Hydrothermal Alteration Assemblages Research Articles

Characterization of a Metamorphosed Volcanic Stratigraphy and VMS Alteration Halos Using Rock Chip Petrography and Lithogeochemistry: A Case Study from King North, Yilgarn Craton, Western Australia

Despite countless advances in recent years, exploration for volcanogenic massive sulfide (VMS) deposits remains challenging. This is particularly the case in the Yilgarn Craton of Western Australia, where outcrop is limited, weathering is deep and extensive, and metamorphism is variable. At Erayinia in the southern Kurnalpi terrane, intercepts of VMS-style mineralization occur along ~35 km strike length of stratigraphy, and a small Zn (-Cu) deposit has been defined at King (2.15 Mt at 3.47% Zn). An extensive aircore and reverse circulation drilling campaign on the regional stratigraphy identified additional VMS targets, including the King North prospect. Through a combination of detailed rock chip logging, petrography (inc. SEM imaging), and lithogeochemistry, we have reconstructed the volcanic stratigraphy and alteration halos associated with the King North prospect. Hydrothermal alteration assemblages and geochemical characteristics at King North (Mg-Si-K enrichment, Na depletion, and high Sb, Tl, Eu/Eu*, alteration index, CCPI, and normative corundum abundance values) are consistent with an overturned VMS system. The overturned footwall stratigraphy at King North is dominated by metamorphosed volcanic rocks, namely the following: garnet amphibolite (tholeiitic, basaltic), biotite amphibolite (andesitic, calc-alkaline), chlorite–quartz schist (dacitic), and narrow horizons of muscovite–quartz schist (dacitic to rhyolitic, HFSE-enriched). The hanging-wall to the Zn-bearing sequence is characterized by quartz–albite schists (metasedimentary rocks) and thick sequences of amphibolite (calc-alkaline, basaltic andesite). An iron-rich unit (>25% Fe2O3) of chlorite–actinolite–quartz schist, interpreted as a meta-exhalite, is associated with significant Cu-Au mineralization, adjacent to a likely syn-volcanic fault. Extensive Mg metasomatism of the immediate felsic footwall is represented by muscovite–chlorite schist. Diamond drilling into the deep hanging-wall stratigraphy at both King North and King has also revealed the potential for additional, stacked VMS prospective horizons in the greenstone belt stratigraphy. The discovery of HFSE-enriched rhyolites, zones of muscovite–chlorite schist, presence of abundant sulfide-rich argillaceous metasedimentary rocks, and a second upper meta-exhalite horizon further expand the exploration potential of the King–King North region. Our combined petrographic and lithogeochemical approach demonstrates that complex volcanic lithologies and VMS alteration signatures can be established across variably metamorphosed greenstone belts. This has wider implications for more cost-effective exploration across the Yilgarn Craton, utilizing RC drilling to reconstruct the local geology and identify proximal halos, and limiting more costly diamond drilling to key areas of complex geology and deeper EM targets.

Evolution of the Hydrothermal System Associated with the ABM Replacement-Style Volcanogenic Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada

Abstract The ABM deposit, Finlayson Lake district, Yukon, Canada, is a bimodal-felsic, replacement-style volcanogenic massive sulfide (VMS) deposit (19.1 Mt @ 6.6 wt % Zn, 0.9 wt % Cu, 2.0 wt % Pb, 1.4 g/t Au, and 148 g/t Ag) hosted by Late Devonian continental back-arc–related volcanosedimentary rocks of the Kudz Ze Kayah formation. The VMS-related hydrothermal alteration associated with the deposit extends >1 km beyond the mineralization. Zones of pervasive sericite and chlorite alteration occur proximal to the massive sulfide lenses (<50 m) both in the hanging wall and the footwall, and zones of pervasive sericite and moderate sericite ± chlorite alteration extend laterally from the mineralization and into the hanging wall and footwall for hundreds to thousands of meters. Geochemical data and petrographic observations indicate that feldspar destruction and formation of white mica and chlorite were the main alteration processes. In both the hanging wall and footwall to the mineralization, base (e.g., Zn, Cu, Pb) and trace metals (e.g., Sb, Tl, Mo) form halos with elevated values up to 100 and 200 m, respectively. The paragenesis and the formation conditions of the hydrothermal alteration were determined through petrography of hydrothermal alteration assemblages and their crosscutting relationships, electron microprobe analyses of the compositions of white mica, chlorite, and carbonate, and illite-chlorite geothermometry. These data suggest that the sericite ± chlorite assemblage was the earliest and most extensive phase of the hydrothermal alteration that formed at temperatures around 215° ± 30°C. Overprinting the sericite-chlorite assemblage is the pervasive sericite assemblage that formed at temperatures around 250° ± 15°C. The pervasive chlorite assemblage formed at temperatures around 320° ± 10°C and overprints the preceding sericite-rich assemblages. Microprobe analyses of white mica and chlorite generally show that Mg-rich varieties are more common proximal to mineralization and formed earlier in the deposit paragenesis than Fe-rich varieties. Mineralogy derived from short-wave infrared data for mica and chlorite, however, shows no clear spatial trends across the deposit due to complex overprinting relationships between alteration minerals.

