Early Hydrothermal Stage Research Articles

Mineralogy and a New Fertility Index of Alunite in the Fanshan Lithocap, Luzong Basin, Anhui Province, China

Alunite is used as a representative mineral for indicating deposits in lithocaps, and lithocaps are generally related to the porphyry–(high-sulfidation) epithermal mineralization system. The study of alunite is of theoretical and exploration significance for prospecting potential underlying porphyry and epithermal deposits. Studies on alunite geochemistry have made breakthroughs, but there is little research on alunite mineralogy, for example, using scanning electron microscopy (SEM) images, differential thermal analysis (DTA), and Fourier-transform infrared spectroscopy (FT-IR). This study mainly focuses on alunite micromorphological characteristics, weight loss changes with temperature, and ionic group structure, aiming to identify the relationship between these features and indications for prospecting. The Fanshan lithocap is located in the northwest part of the Luzong basin, Anhui province of China, and it can potentially be used for exploring porphyry and epithermal deposits. Fanshan alunite is formed in two stages with three types of alunite. IA alunite is formed in the early hydrothermal stage and replaces felsic minerals in the Zhuanqiao Formation, IB alunite is formed in the later hydrothermal stage and fills in open spaces with bladed particles, and II alunite is the product of pyrite oxidation and reaction with other minerals in the supergene stage. Alunite electron microprobe data and energy-dispersive spectroscopy data further confirm temperature decreases with hydrothermal evolution, and the presence of a high-sulfidation epithermal system in the Luzong basin. Aside from the forming environment, SWIR, and geochemistry of alunite, there are other indication indexes; for example, the larger peak values at 3480 cm−1 and smaller peak values at 1080 cm−1 in FT-IR spectra and the deeper exothermic valleys at 750 °C and steeper weight loss slopes in the DTA curve suggest a favorable formation environment for alunite and provide valuable indications for deposit exploration and assessments of mineralization potential.

Mn–Fe-rich genthelvite from pegmatites associated with the Madeira Sn–Nb–Ta deposit, Pitinga, Brazil: new constraints on the magmatic-hydrothermal transition in the albite-enriched granite system

AbstractGenthelvite from pegmatite veins hosted by the albite-enriched granite (ca.1.8 Ga) corresponding to the Sn–Nb–Ta (F, REE, Li, Zr, U, Th) Madeira deposit, Amazonas, Brazil was studied. Genthelvite, the exclusive Be-bearing mineral within the deposit, occurs as massive crystals of up to 4.7 cm in size. Compositions are homogeneous within individual crystals, although there is moderate variation in the overall composition reflecting relatively limited substitutions within the helvine–genthelvite–danalite solid-solution series, with relatively high Zn contents (36.96 to 49.45 wt.% ZnO), lower Mn contents (0.61 to 3.03 wt.% MnO), and variable Fe contents (2.10 to 10.94 wt.% FeO), completing an existing compositional gap in this system. Genthelvite formed in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, at relatively high temperature (>400°C), in a reducing environment. The extremely high concentration of fluorine in the magma and the crystallisation of magmatic galena resulted in an effective reduction of H2S fugacity. This resulted in the stabilisation of genthelvite during the transition from the late magmatic to early hydrothermal stages of the albite-enriched granite evolution. The variability in Fe content within genthelvite is associated primarily with localised variations in the mineral assemblage (e.g. the presence of riebeckite and polylithionite). Genthelvite was altered by low-temperature aqueous fluids rich in F which resulted in the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U and REE into the Zn2+ structural site and the allocation of excess Si, Al, Ti and P in the IVSi and IVBe structural sites. The substantial content of U and REE substituting for Zn, together with Si substituting for Be, is charge balanced by the presence of vacancies at the A site.

Hornblende in ultramafic-mafic rocks of the Khudolaz Complex of the Southern Urals: Crystallization conditions and petrological implications

Subject research. Morphology and chemical composition of amphiboles from ultramafic-mafic rocks of the Khudolaz complex. Methods. Morphological studies were carried out using optical (Carl Zeiss Axioskop 40A) and electron (Tescan Vega Compact) microscopes. The chemical composition of minerals was determined using CAMECA SX 100 and JEOL JXA-8230 electron probe microanalyzers. Results. The rocks under study are dominated by xenomorphic brown titanium hornblende crystals, formed mainly due to the reaction of clinopyroxene with a residual water-saturated melt at 920–1040°C. Euhedral brown hornblende, which crystallized directly from the residual water-saturated melt in the same temperature range, was found in small amounts. Green hornblende originated along the edges and cracks in brown hornblende crystals during the late magmatic stage and early hydrothermal stage (670–830°C) at the subsolidus transformation. At the hydrothermal stage (620–650°C and below), brown and green hornblende were partially replaced by actinolite and cummingtonite. Conclusions. The nature of changes in the composition of brown hornblende indicates similar petrogenesis conditions at the late magmatic stage in all intrusions of the Khudolaz complex. The process of transition from brown hornblende to green hornblende occurred with a gradual increase in oxygen fugacity (ΔNNO from –0.2…+0.4 to +0.9…+2.5), and was accompanied by decrease of Ti, Fe2+, Na and the increase of Si, AlVI, Mg, and K in the mineral structure. The low concentrations of F and Cl in hornblende crystals indicate their formation after migration of halogens from the melt.

