Magmatic-hydrothermal Processes Research Articles

Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China

Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.

Mechanisms for invisible gold enrichment in the Liaodong Peninsula, NE China: In situ evidence from the Xiaotongjiapuzi deposit

Mechanisms for invisible gold enrichment hosted within Paleoproterozoic Fe-rich metamorphic rocks (marble, biotite schist, and quartz-feldspar schist) in the Liaodong Peninsula, NE China are still unclear. In this study, we choose the Xiaotongjiapuzi deposit as a case study, and present a detailed ore deposit geology and dataset of in situ chemical compositions of Fe-sulfides to address this issue. Fe-sulfides have the following paragenesis: metamorphic pyrite (Py0), first-ore stage pyrite (Py1, Py2), and a second-ore stage comprising arsenopyrite (Apy1, Apy2) and two phases of marcasite (Mas(a-b)). Early metamorphic pyrite (Py0) has relatively low concentrations of Au (<0.06–0.17 ppm). The significantly higher concentrations of Au in Py2 (1.55–423 ppm) and Apy2 (105–341 ppm) than those of Py1 (0.068–0.123 ppm) and Apy1 (2.90–7.70 ppm) suggest that gold prefers to incorporate into non-stoichiometric hydrothermal Fe-sulfides. Substitution of As1- and As3+ in Fe-sulfides may account for the coupled geochemical behavior between As and Au, Ag, Pb, Zn, Cu. Replacement led to fractured Py1 becoming enriched in Au, As, Ag, Cu, Sb and Pb, and Py0 being replaced by Au, As, Ag, Cu, Sb and Pb-rich marcasite. In situ sulfur analysis shows that Py0 has highly positive δ34S values (+13.62 to +18.55‰), suggesting it is associated with metasediments (+14 to +20‰). Py1, Py2, Apy1, and Apy2 are isotopically homogeneous and have δ34S values of +3.67 to +7.50‰, indicative of a magmatic phase. Both generations of marcasite, which replaced Py0, have yielded mixed sulfur isotopic signatures (δ34S values of +8.66 to +13.18‰). The gold mineralization in the Xiaotongjiapuzi gold deposit was the result of magmatic-hydrothermal processes. We propose two key factors: i) abundant defects in non-stoichiometric Fe-sulfides, and ii) replacement processes can cause invisible gold enrichment in Fe-sulfides in the Liaodong Peninsula. This provides a new perspective for invisible gold mineralization in other similar regions worldwide.

Gold endowment of the metasomatized lithospheric mantle for giant gold deposits: Insights from lamprophyre dykes

The metasomatized sub-continental lithospheric mantle (SCLM) is increasingly proposed to play a key control in the formation of giant gold (Au) deposits. However, the extent of Au enrichment in metasomatized SCLM and the Au contents of its derivative partial melts remain poorly constrained. Lamprophyres are derived from low-degree melting of metasomatic and fusible components in the SCLM, and could provide fundamental information about the metasomatized source and metal release compared to mantle peridotites and/or pyroxenites. The giant Jiaodong Au province (> 5000 tons, ∼ 120 Ma) in the eastern part of the North China Craton (NCC) is an ideal locality to assess the Au endowment of metasomatized SCLM which may serve as the source of Au, as indicated by the presence of mantle-dominated volatiles in auriferous fluids associated with the deposits. Based on detailed petrological studies, we analyzed Au, S, Cu and platinum-group element (PGE) contents of the SCLM-derived lamprophyres that are temporally (130–121 Ma) and spatially associated with the Jiaodong lode Au deposits. The lamprophyre dykes were emplaced in the deep crust and did not experience significant degassing and sulfide segregation, and the sulfide saturation occurred at a late stage during magma cooling, as demonstrated by sulfide petrography, S contents, Cu/S and Cu/Pd ratios of the lamprophyres. The relatively oxidized (ΔFMQ +1 – +2), hydrous (2–4 wt.% H2O) lamprophyres display variable and generally low Au contents (0.04–3.59 ng/g, mainly 1–3 ng/g, n = 31), as well as low PGE and Cu contents. Their high Au/Cu(N) and Au/Pd(N) ratios together with high Ba/Nb indicate a discernible Au addition to their source, resulting from multi-stage mantle metasomatism related to subduction. However, the average Au contents of the metasomatized SCLM before large-scale Au mineralization are heterogeneous and overall in a range of 0.5–1.5 ng/g. The values are higher than that of depleted SCLM (< 0.5 ng/g) but are still lower than or similar to the fertile upper mantle (1.2–1.7 ng/g). These results indicate that mantle metasomatism related to subduction replenished some amount of Au into previously depleted SCLM, leading to local or regional Au enrichment. However, such processes evidently did not result in remarkable Au enrichment of the mantle source beneath the eastern part of the NCC. We thus propose that the metasomatized SCLM and magmas derived from it, though without anomalously high Au contents, could still provide large amounts of Au for large-scale Au mineralization. Importantly, metasomatic volatiles from the metasomatized SCLM exert a key role, and they enable efficient release of Au from mantle sources, promote Au transportation along trans-lithospheric fault channels and subsequent enrichment and precipitation via magmatic-hydrothermal processes for giant Au deposits.

