Alteration Mineral Assemblages Research Articles

Hydrothermal alteration and elemental mass balance for the Surda Copper Deposit, Singhbhum Shear Zone, Eastern India: implications for both copper and magnetite–apatite mineralization

This paper explores the geological relationships, mineralogical and geochemical characteristics, and quantitative elemental mass balance constraints of the hydrothermal alteration zones within the uraniferous Surda copper deposit in the Singhbhum Shear Zone in eastern India, which is affiliated with an iron oxide–copper–gold system. Five distinct alteration zones are identified: a proximal potassic (biotitization) zone containing magnetite–apatite; a central chloritization zone containing copper, uranium, gold and magnetite; distal silicification (quartz) and albitization zones; and a transitional chlorite–sericitization zone. Evidence suggests these zones originated from mafic precursors linked to a subduction zone. Magnetite mineralization shows both magmatic ( δ 18 O + 2.0 to +4.8‰) and hydrothermal ( δ 18 O + 0.4 to +0.6‰) signatures. Hydrothermal magnetite–apatite formed at temperatures of 440 to 550°C, with continued crystallization at lower temperatures (280 to 360°C) concurrent with copper sulfide and uranium precipitation from an Fe-rich, isotopically heavy fluid ( δ 34 S + 4.55 to +9.66‰). Major elements (Ca, Na, Mg) and some minor and rare earth elements decrease from weakly altered mafic rocks to late alteration zones, reflecting the breakdown of Ca-bearing hornblende, biotite and plagioclase. Potassium decreases from early to late alteration zones due to mineral transformations. Mass-balanced geochemical data align with hydrothermal alteration mineral assemblages, indicating processes such as mineral dissolution, element absorption and sulfide behaviour. Overall, the study highlights the complex hydrothermal alteration processes shaping the Surda copper deposit and provides insights into ore formation mechanisms.

Integrating remote sensing and geophysical data for mapping potential gold mineralization localities at Abu Marawat area, central Eastern Desert, Egypt

Abu Marawat area in the Central Eastern Desert of Egypt is a very promising mineralization district located in the Golden Triangle area. The current study provides an integrated approach from multisource datasets including; remote sensing, airborne geophysical spectrometry and magnetic data supported by field studies and spectroscopic analyses for delineating potential mineralization localities. Several remote sensing techniques were adopted including; Band Ratios, Relative Band Depth, Mineralogical Indices, Spectral Angle Mapper, and Constrained Energy Minimization. These techniques showed that the alteration mineral assemblage is mainly, kaolinite, sericite, and iron oxides, with less abundant chlorite, epidote, and carbonates. In addition, the radiometry data were processed to map the localities with the highest possibility of potassic alteration abundance by integrating the potassium distribution, K/eTh ratio, and the F-parameter maps. The surface and subsurface linear structural features were also mapped using Digital Elevation Model (DEM) and aeromagnetic data, respectively. The surface linear structures were found exhibiting E-W and NE-SW trends, while, the subsurface structures showed dominant NW–SE trend. All the depicted fault trends match well with the local and regional geological and tectonic setting of the study area suggesting structural control on the mineralization in this area. Integration between the results obtained from both the remote sensing and the geophysical data was conducted by a GIS weighted overlay model. The obtained mineralization potentiality map highlights eight potential localities for mineralization. The accuracy of the adopted methodology was demonstrated through fieldwork and spectral analyses; several alteration indicators were observed, including quartz veins, iron oxides, kaolinite, malachite, montmorillonite, chlorite, talc, and sericite alteration indicator minerals. The adopted remote sensing-geophysical approach showed being very effective for mapping the hydrothermal gold-related alteration zones, and is recommended for other similar investigations.

Diagenetic and hydrothermal controls in a modern submarine rear-arc setting: Insights from Izu-Bonin volcanic arc (IODP Site U1437)

International Ocean Discovery Program (IODP) Expedition 350 drilled Site U1437 in a submarine volcano-bounded basin situated in the modern Izu-Bonin rear-arc, NW Pacific Ocean. Subaqueous volcaniclastic sediments and rocks with a maximum deposition age of 15.4 ± 0.8 Ma were recovered from 0 to 1,806.5 m below seafloor (mbsf). In order to document post-depositional processes in such geological setting, we describe variations in bulk and clay mineralogy over the entire volcaniclastic succession. Four alteration stages (1, 2, 3 and 4) were identified through the occurrence and development of diagenetic background mineral assemblages (smectite ± Na-Ca zeolites ± illite) that were further superimposed by hydrothermal alteration. Stages 1 and 2 are characterized by diagenetic reactions linked with low fluid/rock interactions that enabled glass devitrification and subsequent lithification under burial conditions. Stages 3 and 4 are characterized by moderate to pervasive alteration processes that are well developed in coarser-grained rocks, and that may be induced by thermal pulses associated with fluid inputs. Below 1,460 mbsf, infilling and replacement textures overprinted the background alteration and can be directly linked with the development of two hydrothermal mineral assemblages: (i) ordered C/S (chlorite-smectite mixed-layers) ± chlorite ± albite, and (ii) calcite ± chalcedony ± anhydrite ± laumontite. Both assemblages evidence relatively low-temperature (up to 225 °C) hydrothermal activity that affected subaqueous volcaniclastic rocks at Site U1437. These assemblages are comparable with propylitc alteration facies present in ore-bearing hydrothermal systems. The preferential development of alteration mineral assemblages in high-permeability, coarse-grained lithofacies, reflects the significant influence of the physical properties of volcaniclastic rocks with depth on chemical kinetics, in comparison with those imposed by the local geothermal gradient.