The mineralization of Daxiao carbonate-hosted Pb-Zn deposit, northeast Yunnan province, SW China: Constraints from Rb-Sr isotopic dating and H[sbnd]O[sbnd]S-Pb isotopes

The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province located in western Yangtze Block contains more than 400 carbonate-hosted Pb-Zn deposits. Uncertain mineralization age and ore-forming material source causing debates on the genesis of these carbonate-hosted Pb-Zn deposits. To address this issue, the Daxiao Pb-Zn deposit was chosen as a case study. The orebodies of Daxiao deposit are hosted in Mesoproterozoic Heishan formation argillaceous dolomite interbedded with slate. Three main mineralization stages have been identified, including syn-sedimentary stage, hydrothermal stage and supergene stage. The ore minerals consist of galena, sphalerite, chalcopyrite, argentite, smithsonite, anglesite and cerussite in the deposit. The early hydrothermal stage sphalerite yielded a Rb-Sr isochron age of 223.5 ± 2.9 Ma, consistent with the spike of Pb-Zn mineralization in SYG metallogenic province. Quartz in hydrothermal stage yields δDSMOW values of −69.6 to −65.8 ‰ and calculated δ18OFluid values of + 3.4 – +5.6 ‰, showing an affinity of metamorphic water. While the quartz in supergene stage yields δDSMOW values of −68.3 to −65.8 ‰ and calculated δ18OFluid values of −4.1 to −1.7 ‰, indicating abundant basinal brine or meteoric water had mixed into ore-forming fluid during its evolution from hydrothermal stage to supergene stage. Sulfur isotopic composition (δ34S = +6.3 – +19.1 ‰) of different stage sulfides reveal that the ore-forming sulfur originated from sulfates within the wall rocks, and mainly reduced by Thermal-chemical reduction (TSR). Moreover, the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of early hydrothermal stage sphalerite are 18.282 – 18.312, 15.603 – 15.681and 38.287 – 38.311, respectively. Those of late hydrothermal stage galena are 18.315 – 18.491, 15.676 – 15.721 and 38.314 – 38.326, respectively. All of them plot in the overlapping area of Emeishan flood basalts, Kunyang and Huili groups and Sinian to Triassic strata, indicating that these wall rocks jointly contributed the ore-forming metals for these Pb-Zn deposits in SYG metallogenic province. Hence, a metallogenic model for Daxiao and other carbonate-hosted Pb-Zn deposit in SYG metallogenic province was proposed: In the thrustfold systems caused by the subduction of Paleo-Pacific and the closure of Jinshajiang-Ailaoshan Paleo-Tethys ocean in Late Triassic to Early Jurassic. The fluids originated from metamorphic water migrated along deep faults and extracted ore-forming materials from wall rocks (including Proterozoic basement rocks, Sinian to Triassic sedimentary wall rocks and Permian Emeishan flood basalts), and then precipitated ores in the secondary thrust faults or interlayered fractures within these thrustfold systems.

Rock textures and mineral zoning – A clue to understanding rare-metal granite evolution: Argemela stock, Central-Eastern Portugal

The small leucocratic strongly peraluminous P, F, Li-rich granitic system at Argemela, central-eastern Portugal, is an ideal object for the study of relations between chemical composition of granitic rocks, rock-forming minerals and rock and mineral textures, and the associated Sn, Ta and W mineralization. To define the rock and mineral composition and to formulate an evolutionary model, traditional methods of bulk-rock chemical analyses, and EMPA and LA ICP-MS analyses of rock-forming minerals were combined with the study of macrotextures, TIMA-automated mineralogical mapping of typical rock samples, and CL study of internal texture of quartz and mica crystals. The Argemela stock forms a steep cylindrical body 1 km deep, with an elliptical outcrop 250 × 180 m in size, crosscutting Cambrian schists. The stock comprises two intrusions: a geochemically moderately evolved equigranular facies (0.3 wt% F, 1.2 wt% P2O5, 1850 ppm Li, 310 ppm Sn, 24 ppm Ta, 5 ppm W) composed of albite, quartz, muscovite and minor montebrasite followed by a rather inhomogeneous, more evolved porphyritic facies (0.25–1.2 wt% F, 1.3–1.8 wt% P2O5, 1600–4900 ppm Li, 600–1000 ppm Sn, 40–85 ppm Ta, 3–6 ppm W) composed of quartz, mica and subordinate K-feldspar phenocrysts embedded in an albite-quartz-mica-amblygonite matrix. The later facies is rimmed by stockscheider at the contact with slates, and with an up to 1 m thick layered zone with unidirectional solidification textures along contact with the equigranullar facies. The NE-part of the stock is crosscut by numerous thin quartz veinlets ±K-feldspar, phosphates and wolframite, while several aplitic dikes and thicker quartz+phosphate veins were found in two boreholes NW of the granite. Porphyritic facies, the most voluminous part of the system, contains strongly zoned phenocrysts of mica (phengite→lepidolite) and quartz (Ti-enriched to Al, Rb-enriched) indicating crystallization in two magmatic stages, while the equigranular facies and extragranitic aplitic dikes crystallized in one magmatic stage only. Hydrothermal muscovite enriched in Sn and Ta forms late overgrowths on some mica flakes in the porphyritic facies and in aplitic dikes. Based on a combination of chemical and textural observations, a four-stage evolutionary model of the Argemela stock was formulated: (i) an early magmatic stage comprises intrusion and in situ crystallization of aplitic dikes and the equigranular facies, and crystallization of quartz and mica cores in the deeper reservoir prior to intrusion of the porphyritic facies; (ii) in the late magmatic stage, crystal mush + evolved residual melt from the reservoir intruded upwards crystallizing the porphyritic facies including the stockscheider and layered zone. A small part of evolved melt formed rounded enclaves crystallizing as a phenocrysts-free ball facies. (iii) In the early (high-temperature) hydrothermal stage, magmatic fluid caused weak pervasive muscovitization of the porphyritic facies and aplitic dikes accompanied by the crystallization of disseminated cassiterite and columbite. After the transition from ductile to brittle deformation, quartz veinlets also developed, mainly in the NE part of the body. (iv) In the late, low-temperature stage, schist-derived fluid enriched in Ca, Sr and Ba caused local alteration of granite, forming crandallite–goyazite–gorceixite solid solution.