Geochronology, Fluid Inclusions and Isotopic Characteristics of the Dongjun Pb‐Zn‐Ag Deposit, Inner Mongolia, NE China

AbstractThe Dongjun Pb‐Zn‐Ag deposit in the northern part of the Great Xing'an Range (NE China) consists of quartz‐sulfide vein‐type and breccia‐type mineralization, related to granite porphyry. Hydrothermal alteration is well‐developed and includes potassic‐silicic‐sericitic alteration, phyllic alteration and propylitic alteration. Three stages of mineralization are recognized on the basis of field evidence and petrographic observation, demarcated by assemblages of quartz‐pyrite‐arsenopyrite (early stage), quartz‐polymetallic sulfide (intermediate stage) and quartz‐carbonate‐pyrite (late stage). Zircon LA‐ICP‐MS U‐Pb dating indicates that the granite porphyry was emplaced at 146.7 ± 1.2 Ma (Late Jurassic). Microthermometry and laser Raman spectroscopy shows that ore minerals were deposited in conditions of intermediate temperatures (175–359°C), low salinity (0.5–9.3 wt% NaCl eqv.) and low density (0.60–0.91 g/cm3). Ore‐forming fluids were derived largely from magmatic hydrothermal processes, with late‐stage addition of meteoric water, belonging to a H2O‐NaCl‐CO2 ± CH4 system. The δ34SV‐CDT values range from 0.75‰ to 4.70‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ore minerals are in the ranges of 18.240–18.371, 15.542–15.570, and 38.100–38.178, respectively. Data for the S and Pb isotopic systems indicate that the ore‐forming metals and sulfur were derived from Mesozoic magma. Based on the geological characteristics and geochemical signatures documented in this study, we conclude that the Dongjun deposit is a mesothermal magmatic hydrothermal vein‐type Pb‐Zn‐Ag deposit controlled by fractures and related to granite porphyry, in response to Late Jurassic tectonic–magmatic–hydrothermal activity. We further conclude that fluid immiscibility, fluid mixing and fluid‐rock interactions were the dominant mechanisms for deposition of the ore‐forming materials.

Biotite geochemistry and its implication on the temporal and spatial difference of Cu and Mo mineralization at the Xiaokele porphyry Cu-Mo deposit, NE China

The Late Jurassic large Xiaokele porphyry Cu-Mo deposit is newly discovered in the northern Great Xing’an Range, NE China. Both Cu and Mo mineralizations have occurred in the potassic stage and are present in the potassic zone, but only Mo (Cu negligible) mineralization has been developed in the phyllic stage and is present in the phyllic zone. The cause for such temporal and spatial difference of Cu and Mo mineralization at Xiaokele remains unclear. In this study, biotite major element compositions were studied to reveal the magmatic-hydrothermal processes, and the temporal and spatial difference of Cu and Mo mineralization at Xiaokele. Biotite geochemical data show that the magmatic biotites (Bi-I), and the potassic-stage (Bi-II) and phyllic-stage (Bi-III) hydrothermal biotite are Mg-biotites, and that mineralization at Xiaokele occurred under 52–86 MPa (equivalent to 1.95–3.32 km depth). Calculated oxygen fugacity of Bi-I, Bi-II, and Bi-III biotites are all above nickel-nickel oxide (NNO) oxygen buffer, suggesting that these biotites were formed under oxidizing conditions. The decreasing Cl and largely constant F content from Bi-I, Bi-II to Bi-III result in an increasing trend of IV(F/Cl), indicating different elemental behavior between F and Cl during the magmatic-hydrothermal fluid evolution. Given that Cl content is a key Cu-transporting ligand in porphyry type mineral systems, the decreasing Cl content in Bi-III indicates lower Cu transporting capacity in the phyllic stage, resulting in the negligible Cu mineralization in the phyllic zone. Since F is important for Mo transport in hydrothermal systems, we conclude that the constant F content reflects stable Mo transporting capacity in the fluids, resulting in significant Mo mineralization in both the potassic and phyllic zones. Our results show that biotite geochemistry can be used to explain the temporal and spatial distribution of Cu and Mo mineralization in porphyry Cu-Mo systems.