Mineralogy and paragenesis of the Meso-Proterozoic Rohil uranium deposit, North Delhi Fold Belt, Rajasthan, India

The Rohil uranium deposit, located about 70 km south of the Khetri copper deposit in the North Delhi Fold Belt, Rajasthan, India, is albitite-hosted. This deposit is associated with Na-metasomatism, mainly affecting the metasedimentary wall rocks. Quartzite, quartz biotite schist, quartz amphibole schist, and graphite schist underwent variable degrees of albitization, as documented by a wide range of altered wall rocks, including weakly altered metasediments to almost pure albitites. These host rocks consist of metamorphic mineral assemblages such as Ca plagioclase – quartz – biotite – Ca amphibole – graphite – garnet – sillimanite – corundum – ilmenite. The hydrothermal alteration assemblages contain variable amounts of albite, chlorite, biotite, sericite, orthoclase, sphene, rutile, allanite, actinolite, and calcite. The textural relationships between primary host rock minerals and hydrothermal alteration products suggest four main stages of metasomatism: (1) an early pre-albitization stage, (2) a Na-metasomatic stage, that are followed by (3) a Na‐Ca-Mg- and (4) K- metasomatic stage. Albites are of two types: albite-1 is subhedral to euhedral and equidimensional, formed by the alteration of metamorphic plagioclase during the Na-metasomatic stage. Albite-2 is feathery and formed in fractures/cavities by precipitation of alkali-rich fluid during Na-Mg-Ca metasomatic stages. Albitization is pervasive on a regional scale. The hydrothermal metasomatic fluids utilized structures such as shears and breccias in the metasediments as conduits and as the loci of uranium mineralization. Major uranium minerals are uraninite, U-silicate, and brannerite. Uranium-bearing minerals occur as disseminations, in veins and as minute inclusions in biotite and titanite within albitized metasediments. The petrographic studies suggest that the metasomatic alteration of host rocks is characterized by albitization, actinolitization, carbonatization, chloritization, sericitization, biotitization, sulfidation, and silicification. The close association of U, REE-bearing minerals, and albite-2 indicates that the main U-REE mineralization stage was coeval with the Na-Ca-Mg and K-metasomatic stages.

Hydrothermal fluid signatures of the Yulong porphyry Cu-Mo deposit: Clues from the composition and U-Pb dating of W-bearing rutile

AbstractHydrothermal rutile (TiO2) is a widely distributed accessory mineral in hydrothermal veins or alteration assemblages of porphyry deposits and provides important information for further understanding hydrothermal fluid signatures. This study determines the geochemical composition and U-Pb dates of hydrothermal rutile from the Yulong porphyry Cu-Mo deposit in east Tibet, China. Three types of TiO2 polymorphs have been identified based on their Raman spectroscopic, textural, and chemical characteristics. (1) Brookite and anatase pseudomorphs after titanite in a fine-grained matrix, indicating low-temperature hydrothermal fluids destabilizing primary Ti-bearing minerals during argillic alteration (type-I). (2) Elongated and prismatic rutile present in hydrothermal veins or in clusters in accompanying alteration envelope characterized by weak zoning (type-II). And (3) rutile intergrown with sulfides in hydrothermal veins, characterized by well-developed patchy and sector zoning (type-III). In contrast to the type-I and type-II TiO2 polymorphs, tungsten is enriched in backscattered bright patches and sector zones in type-III rutile, which is due to the substitution of W6+ in the Ti4+ octahedral site. The mechanism of the enrichment of tungsten is effectively driven by the halogen-rich (F, Cl) aqueous fluids during hydrothermal mineralization. In situ U-Pb dating of the type-III rutile yields a lower intercept age of 41.8 ± 1.2 Ma, which brackets the timing of the Cu-Mo mineralization. The relationship between rutile textures and composition indicates that W-bearing rutile can serve as a recorder of hydrothermal processes in porphyry Cu deposits.

Ediacaran - Cambrian fluid flow in Archean orogenic gold deposits: Evidence from U–Pb SHRIMP hydrothermal monazite ages of the metaturbidite-hosted Córrego do Sítio and Pilar deposits, Quadrilátero Ferrífero, Brazil

Archean orogenic gold deposits are hosted in the Rio das Velhas greenstone belt in the eastern sector of the Quadrilátero Ferrífero, Southern São Francisco Craton region, southeastern Brazil. These include the gold deposits along the NE-trending Córrego do Sítio lineament, stretching for some 15 km, and the Pilar deposit, object of the present study. These deposits occur in an area close to the boundary of the São Francisco craton, which is commonly interpreted to have preserved the Archean and Paleoproterozoic crust from the effects of Ediacaran-Cambrian Araçuaí orogenic front. The gold deposits were formerly interpreted as exclusively Archean (ca. 2.7 Ga). However, recent geochronological data suggest the imprint of the late Ediacaran-Cambrian Brasiliano Orogeny in host rocks of these deposits, as structural modification or hydrothermal alteration assemblages that postdates Archean mineralization. To elucidate those issues, contextual (thin section) in-situ U–Pb SHRIMP dating was conducted on hydrothermal monazite crystals from the Carvoaria and Cachorro Bravo deposits of the Córrego do Sítio lineament and from the Pilar gold deposit. Hydrothermal monazite in mineralized metapelites from Carvoaria yielded a U–Pb Discordia with intercepts at 2,514 ± 22 Ma and 555 ± 19 Ma. Three younger, age-equivalent crystals are concordant and yielded a U–Pb Concordia age of 539 ± 9 Ma, identical within uncertainties to the lower intercept age. Monazite from the Cachorro Bravo deposit yielded U–Pb Concordia ages of 551 ± 10 Ma and 510 ± 11 Ma. Monazite from the veined (silicified) sulfide-schist from the Pilar deposit is depleted in U, precluding the calculation of robust U–Pb ages, and disclosed a mean 208Pb/232Th average age of 508.2 ± 6.4 Ma. These results reinforce the proposal of a strong Ediacaran-Cambrian imprint related to the final stages of the Brasiliano orogenic event, affecting Archean gold deposits throughout the Quadrilátero Ferrífero. Consequently, the results point to the importance of mapping Brasiliano-related structures that control the spatial arrangement of the gold deposits, such as the Córrego do Sítio lineament, consisting of an important exploration target. This major NE-SW trending strike-slip shear zone hosts several gold deposits and might represent an Archean structure reactivated during the Brasiliano Orogeny that possibly led the large volume of Ediacaran-Cambrian post-collisional hydrothermal fluids among the east sector of the Quadrilátero Ferrífero.

