Metamorphic Host Rocks Research Articles

Mineralization characteristics of Lead-Zinc-Copper deposits in Akdağmadeni Region (Northern Central Anatolia, Türkiye): Integration of field study, geochemical, isotope, and geophysical data

The Akdağmadeni region is one of the important Pb-Zn-Cu metallogenic provinces in Türkiye. Most of the Pb-Zn-Cu deposits in the region are located near granitoid intrusions within metamorphic rocks, and they are typically classified as skarn-type ores associated with granitoids. However, no relationship has been determined between the Başçatak prospect and any granitoid outcrop. This raises the question of whether the Pb-Zn-Cu mineralization in the region is related to granitoids or if magmatic processes remobilized pre-existing mineralization. Observations from field studies suggest that mineralization in the Başçatak prospect is a stratiform type, metamorphosed occurrence that might have occurred earlier than the granitic intrusions. The geochemical data indicated that the granitoids have low-grade, subeconomic Cu potential and no Zn productivity potential, supporting these observations. Geophysical data also show that there is no intrusive body beneath this prospect. Granitoid-related deposits (Karapir - Ortaköy and Akçakışla) exhibit two distinct occurrences around the contact between the granite and the surrounding metamorphic rocks. The first type of occurrence (O-1) is formed at the contact and contains magnetite, pyrrhotite, and chalcopyrite. The second type of occurrence (O-2) is located outside the contact and is rich in sphalerite and galena. Both O-1 and O-2 contain skarn minerals along with ore minerals. The δ34S values of sulfide minerals from the deposits range from −0,7 to 7,5 ‰ (V-CDT). These values overlap with those of both magmatic sulfur and reduced sulfur from seawater-dissolved sulfate, making it difficult to suggest a sulfur source without additional data. Lead isotope compositions of the galenas from all deposits plot above the average crustal growth curve, suggesting an upper crustal and orogenic source similar to Western Mediterranean and Türkiye type materials described in the literature for the lead, source in different time intervals. Furthermore, Pb isotope geochemistry suggests a contemporaneous age with the host metamorphic rocks (Carboniferous – Lower Permian) for the Başçatak prospect. These results support observations indicating a syn-genetic formation for the Başçatak prospect. The age range of granite-related deposits forms two sub-groups:105–77 Ma and 61–50 Ma corresponding to the Upper Cretaceous and Paleocene transition. These data indicate that that the galenas were formed in two different stages in these mineralizations. Stratiform mineralization in the Başçatak prospect likely formed either through exhalative sedimentary processes by hydrothermal fluids or through chemical sedimentary processes under reductive conditions in a marine environment during metamorphism. O-1 appears to have been formed by hydrothermal fluids developed during granitic magmatism. A plausible formation process for O-2 involves the leaching of sulfur, lead and other metals from Başçatak type enrichments in metamorphics, with transportation and deposition within the epidotized calc-schist and marbles outside the granitoid contacts. The uplift of granitic intrusions prepared the channels and depositional environment for O-2 and caused heating of the hydrothermal fluids during this mineralization period.

The composite metamorphic sequence in the Jiajika gneiss dome, Songpan-Ganze orogenic belt, eastern Tibet: P-T-D-t evolution and implications for lithium mineralization

The metamorphic host rocks of pegmatites can help to decipher their geodynamic setting and emplacement conditions. The Jiajika lithium deposit, located in the Songpan-Ganze orogenic belt, eastern Tibet, is one of the largest hard-rock type lithium deposits in China. The lithium-enriched pegmatites in Jiajika are hosted in the amphibolite facies metasedimentary rocks in the mantle of the Jiajika gneiss dome. However, the metamorphic evolution of Jiajika is poorly constrained. In this study, we carried out detailed fieldwork, petrological observation, mineral chemical analysis, phase equilibria modeling, and geochronological studies on the Jiajika gneiss dome and the borehole JSD-1 (with a total depth of 3211 m) of the Jiajika Scientific Drilling Project (JSD). Structural data outlined two subdomes within the Jiajika gneiss dome. Two main stages of regional metamorphism and deformation are documented in Jiajika: an early Barrovian metamorphism (M1) in ca. 237−221 Ma caused by crustal thickening (D1), which has a peak condition of 6.7−8.3 kbar and 649−694 °C; a late Buchan metamorphism (M2) induced by the emplacement of granites and pegmatites during doming (D2) with a peak condition of 3.4−4.3 kbar and 570−612 °C at 210−206 Ma. The superimposition of the M2 isograds upon the M1 isograds resulted in a composite metamorphic sequence around the Jiajika gneiss dome. The overlapping of the Barrovian metamorphism and the Buchan metamorphism is related to crustal thickening during continental collision and later decompression through doming, which facilitated lithium mineralization.