Gold occurrences in copper-magnetite-apatite deposit at Seruwila, Sri Lanka

This study presents petrology and evidence for possible gold occurrences in Seruwila copper–iron oxide–apatite (IOA) deposit, hosted in an ultramafic intrusion which is located at the boundary between the Highland and Vijayan complexes, within the intermediate-granitic basement in north-eastern Sri Lanka. The study is complemented with petrological observations and XRD and SEM analysis, respectively, to investigate the petrology/subsurface geology of the deposit and identify possible gold occurrences in the deposit. The ore-bearing rocks are mainly composed of magnetite and apatite in various proportions, hosted in an ultramafic intrusion with cumulate features within the granitic-intermediate basement. The secondary veins contain magnetite, chalcopyrite, pyrrhotite, and pyrite together with apatite and scapolite, tremolite, diopside, and minor actinolite and calcite, serpentinite, anhydrite, or gypsum. The clinopyroxene euhedral crystals show cumulus textures including grain triple junctions and large dihedral angles (∼120°), showing magmatic origin. Texturally two types of amphiboles are identified as coarse-grained (0.5–1 mm), pale green euhedral amphibole that is free of inclusions, and fine-grained (<0.1 mm) and brownish, occurring as anhedral inclusions in clinopyroxene. The deposit contains varying amounts of sulfides in which pyrite is the potential gold carrier in magmatic–hydrothermal processes. By the results of XRD analysis, it is evident that the presence of Au (111), Au (200), Au (220) and Au (311), although with low count values (50–500), probably due to the low concentration of gold. Therefore, particularly in the samples with veins or veinlets, gold was inferred to be present in pyrite/chalcopyrite as invisible structurally-bonded gold and/or gold nanoparticles. Hence, the results of this study, although at non-ore grade, veinlets of gold-bearing pyrite/chalcopyrite may serve as a promising target for gold occurrence, being a potential site of gold-mineralization in the context of the East-Gondwana.

Protracted lifespan of the late Mesozoic multistage Qianlishan granite complex, Nanling Range, SE China: Implications for its genetic relationship with mineralization in the Dongpo ore field

The Dongpo ore field, containing a series of W-Sn-Mo-Bi-Pb-Zn granite-related deposits, is a typical reduced intrusion-related system centered with the causative zoned Qianlishan granite complex (QGC). The QGC comprises porphyritic granite, equigranular granite and granite porphyry. According to zircon U-Pb age data, the emplacement timing of the QGC was constrained to 155–151.7 Ma. Here, we conducted a set of geochronological analyses with different closure temperatures (including monazite, apatite U-Pb, biotite and muscovite Ar-Ar and zircon fission track dating) on the QGC, revealing that the QGC experienced a protracted magmatic-hydrothermal timespan of 38.8–53.8 my. The protracted lifespan is probably due to radiogenic heat supplied by highly enriched high-heat-producing elements. The mineralizing blossom time (149–154.2 Ma) in the Dongpo ore field is in accordance with the prolonged lifespan (5.7 my: from 149.3 Ma to 155 Ma) of the magmatic-hydrothermal process caused by the QGC over the temperature range of 300–800 °C. This result implies that successive inputs of magma with high heat-producing elements (U, Th, K) play a fundamental role in the metal enrichment process of intrusion-related deposits and the formation of large economic ore fields.