Origin of the Bada porphyry Cu–Au deposit, eastern Tibet: Geology and isotope geochemistry (C–O–S–Pb) constraints

Bada is a newly discovered Cu–Au deposit located in the south part of the Yulong porphyry Cu (–Mo–Au) belt (YPCB), eastern Tibet. The Cu–Au mineralization, mainly hosted by the alkali-rich quartz monzonite porphyry (QMP), with minor by the fine-grained monzonitic porphyry (MP) and surrounding strata of the Nanxin Formation. The hydrothermal alteration assemblages recognized at Bada mainly include potassic, phyllic, propylitic, and argillic. The Cu–Au mineralization occurs within the potassic and phyllic alteration zones. Three main metallogenic stages have been identified: stage I ore-barren quartz ± dolomite ± K-feldspar ± pyrite veins, stage IIa dolomite ± quartz + pyrite veins, stage IIb dolomite ± quartz + polymetallic sulfide veins, and stage III dolomite ± calcite veins. The dolomite δ13CPDB values from stage I to III range from –2.2 – –1.8 ‰, –6.8 – –3.4 ‰, to –4.2 – 1.9 ‰. Their δ18OSMOW values vary from 7.7 – 9.8 ‰, 6.5 – 11.9 ‰, to 7.8 – 11.4 ‰, respectively. The corresponding calculated δ18Ofluid values ranging from 6.0 to 6.3 ‰ for stage I, and –1.1 to 4.3 ‰ for stage II, and –1.7 to 1.9 ‰ for stage III. The C–O isotopic compositions imply that the ore-forming fluids were dominated by magmatic water with the input of meteoric water. The narrow δ34S value range of sulfides (1.43 to 6.61 ‰, mean 4.64 ‰) reflects a predominantly magmatic origin for the sulfur. The Pb isotopic compositions of most sulfide samples coincide with the alkali-rich porphyries in the region, indicating that the alkalic porphyries provided the main source of Pb. Combined with the zircon U–Pb age of the ore-bearing porphyry (ca. 35.0 Ma), the Bada deposit is defined as a porphyry Cu–Au deposit in a late-collisional setting. The study of the genesis of Bada provides a better understanding of the Cu–Au mineralization potential of the southern YPCB.

Evolution of ore-forming fluids in a post-collisional porphyry Cu-Au system: A case study from the Bučim deposit, Republic of North Macedonia

The Bučim porphyry Cu-Au deposit is situated in the westernmost part of the Serbo-Macedonian Massif, Republic of North Macedonia. The deposit is hosted by Precambrian gneisses with tectonically imbedded rare lenses of Palaeozoic amphibolites. The mineralization is spatially associated with intrusions of andesitic to trachyandesitic magmatic bodies. The geochemical signature of the intrusions and their radiogenic age suggest that the Bučim porphyry Cu-Au deposit represents a product of the Oligocene/Early Miocene post-collisional magmatic activity. The Vršnik intrusion, the youngest among four intrusions at the Bučim deposit, has not been subjected to later magmatic overprints and was therefore selected for studying the evolution of ore-forming fluids in post-collisional porphyry Cu-Au systems.The Cu-Au mineralization is spatially associated with hydrothermal alteration assemblages within and around the Vršnik intrusion. The infiltration of hot, near-neutral, oxidized, and highly saline and metal-rich aqueous solutions developed a mineralized potassic alteration zone. The fluid inclusion study suggests the formation temperature and pressure of 600-645⁰C and 0.675 kbar, respectively. This alteration zone is characterized by the replacement of amphibole with biotite and crystallization of hydrothermal biotite, magnetite, and hematite within the fine-grained groundmass of the intrusion. The oxidation of ferrous to ferric iron in this alteration zone has been recognized as a possible trigger for sulfate reduction and the consequent deposition of sulfide mineralization.The fluid inclusion study reflects that the barren phyllic alteration zone represents a product of infiltration of hot and saline fluids with a high metal-transport capability. Anyhow, sericitization and silicification suggest a low pH that might have played a significant role in preventing Cu deposition. The stable isotope composition of carbonates associated with the phyllic alteration zone point to a contribution of magmatic CO2.The mineralized chlorite alteration zone is mainly well developed along the contact between the intrusion and surrounding gneisses and reflects conditions of a high water/rock ratio. The fluid inclusion assemblages composed exclusively of vapor-rich inclusions indicate fluid flashing, probably associated with the transition from the lithostatic to hydrostatic regime at the point when the system cooled down to approximately 400⁰C. The liquid + vapor fluid inclusion assemblages point to dilution and cooling episodes, apparently related to mixing of ore-bearing fluids and meteoric water. The decrease in temperature as well as the decrease in the fluid salinity are associated with the mobility of metals transported in forms of their chloride complexes, including Cu, Au and Ag.The barren argillic alteration zone was developed under the influence of relatively cold and diluted fluids, with very limited capability for transport of metals. The water/rock ratio was relatively high but lower comparing the chlorite alteration zone.The presented study revealed that development of hydrothermal alteration zones and evolution of ore-forming fluids in post-collisional porphyry Cu-Au systems are comparable with those known from subduction-related magmatic arcs.