Targeting high-potential mineral prospects in the Ezzhiliga region, Moroccan central massif, using spectral data from the ASTER sensor

This research paper introduces a comprehensive methodology for assessing hydrothermal alteration zones and structural complexity in the Ezzhiliga region, situated in the Hercynian Central Massif of Morocco. The approach utilizes ASTER imagery as the primary data source. The main objectives were to identify and analyze these geological features and then create a potential map for mineral exploration by integrating fuzzy logic and fractal concentration area analysis. Band ratios (RBD) and principal component analysis (PCA) were employed to detect and map argillic, phyllic, propylitic, and iron oxide alteration zones. Additionally, structural lineaments were extracted from the PC1 imagery to understand the structural pattern of the survey area. The incorporation of hydrothermal alteration zones and structural lineaments was achieved through the application of a fuzzy logic model, resulting in the generation of a mineral favorability map. The fuzzy logic model was customized to combine hydrothermal alteration and lineaments density maps, effectively eliminating false spectral anomalies induced by various interference factors. This map was then analyzed using the fractal concentration-area (C-A) model, which separated the anomaly from the geological background and generated a final mineral potential map. Analysis of fractal concentration-area was employed to define thresholds with greater precision, enhancing the reliability of mineral prospectivity assessments. Furthermore, laboratory analyses were performed to verify the outcomes of the mineral potential map. The obtained results revealed a significant affinity with the field data and indicated that the highly prospective zones are perfectly limited in spatial extent and generally associated with the contact of the Zaër granitic pluton with the metamorphic host rock, except for the anomaly identified to the southeast of the study area, along a major NE-SW trending fault.

Granulites of the Larba Block of the Dzhugdzhur–Stanovoi Superterrane: Reconstruction of the Formation Conditions

Abstract —We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur–Stanovoi superterrane. The Larba block is dominated by basic schists and garnet–biotite–orthopyroxene and garnet–biotite–cordierite–sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P–T parameters. The P–T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7–8 kbar, 800–850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4–5 kbar and T = 630–700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P–T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, TNd(DM) = 2.57–2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8–3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.

THE GENESIS OF GARNET-PYROXENE SYENITES (SVIATONOSSITES) OF THE MALOBYSTRINSKY MASSIF (SLYUDYANKA COMPLEX, SOUTH BAIKAL REGION): RESULTS OF GEOCHEMICAL AND ISOTOPIC STUDIES

This paper deals with the study of syenites and sviatonossites (andradite-bearing syenites) of the Malobystrinsky massif of the Slyudyanka complex (South Baikal region, Siberia), and a large monzonite dike similar in age and composition to the rocks of the massif considered. The studied rocks belong to a series of highly ferriferous and metaluminous A-type granitoids (ASI index <1). They are characterized by SiO2 45–65 wt. %, K2O+Na2O up to 12 wt. %, MgO <4 wt. %, TiO2 up to 2.5 wt. %, and Al2O3 up to 17 wt. %. CaO varies in a wide range, from 2.2 to 14.7 wt. %. The rocks are similar to each other in trace element composition and show patterns with troughs for Th-U, Nb-Ta and Ti. Low-amplitude negative Eu anomaly is observed in the distribution spectra of rare earth elements for the entire rock complex. The obtained Sm-Nd age of sviatonossites in the Malobystrinsky massif is 487.1±6.1 Ma (MSWD=0.99). Our results indicate that syenites and monzonites have εNd(t) –1.9…–2.8, at εSr(t) 21–30, and sviatonossites have εNd(t) –3.8…–4.1 at εSr(t) – 26. Model ages TNd(DM) for all rock types are Mesoproterozoic (1.3–1.4 Ga). Based on the chemical and Sr-Nd isotopic composition of the magmatic rocks studied, it can be assumed that they have been generated by partial melting of lower crustal rocks (amphibolites). Crystallization of andradite garnet in syenite magma can occur due to melt contamination with metamorphic host rocks of the Slyudyanka complex.