Hydrothermal Alteration, Ore Mineralization, and Fluid Inclusions Study of the Mount Muro Low Sulphidation Epithermal Deposit, Murung Raya Regency, Central Kalimantan Province, Indonesia

Abstract Mount Muro is a part of the ‘Central Kalimantan Gold Belt’, which is found in Murung Raya Regency, Central Kalimantan Province, Indonesia. The study area is part of the Mount Muro mineralized district which includes Serujan Project, Bantian Project, and Kerikil Project. In order to optimize the production yield of the Mount Muro mine, it is important to understand the update data and analyses about the ores characteristics along with their mineral associations and the paleo-fluids involved in it. This research aims to elucidate the mineralization model based on their alteration mineral assemblages, ore minerals formation, and their hydrothermal fluids characteristics. This research is dealing with characteristics of wallrock hydrothermal alteration using X-Ray Diffraction (XRD) and thin section to identify the associations of hydrothermal alteration minerals, ore microscopy to identify the type, associations, and paragenesis of the ore minerals, and fluid inclusions studies which include fluid inclusions petrography and microthermometry measurements to identify paleo-fluid characteristics in the research location. Four alteration types based on their mineral assemblages are identified, namely argillic, silicification, propylitic, and sericitization alterations. Silicification is marked by abundant quartz ± illite ± smectite. Propylitic alteration is typified by chlorite + smectite + calcite + quartz ± epidote ± illite ± adularia. Argillic alteration is presence by clay minerals such as illite + smectite + kaolinite + palygorskite. Sericite + chlorite + illite + smectite assemblage is key minerals of sericitization alteration. Metalliferous and ore minerals are found as native gold, acanthite-argentite, native silver, electrum, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, tennantite, chalcosite, covellite, goethite, and hematite. Based on fluid inclusion studies on the quartz veins, four fluid inclusion types are identified, namely type I (monophase, liquid rich) where L = 100%, type II (liquid rich, L+V) where L>50%, type III (vapor rich, V+L) where V>50%, type IV (vapor rich, V) where V = 100%. Homogenization temperature (Th) mean of banded vein ranged from 279°C - 284°C, with salinity mean 4.2 - 2.6 wt% eq. NaCl and formed around 806 - 711 meters depth under paleosurface. Homogenization temperature (Th) mean of cockade vein 278°C, with salinity mean 2.3 - 2.4 wt% eq. NaCl and formed around 728 - 726 meters depth under paleosurface. Homogenization temperature (Th) mean of massive barren vein is 267°C, with salinity 2.2 wt% eq. NaCl and formed around 607 meters depth under paleosurface. Binary plot between Th and salinity result and coexisting liquid-rich and vapor-rich fluid inclusions confirm that boiling processes took major place during the ore mineralization. Based on ore mineralogy, hydrothermal alteration mineral assemblage, and fluid inclusions characteristics, deposit is categorized into a deep low sulphidation epithermal deposits type, specifically in transition of precious metal to basemetal zone.

Characteristics of pyrophyllite and hydrothermal alteration at Argotirto area, Malang Regency, East Java, Indonesia

Abstract Argotirto area located in South Malang, East Java province, has abundant non-metallic mineral resources, e.g., pyrophyllite. The presence of pyrophyllite indicates that the area has undergone hydrothermal alteration processes. This research aims to determine characteristics of pyrophyllite and hydrothermal alteration at Argotirto and its surrounding area. The methods used in this research comprises geological and hydrothermal alteration mapping, thin section and X-ray diffraction (XRD) analyses from rock samples. There are five lithological units from the oldest to the youngest, namely altered basalt, altered andesite breccia, altered dacite intrusion, altered dacitic tuff breccia, and limestone units. In hand specimen, pyrophyllite has white to tan colour, massive and waxy/soapy textures. Furthermore, in thin section, pyrophyllite selectively replaces feldspar phenocrysts or it is disseminated in the matrix and intergrown with quartz. In the research area, there are four alteration zones, such as pyrophyllite-dickite-sericite-illite, pyrophyllite-vermiculite-alunite-tridymite, pyrophyllite-cristobalite-sericite-illite-smectite, and pyrophyllite-halloysite-sericite-illite zones. According to hydrothermal alteration mineral assemblages, it can be interpreted that those minerals are formed at low pH (approximately pH 4-5) and relatively low temperatures (100-<250°C).

Application of geological and short wavelength infrared (SWIR) spectroscopy mapping in the Mailong gold deposit, East Kunlun: Implications for exploration targeting

Widespread regolith cover makes it difficult to identify the anomalous size of their mineralogical or geochemical footprint at the surface in gold exploration. Short wavelength infrared (SWIR) spectroscopy with effective identification of clays and other hydrous minerals shows potential application in gold exploration. The detailed geological and short wavelength infrared (SWIR) spectroscopy mapping was conducted in the newly-discovered orogenic-type Mailong gold deposit (13.06 t @ 6.29 g/t Au), East Kunlun, to determine alteration stage and mineral assemblages and discuss the potential values of SWIR spectral in gold exploration.In the Mailong area, magmatic rocks consist of granodiorite, biotite granodiorite, plagiogranite and mafic dikes, and the gold mineralization shows a close spatial and temporal relationship with mafic dikes. The earliest chloritization (chlorite-quartz) is widely distributed in granodiorite, biotite granodiorite and plagiogranite. The following potassic alteration (hydrothermal K-feldspar and quartz) is developed along high-angle fault/fracture, which cut through granodiorite, biotite granodiorite and plagiogranite. The subsequent sulfide alteration (chlorite-muscovite-sulfide-carbonate) overprints the former potassic and chloritization alteration, and shows spatial association with mafic dikes. Native gold occurs in fracture arsenopyrite or associated with minerals such as pyrite, and chalcopyrite during the early sulfide stage. Supergene alteration is characterized by limonite and malachite. At Mailong, the SWIR data indicate that the most abundant alteration minerals (chlorite, white micas and ankerite) mainly occurred in the northern area. White mica Al-OH absorption occurs at longer wavelengths (Pos2200 = 2202.5 ∼ 2203 nm) and higher Illite crystallinity (IC = 1.2 ∼ 3.1) probably reflecting a higher temperature and pH conduit at the bottom of ZK0001 and ZK0801. The sharp shift of IC values near the bottom reflects the control of fault. This area is also situated at the V-shaped intersections of the three granitic intrusions and structural transection, indicating more of a possible hydrothermal fluid channel than a mineralization center. Low-grade Au mineralization (Au = 0.1 ∼ 0.5 g/t) is mainly distributed within hydrothermal channels with higher Pos2200 (mainly 2022.5 ∼ 2203 nm) and IC values (mainly 1.2 ∼ 3.1), while high-grade Au endowment associated with mafic dikes has low Pos2200 (mainly 2201 ∼ 2202.5 nm) and IC values (mainly 0.43 ∼ 1.2). Meanwhile, an equation of distance = -15*IC + 31 (R2 = 0.47) can be summarized for low-grade orebodies vectoring. Integrating geological mapping and spectral data, target areas for high-grade ore bodies can be outlined using the criteria of white mica, shorter wavelength of Al-OH absorption position (Pos2200 = 2201 ∼ 2202.5 nm), lower IC values (IC = 0.43 ∼ 1.2), and chlorite Fe-OH absorption position Pos2250 (>2247 nm) along with occurrences of mafic dikes. This study demonstrates that utilizing the detailed SWIR identification in conjunction with accurate geological mapping has great potential for greenfield exploration of orogenic gold deposits.