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

Mineralogy and geochemistry of the Zhuxi Nb-rich trachytic rocks, South Qinling (China): Insights into the niobium mineralization during magmatic-hydrothermal processes

Series of trachytic rocks, containing significantly high Nb contents, were identified in the Zhuxi region of South Qinling Orogenic Belt (SQB). An integrated investigation, mainly focusing on Nb variations in rocks and minerals, was conducted to constrain the enrichment mechanism of niobium. The Zhuxi trachytic rocks are predominantly composed of trachyte ignimbrites and trachytes, with pronounced Nb contents of 213–627 ppm and 116–384 ppm, respectively. Apatite U–Pb dating has revealed that the rocks were formed during the Silurian (434 ± 10 Ma), consistent with the associated alkali basaltic rocks. The trachytic rocks are chemically characterized by high total alkali, HFSEs (Nb, Ta, Zr, and Hf), and LREE, along with a marked depletion of Ba, Sr, P, and Ti, similar to the oceanic island basalt (OIB)-type series. The Zhuxi trachytic rocks are most probably generated by partial melting of a metasomatic lithospheric mantle followed by extensive fractional crystallization. The whole rock fluorine contents gradually decreased with decreasing Nb contents from trachyte ignimbrites (F = 123–3090 ppm) to trachytes (F = 90–1820 ppm), indicating that substantial quantities of halogen played an important role in Nb enrichment.The major mineral reservoirs of niobium in Zhuxi trachytic rocks are columbite, titanite, and aeschynite of magmatic and hydrothermal origins. The magmatic titanite generally contains lower Nb2O5 (0.71–1.68 wt%) than the hydrothermal titanite (0.67–7.27 wt%). In contrast, the Nb2O5 contents are indistinguishable in magmatic and hydrothermal columbites (70.16–74.44 wt%). Aeschynite is essentially a hydrothermal product, comprising high Nb2O5 contents (25.64–39.67 wt%). The high Nb contents in early-crystallized magmatic columbites and titanites indicate that the parental magma was initially Nb-enriched. The pronounced Nb contents in hydrothermal mineral phases (such as titanite) reveal that hydrothermal processes likely led to further Nb enrichment.

Tourmaline composition and boron isotope signature as a tracer of magmatic-hydrothermal processes

Abstract This study presents a petrogeochemical and boron isotope study on tourmaline from the barren Damai, and the contemporaneous but ore-bearing Dewulu and Meiwu intrusions, to better understand the origins, sources, and fluid evolution of magmatic-hydrothermal ore systems and provide ore formation implications for gold, copper, and iron deposits in the Xiahe-Hezuo polymetallic district in the West Qinling, China. Tourmaline from all three intrusions shows similar compositions and encompasses Na-Fe schorl and Na-Mg dravite. Tourmaline at Dewulu is primarily found in tuffaceous breccias and a quartz diorite porphyry. In the tuffaceous breccia body, tourmaline occurs as fine-grained anhedral masses that fill voids and cement fragments of breccia and sickle quartz. Tourmalines in breccia are texturally similar to those formed in typical breccia pipes, which are attributed to explosion or collapse induced by a transition from magmatic to hydrothermal Si- and B-rich fluids. They display element substitutions controlled by Fe2+Mg–1, indicating a reduced environment. Values of δ11B are –6.6 to –4.0‰, representing the primary magmatic-hydrothermal fluids. Tourmaline from the Dewulu quartz diorite porphyry is coarse-grained, euhedral, and found in quartz-sulfide veins. The tourmaline displays oscillatory zoning textures but lacks correlative variations of major elements. The Fe2+Mg–1 and Fe3+Al–1 substitution mechanisms are dominant, demonstrating more oxidized fluids. The δ11B values in the cores, ranging from –7.1 to –5.6‰, suggest that the tourmalines in the quartz veins were inherited from magmatic-hydrothermal fluids that precipitated the fine-grained tourmaline in the tuffaceous breccia body. A large δ11B isotopic fractionation that decreases from cores (–5.6‰) to rims (–10.7‰) indicates significant fractionation during degassing occurred, increasing oxygen fugacity of the residual liquid. The Meiwu locality hosts fine-grained euhedral tourmalines coexisting with magnetite. Their composition is controlled by substitution between Al3+ by Fe3+ and has the lightest δ11B values ranging from –11.4 to –10.0‰. They are interpreted to result from skarn formation under oxidized conditions. In contrast, X☐Al(NaMg)–1 is the dominant substitution mechanism for Damai tourmalines and attributed to (geochemically) reduced fluids with a low salinity. We conclude that tourmalines with low Fe values, substitution mechanisms dominated by Fe3+Al–1, and large shifts of B isotopic composition are potentially an ore-forming indicator in the Xiahe-Hezuo polymetallic district.