The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

In the Brioude-Massiac district (French Massif Central: FMC), a network of W-As-Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: (i) an early deep-seated wolframite-löllingite stage formed between 12 to 9 km, at up to 400 °C; (ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 480–300 °C; (iii) a late stage controlled by fluid-overpressure potentially triggered by fault-valve mechanism, at a depth of 7 to 5 km, and a temperature estimated between 266 to 240 °C, is marked by micro-fracturing infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis and apatite LA-ICP-MS U/Pb dating demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321–316 Ma, during, or just after, the emplacement of the peraluminous dykes estimated around 329–315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid could have then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in pressure, associated with the decrease of the temperature, and sulfide-fugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the FMC). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the “Or300” event. The genetic classification of the Bonnac mineralization is equivocal. The W-As-Bi-Au-quartz veins exhibit the features of both an “orogenic gold” deposit at a relatively deep emplacement level (mesozonal), and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of economic high-grade gold content in tungsten vein-type, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central.

Magmatic fractionation and the magmatic-hydrothermal transition in rare metal granites: Evidence from Argemela (Central Portugal)

Rare Metal Granites (RMGs) are highly evolved intrusions with specific geochemical signatures that are the result of combined magmatic and hydrothermal processes. In most examples, the early evolution is overprinted by late subsolidus transformations and the role of magmatic and magmatic-hydrothermal fractionation processes remains unclear. The origin of fluids involved in magmatic-hydrothermal systems is also an open question. The Argemela RMG (Portugal) is a small subvolcanic intrusion, which exposes in remarkable continuity a sequence of magmatic, magmatic-hydrothermal and early hydrothermal processes. The intrusion comprises a main granitic facies and a border unit made of a complex alternation of aplitic and pegmatitic facies plus three generations of intragranitic veins. The granitic mineral assemblage includes quartz, K-feldspar, albite, muscovite and accessory montebrasite, cassiterite and columbite-tantalite. The whole-rock chemistry is strongly peraluminous, with very high P2O5, Li and F enrichments, elevated rare metal (Sn, Nb, Ta, W) concentrations and strong depletions in Ca, Fe, Ti, Mg. Trace element zonation in minerals, mainly muscovite and quartz, record crystallization/fractionation processes along the magmatic-hydrothermal transition. Three main stages are recognized, magmatic (crystal fractionation and accessory/minor mineral saturation), magmatic-hydrothermal (fluid/melt element partitioning) and early hydrothermal, corresponding to the segregation and collection of magmatic fluids in the intragranitic vein system. Those are followed by limited metasomatic transformations and a late sulphide stage. Stable isotope (O, H) data demonstrate a magmatic origin for the early fluids and a granite-buffered signature for the vein-forming fluids. The fractionation of rare elements (Sn, Nb, Ta, Li, W, Mn, B, Rb, Cs) from the magmatic to the early hydrothermal stage is quantitatively modelled from the combination of three mechanisms, crystal fractionation, magmatic crystallization of accessory/minor phase (MCMA) and fluid/melt partitioning. Sn and W show a markedly contrasted behaviour with Sn precipitated as disseminated mineralization within the intrusion and W partitioned toward the magmatic fluid and extracted from the intrusion. Geochemical trends along the magmatic-hydrothermal transition do not require phases with unusual properties, such as immiscible hydrous melts to be explained and they do not necessarily result in an amplification of initial rare metal melt enrichments. The Argemela RMG stresses the importance of MCMA to generate magmatic rare element pre-concentrations, which later can be hydrothermally redistributed to form large economic deposits.

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

Late Cretaceous granitic magmatism and Sn mineralization in the giant Yinyan porphyry tin deposit, South China: constraints from zircon and cassiterite U–Pb and molybdenite Re–Os geochronology

The Yinyan porphyry tin deposit in western Guangdong is spatially associated with quartz porphyry and granite porphyry. LA–ICP–MS zircon U–Pb dating defined an emplacement age of 78.5 ± 0.4 Ma for the quartz porphyry and 79.2 ± 0.9 Ma for the granite porphyry. LA–ICP–MS cassiterite U–Pb dating yielded Tera–Wasserburg lower intercept ages of 78.5 ± 0.6, 78.6 ± 1.2, and 78.2 ± 0.7 Ma, for cassiterite from a cassiterite–sulfide vein, cassiterite–sulfide ore, and a cassiterite–topaz–quartz stringer, respectively. Re–Os dating of molybdenite from seven different veins yielded an isochron age of 77.0 ± 0.5 Ma. All these new age data are indistinguishable within analytical uncertainty and, therefore, indicate a genetic relationship between the Sn mineralization and the porphyry magmatism in the Yinyan deposit. The REE tetrad effect and very low Nb/Ta and Zr/Hf ratios indicate that the quartz porphyry and the granite porphyry are highly evolved. The U–Pb dated cassiterite is enriched in Fe, W, and U and in high field strength elements (HFSEs) such as Zr, Hf, Nb, and Ta. The high Fe, Nb, and Ta contents may be responsible for the dark luminescence of cassiterite in CL images. The Zr/Hf ratio of cassiterite may potentially be used to distinguish the mineralization type. Cassiterite from pegmatites has lower Zr/Hf ratios (~ 5–6) in comparison with granite/greisen-related (~ 9–30) cassiterite. Cassiterite from the early hydrothermal stage typically contains higher amounts of Ti, Nb, Ta, Zr, and Hf than that from the late hydrothermal stage. In combination with published geochronological data of other Sn–W deposits in the western Guangdong Province, two Sn–W metallogenic events at ca. 85 and 77–80 Ma have been identified. These two metallogenic events are part of a larger-scale 75–100 Ma Sn–W mineralization event in South China, which we suggest was probably related to the subduction of the Neo-Tethyan oceanic plate.