U-Pb zircon-titanite-apatite age constraints on basin development and basin inversion in the Kiruna mining district, Sweden

To constrain the tectonothermal evolution of the type locality for iron oxide-apatite deposits, we have obtained U-Pb zircon, titanite, and apatite age data for the Kiruna mining district in northernmost Sweden. The results indicate that the host basin initiated in an overall extensional regime as indicated by the deposition of alluvial conglomerates and greywackes. A volcanic intercalation in a conglomerate unit approximately 1 km northwest of the Luossavaara iron oxide-apatite deposit yields a U-Pb zircon concordia age of 1887 ± 3 Ma representing the timing of the earliest Orosirian volcanism in the central Kiruna mining district coinciding with the onset of basin development. In-situ analysis of titanite sitting on hydrothermally altered fracture planes within a cataclastic fault damage zone (c. 270 m from the fault core system associated with the Luossavaara iron oxide-apatite deposit) yields complex U-Pb data. Applying a strict discordance filter yields seven analyses (<1.5 % relative discordance which included six common Pb-corrected analyses and one analysis where no significant common Pb was detected) that yield a 207Pb/206Pb weighted average age of 1889 ± 26 Ma that we interpret as the most probable minimum age of fault initiation. The overlap in age of the host volcanic rock and the fault implies that during basin development, contemporaneous volcanism and syn-volcanic faults may have played an important role during iron ore emplacement. A U-Pb lower intercept age of apatite from the Nukutus iron oxide-apatite deposit provides a first indication that the area was subsequently buried and metamorphosed under upper greenschist-facies conditions and later tectonically exhumed and cooled below the apatite closure temperature at 1805 ± 26 Ma. Further temporal constraints on exhumation and basin inversion is given by syn-tectonic titanite along a brittle-ductile reverse shear zone to the east of the study area that shows limited presence of significant common Pb and is part of a sodic-calcic + Fe + Cl hydrothermal alteration assemblage. Titanite grains that show a well-developed zonation yield a U-Pb concordia age of 1812 ± 3 Ma, which we interpret as the onset of basin inversion. Titanite grains from the same sample that show less pronounced zonation yield a U-Pb concordia age of 1802 ± 8 Ma, tentatively indicating that the tectonothermal activity lasted up to c. 20 m.y.

Comments on “Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur Granite associated with vein type Gogi uranium deposit, Bhima basin, Eastern Dharwar Craton, India: Implication for physico-chemical conditions of ore formation” by Mohd Qaim Raza and Nurul Absar. [Ore Geol. Review, 2021, 128, 1–20

Comments on “Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur Granite associated with vein type Gogi uranium deposit, Bhima basin, Eastern Dharwar Craton, India: Implication for physico-chemical conditions of ore formation” by Mohd Qaim Raza and Nurul Absar. [Ore Geol. Review, 2021, 128, 1–20

New constraints on genesis of the Qiangma gold deposit, southern North China Craton: Evidence from rutile U-Pb dating, pyrite trace element composition and S-Pb isotopes

The Xiaoqinling district, situated at the southern North China Craton, is the second largest gold province in China. The genesis of gold deposits in this district, however, has been debated for decades because of lacking unequivocal constraints on mineralization age and gold source(s). In this study, we present ore-related hydrothermal rutile U-Pb dating, pyrite trace element compositions, and S-Pb isotopes of the Qiangma Au deposit that is one of the largest gold deposits in the Xiaoqinling district, to provide new constraints on genesis of the regional gold mineralization. The Qiangma Au deposit is formed by four paragenetic stages consisting of milky quartz—coarse-grained pyrite, grey quartz—fine-grained pyrite, grey quartz—polymetallic sulfides, and quartz-calcite. The gold mineralization is accompanied by intensively hydrothermal alteration assemblage comprising quartz, sericite, sulfides, and calcite. Hydrothermal rutile texturally intergrown with gold-bearing pyrite and sericite yields a U-Pb age of 128.1 ± 7.4 Ma (2σ, MSWD = 0.9), representing the timing of gold mineralization at Qiangma. This age is consistent with the ages of regional felsic to mafic magmas and metamorphic core complexes as results of late Mesozoic tectonic reactivation of the North China Craton. Petrographic investigation and LA-ICP-MS trace element analysis show that gold mainly occurs as native gold with minor amounts of lattice gold within pyrite, which has no correlation with As and Te. Pyrite from the four stages have sulfur isotopes mainly between 1.8 and 5.8 ‰ (mean of 3.7 ‰), suggesting a magmatic affiliation. Pyrite from quartz-polymetallic sulfides stage has lead isotopes of 17.51–17.71 for 206Pb/204Pb, 15.39–15.49 for 207Pb/204Pb, and 38.01–38.53 for 208Pb/204Pb, which are distinctly different from those of Mesoproterozoic clastic and carbonate rocks and late Mesozoic granites, but are broadly consistent with late Mesozoic subcontinental lithospheric mantle derived mafic dikes. Collectively, our new lines of evidence suggest that the Qiangma Au deposit, as well as some gold deposits in the Xiaoqinling district, is most likely related to mantle-derived magmas generated in an extensional setting related to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton.

Reservoir characterization of the Paka geothermal system in Kenya: Insights from borehole PK-01