Hydrothermal alteration and geochemical proximity indicators to ore at the Metsämonttu Zn–Pb–Cu–Au–Ag deposit, Uusimaa belt, southern Finland

The Paleoproterozoic Metsämonttu Zn–Pb–Cu–Ag–Au deposit (1.5 Mt at 3.5 wt% Zn, 0.8 wt% Pb, 0.3 wt% Cu, 13.2 wt% S, 25 g/t Ag, and 1.4 g/t Au, production 1952–1974) is the largest past-producing mine in the Aijala–Orijärvi area (Orijärvi formation, Aijala member) within the Uusimaa belt, southern Finland. The Aijala member is characterized by 1.9–1.88 Ga felsic-dominated volcanic-sedimentary supracrustal rocks with intercalated sedimentary carbonates and iron formations. The area is underexplored, and little deposit-scale research has been carried out, despite the lateral continuum to the world-class ore district of Bergslagen in south-central Sweden. To better understand the Metsämonttu VMS-related alteration system, we reassessed the previously described metamorphic mineral assemblages and their protolith rock compositions by using mobile and immobile element geochemistry. This resulted in the definition of four metamorphosed alteration mineral assemblages and eight chemostratigraphic rock units. The chemostratigraphic results suggest that the lithological and/or structural setting of the Metsämonttu succession is more complex than previously considered. The stratigraphic footwall is mainly characterized by an extensive cordierite + anthophyllite ± biotite ± phlogopite + pyrite ± pyrrhotite (Mg–Fe–S) assemblage dominated by mafic rocks, designated Mafic B1 and Mafic B2, and Andesite A1. The main sulfide mineralization is hosted by tremolite + diopside ± biotite ± phlogopite ± chlorite skarn (Ca–Mg–K). A sericite-bearing muscovite + quartz ± biotite ± phlogopite + pyrite (K–Si–S) assemblage is composed of felsic to mafic protoliths (Mafic B1–B2, Andesite A1, Dacite A1, Dacite B1, Dacite C1) and extends several tens of meters into the stratigraphic hanging-wall. A quartz + pyrite ± muscovite (Si–S) assemblage represents the immediate ore-proximal alteration and is derived from rocks with a rhyolitic composition (Rhyolite A1).Limited drill core samples near Metsämonttu mineralization, along with restricted surface alteration, pose challenges in studying geochemical variations from distal to ore proximal areas and limits the ability to model the shape and size of the alteration zone. Large mass changes suggest that the alteration was hydrothermal, and due to several protolith compositions, the alteration is interpreted to be predominantly discordant to stratigraphy. A 60-m-wide alteration halo surrounds the Metsämonttu deposit. Major and trace elements, namely MgO, Na2O, K2O, SiO2, Fe2O3, Cu, Zn, Pb, S, Ag, Tl, Hg, Se, Te, Sn, Sb, Rb, and Sr, and the indices modified alteration index (MAI), Ishikawa alteration index (AI), chlorite‑carbonate-pyrite index (CCPI), S/Na2O can be used for chemical vectoring, which can assist regional or near-mine exploration. Elements MnO, Ba, Cd, Bi, As, Ni, Co, W, Ga, Mo, and indices Hashigushi index and advance argillic alteration index (AAAI) were less useful as geochemical exploration indicators.

Physico-chemical constraints associated with hypogene hydrothermal alteration and supergene oxidation at the Farallón Negro intermediate-sulfidation epithermal Au-Ag deposit, NW Argentina