Metallogenic model of the Jinchang Au-Ni deposit in the Ailaoshan belt, SW China, determined on the basis of pyrite trace element contents, in-situ sulfur isotope composition and PGE geochemistry

The Ailaoshan metallogenic belt in southwestern China contains numerous shear zone-related Au deposits with Au reserves greater than 200 tonnes. However, the source of ore-forming fluids and metals is controversial, hampering the construction of genetic models. In this study, we selected the Jinchang Au-Ni deposit in the northern segment of the Ailaoshan belt for detailed investigation of pyrite chemistry and in-situ sulfur isotope compositions using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The petrographic and pyrite chemistry analyses revealed four types of hydrothermal pyrite. Py1 pyrite is euhedral to subhedral and has in-situ sulfur isotope (δ34S) values of −5.30 to 6.26‰ (median: −2.93‰) and low concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. Py1 is interpreted to have formed during the early hydrothermal stage. Py2 pyrite is subhedral to anhedral and porous with variable size. Although Py2 and Py3 show different textural relationships, we conclude that both of them formed during the intermediate hydrothermal stage as evidenced by similar sulfur isotope compositions and trace element contents, and common intergrowth textures. Py2 and Py3 pyrite have in-situ δ34S values ranging from −11.6 to 1.29‰ (median: −4.45‰) and relatively elevated concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. In Py2 and Py3 pyrite, Ni concentrations correlate with Co concentration, whereas Au exhibits moderate positive correlations with Ni, Co, and As, indicating that Au, Ni and other metals were predominantly sourced from the Jinchang ultramafic rocks. Py4 pyrite is euhedral in shape and has relatively low in-situ δ34S values of −23.5 to −4.34‰ (median: −8.33‰). The metal concentrations in Py4 pyrite are lower than those in Py2 and Py3 pyrite. In general, the metal concentrations in the different pyrite populations display systematic changes, with low levels in Py1, higher levels in Py2 andPy3, and low levels again in Py4.The average total platinum-group element (PGE) concentration of the Jinchang Ni-bearing ultramafic rocks is 26.3 ppb, similar to the average value of the primitive mantle of 22.0 ppb. The Au-Ni ores from the Jinchang deposit have an average Pd/Pt ratio of 5.98, similar to that of hydrothermal Ni ores but higher than that of typical magmatic Ni ores. In contrast, the Jinchang ultramafic rocks have low Pd/Pt ratios between 0.57 and 1.13, suggesting that the PGEs in the Jinchang intrusion are of magmatic origin. Similarly, the Rh/Ir ratios of Au-Ni ores are consistent with those of hydrothermal Ni ores but differ from those of magmatic Ni ores.The pyrite chemistry, in-situ sulfur isotope compositions, and PGE geochemistry suggest that the ore-forming fluids and metals associated with the Jinchang Au-Ni deposit were dominantly derived from the Jinchang ultramafic intrusion. Mesozoic felsic magmatism or Cenozoic regional strike-slip activity may have triggered migration of the ore-forming fluids, leaching of metals from the Jinchang intrusion and precipitation of these metals at the boundary between the intrusions and carbonaceous rocks.

Genesis and age of the Toudaoliuhe breccia-type gold deposit in the Jiapigou mining district of Jilin Province, China: Constraints from fluid inclusions, H–O–S–Pb isotopes, and sulfide Rb–Sr dating