A polygenetic origin for the Sikhoran ultramafic-mafic complex in South Iran

The nature and geodynamic setting of the Sikhoran ultramafic–mafic complex, southwest Iran, is debated, with multiple competing theories having been proposed in recent years. The Sikhoran ultramafic–mafic complex is located in the southern part of the Sanandaj-Sirjan metamorphic-magmatic zone (SSMMZ) and is comprised of porphyroclastic, transitional and layered ultramafic-mafic sequences of Carboniferous age. These units are cut by isotropic gabbros, diabasic and pegmatoid gabbroic dikes with ages ranging from Permian to the Cretaceous. These bodies also intrude into Upper Paleozoic metamorphic rocks, where they induced high-temperature contact metamorphism during the Late Carboniferous. Here we present new textural descriptions, mineral and whole-rock geochemical analyses, and the results of zircon UPb dating of magmatic mafic rocks and metamorphic host rocks from this region in order to reconstruct the petrogenesis of the Sikhoran ultramafic–mafic complex. Zircon UPb ages from layered gabbros indicate Late Carboniferous (320.8 ± 6.4 Ma) crystallization, which is similar to the age of the Upper Paleozoic host gneisses (334.6 ± 4.9 Ma). By contrast, isotropic gabbros that cross-cut the complex have a crystallization age of 178.3 ± 2.3 Ma, and anatectic quartz diorite/plagiogranite produced from partial melting of hosting amphibolite in contact with the gabbros yielded Late Triassic-Early Jurassic ages (187.2 ± 2.6 Ma). These data suggest a polygenetic origin and a range of tectonic settings for various parts of the ultramafic-mafic complex. Geochemical modeling shows that mantle plume-related melting (∼5–40%) of metasomatized mantle could have formed the ultramafic-mafic layered cumulate complex during the Late Carboniferous. A geochemical transition from island arc tholeiite (IAT) isotropic gabbros to cross-cutting enriched mid-ocean ridge basalts (E-MORB)–diabasic dikes occurred between the Late Triassic-Early Jurassic to the Late Cretaceous. The mantle-derived ultramafic parts of the Sikhoran ultramafic-mafic complex have similarities with other known ultramafic-mafic mantle plumes (e.g., Tinaqulllo, Lherz, and Ronda), and its layered ultramafic-mafic segment has similarity with layered intrusions (e.g., Pulur complex in Turkey). The highest observed crystallization temperature in porphyroclastic, transitional and layered ultramafic mantle sections of ∼1140 °C is considered as the mantle potential temperature, since the highest crystallization temperatures are observed in the most forsteritic olivine crystals. Based on amphibole chemistry, the crystallization conditions of transitional to layered ultramafic rocks at 900–1005 °C occurred at high H2O contents in a melt between nickel–nickel oxide (NNO) −8.5 and NNO −9.0, which lies along the NNO−2 buffer. These results are consistent with a model in which the opening of the Zagros Neotethys Oceanic basin in the Esfandagheh region initiated in the Late Carboniferous, alongside the formation of an extensional environment that allowed ascent of a subcontinental mantle plume. Subduction initiation of oceanic lithosphere began during the Late Triassic-Early Jurassic and ended in the Late Cretaceous.

The Mal’dzhangarka Carbonatite Massif (Anabar Shield): The Age of Magmatism and Mineralization (U–Pb and Re–Os Isotope Systems)

Abstract—We present the first results of a comprehensive isotope-geochemical study of dolomitic carbonatites of the Mal’dzhangarka massif located in the southeast of the Billyakh melange zone (southeastern periphery of the Anabar Shield). Zircon grains separated from core samples from a depth of 6–30 m have a three-phase structure. All of them were trapped from the host metamorphic rocks and mark the age of the main stage of high-gradient metamorphism in the region, 2027 ± 9 Ma. Pyrochlore containing 38–705 ppm U and 5–21 ppm radiogenic Pb, with weak metamictization of the crystal lattice and an undisturbed U–Pb system, made it possible to estimate the concordant age of rare-metal mineralization in the massif, 167 ± 4 Ma, which is probably close to the crystallization age of the host carbonatites. The estimated age corresponds to one of the stages of kimberlite–carbonatite magmatism on the eastern slope of the Anabar dome. The Re–Os isotope system of pyrite from superposed late carbonate–sulfide veinlets in the carbonatites testifies to a close (within the error of determination) age, 179 ± 14 Ma, and a low initial Os isotope ratio, which indicates the contribution of mantle material to the formation of this isotope system.

Trace-element signatures of pegmatite and post-pegmatite minerals from the Govedarnika Pb-Zn deposit, Laki district, Central Rhodopes

The current study presents mineralogical and geochemical characteristics of pegmatite bodies from the Govedarnika Pb-Zn deposit, situated in Laki district, South Bulgaria. The studied pegmatites are composed mainly of K-feldspar, plagioclase, quartz and accessories of titanite, apatite, allanite and zircon. Intensive hydrothermal alteration resulted in specific mineral assemblage of chlorite, hematite, epidote and carbonates. According to the mineral relationships, it preceded or coincided with the time of base metal ore formation and affected both pegmatites and host metamorphic rocks. Both pegmatite accessories and post-pegmatite hydrothermal silicates often show enhanced contents of some strategic elements such as Sr, Li, V, Ga, Ni and REE. Pilot U-Pb geochronology of titanite from the studied pegmatites yielded a lower intercept age of 39.9±2.3 Ma.