The Farallón Negro deposit is located in the Farallón Negro Volcanic Complex (FNVC; ∼9.40 to 4.88 Ma) in NW Argentina and includes intermediate-sulfidation, epithermal Au-Ag mineralization in the Alto de la Blenda deposit. Detailed mineralogical and geochemical investigations of the Esperanza-Esperanza Sureste, Cecilia veins, both hosted in the Alto de la Blenda monzonite, and the Laboreo vein, hosted in the volcaniclastic Morada Central andesitic breccia, reveal four hypogene vein stages. Each of these four stages are comprised of a texturally early quartz, followed by a sulfide-sulfosalt and a late Mn-carbonate sub-stage. Stage 1 is characterized by a quartz, base metal and Mn-carbonate sub-stage showing high δ18OVSMOW (δ18OQuartz = 13.7 ± 0.4‰ and δ18OMn-Calcite = 11.6 ± 0.2‰) and low δ13CVPDB values (δ13CMn-Calcite = –4.7 ± 0.2‰). Stage 2 includes most gold and silver minerals in a tennantite-tetrahedrite-sphalerite-pyrite-galena±electrum sub-stage. Fluid inclusions trapped in vein-anhydrite, only associated with stage 2, record the highest temperatures (TEntrapment = 327–335°C) and highest salinity (3.8NaCleq.wt.%) when compared to all other stages. Stage 2 quartz (δ18OQuartz = 9.9 to 11.9‰) and carbonate display lower δ18OVSMOW (δ18OMn-Calcite = 7.9 to 10.3‰), as well as lower δ13CVPDB values (δ13CMn-Calcite = -5.2 to 4.7‰) than all other stages. Stages 3 and 4 consist of barren quartz and carbonate sub-stages that show lower temperatures (TEntrapment = 187–262°C) and higher δ18OVSMOW (δ18OQuartz = 13.4 to 13.9‰ and δ18OMn-Calcite = 10.7 to 11.8‰) and δ13CVPDB values (δ13CMn-Calcite = –3.5 to −3.9‰) when compared to earlier stages.Petrographic, sulfide mineral chemistry, microthermometric, and stable isotope data combined with distinct alteration mineral assemblages allow the reconstruction of a model for the Alto de la Blenda deposit. During the hypogene hydrothermal fluid flow stage, around 7.16 Ma, at ∼2.2 km depth (PHYDROSTATIC = 220 bars), the high temperature alteration mineralogy, low fluid salinities and 18O and 13C isotope compositions indicate cooling of ascending magmatic-hydrothermal fluids. These fluids were derived from a contracted magmatic vapor phase that exchanged with unexposed sedimentary rocks of the Sierras Pampeanas. Hypogene quartz-R3 illite/smectite-chlorite-calcite-pyrite alteration in the andesitic-breccia and quartz-illite-R3 illite/smectite-chlorite-calcite-pyrite alteration in the monzonite are temporally and spatially to the hydrothermal vein minerals deposited between 187 and 335°C. During the post-hydrothermal supergene weathering event, around 2.70 Ma, sulfuric acids generated by meteoric fluids and supergene oxidation of hypogene sulfides, particularly pyrite, descended along normal fault zones and related vein and fracture networks to at least 450 m depth below the surface level, forming the supergene alteration assemblage of dioctahedral smectite-kaolinite-Mn-/Fe-oxides/-hydroxides-gypsum±interstratified chlorite/smectite±interstratified R1 illite/smectite in both wall rock types at temperatures <50°C.The dominant hypogene alteration assemblage, present in the monzonite and volcaniclastic andesitic breccia, may be indicative of deeper-seated, transitional magmatic-hydrothermal systems (>1 km depth), generally controlled by fluid cooling, as is the case at Farallón Negro. Pervasive supergene alteration can conceal these systems but the identified supergene alteration assemblage of dioctahedral smectite-kaolinite-interstratified chlorite/smectite-interstratified R1 illite/smectite that replaces the hypogene alteration assemblage along the permeable fault, vein and fracture zone network may present a useful vectoring tool towards concealed, deeper-seated, epithermal Au-Ag mineralization in arid climates.

Reaction path model of the formation of abiotic immiscible hydrocarbon fluids in subducted carbonated serpentinites, Lanzo Massif (Western Italian Alps)

Fluids generated from subducted slabs participate in the cycling of deep carbon in the crust and upper mantle. In these fluids, aqueous carbon species vary in oxidation state between +IV and -IV depending on whether the fluids are oxidizing or reducing, respectively. Most studies of subduction-zone fluids have focused on oxidized carbon species. However, recent studies of natural samples have demonstrated the occurrence of deep, reducing fluids, generated in both subducted oceanic upper mantle and crustal rocks. CH4-H2-rich fluid inclusions in subducted carbonated serpentinites have demonstrated the existence of abiotic, immiscible, hydrocarbon fluids at upper mantle conditions. To investigate the formation of such immiscible hydrocarbon fluids during the evolution of subducted carbonated serpentinites, we used equilibrium constants from the Deep Earth Water model to carry out predictive chemical mass transfer modeling to simulate the alteration reactions. A novel feature of the models was the inclusion of an immiscible hydrocarbon fluid containing six components (CH4,f,C2H6,f,C3H8,f,isoC4H10,f,CO2,f,H2,f). This feature enabled prediction of the formation of a separate immiscible fluid in equilibrium with aqueous species and minerals.We developed a predictive reaction path model of invasive H2,f reacting with carbonated serpentinites and interstitial aqueous fluids for comparison with the natural samples from the Lanzo Massif, western Italian Alps. Over a range of temperatures and pressures, immiscible hydrocarbon fluids formed in association with altered mineral assemblages. Reaction progress caused the transformation of carbonated serpentinites and the formation of clinopyroxene, brucite, graphite, and hydrocarbon fluids, along with changes of pH, logfO2, and aqueous species. CH4,f was the most abundant hydrocarbon species in all the models. The overall results at 2.0 GPa and 400 to 450 °C were consistent with the natural samples from the Lanzo Massif. Interestingly, large amounts of H2O formed due to oxidation of H2. More hydrocarbons and H2O formed in models with lower fluid/rock mass ratios or with more reactant H2. Models at different pressure and temperature conditions showed similar results with some variation in the relative stabilities of aragonite, graphite and olivine solid solution, and associated differences in mineral sequences, hydrocarbon fluids, values of aqueous species, and the final logfO2 and pH. Our models strongly support the laboratory and field evidence that reduction of carbonated serpentinites by infiltrating H2 fluids can cause the formation of immiscible, abiotic hydrocarbon fluids in subduction zones.