The Jiapigou mining district (JMD) in the northeastern part of the North China Block mainly contains quartz vein- and altered rock-type gold deposits that have been prospected and mined for over 200 years. Recently, a breccia-type gold deposit, known as Toudaoliuhe, was discovered in the JMD, that has attracted attention for prospecting and exploration. In this paper, we report the ore geology, gold-bearing sulfide (i.e., pyrite, galena, and sphalerite) Rb–Sr age, fluid inclusions (FIs), and H–O–S–Pb–Sr isotope data of the Toudaoliuhe gold deposit to determine its genesis and ore-forming mechanisms. Gold orebodies mainly occur in the breccia cements, as well as in several quartz veins controlled by NW-trending brittle faults. There were three hydrothermal stages: the early (quartz–pyrite), main (quartz–polymetallic sulfide), and late (quartz–carbonate) stages. Gold mineralization occurred in the main hydrothermal stage. Three types of FIs were identified: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type), and pure CO2 (PC-type). The FIs in quartz from the early hydrothermal stage are predominantly of the C- and W-types (with traces of PC-type), and have homogenization temperatures of 299.8–340.0 °C and salinities of 6.5–14.8 wt% NaCl equivalent (E). The FIs in the quartz and sphalerite forming the main hydrothermal stage are also predominantly of the C- and W-types (with trace PC type), and have homogenization temperatures of 169.1–298.7 °C and salinities of 5.7–16.5 wt% E. The FIs in the quartz and calcite from the late hydrothermal stage were solely of the W-type, and have homogenization temperatures of 126.0–210.2 °C and salinities of 2.1–11.4 wt% E. The ore-forming fluids are characterized by moderate temperature and low salinity, suggesting a CO2–H2O–NaCl system. The H–O–S–Pb–Sr isotopic results suggest that the ore-forming fluids were dominated by magmatic water, and that the ore-forming materials were mainly extracted from the Middle Jurassic magmatic reservoir (177–163 Ma), sourced mainly from the lower crust with trace mantle components. Fluid boiling was the dominant mechanism for gold and associated sulfide precipitation. The Rb–Sr isochron age of the gold-bearing sulfides at 177.7 ± 1.7 Ma indicates that gold mineralization occurred in the early Middle Jurassic. We therefore proposed that the Toudaoliuhe is a mesothermal gold deposit that formed in an extensional setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Fractionation of Rare Earth Elements in Greisen and Hydrothermal Veins Related to A-Type Magmatism

This study focuses on concentrations and fractionation of rare earth elements (REE) in a variety of minerals and bulk materials of hydrothermal greisen and vein mineralization in Paleoproterozoic monzodiorite to granodiorite related to the intrusion of Mesoproterozoic alkali- and fluorine-rich granite. The greisen consists of coarse-grained quartz, muscovite, and fluorite, whereas the veins mainly contain quartz, calcite, epidote, chlorite, and fluorite in order of abundance. A temporal and thus genetic link between the granite and the greisen/veins is established via high spatial resolution in situ Rb-Sr dating, supported by several other isotopic signatures (δ34S, 87Sr/86Sr, δ18O, and δ13C). Fluid-inclusion microthermometry reveals that multiple pulses of moderately to highly saline aqueous to carbonic solutions caused greisenization and vein formation at temperatures above 200–250°C and up to 430°C at the early hydrothermal stage in the veins. Low calculated ∑REE concentration for bulk vein (15 ppm) compared to greisen (75 ppm), country rocks (173–224 ppm), and the intruding granite (320 ppm) points to overall low REE levels in the hydrothermal fluids emanating from the granite. This is explained by efficient REE retention in the granite via incorporation in accessory phosphates, zircon, and fluorite and unfavorable conditions for REE partitioning in fluids at the magmatic and early hydrothermal stages. A noteworthy feature is substantial heavy REE (HREE) enrichment of calcite in the vein system, in contrast to the relatively flat patterns of greisen calcite. The REE fractionation of the vein calcite is explained mainly by fractional crystallization, where the initially precipitated epidote in the veins preferentially incorporates most of the light REE (LREE) pool, leaving a residual fluid enriched in the HREE from which calcite precipitated. Fluorite occurs throughout the system and displays decreasing REE concentrations from granite towards greisen and veins and different fractionation patterns among all these three materials. Taken together, these features confirm efficient REE retention in the early stages of the system and minor control of the REE uptake by mineral-specific partitioning. REE-fractionation patterns and fluid-inclusion data suggest that chloride complexation dominated REE transport during greisenization, whereas carbonate complexation contributed to the HREE enrichment in vein calcite.

Mineralogy of the Aktogai giant porphyry Cu deposit in Kazakhstan: Insights into the fluid composition and oxygen fugacity evolution

It has been proposed that most giant porphyry Cu deposits are closely associated with oxidized magmas. Most recently, many methods, including zircon trace elements, Mn content in apatite and magnetite-ilmenite mineral pair, were used to reveal the primary magmatic oxygen fugacity of the porphyry Cu deposits. Hydrothermal alterations are products of water-rock reactions and can form various hydrothermal minerals which can be used for exploration. The Aktogai Group, including Aktogai, Aidarly and Kyzilkia, contains more than 12.5 Mt Cu @ 0.4 wt% and 80 t Au @ 0.007 g/t–0.4 g/t. Chlorites show significant variations in the Aktogai deposit, which are responsible for degrees of Tschermak and Di-trioctahedral substitution. Chlorites are divided into type I and II based on Fe/(Fe + Mg) ratios of chlorites which are predominately controlled by hydrothermal fluid composition and oxygen fugacity. The chlorite I have higher Fe/(Fe + Mg) ratios than chlorite II. The formation temperatures of chlorite are 248–322 °C based on empirical geothermometry T5. Sericites belong to muscovite and phengitic muscovite, and their compositions are controlled by Tschermak and Fe3+VI = Al3+VI substitution. Water-rock reaction leaches out some elements into hydrothermal fluids, therein Si4+ and K+ precipitate as quartz and sericite at all alteration stages, but Fe2+ and Ca2+ precipitate as specific minerals (such as chalcopyrite and calcite) at main mineralization or late stage. The chlorite I form under a Fe-rich condition with high water/rock ratio, while chlorite II form under a low water/rock ratio and Fe-poor condition which may be responsible for the sulfides and epidote precipitation. According to zircon trace elements, Mn content in apatite, magnetite-ilmenite mineral pair, and Fe/(Fe + Mg) and Fe3+/TFe ratios of chlorites, the oxygen fugacity changes from high at magmatic and early hydrothermal stages, through low at main mineralization stage, to slightly high at late or post mineralization stage. Our results, combined with previous works suggest that Fe contents and Fe/(Fe + Mg) ratios of chlorites are good indicators for exploration.