Trachytic and basaltic volcanic material erupted from ∼390 ka to Holocene coalesced to form the 136 km2 active volcanic edifice of Paka on top of an older ∼580 ka plateau eruptive sequence. The latter constrains the maximum age for the Paka geothermal system manifested mainly as surface fumaroles and hot altered grounds. To date, three exploration wells have been drilled to >2500 m depth. Results from analyses of well PK-01 show that the upper part of the well (0–1050 m) is exclusively a trachyte lava sequence while the lower part (1050–2552 m) penetrates sequences of trachyte, intermediate, basaltic and syenite intrusive rocks. Temperature attained in the well based on hydrothermal alteration assemblage vary from ∼180 °C at 400 m depth where quartz was first observed in the drill cuttings to >220 °C within the chlorite-illite zone at 700 m and 280 °C below 1450 m in the actinolite-epidote zone. Fluid inclusion homogenization temperatures (Th) range widely between 211 and 286 °C at 1390 m and 209–271 °C at 2550 m. Fluid inclusion indicate that the reservoir fluid from the paleo system was bicarbonate-rich and of very low salinity according to ice-melting temperature, which is comparable to aqueous fluid discharge from the well. Stabilised well temperature increases with depth to a maximum of 245 °C at 1800 m depth where a reversal occurs and becomes nearly isothermal at 230 °C, then increases slightly below 2200 m before an inversion to about 220 °C. The main high-temperature geothermal system is hosted within the lower part of the well (1050–2552 m). It is probably steam-dominated above 900 m depth where calculated formation temperatures intersect the boiling point curve for pure water, and liquid-dominated below 900 m. Suitability modelling from surface data integration indicates that the most suitable area (P90) for geothermal development is within an area of 12 km2 in the summit area and Northeast of the Paka caldera.

Sericite 40Ar/39Ar and zircon U-Pb dating of the Liziyuan gold deposit, West Qinling orogen, central China: Implications for ore genesis and tectonic setting

The West Qinling orogen in central China contains several dozens of gold deposits. However, absolute ages of gold mineralization and its relations to regional tectonism, magmatism, and metamorphism remain not tightly constrained for most of these gold deposits. Here we integrate 40Ar/39Ar dating of sericite from various hydrothermal alteration assemblages and U-Pb dating of zircon grains from magmatic rocks within and surrounding the Liziyuan gold deposit in the northern belt of the West Qinling orogen to provide new insights into the age and tectonic setting of gold mineralization. The Liziyuan gold deposit consists of six ore camps (Yuzigou, Liushagou, Jiancaowan, Kuangou, Suishizi, and Yingfang) that are hosted in Ordovician greenschist facies metamorphosed sedimentary-volcanic rocks and, less significantly, the Middle Triassic Tianzishan monzogranite pluton intruding the metamorphic rocks. Gold mineralization consists of auriferous quartz-pyrite veins and subordinate sulfide disseminations in hydrothermally altered wall rocks, with minor calcite-polymetallic sulfide veins in the monzogranite-hosted orebodies. Ore-related alteration assemblages are dominated by quartz, sericite, pyrite, chlorite, and calcite, with gold occurring mostly as native gold grains enclosed in or filling microfractures of pyrite and quartz. Barren alteration assemblages, dominated by K-feldspar, quartz, sericite, and pyrite, are locally developed along eastern margin of the Tianzishan monzogranite. Two sericite aggregates extracted from gold ores in the Liushagou camp yield well-defined 40Ar/39Ar plateau ages of 200.1 ± 1.2 and 198.8 ± 1.1 Ma (2σ), whereas those from barren alteration assemblages in the Tianzishan monzogranite, which hosts the Suishizi ore camp, give rise to older 40Ar/39Ar plateau ages of 211.9 ± 1.2 and 211.3 ± 1.3 Ma (2σ). The Tianzishan monzogranite pluton and several dioritic to granitic stocks and dykes in and surrounding the gold mines have zircon U-Pb ages ranging from 241.3 ± 1.2 to 212.4 ± 0.9 Ma (2σ). These new isotopic ages suggest that the Liziyuan gold deposit formed at the Triassic-Jurassic boundary, arguably representing the youngest orogenic-type gold mineralization event over the northern belt of the West Qinling orogen. Gold mineralization postdates magmatism in the Liziyuan gold deposit at least by about 10 million years, precluding a possible genetic relation between the two. Rather, the ore genesis was likely related to metamorphic fluids released from Paleozoic sedimentary and volcanic rocks in relation to the Qinling orogenesis. In contrast, the barren hydrothermal alteration assemblages were likely caused by fluids exsolved from a magma presumably represented by the ca. 212 Ma diorite dyke. Results presented here, when combined with independent geological and geochemical data, suggest that the Liziyuan gold deposit formed most likely under a post-collisional extensional setting related to the West Qinling orogeny.

The Relation between Trace Element Composition of Cu-(Fe) Sulfides and Hydrothermal Alteration in a Porphyry Copper Deposit: Insights from the Chuquicamata Underground Mine, Chile

Porphyry Cu-Mo deposits are among the world’s largest source of Cu, Mo, and Re, and are also an important source of other trace elements, such as Au and Ag. Despite the fact that chalcopyrite, bornite, and pyrite are the most common sulfides in this deposit type, their trace element content remains poorly constrained. In particular, little is known about minor and trace elements partitioning into Cu-(Fe) sulfides as a function of temperature and pH of the hydrothermal fluid. In this study, we report a comprehensive geochemical database of chalcopyrite, bornite, and pyrite in the super-giant Chuquicamata porphyry Cu-Mo deposit in northern Chile. The aim of our study, focused on the new Chuquicamata Underground mine, was to evaluate the trace element composition of each sulfide from the different hydrothermal alteration assemblages in the deposit. Our approach combines the electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of sulfide minerals obtained from six representative drill cores that crosscut the chloritic (propylitic), background potassic, intense potassic, and quartz-sericite (phyllic) alteration zones. Microanalytical results show that chalcopyrite, bornite, and pyrite contain several trace elements, and the concentration varies significantly between hydrothermal alteration assemblages. Chalcopyrite, for example, is a host of Se (≤22,000 ppm), Pb (≤83.00 ppm), Sn (≤68.20 ppm), Ag (≤45.1 ppm), Bi (≤25.9 ppm), and In (≤22.8 ppm). Higher concentrations of Se, In, Pb, and Sn in chalcopyrite are related to the high temperature background potassic alteration, whereas lower concentrations of these elements are associated with the lower temperature alteration types: quartz-sericite and chloritic. Bornite, on the other hand, is only observed in the intense and background potassic alteration zones and is a significant host of Ag (≤752 ppm) and Bi (≤2960 ppm). Higher concentrations of Ag and Sn in bornite are associated with the intense potassic alteration, whereas lower concentrations of those two elements are observed in the background potassic alteration. Among all of the sulfide minerals analyzed, pyrite is the most significant host of trace elements, with significant concentrations of Co (≤1530 ppm), Ni (≤960 ppm), Cu (≤9700 ppm), and Ag (≤450 ppm). Co, Ni, Ag, and Cu concentration in pyrite vary with alteration: higher Ag and Cu concentrations are related to the high temperature background potassic alteration. The highest Co contents are associated with lower temperature alteration types (e.g., chloritic). These data indicate that the trace element concentration of chalcopyrite, bornite, and pyrite changed as a function of hydrothermal alteration is controlled by several factors, including temperature, pH, fO2, fS2, and the presence of co-crystallizing phases. Overall, our results provide new information on how trace element partitioning into sulfides relates to the main hydrothermal and mineralization events controlling the elemental budget at Chuquicamata. In particular, our data show that elemental ratios in chalcopyrite (e.g., Se/In) and, most importantly, pyrite (e.g., Ag/Co and Co/Cu) bear the potential for vectoring towards porphyry mineralization and higher Cu resources.