New field and petrographic data about igneous rocks from Barnard Point, Livingston Island, Antarctica

The Southeastern area of Livingston Island is built up by igneous rocks of the Eocene Barnard Point Pluton and related or younger dykes. Our new field and petrographic studies in the area show that the pluton consists of several main rock varieties namely gabbro, gabbrodiorite, granodiorite and granites, whose relationships evidences magma mixing processes. The internal parts of the former chamber contain levels enriched in mafic microgranular enclaves forming pronounced magmatic foliation, which towards western contact of the pluton transits to a subvertical N-S trending solid-state foliation and both parallelize the main foliation of the metamorphic host rocks. The latter evidences for a room creation and magma emplacement in tectonically active setting.

Hydrothermal evolution and origin of the Suixian molybdenum deposit in the Tongbai orogenic belt, China: Constraints from geology, fluid inclusions and multiple isotopes (H[sbnd]O[sbnd]C[sbnd]S[sbnd]Pb)

The Suixian Mo deposit is a recently discovered deposit in the northern Hubei Province, Tongbai orogen. The Mo orebodies of this deposit are all hosted in the metamorphic rocks (leuco-gneiss, amphibolite and marble) of the Neoproterozoic Tianwangzhai Group, and these orebodies have no direct contact with the granite surrounding the host rocks. The mineralization types are dominated by disseminated ore and fine Mo-bearing quartz-sulfide veins. Hydrothermal alteration consists of abundant feldspathization, epidotization and chloritization, together with less garnetization, tremolitization, and muscovitization. A detailed study on fluid inclusions and multiple isotope (CHOSPb) analyses, together with geologic features, are carried out to investigate the fluid composition, fluid origin and evolution, the source of ore-forming materials, and the ore genetic type. In line with microthermometric measurements and laser Raman analysis on fluid inclusions from three hydrothermal stages, the ore-forming fluid in this deposit is characterized by medium–high temperatures and salinities, as well as richness in CO2 and CH4. Three hydrothermal stages are identified in this deposit, including (I) feldspar-quartz-amphibole-muscovite stage, (II) quartz-sulfides stage, and (III) quartz-calcite-scheelite-fluorite stage. From stages I, II to III, the temperatures change from 290 to 354 °C, through 229–312 °C, to 173–269 °C, with decreasing salinities of 12.6 to 16.3 wt% NaCl equiv., 4.5 to 13.4 wt% NaCl equiv., and 0.2 to 6.9 wt% NaCl equiv., respectively. Most of sulfur isotope data of sulfides (chalcopyrite, pyrite, pyrrhotite and molybdenite) show a narrow δ34S range from −2.0 to 3.0 ‰, suggesting a dominant magmatic source, but some data show either positive or negative values (δ34S: −13.3 to −10.6 ‰ and 9.8 to 11.2 ‰) which may indicate small contribution from the restricted wall rocks. The Pb isotope data (molybdenite, pyrite and pyrrhotite: 206Pb/204Pb: 17.239 to 18.483; 207Pb/204Pb: 15.378 to 15.717; 208Pb/204Pb: 37.889 to 38.850) imply that the lead of sulfides pertains to a mixture of magma (derived from the lower crust) and upper crustal materials (host metamorphic rocks). Based on evident CO2 and CH4 in fluid inclusions, the gradually depleted carbon isotopic compositions (δ13C: −5.4 to −9.9 ‰ at stage I, −14.8 to −19.7 ‰ at stage II, and −17.0 to −24.2 ‰ at stage III) and the HO isotope data, it is suggested that the ore-forming fluids were mainly magmatic fluids with mixture of some organic-containing water, and the thermochemical reaction between the fluids and organic matters (e.g., CH4) from host metamorphic rocks and fluid immiscibility are two important mechanisms for Mo mineralization. Compared with typical Mo deposits in the same orogen, the Suixian Mo deposit belongs to the magmatic hydrothermal vein-type deposit associated with intrusions rather than porphyry or skarn type deposit.