Fluid Evolution and Ore‐forming Processes of the Jiama Cu Deposit, Tibet: Evidence from Fluid Inclusions

AbstractThe Jiama deposit is a large copper deposit in Tibet. Mineralization occurs in three different host rocks: skarn, hornfels and porphyry. A detailed fluid inclusion study was conducted for veins in the different host rocks to investigate the relationship between fluid evolution and ore‐forming processes. Based on examination of cores from 36 drill holes, three types of veins (A, B and D) were identified in the porphyries, four types (I, II, III and IV) in the skarn, and three (a, b and c) in the hornfels. The crosscutting relationships of the veins and that of the host rocks suggest two hydrothermal stages, one early and one late stage. Fluid inclusions indicate that the Jiama hydrothermal fluid system underwent at least two episodes of fluid boiling. The first boiling event occurred during the early hydrothermal stage, as recorded by fluid inclusions hosted in type A veins in the porphyries, type a veins in the hornfels, and wollastonite in the skarns. This fluid boiling event was associated with relatively weak mineralization. The second boiling event occurred in the late hydrothermal stage, as determined from fluid inclusions hosted in type B and D veins in the porphyries, type I to IV veins in the skarns, and type b and c veins in the hornfels. This late boiling event, together with mixing with meteoric water, was responsible for more than 90% of the metal accumulation in the deposit. The first boiling only occurred in the central part of the deposit and the second boiling event took place across an entire interlayered structural zone between hornfels and marble. A spatial zoning of ore‐elements is evident, and appears to be related to different migration pathways and precipitation temperatures of Cu, Mo, Pb, Zn, Au and Ag.

Diversity of Ti–Sn–W–Nb–Ta oxide minerals in the classic granite-related magmatic–hydrothermal Cínovec/Zinnwald Sn–W–Li deposit (Czech Republic)

The diversity of Ti–Sn–W–Nb–Ta oxide minerals within a complex granite-related magmatic–hydrothermal system was studied by electron microprobe on the example of the Cinovec (Zinnwald) granite cupola containing a well-known Sn–W–Li deposit. The Cinovec granite cupola is a late Variscan ( ca . 322 Ma), strongly fractionated A-type pluton composed of upper zinnwaldite granite and lower biotite granite. The zinnwaldite granite contains numerous subhorizontal quartz veins and greisen bodies, both composed of quartz, zinnwaldite and topaz and minor amounts of fluorite and Ti–Sn–W–Nb–Ta oxide minerals. Common Nb–Ta-rich rutile and rare cassiterite, columbite-(Fe) and -(Mn), and minerals of the pyrochlore-supergroup were found in biotite granite in deeper parts of the pluton, whereas common columbite occurs with rare W-rich ixiolite, qitianlingite, pyrochlores, and scheelite in some varieties of zinnwaldite granite. In the uppermost part of the cupola, Ta-rich cassiterite, microlite and pyrochlore are frequently observed. The greisens host predominantly wolframite, scheelite and Ta-poor cassiterite, which are accompanied by columbite-group minerals, rare W-rich ixiolite and qitianlingite. Throughout the pluton, the molar Mn/(Fe + Mn) ratio in columbite increases from 0.15–0.40 in the deeper part of the zinnwaldite granite (depths of 500–635 m) to 0.5–0.9 in granite and greisen in the uppermost part of the pluton (depths of 0–180 m), but no systematic changes in the Ta/(Nb + Ta) ratio were found. On the scale of individual zoned crystals, the rims are always Ta-enriched, whereas the Mn/(Fe + Mn) values in the rims are equal or lower. The Nb,Ta-rich rutile, interiors of the columbite crystals and Ta-rich cassiterite disseminated in the granites are interpreted as products of magmatic crystallization. The Ta-rich rims of columbite-(Fe) to -(Mn), Ta-poor cassiterite and all wolframite found in the greisen should be attributed to the early hydrothermal (greisenization) stage, whereas W-rich ixiolite, qitianlingite, and pyrochlore-supergroup minerals appear to belong to the later, hydrothermal event at comparatively lower temperature.

Geology, mineralization, stable isotope, and fluid inclusion characteristics of the Vostok-2 reduced W-Cu skarn and Au-W-Bi-As stockwork deposit, Sikhote-Alin, Russia