Evolution of Archean Sanukitoids from the Otto Stock by Magma Mixing and Na–K Metasomatism: Evidence from Petrological Observations and Lithium Isotope Geochemistry

Abstract Petrogenetic models for the genesis of Archean sanukitoids are diverse in the literature but most point to the initiation of plate tectonics and the construction of the first proto-continents during the Late Archean. These rocks include diorites, monzonites, monzodiorites, quartz monzodiorites, trachyandesites, granodiorites and tonalites, and are defined as having SiO2 = 55–60 wt%, Na2O + K2O = 6 wt%, MgO &amp;gt; 6 wt%, Mg# &amp;gt; 60, Ni–Cr &amp;gt; 100 ppm, Sr and Ba = 600–1800 ppm, chondrite-normalized Ce and Yb values of 80–250 and 4–10, respectively, and no Eu anomalies. Petrogenetic models include the partial melting of mantle peridotite previously metasomatized by fluids of crustal or mantle origin, partial melting of subducted slabs and assimilation of peridotite, and partial melting of undepleted peridotite with subsequent mixing with tonalite–trondhjemite–granodiorite (TTG) crustal partial melts and addition of carbonatite, all of which may or may not include subsequent differentiation by fractional crystallization. Here, extraordinary textural relations unequivocally show that at the Late Archean Otto stock, Abitibi, Canada, sanukitoids resulted from the assimilation of clinopyroxenite by monzonitic magmas and coeval magmatic–hydrothermal Na–K metasomatism. The metasomatized monzonites (± quartz), melanogranites, granodiorites, and sanukitoidal melanoporphyries were subsequently cut by swarms of lamprophyre dykes hosting a different set of hydrothermal alteration assemblages, including propylitic alteration, biotitization, chloritization, and pyritization, along with associated enrichments in Au. These alterations are shown to be associated with the nearby Cadillac–Larder Lake Fault Zone and the world-class Kirkland Lake gold deposit. The Li isotopic compositions of these rocks correlate with Au concentrations and range from magmatic values of ∼ +4 ‰ up to +10·4 ‰ in Na–K metasomatized and Au-depleted rocks, and from magmatic values of ∼4 ‰ down to 0·6 ‰ in rocks hosting propylitic, chloritic, and biotitic alteration and Au enrichments of up to 2·9 ppm Au.

The Kirganik alkalic porphyry Cu-Au prospect in Kamchatka, Eastern Russia: A shoshonite-related, silica-undersaturated system in a Late Cretaceous island arc setting

The Kirganik prospect is a silica-undersaturated, shoshonite-related, alkalic, porphyry Cu-Au system developed within the Late Cretaceous island arc of Central Kamchatka. It is associated with a potassic volcano-plutonic suite of rocks that range from monzogabbro- and monzodiorite- to monzonite- and syenite-porphyry, followed by a second, compositionally overlapping magmatic cycle of trachytic syenite-porphyry, and possibly other, more differentiated monzonitic phases. The rocks exhibit a subduction-related, island arc rock affinity, with the magma generation involving low degree (~1–3 vol%) partial melting of metasomatized, K-rich lithospheric mantle, followed by amphibole fractionation in a deep, probably lower crustal, magma chamber, with subsequent fractionation of accessory minerals, clinopyroxene and possibly biotite in shallower magma chambers. An early calc-potassic alteration assemblage of K feldspar-pyroxene-apatite-magnetite is closely related to small syenite intrusions. This stage is overprinted by more extensive potassic alteration zones of similarly quartz-free biotite-K feldspar-magnetite, some of which contain abundant Cu-sulfides (mostly bornite and chalcopyrite, with minor chalcocite), minor native gold and PGE minerals. Subsequent sodic-potassic to calcic-sodic-potassic alteration comprising albite-magnetite-calcite-epidote, and propylitic assemblages of chlorite-magnetite-epidote ± pyrite are much more weakly developed. These hydrothermal alteration assemblages form lens-shaped, replacement-style zones, rather than stockworks of well-defined veinlets/stringers, particularly quartz-sulfide veinlets, which are absent. Apatite from calc-potassic and potassic alteration assemblages contains low salinity (5–10 wt% NaCl-eq.) liquid-gaseous fluid inclusions indicating a homogenous, low salinity, supercritical aqueous fluid exsolved directly from a cooling and degassing pluton, with no boiling and unmixing into hypersaline and gaseous phases. Under inferred high temperatures of calc-potassic and potassic alteration (450–550 to 400–420 °C) and significant (2.1–2.2 to 1.5–1.8 kbar) pressure, this suggests a deep (>5 km?), possibly root-level of a larger, shallower, since eroded, porphyry Cu-Au system. The current geological setting and distribution of alteration and mineralization can be explained by a sharp, up to 90°, post-mineral tilting of the initial narrow, subvertical, 'finger-like' intrusions and related mineralized system, followed by deep exhumation. As a consequence, a significant part (>1.5 km) of the deep porphyry system is currently exposed at surface.

Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur granite associated with vein-type Gogi uranium deposit, Bhima Basin, Eastern Dharwar Craton, India: Implications for physico-chemical conditions of ore formation

Eastern Dharwar craton (EDC) is a major uranium province in India hosting number of unconformity related, vein-type and carbonate hosted strata-bound uranium deposits. Gogi uranium deposit (GUD) is a structurally controlled hydrothermal ore deposit (~1.35 Ga), hosted in a reverse fault zone between the foot-wall Mesoproterozoic Bhima Group carbonates and the hanging wall basement granites (~2.5 Ga) of the EDC. The hanging wall granites, locally known as Shahapur granite show extensive chloritisation and epidotisation, and constitute the propylitic outer zone of the main hydrothermal system of the GUD. We conducted systematic mineralogical and geochemical studies on hydrothermally altered assemblages to understand physico-chemical conditions of hydrothermal fluid. Quartz, epidote, chlorite, titanite, fluorite and hematite are the main hydrothermal mineral phases. Chlorites occur in two different forms, as pseudomorphic replacement of hornblende and as vein/cavity fillings. Epidotes occur as coarse prismatic grains in the main veins, or as replacement of albite in the wall rock. Mineralogical, textural data and geochemical mass balance calculation indicate that about 17.9% of total albites (Modal abundance = ~48%) and all the hornblendes (modal abundance = ~7.5%) of the parent rock were broken down and reacted to form quartz, chlorite, epidote, secondary titanite and hematite. The hydrothermal reaction resulted in creation of 13 vol% secondary porosity in the host rock and acidification of the evolved fluid. Thermodynamic calculation on chlorite compositions indicates ~ 250 °C temperature of formation and oxidising nature of ambient fluid, with log fO2 ranging between −37 and −31 (avg. −35). Available fluid inclusion data, in conjunction with chlorite geothermometric results, show fluid entrapment pressure between 625 bar and 792 bar, which is consistent with emplacement of hydrothermal fluid in low-pressure brittle deformation zone. We surmise that U6+ was liberated from basement granite by reaction with oxidised acidic fluid, transported as chloride, carbonate complexes and precipitated in high permeability fault-zone, possibly because of mixing with reduced- basin derived fluids.

Overprinting porphyry‐type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect, southwestern Sto. Tomas II (Philex), Baguio District, Philippines

AbstractThe Southwest prospect is located at the southwestern periphery of the Sto. Tomas II porphyry copper–gold deposit in the Baguio District, northwestern Luzon, Philippines. The Southwest prospect hosts a copper‐gold mineralization related to a complex of porphyry intrusions, breccia facies, and overlapping porphyry‐type veinlets emplaced within the basement Pugo metavolcanics rocks and conglomerates of the Zigzag Formation. The occurrences of porphyry‐type veinlets and potassic alteration hosted in the complex are thought to be indications of the presence of blind porphyry deposits within the Sto. Tomas II vicinity. The complex is composed of at least four broadly mineralogically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. These intrusions were accompanied with at least five breccia facies that were formed by the explosive brecciation, induced by the magmatic–hydrothermal processes and phreatomagmatic activities during the emplacement of the various intrusions. Hydrothermal alteration assemblages consisting of potassic, chlorite–magnetite, propylitic and sericite–chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Elevated copper and gold grades correspond to (a) chalcopyrite–bornite assemblage in the potassic alteration in the syn‐mineralization early‐mineralization diorite (EMD) and contemporaneous veinlets and (b) chalcopyrite‐rich mineralization associated with the chalcopyrite–magnetite–chlorite–actinolite±sericite veinlets contemporaneous with the chlorite–magnetite alteration. Erratic remarkable concentrations of gold were also present in the late‐mineralization Late Diorite (LD). High XMg of calcic amphiboles (&gt;0.60) in the intrusive rocks indicate that the magmas have been oxidizing since the early stages of crystallization, while a gap in the composition of Al between the rim and the cores of the calcic amphiboles in the EMD and LD indicate decompression at some point during the crystallization of these intrusive rocks. Fluid inclusion microthermometry suggests the trapping of immiscible fluids that formed the potassic alteration, associated ore mineralization, and sheeted quartz veinlets. The corresponding formation conditions of the shallower and deeper quartz veinlets were estimated at pressures of 50 and 30 MPa and temperatures of 554 and 436°C at depths of 1.9 and 1.1 km. Temperature data from the chlorite indicate that the chalcopyrite‐rich mineralization associated with the chlorite–magnetite alteration was formed at a much lower temperature (ca. 290°C) than the potassic alteration. Evidence from the vein offsetting matrix suggests multiple intrusions within the EMD, despite the K‐Ar ages of the potassic alteration in EMD and hornblende in the LD of about the same age at 3.5 ± 0.3 Ma. The K‐Ar age of the potassic alteration was likely to be thermally reset as a result of the overprinting hydrothermal alteration. The constrained K‐Ar ages also indicate earlier formed intrusive rocks in the Southwest prospect, possibly coeval to the earliest “dark diorite” intrusion in the Sto. Tomas II deposit. In addition, the range of δ34S of sulfide minerals from +1.8‰ to +5.1‰ in the Southwest prospect closely overlaps with the rest of the porphyry copper and epithermal deposits in the Sto. Tomas II deposit and its vicinity. This indicates that the sulfides may have formed from a homogeneous source of the porphyry copper deposits and epithermal deposits in the Sto. Tomas II orebody and its vicinity. The evidence presented in this work proves that the porphyry copper‐type veinlets and the adjacent potassic alteration in the Southwest prospect are formed earlier and at a shallower level in contrast with the other porphyry deposits in the Baguio District.