Characteristics of the Kematu epithermal gold deposit at Tboli, Mindanao, Philippines

AbstractThe Kematu Au‐Ag epithermal deposit at Tboli area, Province of South Cotabato, Mindanao, Philippines, contains an inferred mineral resource of 3.8 million tonnes, at 8.7 g/t Au and 27 g/t Ag in 2012. The deposit is spatially and genetically linked to the volcanism related to the Parker Volcanic Complex of Pliocene to Holocene age. Local extension of the northwest‐trending splays of the faults in the Tboli area are associated with epithermal mineralization, including the Kematu Au‐Ag deposit. The study aimed to provide a detailed description of the characteristics and physicochemical processes involved in the formation of the Kematu Au‐Ag deposit on the basis of field occurrence, mineralogy, ore composition, mineral chemistry, and sulfur isotope data. The deposit is characterized by Au‐Ag bearing quartz‐adularia‐clay‐sulfide veins hosted in argillic‐altered andesite porphyry and coarse tuff of the Parker Volcanic Complex. The mineralization in the Kematu deposit is in the form of veins as well as open space fill and dissemination. Based on the observation of the ore bodies and petrographic analysis, three paragenetic stages with different vein characteristics have been identified. Stage 1 is dominantly quartz, adularia, and clay mineral veins, with electrum, sphalerite, pyrite, chalcopyrite, tetrahedrite‐tennantite, and galena. Stage 2 is coarse‐grained sulfide‐quartz‐adularia veins and in‐fill of vugs, consisting of sphalerite, pyrite, chalcopyrite, galena, tetrahedrite‐tennantite, and electrum. Stage 3 is massive calcite veins, with minor amounts of electrum, sphalerite, pyrite, chalcopyrite, and galena. The alteration mineral assemblage of the host rocks is chlorite‐illite‐adularia, indicative of neutral pH conditions. The FeS content of the Fe‐poor sphalerite in the stage 2 ranges from 0.03 to 1.07 mol%. Sulfur isotope compositions (δ34SCDT) of sulfides in all the stages range from 0.1‰ to 1.9‰ for pyrite, and −5.3‰ to 0.5‰ for galena, sphalerite, and chalcopyrite, where those in the stage 2 tend to have relatively lower values compared to those in the stages 1 and 3. The relatively narrow and close to zero δ34SCDT values suggest that the sulfur was mainly derived from a magma. The mineral assemblages of the stages 1, 2, and 3, and the sulfur fugacity (log fS2 = −12.5 to −8.8) and temperature (200–280°C) of ore formation based on the FeS mol%, electrum composition (at.%), and alteration mineral assemblage suggest that the Kematu deposit is of intermediate sulfidation type.

Unveiling an early Paleozoic porphyry-epithermal gold system in Songshunangou district, North Qilian, northwest China: Geological and geochronological constraints

The Songshunangou district is located in the central part of the North Qilian orogenic belt and hosts both the Eastern and Western deposits. The Western deposit is a porphyry-Au deposit, characterized by concentrated high-grade Au mineralization within a quartz-dioritic porphyry and occurs along the contact to basaltic andesite. Alteration is concentric with potassic alteration associated with high-grade Au mineralization in the core zone, while surrounding propylitic alteration is associated with an only lower-grade Au mineralization. Barren sericite-chlorite alteration is restricted to shallow zones above or partially overprinted the potassic alteration. Mineralization, including chalcopyrite, bornite, pyrite, magnetite, native gold with minor galena, sphalerite, hematite, and covellite, is observed as quartz-sulfide stockworks and fine-grained disseminations in different alteration zones. LA-ICP-MS zircon U–Pb dating of samples from the ore-bearing quartz-dioritic porphyry, the post-mineralization dioritic porphyry and the granodioritic porphyry in the Western deposit yield mean 206Pb/238U ages of 456.3 ± 2.0 Ma to 454.9 ± 2.4 Ma, and 449.5 ± 4.5 Ma to 448.3 ± 1.4 Ma, respectively. Pyrite from quartz – chalcopyrite – bornite veinlets in the Western deposit yields a Re-Os isochron age of 454.6 ± 3.7 Ma. In the Eastern deposit, which is an intermediate-sulfidation epithermal deposit hosted in basaltic andesite, sericite-chlorite alteration is associated with the mineralization. The mineralization is predominantly found within veins and disseminated, primarily composed of galena, sphalerite, native gold, tetrahedrite, and minor chalcopyrite, pyrite, and covellite. The alteration mineral assemblage comprises sericite, chlorite, Mn-calcite, quartz, calcite, and minor epidote. A Mn-calcite – quartz – polymetallic vein from the Eastern deposit yields a calcite Sm-Nd isochron age of 453.2 ± 5.8 Ma, suggesting that intermediate-sulfidation mineralization was coeval with the emplacement of the ore-bearing quartz-dioritic porphyry and the porphyry mineralization in the Western deposit. Based on these geologic features and geochronological data, we conclude that an early Paleozoic porphyry-epithermal Au system developed in the Songshunangou district, which formed in an arc setting associated with the northward subduction of the North Qilian Ocean beneath the North Qilian orogenic belt. The occurrence of an early Paleozoic porphyry-epithermal Au system in the North Qilian orogenic belt provides new insights for regional mineral exploration.

Magmatic fluids responsible for lode gold mineralization in the giant Linglong deposit at Jiaodong, North China Craton: Constraints from Li[sbnd]O isotopes

The Jiaodong Peninsula, located in the eastern margin of the North China Craton, contains numerous world-class lode gold deposits that combined have a proven gold reserve of >5000 t. Despite a large amount of research, the source and evolution of hydrothermal fluids responsible for the gold mineralization in this region remains highly debated. The Linglong gold deposit is one of the largest deposits in the Jiaodong Peninsula. Gold lodes at Linglong are predominantly hosted within the Late Jurassic Linglong granite and structurally controlled by NE-trending faults. They are closely associated with large-scale hydrothermal alteration, which is paragenetically characterized by hydrothermal K-feldspar in pre-ore stage I, and sericite-quartz-pyrite in syn-ore stages II and III. The syn-ore alteration mineral assemblage is distributed within and around the gold lodes, whereas the K-feldspar alteration remains mainly distal to the gold lodes. From the unaltered to progressively intensifying K-feldspar-altered Linglong granites, there are decreasing trends for both whole-rock Li concentrations and δ7Li values, which are attributed to Rayleigh fractionation. The two values increase covariantly within the sericite-quartz-pyrite alteration zone towards the mineralization center, suggesting that further fluid-rock interaction and buffering of the ore fluid accounts for the observed Li elemental and isotopic variations. Fluid inclusion assemblages in quartz of stages I-III have homogenization temperatures of 349°-312 °C, 318°-272 °C, and 307°-246 °C, respectively. In-situ oxygen isotopic analysis on stage I quartz reveals δ18OQ values of 10.9–14.7 ‰, corresponding to δ18OW values of 4.3–9.2 ‰ for the equilibrated fluid. The δ18OW values of the equilibrated fluids of stages II and III are 5.6–8.9 ‰ and 5.2–8.8 ‰, respectively. The oxygen isotopic compositions indicate that magmatic fluid is prevailing for the K-feldspar and sericite-quartz-pyrite alterations. A binary mixing model using Li concentrations and δ7Li values further implies that the hydrothermal fluid was mainly sourced from coeval mafic magmas derived from the metasomatized lithospheric mantle. Our study highlights that lithium and in-situ oxygen isotopes can be used effectively to fingerprint the source of hydrothermal fluids and fluid-rock interactions.