Zinc isotopic fractionation between aqueous fluids and silicate magmas: An experimental study

Fluids in magmas play a key role in magma differentiation and transportation of economic metals for ore deposits. As a chalcophile and incompatible element, zinc and its isotopes have been increasingly applied to study the magmatic-hydrothermal processes. However, zinc isotopic fractionation between aqueous fluids and magmas has not been well constrained. Here we experimentally determined equilibrium fractionation factors of Zn isotopes between aqueous fluids and silicate magmas (Δ66Znfluid-magma = δ66Znfluid – δ66Znmagma). The results reveal that aqueous fluids are isotopically heavier than the coexisting silicate magmas. No correlation between Δ66Znfluid-magma and temperature or chlorine contents in fluids is observed under our experimental conditions. Instead, Δ66Znfluid-magma is negatively corresponded with NBO/T of the melt (the ratio of non-bridge oxygen and tetrahedron ions), and positively correlated with the molar ratio of Al/(0.5 K + Ca + Fe) in the bulk magmas, suggesting the controlling of silicate composition on Zn isotope fractionation. Our data therefore indicate that the isotopically heavier Zn in the fluids exsolved from magmas may account for the higher δ66Zn of pegmatites and high-silica granitic rocks. Moreover, involvement of magmatic fluids can explain the highly variable and remarkably heavier Zn isotopic signatures of fumaroles, thermal spring waters, and seafloor hydrothermal fluids compared to the igneous rocks. This study provides information that can be used to guide research using Zn isotopes to trace fluid activity and magmatism.

Mineralization of the Tuwu Porphyry Cu Deposit in Eastern Tianshan, NW China: Insights From In Situ Trace Elements of Chlorite and Pyrite

The Tuwu porphyry copper deposit is located on the Dananhu-Haerlik island arc in eastern Tianshan, NW China. Based on geology, petrology, and in situ trace element studies of pyrite and chlorite, we redefined the characteristics of hydrothermal fluids and the following three mineralization stages: premineralization stage (stage Ⅰ), porphyry metallogenic stage (stage Ⅱ), and superimposed transformation stage (stage Ⅲ). Pyrite stage Ⅰ (Py-I) has the highest Co/Ni ratios, and the precipitation crystallization of chlorite (Chl-I2) has the similar rare earth element distribution patterns with those of volcanic rocks Carboniferous Qieshan (CQ), indicating intense volcanic hydrothermal activity. The Co/Ni ratios of Py-II1 and Py-II2 (stage Ⅱ) tend to decrease over time. Moreover, the rare earth element (REE) distribution patterns of Chl-II have similar LREE enrichment, and the Eu anomalies in Chl-II1, Chl-II2, and Chl-II3 range from positive to negative. The initial ore-forming fluid was mainly magmatic hydrothermal fluid, and with the late-stage addition of meteoric water and continuous sulfide precipitation, the trace element composition of the fluid changed, and the whole system became more oxidizing. Py-III (stage Ⅲ) has the lowest Co/Ni ratios, and the REE distribution pattern of Chl-III is characterized by LREE enrichment. Moreover, the Chl-III shows obvious shear deformation characteristics. The results indicate that the host rocks experienced intensely superimposed reformation. By combining and integrating our results with the regional evolution processes in the eastern Tianshan, we propose that the Tuwu porphyry deposit has undergone magmatic hydrothermal and metamorphic hydrothermal processes. Volcanism (stage Ⅰ) provided the space and initial conditions for the emplacement of the metallogenic body. With the emplacement of the plagiogranite porphyry (stage Ⅱ), the main copper mineralization occurred in the porphyry and surrounding rocks. After porphyry mineralization (stage Ⅲ), regional ductile shearing and collisional compression led to a copper reaction, and its accumulation along the faults formed an ore shoot.

Platinum-group elements and gold distribution in ores of the Haftcheshmeh porphyry Cu-Mo-Au deposit, NW Iran