Chapter 8 Scottish mineral Geological Conservation Review sites – Replacement minerals

This is, the eighth and final paper of a series that describes the Scottish mineral Geological Conservation Review (GCR) sites within both mainland Scotland and the associated Isles. It features locations where the mineralogy of the original rock unit has been either largely or completely replaced as a consequence of their interaction with either fluids or gaseous components derived from external sources. At both Lunda Wick and the Queyhouse Quarry, Unst, deformation of ultrabasic rocks located within or close to Cambrian-age obduction-related shear zones has resulted in the formation of new mineral species including talc, actinolite, chlorite and biotite. Within the former mine site at Craignure, copper and nickel mineralisation is a consequence of the metasomatic alteration of metabasic igneous intrusions within the youngest part of the Dalradian Supergroup sequence resulting in the formation of pyrrhotite, pentlandite and chalcopyrite. The Lecht Mine site involves Dalradian and younger rocks, where iron and manganese oxides, derived from the metamorphic host rocks, have replaced both clasts and matrix within Devonian-age intrusive breccia pipes. The predominant replacement mineral within this setting is cryptomelane, but also includes ramsdellite, pyrolusite, chalcophanite, lithiophorite, woodruffite and caconexite. These replacement minerals also contain enhanced concentrations of barium, zinc, thallium, molybdenum and silver.

Telluride-bearing Au-Ag mineralization in the Singu-Tabeikkyin gold District, Mogok metamorphic Belt, Myanmar: New constraints on an intermediate-sulfidation epizonal orogenic ore system

The Singu-Tabeikkyin gold district is located in the Mogok Metamorphic Belt in Myanmar. Gold mineralization in this district occurs in a set of mainly N − S trending quartz veins, which are associated with minor replacement zones and skarns. This study investigates the field geology, ore mineralogy, mineral chemistry, and fluid inclusion characteristics of three gold prospects (CG-1, CG-2, and CG-3) containing Au-Ag along with telluride assemblages. Gold has formed as two generations, and only the second gold mineralization stage is associated with major telluride assemblages. The tellurides are associated with chalcopyrite, a late generation of sphalerite, and electrum. The telluride phases identified in the study are hessite (Ag2Te), petzite (Ag2AuTe3), altaite (PbTe), tellurobismuthite (Bi2Te3), and tetradymite (Bi2Te2S), and unnamed Au-Ag telluride phases. Liquid-rich two-phase fluid inclusions have homogenization temperatures in the range of 174–310 °C, and salinities of 1–12 wt% NaCl equiv. The chlorite geothermometry in this study yielded a variable temperature range from as low as 250 °C to 380 °C, in general agreement with the fluid inclusion temperature range. The thermometric data, in conjunction with the analysis of telluride-sulfide phase stability, constrain the conditions of ore formation to logƒTe2 and logƒS2 values of − 17 to − 9.5, and of − 13.4 to − 12.5. The geological and mineralogical data and the reconstructed conditions of ore formation as well as high-grade metamorphic host rocks of gneiss, marble and calc-silicates suggest that the telluride bearing gold-silver mineralization in the Singu-Tabeikkyin gold district forms part of an intermediate-sulfidation epizonal orogenic Au system.

Geology along the Bedretto tunnel: kinematic and geochronological constraints on the evolution of the Gotthard Massif (Central Alps)

The unlined Bedretto tunnel crosses large parts of the pre-Triassic basement of the Gotthard massif (Central Alps), giving the possibility to study late-Variscan plutonic rocks (Rotondo granite) and their Caledonian (poly-)metamorphic host rocks (Tremola and Prato series). The Rotondo granite consists mostly of an equigranular, fine-grained granite and to a lesser extent of a porphyritic granite. Commonly, the Rotondo granite is massive or only slightly foliated. Ductile deformation is localized along discrete shear zones composed of granitic or quartz-biotite-rich lithologies. This paper reviews the geology of the Bedretto tunnel with emphasis on the Rotondo granite and presents constraints based on kinematic, microstructural, and U–Pb geochronological evidence, which can be summarized as follows: (1) Both granitic and quartz-biotite-rich shear zones (QB-SZ) in the Rotondo granite generally dip moderately to steeply towards north and are related to top-to-south reverse shearing, indicating south-verging backthrusting during the exhumation of the Gotthard massif. (2) Zircons from both the equigranular and porphyritic Rotondo granite show overlapping {}^{206}hbox {Pb}/{}^{238}hbox {U}-age ranges of 285–319 Ma and 280–335 Ma, respectively, which indicate that both are part of the same late-Variscan magmatic episode. Almost no older inherited cores are reported. (3) In zircons from a QB-SZ, 30% of the concordant age spots scatter between 339 and 589 Ma. This suggests that the parent material of the QB-SZ is unrelated to the magmatic episode that formed the Rotondo granite, but rather that the QB-SZ represent sheared xenoliths within the granite.