Fluid-rock interaction modeling to constrain Au enrichment: Implication for the giant Jiaodong Au mineralization, eastern North China Craton

The devolatilization model of the metasomatized lithospheric mantle without pre-enriched gold has been proposed to account for the giant gold mineralization. An excellent example is the world-class Jiaodong gold province with >5000 tonnes Au resources in the eastern North China Craton. The auriferous fluid transport and gold enrichment during wallrock alterations are two vital processes to determine the giant gold mineralization formation in this province. However, the effects of the fluid-rock interaction with alterations on the auriferous fluid transport and gold enrichment still keep poor understanding, which leads the above model to be imperfect. The giant Jiaojia goldfield in this province recorded a wallrock alteration evolution from K-feldspar alteration to pyrite-sericite-quartz alteration, and some parts of the latter can become gold orebodies when the gold grade is >1 ppm. This study conducts thermodynamic fluid-rock interaction modeling to reveal auriferous fluid transport and coupled relationship between gold enrichment and alteration mineral assemblage based on the alteration-mineralization and ore fluid characteristics of the goldfield. The modeling of fluid-rock interaction with cooling indicates the transformation of Au-Cl complexes to Au-S complexes combined with the total sulfur concentration decrease by pyrite precipitation when cooling from ∼460 °C can trigger the dispersive gold precipitation, which should hinder the gold long-range transport to lower ambient temperature. The high oxygen fugacity at >400 °C can enhance Au-Cl complexes stability, and the low pH can maintain high total sulfur concentration in the auriferous fluid, both of which facilitate the long-range gold transport to a lower-temperature environment. The auriferous fluid would acquire higher pH by the buffering of feldspars or sericite, which was beneficial for the high-efficiency precipitations of pyrite and gold. The ankerite-siderite assemblage without pyrophyllite in the pyrite-sericite-quartz alteration zone indicates that a cumulative fluid to rock mass ratio (f/r) of 3.8–4.8 should be needed for the transformation from K-feldspar alteration to pyrite-sericite-quartz alteration according to the fluid-rock interaction modeling at 300 °C and 2000 bar. In the case of auriferous fluids with ≤200 ppb Au concentration, the single fluid-rock interaction can only elevate the gold grade to ≤0.69–0.87 ppm in the pyrite-sericite-quartz alteration zone at f/r 3.8–4.8. Therefore, the fracture-induced fluid flow coupled with fluid-rock interaction is proposed to the prerequisite to elevate the gold grade to >1 ppm in the pyrite-sericite-quartz alteration zone. The metasomatized lithospheric mantle volume for the required ore fluid and Au in the Jiaodong province is estimated according to the modeling results and alteration-mineralization characteristics, which provides a link between the mantle without abnormal Au enrichment and the alteration-mineralization processes.

Alteration and Mineralization in the Coppo Village, Barru District, South Sulawesi Province

Hydrothermal alteration can be used to interpret and identify undiscovered mineralization, and can lead to the discovery of such mineralization. In the research area, it was found that there were outcrops that were altered with the physical characteristics of weathered outcrops. The research location is in the Camba Formation of Volcano Rock Members, and there are indications of sulfide mineralization, megascopically this study took 3 (three) alteration samples, selected based on the differences in color and texture of the rocks found. Samples we reanalyzed to determine alteration mineral assemblages using petrographic and XRD analysis methods, then to determine the type of alteration in sulfide deposits in the study area using the classification by Corbett and Leach, 1996. The results of the petrographic analysis showed that sample 1 consisted of chlorite, epidote, quartz and mineral assemblages. opaque minerals, sample 2 consists of plagioclase minerals, kfeldspar, clay minerals and quartz, sample 3 consists of epidote minerals, calcite and opaque minerals. The results of the XRD analysis showed that sample 1 consisted of the minerals quartz, chlorite, epidote and pyrite, sample 2 consisted of the minerals kaolinite, illite and pyrite, sample 3 consisted of the minerals quartz, chlorite, dolomite, epidote and auricuprite. The results of the analysis of alteration minerals assemblages were classified using Corbett and Leach, 1996 and it was found that the types of alteration in the research area were propylitic and argillic alteration types.

Trace element geochemistry of magnetite from Mabianshan skarn iron deposit in the Lu-Zong volcanic basin, Eastern China

The composition of host rocks is essential in determining the composition of hydrothermal magnetite formed via fluid-rock interactions in skarn deposits. The HuangLong ore field in the Luzong volcanic basin, Eastern China, hosts several stratabound iron (Fe) skarn deposits, including Mabianshan and Longqiao. In this paper, we analyse the composition of different lithologies of carbonate strata and trace element concentrations in magnetite from Mabianshan, which has complex lithology carbonate strata, and compare it with magnetite from Longqiao, which has uniform lithology carbonate strata. The objective is to investigate the influence of lithology variation on magnetite trace elements and enhance the indicative significance of magnetite trace elements in prospecting and exploration. Our results demonstrate that the lithology of ore-bearing carbonate strata varies greatly due to inversion and different degrees of denudation in the ore field, resulting in variations of Al, Si, Mg, Mn, and Ca in the protolith across different areas. The alteration mineral assemblage of the Mabianshan deposit primarily included epidote, chlorite, and magnetite, while only a few diopside-magnetite relatively high-temperature assemblages developed in the southeastern part of the deposit. Moreover, higher concentrations of Ti and V in magnetite samples from the southeastern area of the deposit suggest that the heat sources related to Fe mineralization may be located there. Compared with the regular spatial variations of Mg, Mn, Al, and Si in the magnetite from Longqiao, the magnetite from Mabianshan does not show evident spatial variations due to the complex lithology of the magnetite protolith in different regions. The protolith lithology of magnetite from Longqiao is relatively stable, mainly dolomitic limestone. However, the protolith lithology of magnetite from Mabianshan is complex and includes dolomitic marlstone, dolomitic limestone, dolomite, and Fe-Mn bearing dolomite, resulting in a significant increase of Si, Al, Mg, and Mn in magnetite. Therefore, it is difficult to effectively trace the temporal-spatial evolution of the hydrothermal fluids using these elements in magnetite. Special attention should be paid to the uniformity of the protolith lithology of the ore when using these elements in magnetite to trace the fluid source.