The large (>180KtWO3 and at least 10–15 t Au) Vostok-2 deposit is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in late to post-collisional tectonic environment after cessation of active subduction. The deposit is related to an ilmenite-series high-K calc-alkaline plutonic suite that, by its petrologic signatures, is transitional between those at W-dominant and Au-dominant reduced intrusion-related deposits. Consistently, besides large W-Cu skarns of the reduced type, the deposit incorporates quartz stockworks with significant Au-W-Bi mineralization also formed in a reduced environment. The hydrothermal stages include prograde and retrograde, essentially pyroxene skarns, hydrosilicate (amphibole, chlorite, quartz) alteration, and phyllic (quartz, sericite, albite, apatite, and carbonate) alteration assemblages. These assemblages contain abundant scheelite associated with pyrrhotite, chalcopyrite and, at the phyllic stage, also with Bi minerals, As-Bi-Sb-Te-Pb-Zn sulfides and sulfosalts, as well as Au mineralization. The fluid evolution included hot, high-pressure (420–460°C, 1.1–1.2kbar), low-salinity (5.4–6.0wt% NaCl-equiv.) aqueous fluids at the retrograde skarn stage, followed by lower temperature cyclic releases of high-carbonic, low salinity to non-carbonic moderate-salinity aqueous fluids. At the hydrosilicate stage, a high-carbonic, CH4-dominated, hot (350–380°C) low salinity fluid was followed by cooler (300–350°C) non-carbonic moderate-salinity (5.7–14.9wt% NaCl-equiv.) fluid. At the phyllic stage, a high-carbonic, CO2-dominated, moderately-hot (330–355°C, 0.9kbar) low salinity fluid was followed by cooler (230–265°C) non-carbonic moderate-salinity (6.6–12.0wt% NaCl-equiv.) fluid. A homogenized magmatic source of water (δ18OH2O=+8.3 to +8.7‰), and a sedimentary source of sulfur (δ34S=−6.9 to −6.2‰) and carbon (δ13Cfluid=−20.1 to −14.9‰) at the hydrosilicate stage are suggested. A magmatic source of water (δ18O=+8.6 to +9.2‰) and a sedimentary source of sulfur (δ34S=−9.3 to −4.1‰) but a magmatic (mantle- to crustal-derived) source of carbon (δ13Cfluid=−6.9 to −5.2‰) are envisaged for fluids that formed the early mineral assemblage of the phyllic stage. Then, the role of sedimentary carbon again increased toward the intermediate (δ13Cfluid=−16.4 to −14.5‰) and late (δ13Cfluid=−16.3 to −14.7‰) phyllic mineral assemblages. The magmatic differentiation was responsible for the fluid enrichment in W, whereas Au and Bi could also have been sourced from mafic magma. The decreasing temperatures, together with elevated Ca content in non-boiling fluids, promoted scheelite deposition at the early hydrothermal stages. The most intense scheelite deposition at the phyllic stage was caused by CO2 removal due to boiling of CO2-rich fluids; further cooling of non-boiling fluids favoured joint deposition of scheelite, Bi and Au.

THE OCCURRENCE OF HIGH-TEMPERATURE SKARNS FROM ORAVIŢA (BANAT, ROMANIA): A MINERALOGICAL OVERVIEW

A shallow-level dioritic pluton of Upper Cretaceous age caused extensive contact metamorphism of a Mesozoic calcareous sequence at Oraviţa-Ciclova in Romania. The peak metamorphic assemblage (stage I) includes wollastonite, akermanite-rich melilite, Ti-rich calcic garnet, and pyrrhotite. A second, subsequent paragenesis (stage II) is due to local decreases in fluid pressure and includes gehlenite, Ti-poor calcic garnet, monticellite, Al-rich diopside, magnetite, perovskite, and probably a Ca-Si mineral (kilchoanite?). An early hydrothermal stage (stage III) produced retrogression of the first two stages and a paragenesis including vesuvianite, clintonite, fukalite, katoite, and hydroxylellestadite. A second, late hydrothermal event (stage IV) resulted in the formation of clinochlore, talc, epidote, zeolites (scolecite and thomsonite-Ca), thaumasite, and tobermorite. A last, weathering, paragenesis corresponding to stage V includes allophane (C-S-H gels) and probably portlandite, unpreserved because of its transformation to aragonite, then calcite. The peak metamorphic conditions during the prograde evolution are estimated to have been 750–800 °C for a fluid pressure of up to 0.5 kbar. The main constants determined from chemical, optical, and X-ray powder analyses of these mineral species are reported here. This work documents the first occurrence of katoite and the second occurrences of perovskite, fukalite, and hydroxylellestadite in the Romanian skarns.

Immiscibilty between silicate magma and aqueous fluids in Egyptian rare-metal granites: melt and fluid inclusions study

Rare-metal granites of Nuweibi and Abu Dabbab, central Eastern Desert of Egypt, have mineralogical and geo- chemical specialization. These granites are acidic, slightly peraluminous to metaaluminous, Li-F-Na-rich, and Sn-Nb- Ta-mineralized. Snowball textures, homogenous distribution of rock-forming accessory minerals, disseminated mineraliza- tion, and melt inclusions in quartz phenocrysts are typical features indicative of their petrographic specialization. Geochemical characterizations are consistent with low-P-rare metal granite derived from highly evolved I-type magma in the late stage of crystallization. Melt and fluid inclusions were studied in granites, mineralized veins, and greisen. The study revealed that at least two stages of liquid immiscibility played an important role in the evolution of magma-hydrothermal transition as well as mineral deposition. The early stage is melt/fluid case. This stage is represented by the coexistence of type-B melt and aqueous-CO2 inclusions in association with topaz, columbite-tantalite, as well as cassiterite mineral inclu- sions. This stage seems to have taken place at the late mag- matic stage at temperatures between 450 °C and 550 °C. The late magmatic to early hydrothermal stage is represented by vapor-rich H2 Oa nd CO2 inclusions, sometimes with small crystallized silicic melt in greisen and the outer margins of the mineralized veins. These inclusions are associated with beryl, topaz, and cassiterite mineralization and probably trapped at 400 °C. The last stage of immiscibility is fluid-fluid and represented by the coexisting H2O-rich and CO2-rich inclu- sions. Cassiterite, wolframite ± chalcopyrite, and fluorite are the main mineral assemblage in this stage. The trapping tem- perature was estimatedbetween 200 °C and 350 °C. The latest phase of fluid is low-saline, low-temperature (100-180 °C), and liquid-rich aqueous fluid.