U-PB ГЕОХРОНОЛОГІЯ КРИСТАЛІЧНИХ ПОРІД ВІЛЬШАНСЬКОГО КАР'ЄРУ (СЕРЕДНЄ ПОБУЖЖЯ, УКРАЇНСЬКИЙ ЩИТ)

Geochronological studies of granitoids from the Vilshansky mine of Middle Pobuzhzhya were carried out. Zircons and monacites from plagiogneiss, plagiogranites and migmatites of the Bug complex were analyzed. As a result, it was established that the rock association, which consists of migmatites with numerous xenolites of biotite gneisses, plagiogneises, occasionally graphite-containing gneisses, amphibolites and biotites (Khashchuvate-zavalivska suite of the Bug series) and intersected with the veins of aplito-pegmatoid granites, was formed as a result of a large-scale manifestation of graniteization processes, which began not later than 2.02 billion years ago and ended about 2.0 billion years ago. The age of metamorphic host rocks corresponds to the Archean values of uranium-lead dating.

Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia

AbstractThe Luk Ulo Metamorphic Complex, Central Java is a product of the Cretaceous subduction and accretion, and includes diverse types of protoliths. Two‐types of primary mineralization have been recognized in this area, namely, (1) seafloor basalt‐hosted massive sulfide mineralization and (2) low‐grade metamorphic rocks‐hosted vein type mineralization. Later erosion of these types of primary mineralization formed placer gold deposits along rivers. However, the source has never been identified. Thus, this study aims at understanding the source of placer gold, the characteristics of the primary mineralization, and the tectonic evolution of the study area on the basis of mineralogy, mineral chemistry, whole‐rock geochemistry, and sulfur isotope analyses. Volcanogenic massive sulfide (VMS)‐type mineralization was identified in the seafloor basalt and few deep‐sea sedimentary rocks, and both the ores and host rocks preserved pre‐metamorphic textures and minerals. The characteristics of this VMS‐type mineralization include (1) crustiform quartz veins with pyrite cutting the host rocks, (2) zonation of local silicification to interlayered chlorite/smectite‐chlorite‐laumontite‐calcite‐epidote alteration from central to outer zone, (3) pyrite‐dominated ores with minor amounts of arsenian pyrite, chalcopyrite, and marcasite, (4) unmetamorphosed host rocks and ores, and (5) sulfur isotope signature with a median δ34S of +3.1‰ suggesting sulfur derived from magmatic source and/or sulfur extracted from basaltic rocks with a small contribution of biogenic sulfur. On the other hand, low‐grade metamorphic rocks‐hosted vein type mineralization was identified as orogenic‐type gold mineralization, and the mineralized veins formed after the peak of metamorphism. It is characterized by (1) pyrite‐arsenian pyrite ores with minor amounts of arsenopyrite, galena, tetrahedrite, chalcopyrite, and sphalerite, (2) quartz‐illite‐graphite alteration assemblage, (3) mineralized veins cross‐cutting the foliation of metamorphic host rocks, (4) high antimony contents of pyrite (up to 1.7 wt%) and rutile (up to 160 ppm), (5) relatively high ore‐forming temperature (423 ± 9°C, calculated from arsenopyrite and graphite geothermometers), and (6) remobilized‐sedimentary sulfur signature of the ores with a median δ34S of −9.8‰. Several lines of evidence suggest that placer gold was likely derived from the erosion of orogenic‐type gold ores in the surrounding areas. This evidence includes the presence of gold‐bearing ores hosted by low‐grade metapelites and metagranitoid with characteristics of orogenic‐type gold mineralization, whereas the VMS‐type ores are barren in gold. The occurrence of the mid‐oceanic ridge‐ and accretion zone‐related mineralization in this area reflects the subduction and amalgamation of oceanic and continental crustal blocks during the Cretaceous period. Discovery of gold mineralization hosted in the Cretaceous basement rocks of the Sunda arc indicates the importance to broaden the gold exploration targets to include not only young volcanic rocks, but also relatively old metamorphic basement rocks.

Hypozonal gold mineralization in shear zone-hosted deposits driven by fault valve action and fluid mixing: the Nalunaq deposit, Greenland

Abstract The Nalunaq deposit, Greenland, is a hypozonal, shear zone-hosted, Au deposit. The shear zone has previously been interpreted as having undergone four stages of deformation, accompanied by fluid flow and vein formation. Coupled with previous trapping T estimates, fluid inclusion data are consistent with the trapping of fluids with salinities between 28 and 45 wt% NaCl equiv., from 300 to 475°C during D 2 and D 3 , with pressure varying between c. 800 and 100 MPa. The range reflects pressure cycling during seismic slip-related depressurization events. D 4 fluids were lower salinity and trapped from 200 to 300°C, at c. 50–200 MPa during late-stage normal faulting. The variation in major element chemistry is consistent with the ingress of hypersaline, granitoid equilibrated fluids into the shear zone system and mixing with fluids that had reacted with the host metamorphic rocks. D 4 -stage fluids represent the ingress of meteoric fluids into the system. Gold contents in inclusion fluids range from c. 300 to 10 mg kg −1 . These data are consistent with the high- P–T solubility of Au as AuHS(H 2 S) 3 0 complexes, and Au deposition by decompression and cooling. The high salinities also suggest Au transport as chloride complexes may have been possible. Gold distribution was modified by the release of chemically bound or nanoscale Au during sulfide oxidation at the D 4 stage.