The Haftcheshmeh porphyry Cu-Mo-Au deposit is located in the Arasbaran metallogenic belt, northwest Iran, with mineralization occurring mainly in gabbro-diorite and granodiorite porphyry. Pyrite and chalcopyrite are the most abundant sulfides that occur as disseminated and veinlet type ores in the potassic alteration zone. Platinum-group elements (PGEs), Au, and Cu concentrations of selected pyrite- and chalcopyrite-bearing samples were analyzed using in situ LA-ICP-MS and bulk rock Lead Fire Assay. Relatively high contents of PGEs and other precious elements are present in sulfide minerals, especially in pyrite that is the principal carrier of these elements in the deposit. Palladium is the most abundant of the PGEs in the sulfides. The contents of Pd are systematically higher than Pt in this deposit, consistent with other porphyry deposits, although Os, Rh, and Ir abundances are in general equal to or below the limits of detection in pyrite and chalcopyrite. The most important factors related to potential PGEs and chalcophile elements enrichment in the Haftcheshmeh deposit is the oxidized mafic endmember components of source magmas, i.e., high ƒO2 conditions, possibly with a semi-metal collector melt involved, and the high saline magmatic hydrothermal fractionation process. All these mechanisms seem to have enhanced the concentration of PGEs, Au, and other chalcophile metals. PGEs and other chalcophile elements were transported by exsolved high-T, saline magmatic hydrothermal fluids and then low T reaction and reduction to enhance saturation of sulfide phases during mineralization associated with potassic alteration.

Theoretical estimates of equilibrium sulfur isotope effects among aqueous polysulfur and associated compounds with applications to authigenic pyrite formation and hydrothermal disproportionation reactions

Inorganic polysulfur compounds (polysulfides, Sx2-; polysulfur radical ions, Sx·-; thiosulfate, S2O32-; polythionate, SxO62-; elemental sulfur, e.g. S8) participate in numerous geochemical processes related to the sulfur cycle. These include authigenic pyrite formation in sediments undergoing early stages of diagenesis, reactions associated with magmatic-hydrothermal processes, and numerous other aquatic sulfur redox processes (e.g., pyrite and sulfide oxidation). Sulfur isotope fractionations among many of these and associated compounds (e.g., H2S, HSO4-) are either unknown or unconstrained over wide ranges of temperatures. We present theoretical estimates of equilibrium sulfur isotope fractionation factors among aqueous polysulfur compounds (including select polysulfides, polysulfur radical anions, and polythionates) and select aqueous sulfide and sulfate compounds that correspond to all three stable isotope ratios of sulfur (33S/32S, 34S/32S, 36S/32S). Our estimates are based on electronic structure calculations performed at the B3LYP/6–31+G(d,p) level of theory and basis set implemented in concert with an explicit solvation model whereby molecules are encapsulated in water clusters of varying size (30–52 H2O) to simulate the aqueous solvation environment. These calculations yield relatively small magnitude fractionation factors between aqueous polysulfides, polysulfur radicals, and reduced sulfur moieties in polythionates relative to the aqueous sulfide compounds but reveal numerous crossovers that result in non-intuitive temperature dependencies. Our predictions of 34S/32S-based fractionation factors among aqueous sulfur compounds generally agree with previous experimental constraints where available within estimated uncertainties (e.g., HSO4-/H2S(aq), H2S(aq)/HS-, HSO4-/S0, H2S(aq)/S0). We use our calculations to explore equilibrium isotope fractionations among polysulfur and sulfide compounds that are precursors to authigenic pyrite in the framework of established mechanisms (e.g., the polysulfide mechanism). We examine possible explanations for why pyrite formation may be associated with relatively small isotope fractionation with respect to precursor aqueous sulfur compounds. We additionally use our theoretical calculations to constrain multiple sulfur isotope (33S/32S, 34S/32S, 36S/32S) mass balance models associated with the abiotic hydrolytic disproportionation of intermediate sulfur compounds (SO2, S8, S3·-) relevant to hydrothermal-magmatic-volcanic systems in order to illustrate the potential for subtle but potentially resolvable effects expressed in values of Δ33S and Δ36S associated with these processes. We apply a SO2 disproportionation mass balance model based on previous work but newly constrained by our theoretical calculations to (hyper-) acid crater lakes associated with active volcanoes, and newly highlight the potential for the utility of multiple sulfur isotope analyses in volcanic gas monitoring and constraining sulfur cycling processes in such systems.