Aspects of hydrothermal alteration assemblages around Bakwani-Bagwa, Northwestern Nigeria; Insights from multifractal analysis of measured thresholds for mineral exploration.

This study is aimed at identifying highly altered zones by applying concentration area (C-A) fractal model technique on Landsat ETM+ image of area around Bakwani-Bagwa, Northwestern Nigeria. Multifractal analysis was used to quantify degree of alteration on satellite images processed using crosta technique. A log–log plot was used to display the multifractal characteristics for clay and iron altered images and identify threshold values. Maximum and minimum threshold values obtained from fractal analysis are 1.74 and 4.01 for clay altered images and 0.607 and 3.04 for iron altered images. High clay and iron alteration zones accounts for 5.89% and 7.84%, covering an area of 22.28km2 and 29.65km2 respectively. Delineating highly altered zones from these images reveals higher alteration largely within silicified sheared rocks and areas around faults. The current study proved the multifractal processing techniques to be valuable mapping tools for geological reconnaissance which can be extended to a large area of the Nigerian basement and provide preliminary hydrothermal alteration maps that are accurate as well as cost and time effective.

4D history of the Nimbus VHMS ore deposit in the Yilgarn Craton, Western Australia

Due to its unique mineralogy in the Archaean Yilgarn Craton, the origin of the high-grade Nimbus Ag-Zn-(Au) deposit has been contentious for a number of years. Recent interpretations of the deposit as a shallow water and low temperature volcanic-hosted massive sulphide (VHMS) deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), were based on detailed studies of its volcanology, mineralogy, hydrothermal alteration assemblages, geochemistry, multiple S isotopes and trace element content of sulphide. However, this model has been questioned in favour of a late, epithermal fault-hosted system. Effective greenfields exploration for similar deposits elsewhere requires a robust deposit model, and well understood timing of ore formation. We present a comprehensive multi-disciplinary study to further constrain the processes involved in the evolution of Nimbus and generate a 4D evolutionary model of the system.Host rock dacite formation is well constrained by SHRIMP U-Pb zircon geochronology to c. 2703 Ma. Re-Os ages for the first sulphide phase, gives an imprecise maximum age to the ore formation of 2682 ± 110 Ma. High precision Pb-isotope variations in ore-stage and late galenas track changes in the sulphides over time, complemented by new geochronology. The Pb-isotope data indicates that the polymetallic ore precipitation happened at the same time as the volcanism, with the second generation indicating minor, local remobilisation of Pb in late quartz-carbonate veins at c. 2630 Ma. 40Ar/39Ar dating of sericite-altered plagioclase from foliated dacite, and U-Pb SHRIMP dating of syn- to post-deformation monazites have yielded ages of ~2630 Ma. The late event at c. 2630 Ma is most likely related to widespread late low-Ca granite emplacement across the Eastern Goldfields.This study provides arguments that suggest that Nimbus is a syn-volcanic deposit. The ore timing is consistent with the model of replacement-type VHMS deposit, with the age of the main mineralisation similar to the age of the dacitic host-rocks at 2703 Ma. The late event modifies the pre-existing ore and overprints the previous ore-related alteration ages from the altered plagioclase. High precision, double spiked, Pb isotope constraints from late quartz-carbonate veins provides an opportunity to vector to hidden deposits.

Geology, mineralogy and stable isotope geochemistry of the Dzuunmod area in northern Mongolia: Constraints for gold ore genesis and sources

The Dzuunmod area located in the North Khentii Gold belt (NKGB) of northern Mongolia includes lode gold deposits such as Gatsuurt, Sujigtei and Boroo with several minor gold deposits and occurrences. They show similar hydrothermal alteration assemblages (sericitic, siliceous and potassic) and ore mineral assemblages (pyrite and arsenopyrite with minor amount of galena, sphalerite and chalcopyrite). Gold occurs as native form and invisible gold in pyrite and arsenopyrite. The major sulfide minerals are separated into earlier non-auriferous stage and later auriferous grains containing invisible gold. Native gold postdates the major sulfide mineralization.Gold and arsenic content of pyrite grains indicates that gold exists mainly as solid solution form (Au+1) in the Gatsuurt and Boroo deposit whereas gold nanoparticle (Au0) is present in the Sujigtei deposit. High Co/Ni and Mo/Ni ratios of pyrite grain suggest a post-sedimentary or hydrothermal origin and the ore-forming fluid was significantly affected by fluid-host rock interactions during mineralization processes.Large variation of δ34S values of pyrite and arsenopyrite from −2.6‰ to 17.2‰ indicates that sulfur seems to be mainly derived from a source with heterogeneous sulfur isotope composition, even though the role of magmatic sulfur as one of possible sulfur sources cannot be ruled out. Consistent with geological evidence, relatively positive δ34S values suggest that sulfidation plays an important role for gold and sulfide precipitation. The calculated δ18O values of hydrothermal fluid from the measured δ18O values of quartz samples (from 14.7‰ to 17.7‰) indicate a metamorphic derivation of ore-forming fluid.Gold mineralization processes in the Dzuunmod area seem to occur several times by multiple input of hydrothermal fluid and fluid-host rock interactions. The gold deposits in the Dzuunmod area are considered to be orogenic gold type influenced by fluid-host rock interactions in the deposit area.