Two episodic Au–Mo mineralization in the Laowan district from the Tongbai orogenic belt of China: Constraints from U–Pb dating of zircon, rutile, and REE phosphate, and Re–Os dating of molybdenite

The Laowan super-large (208 t Au) gold deposit is located in the Tongbai orogenic belt, along with a newly-discovered molybdenite deposit at its depth in the northern periphery. The gold deposit is a fault-controlled lode gold deposit featuring sulfide quartz vein-type and altered rock-type mineralization hosted within the metamorphic rocks, whereas the molybdenum deposit is a typical porphyry-type deposit. These two deposits have their respective paragenetic sequences and mineralization types without spatial overlapping in the ore field. Here, we present U–Pb dating of zircon, hydrothermal rutile, and REE phosphates, and Re–Os dating of molybdenite to constrain the relationship between the Au and Mo mineralization, and their relation to Mesozoic granitic magmatism. Textural features indicate that rutiles and associated phosphate minerals are of hydrothermal origin, generated by re-precipitation and dissolution of titanite crystals hosted in the metamorphic host rocks due to hydrothermal alteration during Au and Mo mineralization. LA-ICP-MS U–Pb dating of rutile from the auriferous altered rocks yields a 206Pb/238U age of 120.0 ± 6.7 Ma (MSWD = 1.4), consistent with the ages of the associated monazite (125.0 ± 2.4 Ma) and xenotime (126.3 ± 2.0 Ma), representing the timing of the gold mineralization, coeval with Laowan monzogranite (zircon U–Pb: 128.8 ± 0.7 Ma). Rutile from the molybdenum deposit show an older 206Pb/238U age of 141.0 ± 11 Ma (MSWD = 1.1), consistent with the molybdenite Re–Os age (141.5 ± 1.7 Ma; MSWD = 0.27) and the zircon U-Pb age (141.6 ± 0.8 Ma; MSWD = 1.5) of the Songpa porphyry. Hence, the Au and Mo mineralization in the Laowan district represent two episodic mineralization events related to the late Mesozoic granitic magmatism. Our study also suggested that multiple isotopic dating is helpful to precisely constraint the ages for complex hydrothermal deposits.

Minerály kontaminovaných granitových pegmatitů z lomu Pohled u Havlíčkova Brodu (moldanubikum), část II: prvky a sulfidy

In the Pohled quarry near Havlíčkův Brod town (central part of Czech Republic), texturally and mineralogically simple contaminated anatectic pegmatites form dikes or irregular bodies cementing breccia of host metamorphic rocks (paragneisses, amphibolites) belonging to the Monotonous (Ostrong) Group of the Moldanubicum of the Bohemian Massif. They exhibit signs of intense hydrothermal overprint and also the presence of abundant disseminations, nests and veinlets of ore minerals. A detailed mineralogical study revealed the presence of an extraordinary rich ore assemblage (20 species in total, including one unnamed phase). The oldest minerals are sphalerite (rich in Fe), löllingite, Fe-Co-Ni sulphoarsenides (cobaltite, glaucodot, arsenopyrite, gersdorffite), pyrrhotite, galena and chalcopyrite, in later portion accompanied by inclusions of Bi-minerals (native bismuth, bismuthinite, joséite-A, joséite-B, ikunolite and a Pb-Bi sulphosalt). The composition of the Pb-Bi sulphosalt is equal to Ag,Fe-substituted eclarite; its identity was confirmed also by Raman spectrum. Pyrite is very abundant phase, present probably in several generations. The Fe-Co-Ni thiospinels disseminated in younger chlorite, and represented by siegenite, violarite, grimmite and an unnamed NiFe2S4 phase, are the youngest ore minerals. The mineral association as well as chemical composition of most ore minerals are well comparable to those of local polymetallic ore veins and Alpine-type veins, which give evidence for identical origin of all these ore mineralizations. The formation of pegmatite-hosted ore assemblage was long-lasting multiphase process, which took place at temperatures between ca. 350 and <120 °C during changing fugacities of sulphur, tellurium and oxygen. A distinct enrichment in cobalt and nickel of ore mineralization hosted by pegmatites (in comparison with hydrothermal veins) is explained in terms of pronounced interactions of fluids with amphibolites and serpentinites.