Compositional trends in magmatic and hydrothermal silicates of the Petráčkova hora intrusive complex, Bohemian Massif - link between the magmatic processes and intrusion-related gold mineralization

Mineralized zones of the Variscan (348–344 Ma) Petrackova hora gold deposit locally exhibit high-temperature (~600 to ~500 °C) hydrothermal alterations: K-alteration-1 (selectively pervasive) and K-alteration-2 (fracture-related) in the granodiorite and granodiorite porphyries, and silicification-1 and -2 in the country rocks (biotite hornfels and feldspathic quartzite). Alteration processes led to partial recrystallization of primary magmatic silicates, and to crystallization of new hydrothermal phases (K-feldspar, biotite and hornblende). Electron microprobe analyses were used to trace magmatic to hydrothermal evolution of the mineral chemistry. The K-feldspar phenocrysts in the granodiorite and granodiorite porphyry exhibit complex magmatic crystallization and resorption textures. Compositionally, K-feldspars differ especially in the barium admixture. It decreases from early (5–0.8 % celsian or 2.6–0.4 wt. % BaO; phenocrysts) to late magmatic (1.1–0 % celsian; groundmass) and hydrothermal (1.2–0 % celsian) K-feldspars. No significant differences were found between the chemistry of magmatic and early hydrothermal amphiboles. They all correspond to magnesiohornblende or actinolite. Tetrahedral silica (SiIV) ranges from 6.7 to 7.8 atoms per formula unit, however; values greater than 7.2 are the most common. They exhibit positive SiIV–Mg/(Mg + Fe2+) correlation. Late hydrothermal amphibole associated with silicification-2 is also magnesiohornblende; however, it exhibits negative SiIV–Mg/(Mg + Fe2+) trend. Differences in the biotite composition are less pronounced in comparison to amphiboles and comprise especially a decrease in the Ti content and an increase in the Mg/(Mg + Fe2+) ratio from the magmatic to the hydrothermal biotites. Some of the fine-grained biotite aggregates (recrystallized magmatic or hydrothermal biotite) are depleted in potassium, probably due to alteration demonstrated by the presence of submicroscopic intergrowths with clay/chlorite minerals (unrecognizable in back-scattered electron mode). Broad similarities in the chemistry of magmatic and early hydrothermal silicate minerals support the high-temperature nature of the early hydrothermal stage, and point to probable admixture of exsolved magmatic fluid phase in the early hydrothermal fluids.

THE ORIGIN AND ZONING OF HYPOGENE AND SUPERGENE Fe-Mn-Mg-Sc-U-REE PHOSPHATE MINERALIZATION FROM THE NEWLY DISCOVERED TRUTZHOFMUHLE APLITE, HAGENDORF PEGMATITE PROVINCE, GERMANY

An aplite containing Fe–Mn–Mg–Sc–U–REE phosphates, some Cu–Pb–Zn sulfides, barite, and U–Nb–Ta–Ti–Fe–Mn oxides was recently discovered near Trutzhofmuhle (THM) at the western border of the Hagendorf Pegmatite Province, Germany. We describe the sequence of phosphate crystallization in six stages of mineralization (I to VI) covering the time span from the Late Carboniferous through the Recent supergene alteration, and six sequences (1a/1b to 5) reflecting the reaction of phosphate-bearing solutions with the gneissic country-rocks (exo-aplitic) and with intra-aplitic rock-forming minerals that formed during crystallization. Age dating was carried out on columbite-(Fe) and torbernite using laser-ablation techniques. Precipitation of columbite-(Fe) and early magmatic phosphates (Mn-rich apatite, monazite) in the THM aplite is correlated with a thermal event around 302 Ma postdating the intrusion of the post-kinematic Flossenburg granite. The sequences 1a and 1b, containing the lazulite solid-solution series, gordonite and childrenite–eosphorite series, reflect late magmatic and early hydrothermal exo-aplitic processes. The late magmatic and early hydrothermal stages of the intra-aplitic sequences 2 to 5 are characterized by triplite, wolfeite, triploidite, an unnamed K–Ba–Sc–Zr phosphate, an unnamed Zr–Sc phosphate–silicate, phosphoferrite, Mn-rich vivianite, and lermontovite – vyacheslavite (?). Complexing agents such as fluorine and phosphate control the formation of Sc phosphates and silicates. In contrast with the neighboring Hagendorf pegmatite, the magmatic and hydrothermal phosphate mineralization of the THM aplite does not contain any Li-bearing phosphates and is very low in F. Rockbridgeite, whitmoreite, “ferrolaueite”, Al-bearing rockbridgeite, mitridatite, “metamitridatite”, kolbeckite and strunzite appear during late hydrothermal processes and weathering. Kolbeckite formed at the transition from hypogene to supergene processes. Its morphology varies from a rather simple combination of faces (platy kolbeckite I) under hydrothermal conditions to complex mineral aggregates (stubby kolbeckite II) produced under weathering conditions. The latest supergene alteration consists of wavellite, beraunite, cacoxenite, strengite, P- and Mn-bearing “limonite”, autunite, Sc-bearing vochtenite, Sc-bearing churchite-(Y) and diadochite. The latter phosphates with predominantly Fe, Al and U in close association with kaolinite are the representatives of supergene alteration, which is related in time and space to the Miocene peneplanation between 4.8 and 6.9 Ma. The boron- and phosphate-bearing THM aplite is not directly linked to any of the granitic plutons nearby, and is not easily classified within the scheme of rare-element pegmatites.