Orogenic gold mineralization in the Huangling region and its intimate tectonic linkage to the Neoproterozoic orogeny of South China

Gold mineralization types were rarely exposed in the Neoproterozoic accretionary-to-collisional orogen, particularly in China, throughout Earth’s history. The Huangling region in South China contains more than 70 gold deposits/occurrences hosted in Archean-Paleoproterozoic metamorphic rocks and Neoproterozoic granitic complexes. However, the absolute age of hydrothermal mineralization, genesis, and their relationship with regional-scale processes, such as tectonism, magmatism, and metamorphism, remains unclear. Therefore, in this study, we investigated three gold deposits (Shuiyuesi, Hongwucun, and Guaizigou) hosted in different lithological units from the northern, central, and southern regions of Huangling, respectively. We conducted detailed fieldwork and analyzed the hydrothermal apatite and monazite geochronology and Sr-Nd isotopic composition of these deposits. The gold mineralization is mainly hosted within sulfide-quartz veinlets, and it is structurally controlled by second-order NW- or NNW-striking faults of the first-order NW trending main fault zone. Gold is present in various forms, such as native gold, electrum, and petzite or in fine-grained pyrite as invisible gold co-precipitated with alteration mineral assemblages that include sericite, quartz, and hydrothermal apatite, and monazite. LA-ICP-MS U-Pb dating of the hydrothermal monazite and apatite for the Shuiyuesi deposit yielded two ages: 831 ± 2 Ma and 830 ± 23 Ma. These ages are consistent with the weighted mean 206Pb/238U age of 834 ± 3 Ma for apatite, obtained from the southern Guaizigou deposit within margin of error. Monazite and apatite dating of the Hongwucun gold deposit yielded younger ages of 810 ± 3 Ma and 800 ± 10 Ma, respectively. The new dates suggest that the gold deposits in the Huangling region underwent two distinct metallogenic episodes of gold mineralization during the Neoproterozoic, coeval with the temporal post-collisional mafic dykes, which postdated the regional metamorphism by ∼100 m.y, as indicated in the Huangling region after the peak period of collisional orogeny and in response to an extensional setting, possibly due to asthenosphere wedging. The Sr-Nd isotope of apatite and monazite (87Sr/86Sr: 0.69555–0.76244, εNd(t): −22.1–−2.9) and collective geochemical data indicate that ore-forming fluids are mainly consistent with magmatic sources with significant contribution from mantle materials. The results of the study suggest that the Huangling gold mineralization province is genetically related to Neoproterozoic post-collisional magmatism. Additionally, the region shares many characteristics of large orogenic gold provinces around the world, including alteration types, element associations, occurrence states of gold, and the evolution of orogenic belts, offering valuable new insights into the gold metallogeny of the early Neoproterozoic period, which is generally not known for its abundance of Au deposits.

Geology, alteration system and uranium mineralization of Tamusu large uranium deposit, China

Tamusu uranium deposit is located in Yingejing sag in the Bayingobi basin. A rift type structural slope zone developed northwest of the sag. Based on the study of the lithology, lithofacies, whole rock geochemistry, and fluid inclusion characteristics of the deposit, a paragenetic association sequence of altered minerals in the deposit was established, and the characteristics of the source, temperature, salinity, and evolution of the ore-forming fluid in the deposit were discussed. The stratigraphic assemblage structure of the lower member of the Bayingobi Formation (falling-stage system tract, lowstand system tract, and transgressive system tract) laid the foundation for uranium mineralization. The uranium mineralization period of ore deposit can be divided into three stages. The altered mineral assemblages of Stage 1 are mainly composed of hematite, dolomite, iron-bearing dolomite, ankerite, fluorite, euhedral pyrite, and metal sulfide. Stage 2 altered mineral assemblages are dominated by limonite, euhedral-subhedral pyrite, and gypsum. The altered mineral assemblages of Stage 3 are mainly composed of layered gypsum, celestite, and barite. The early uranium ore-forming fluid mainly originated from the deep basin and percolated along the fault and sand body of the lowstand system tract to form uranium orebodies. The fluid inclusion temperature of ore-forming fluid was 211–140 ℃, and the salinity was 4.49%–16.24%. The temperature gradually decreased as the ore-forming fluid evolved, and CO2 was released, causing dolomite, calcite, fluorite, and other minerals to precipitate in the sandstone pores. The late ore-forming fluid mainly originated from supergene oxygen water, which migrated into the basin along the erosion window at the edge of the sag. The fluid inclusion temperature of ore-forming fluid was 100–130 ℃, the salinity was 2.24%–7.02%, and the average was 4.49%. In the late stage of the supergene fluid, the fluid inclusion temperature was 55–79 ℃, and the salinity was 4.96%–5.11%. The ore-forming fluid of the deposit gradually decreased in temperature from the early stage to the late stage of mineralization. In addition, the salinity gradually evolved from high in the early stage to low and evolved to high in the late stage. Sulfophile, lithophile, and rare earth elements precipitated first in the process of fluid action, followed by U, and Sc increased with the U content. A uranium metallogenic model of the deposit was established based on the relationship between the structural, lithology, mineralization alteration, fluid evolution, and uranium mineralization of the basin.