Graphite paradox in Baikal geyserite paleovalley, Russia

Abstract Natural graphite, a polygenic mineral, is a product of regional, contact, impact metamorphism, and magmatic or fluid deposition. In fluid-deposited graphite, aqueous C-O-H systems play a special role in determining the characteristics of hydrothermal products by shifting the chemical equilibrium. From this viewpoint, the recently discovered carboniferous mineralization in the Baikal hydrothermalites has attracted increasing interest with regard to graphite crystallization under the influence of low-pressure low-temperature (LPLT) carboniferous H2O-rich fluids. Herein, we studied graphite mineralization in the geyserites and travertines of the Baikal geyserite paleovalley (Eastern Siberia, Russia) by applying a multitude of mineralogical studies. Optical, scanning, transmission electron, and atomic force microscopy, energy-dispersive spectroscopy, Raman spectroscopy, and carbon isotopic composition analyses of graphite, carbonate carbon, and oxygen in both the hydrothermalites and host rocks were conducted. The obtained results revealed several peculiar features regarding the graphite in geyserites and travertines. We found that Baikal graphite, earlier predicted to be a product of hydrothermalites, generally occurs as a relict graphite of the host metamorphic rocks with partial in situ redeposition. The newly formed LPLT fluid-deposited graphite is characterized by micrometer- and submicrometer-sized idiomorphic crystallites overgrown on the relict metamorphic graphite seeds and between calcite sinter zones during the last stage of travertine formation. The results present additional valuable data for understanding the mechanism, range of the formation conditions, and typomorphism of fluid-deposited graphite with probable crystallization from carbon solution in the C-O-H system at LPLT conditions.

Multiple episodes of gold mineralization in the East Kunlun Orogen, western Central Orogenic Belt, China: Constraints from Re-Os sulfide geochronology

The Gouli goldfield (>110 t Au), located in the East Kunlun Orogen, western Central Orogenic Belt of China, is one of the most important goldfields in this area. In the last decade, a number of orogenic gold deposits (e.g., Guoluolongwa and Annage) have been shown to be hosted by rock units of different lithology and ages. Rhenium-osmium (Re-Os) geochronology of sulfides from gold-bearing veins was performed to define the chronologic relationships between gold mineralization present in the metamorphic rocks (Proterozoic and Silurian) of the East Kunlun Orogen. Sulfides (pyrite and chalcopyrite) from pyrite-quartz vein and polymetallic sulfides-quartz vein in the Guoluolongwa gold deposit yield Re-Os isochron dates of 374 ± 15 Ma (MSWD = 4.6; initial 187Os/188Os ratio (Osi) = 0.06 ± 0.22) and 354 ± 7 Ma (MSWD = 0.18; Osi = 0.13 ± 0.01), respectively. Similar ages are also revealed by the pyrite mineral separates from the Annage gold deposit (383 ± 8 Ma and 349 ± 6 Ma). These ages are interpreted to record the timings of the formation of the two vein types in these deposits, which are nominally separated by ~20 Ma.The new Re-Os ages presented here identify the first two Late Paleozoic (Devonian and Early Carboniferous) gold-mineralizing events in the East Kunlun Orogen and thus indicate at least two mineralization epochs in this area given ages (Late Triassic) of other gold systems and field observations. Considering the geological background and temporal distribution of gold deposits in adjacent areas (western Qinling and Qaidam-Qilian), we suggest that gold deposits in the western Central Orogenic Belt were formed in collisional/post-collisional settings being controlled by common tectonic-magmatic activities related to the evolution of both the Prototethys Ocean (Proterozoic – Paleozoic) and Paleotethys Ocean (Paleozoic – Early Cenozoic).Further, the initial Os (Osi) obtained from the Re-Os isochron suggest that for the two vein types in the Guoluolongwa gold deposit the Os and by inference the ore metal (Au) were derived from a mantle-like source (Osi values = ~0.12–0.13), which should be related to the contemporaneous mantle-like magmatism. In contrast, the pyrite-quartz vein in the Annage gold deposit possesses a significantly radiogenic Osi value (3.65 ± 0.51). Given the similar timing of mineralization between the Guoluolongwa and Annage deposits, it is considered that the ore metal likely has a similar origin, i.e., a mantle-like source, however at Annage the hydrothermal fluid interacted with the Proterozoic metamorphic host rocks and leached radiogenic Os that masks any evidence of a mantle-like source.