively. The sulfide Sr and Nd isotopes are featured by their narrow compositional ranges and crustal affinities (i.e., (87Sr/86Sr)i = 0.70959–0.72216 and calculated εNd(t) = −3.20000 to −1.86000). All the Pb, Sr and Nd isotope compositions of the Sarekuobu ore sulfides are similar to those of the Kangbutiebao Formation wall rocks, indicating that the metals were mainly wall rock-derived. The structurally-controlled nature, the wall-rock metamorphism and alteration styles (e.g., silicic, chlorite, epidote and carbonate), and the abundance of CO2-rich and low-salinity fluid inclusions of the Sarekuobu deposit are similar to typical orogenic Au deposits. We therefore propose that the Sarekuobu Au deposit is a typical metavolcanic-hosted orogenic Au deposit, and was likely formed in a Triassic collisional-related setting.

Fluid evolution in the southern part of the Proterozoic Vazante Group, Brazil: Implications for exploration of sedimentary-hosted base metal deposits

The Proterozoic Vazante Group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical, fluid inclusion and isotopic studies of the Upper Morro do Pinheiro and Lower Pamplona members from the Serra do Poço Verde Formation in the southern part of the paleo-basin. These units are the main hosts for the hypogene, structurally-controlled zinc silicate deposits in the Vazante Zinc District, including the Vazante mine, which is considered to be the largest willemitic (Zn2SiO4) deposit in the world, with estimated total resources of 40–60Mt at 20% Zn. Five hydrothermal alteration types were identified in the Southern Extension of the Vazante Group. (I) Early stage alteration comprises dolomite substitution bands and nodules, associated with moderate salinity H2O-NaCl-CaCl2 fluids, with temperatures around 90°C, interpreted as late-diagenetic. The calculated C and O isotopic compositions of the fluids suggest meteoric and/or connate origin and interaction with organic carbon. (II) Pre-ore stage alteration is evidenced by dog-tooth dolomite and quartz with minor Fe-oxi/hydroxides which fills dissolution voids formed by H2O-NaCl-CaCl2±MgCl2 fluids at temperatures around 100° to 150°C. The C isotopic data also indicate interaction with organic carbon. The two first stages are poor in ore-related elements. (III) Ore stage encompasses four phases of mineral precipitation. The first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite. The second is the main phase of the ore stage, composed of massive red dolomite, massive hematite and willemite with enrichment mainly in Fe2O3, SiO2, Ag, As, Cd, Cu, Ni, Pb, Se and Zn. The third phase comprises white dolomite, hematite and traces of willemite with enrichment in MnO, Cd, Ni and Pb. The calculated C and O isotopic compositions of the fluids (at 180°C) associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock. The fourth phase corresponds to Zn-chlorite and quartz which are associated with H2O-NaCl fluids of variable salinities and distinct temperatures (90–140°C and 170–190°C). (IV) Pyrite-bearing vein characterized by pyrite, sphalerite, white dolomite, fine hematite and late quartz, with C and O isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage. These data suggest a co-genetic relationship of these two alteration styles. (V) The late sulfide stage characterized by rare galena-bearing stringers with sphalerite, chalcocite, greenockite, covellite and white dolomite that cross-cut the main ore stage phases. The ore-related fluid compositions associated with the silicate zinc mineralization in the southern part of the Vazante Group are also similar to the fluids reported in previous work for the sulfide zinc-lead deposits in the northern part, indicating favorable conditions for metal transport during the Brasiliano Orogeny. Sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water. This finding implies that carbonate sequences in other districts with sulfide lead-zinc deposits may also host hypogene silicate zinc deposits.

Geochemistry and mineralogy of contrasting supergene gold alteration zones, southern New Zealand

Supergene alteration of gold deposits has had important geochemical effects that changed the mineralogy and textures of near-surface exposures and can affect the economics of gold mines by enhancing gold liberation and increasing gold grades. This study links observed mineralogy with 655 water analyses from environments relevant to supergene processes in the Otago Schist goldfield of southern New Zealand, and these analyses are used as proxies for past supergene processes. There are two contrasting supergene alteration styles in otherwise identical host rocks and mineralisation zones in this area. Long-term groundwater alteration of sediments and underlying basement near to a regional unconformity developed progressively since the Cretaceous. This alteration has affected orogenic gold deposits exposed at the unconformity, and has resulted in supergene enrichment of gold with localised formation of centimetre scale gold nuggets. Alteration processes occurred under circumneutral pH conditions (pH 6–8.5), and was accompanied by extensive oxidation and clay alteration of basement rocks and sediments. Gold was mobilised as thiosulphate and bisulphide complexes and was reprecipitated by redox changes near the unconformity. The chemical processes of supergene enrichment were primarily abiotic, with minor bacterial facilitation of late stage gold mobility at the micron scale. In contrast, supergene alteration of eroding freshly-exposed mountain outcrops has occurred in the late Cenozoic without Au enrichment apart from liberation and minor particle size enhancement. The ambient pH of the associated waters was also circumneutral, but localised and temporary acidification (to pH ∼3) has occured on the metre scale or less in sulphidic outcrops. This supergene alteration was dominated by oxidation but some low redox minerals, senarmontite and secondary stibnite, have formed during the early stages of oxidation or subsequent reduction. Sulphide decomposition, with associated localised acidification and gold remobilisation was probably enhanced by bacterial processes in these exposed outcrops.