Mineral Chemistry of the Eagle Lake Granite Porphyry, Southwestern New Brunswick, Canada: Implications for Cu-Mo-Au Mineralization

Abstract The oxidized I-type Eagle Lake Granite stock in southwestern New Brunswick, Canada, is texturally divided into porphyritic and equigranular phases. The porphyritic granite consists of phenocrysts (i.e., plagioclase, K-feldspar, quartz, and biotite) and microcrystalline groundmass with minor magnetite–ilmenite, titanite, apatite, and zircon. The equigranular phase has a similar primary mineral assemblage to the porphyritic phase. Their common magnetite-ilmenite-titanite assemblage reflects co-crystallization (magnetite series) from a magma imparting some redox control. However, these granite phases show minor potassic to propylitic alteration mineral assemblages with very minor sulfides, suggesting localized fluid–rock reaction. The composition of plagioclase varies between albite and oligoclase, and K-feldspar is orthoclase commonly displaying considerable turbidity. The An% versus Al/(Ca+Na+K) data indicate that these feldspars are slightly aluminous, reflecting cryptic alteration. Biotite is rich in Fe, plotting near the boundary of primary and re-equilibrated biotite; these biotites formed at temperatures ranging from 670 to 725 °C, based on Ti-in-biotite thermometry. Secondary biotite grains are also locally evident, formed from magmatic-hydrothermal fluids. Secondary fine-grained biotite associated with fine-grained magnetite-pyrite indicates potassic alteration and related Cu±Mo±Au mineralization. Some of these various biotites are partially altered to chlorite at ∼301–361 °C. Like biotite, apatite occurs as both igneous and hydrothermal phases. Based on the concentration of F (4.21–2.90 wt.%), all these apatites are fluorapatites with content of light rare earth elements about 7000 ppm and Eu/Eu* = 0.16.

Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu–Ag mineralisation in the central European Kupferschiefer

Abstract. The analysis of hydrothermal alteration in exploration drill cores allows for fluid–rock interaction processes to be traced, for fluid flow paths to be identified, and thus for vectors in mineral systems to be determined. Hyperspectral imaging techniques are increasingly being employed to fill the scale gap between lab-based petrographic or geochemical analyses and the typical size of exploration targets. Hyperspectral imaging permits the rapid, cost-efficient, and continuous characterisation of alteration mineralogy and texture along entire drill cores, with a spatial sampling of a few millimetres. In this contribution, we present the results of an exploratory study on three mineralised drill cores from the Spremberg–Graustein Kupferschiefer-type Cu–Ag deposit in the Lusatia region of Germany. We demonstrate that hyperspectral imaging is well-suited to recognising and tracking the effects of hydrothermal alteration associated with strata-bound hydrothermal mineralisation. Micro X-ray fluorescence spectrometry was used to corroborate the alteration mineral assemblages identified in hyperspectral data acquired in the visible, near- (400 to 970 nm), shortwave (970 to 2500 nm), mid-wave (2700 to 5300 nm), and longwave infrared (7700 to 12 300 nm). We identified two main shortcomings of the technique, namely the overlapping of some mineral features (e.g. carbonate and illite absorption in the shortwave infrared) and the darkness of the organic-matter-rich dolostones and shales that results in low reflectance. Nevertheless, spectral features associated with iron oxide, kaolinite, sulfate, and carbonates were successfully identified and mapped. We identified different markers of hydrothermal alteration spatially associated with or stratigraphically adjacent to Cu–Ag mineralisation. Importantly, we can clearly distinguish two mineralogically distinct styles of alteration (hematite and ferroan carbonate) that bracket high-grade Cu–Ag mineralisation. Intensive hydrothermal alteration is characterised by the occurrence of well-crystallised kaolinite in the sandstone units immediately below the Kupferschiefer horizon sensu stricto. Proximal Fe-carbonate and kaolinite alteration have not previously been documented for the high-grade Cu–Ag deposits of the central European Kupferschiefer, whereas hematite alteration is well-known in Kupferschiefer-type ore deposits. The latter marks the flow path of oxidising, metal-bearing hydrothermal fluids towards the site of hydrothermal sulfide mineralisation. In contrast, ferroan carbonate alteration in carbonate rocks located above the main mineralised zone is interpreted as a mark of hydrothermal fluid discharge from the mineralising system. Although this study is limited to a small number of drill cores, our results suggest that hyperspectral imaging techniques may be used to identify vectors towards high-grade Cu–Ag mineralisation in Kupferschiefer-type mineral systems.

The thermal conductivity of unlithified granular volcanic materials: The influence of hydrothermal alteration and degree of water saturation

The thermal properties of volcanic materials are required for modelling and for understanding volcanic surface heat fluxes and timescales for cooling magma. However, compared to volcanic rocks, there are relatively few thermal property data for unlithified granular volcanic materials. Here, we measured the thermal properties of a suite of hydrothermally altered powders from La Soufrière de Guadeloupe (Eastern Caribbean) as a function of water saturation degree. Our data show, under dry conditions, that thermal conductivity and thermal diffusivity decrease, and that specific heat capacity does not change systematically, as a function of the degree of alteration of the unlithified granular material. For example, thermal conductivity decreases from ∼0.3 to ∼0.2 W·m−1·K−1 as the quantity of alteration minerals in the samples increases from ∼10 to ∼70 wt%. We interpret the decrease in thermal conductivity with increasing alteration as the result of the lower thermal conductivity of the alteration mineral assemblage relative to the unaltered assemblage. Our data also show that thermal conductivity increases from ∼0.2–0.3 to ∼0.8–1.1 W·m−1·K−1 as saturation degree increases from dry to at, or close to, complete saturation, due to the higher thermal conductivity of water compared to air. We show that an empirical model for variably saturated granular media is in general agreement with our data and provides a framework to predict the thermal conductivity of unlithified granular volcanic materials as a function of saturation degree. The data and modelling provided herein can help improve heat flux calculations designed to inform on volcanic and geothermal processes.