Molybdenum isotope fractionation at upper-crustal magmatic-hydrothermal conditions

Molybdenum isotopes are an established proxy for paleoredox conditions in low-temperature surface systems. However, the mechanisms behind demonstrated Mo isotope fractionation during igneous and hydrothermal processes at elevated temperatures are still controversial. This study focusses on a comprehensive dataset documenting the late stage magmatic-hydrothermal evolution of Mo isotope systematics in miarolitic cavities and their host granite from a shallow arc-related intrusive system, the Torres del Paine laccolith in Chile. Molybdenum isotopic compositions (δ98MoSRM3134) were measured for (i) granitic bulk with or without petrographic signs of fluid loss, (ii) magmatic-hydrothermal fluids, and (iii) successively crystallised hydrothermal minerals and range from −1.6 to +1.8‰. The observed variability in δ98MoSRM3134 for individual miarolitic cavities approaching closed system conditions are smaller than the overall range in our dataset but still exceed 1.5‰. The Mo isotopic signature of magmatic fluids was directly measured for the first time by bulk dissolution of magmatic fluid inclusion bearing quartz. Absolute values for magmatic-hydrothermal fluids vary between +0.6 to +1.8‰ δ98MoSRM3134, which is significantly heavier than the granitic bulk rock signatures of −0.1 to +0.46‰ δ98MoSRM3134. Hydrothermal minerals in contrast exhibit variably light δ98MoSRM3134 between −1.6 and +0.6‰. Isotopic differences Δ98Mofluid-mineral between fluid and hydrothermal minerals coexisting in the sampled cavities are largest for plagioclase with 1.9–2.2‰ Δ98Mo, and amount to 1.6–1.9‰ Δ98Mo and 1.5–1.9‰ Δ98Mo for alkali feldspar and biotite, respectively. Smaller values of 1.2–1.5‰ Δ98Mofluid-siderite, 0.8–1.9‰ Δ98Mofluid-molybdenite, 0.4–1.2‰ Δ98Mofluid-titanite and 0.4–1.3‰ Δ98Mofluid-allanite are obtained for higher Mo concentration minerals. Given that fluid-mineral pairs coexisted in equilibrium the ranges in Δ98Mofluid-mineral values we report offer first constraints on the extent of hydrothermal Mo isotope fractionation. The magnitude and direction of these values agrees well with fractionation factors calculated based on an ionic bond-strength model for the incorporation of Mo6+ in hydrothermal minerals for crystallisation temperatures in miarolitic cavities (650–450 °C). This implies that significant fractionation effects can arise during hydrothermal processes even without changes in Mo redox state from oxidised fluid.We can summarise the Mo isotope evolution during magmatic-hydrothermal processes as follows: First, Mo is transferred into the fluid phase exsolving from solidifying magma during late stage igneous evolution. The exsolved fluid subsequently precipitates hydrothermal minerals upon cooling, which dominantly incorporate light Mo isotopes (at variable KDMo(fluid-mineral)). With progressive hydrothermal crystallisation, the remaining fluid evolves to increasingly higher δ98MoSRM3134 along with decreasing Mo concentration.Our data demonstrate that large variability in Mo isotopic signatures can be produced solely by primary magmatic-hydrothermal isotope fractionation processes at elevated temperatures. The generated large range in δ98Mo signatures implies that (i) Mo isotopic signatures of evolved samples cannot be employed for tracing sources or precursor processes unless isotopic fractionation during magmatic-hydrothermal stages is quantified and can be corrected for; and (ii) mass balance models for the global Mo cycle used in paleoredox reconstruction need to account for potentially heterogeneous and isotopically fractionated continental contributions from evolved or hydrothermally overprinted rocks.

Significant boron isotopic fractionation in the magmatic evolution of Himalayan leucogranite recorded in multiple generations of tourmaline

The late-stage evolution of leucogranite magmas and related magmatic-hydrothermal processes which could lead to rare-metal mineralization are still poorly studied in the Himalayas. Since tourmaline is among the minerals that are present in evolved granites of the Himalayas, the geochemistry of this mineral, and in particular the boron isotopic composition, is a powerful tool to trace evolution processes and sources of these magmas. This study focuses on the analysis of tourmalines in the Gurla Mandhata tourmaline leucogranites (GMTL) from the northwest of the Himalayan orogen. Two generations of tourmaline show clear differences in texture, major element, and B isotopic compositions. Early-stage tourmalines have high-Mg# [Mg/(Mg + Fe) 0.39–0.45] and occur as inclusions in late-stage tourmalines [Mg# <0.34] and other minerals. The δ11B values are in the range of −7 ~ −8‰ for the early-stage tourmaline and in the range of −12 ~ −15‰ for the late-stage tourmaline. The occurrence of tourmalines with contrasting B isotopic compositions from the same leucogranite body (even in a same specimen) can be explained either by mixing of magmas from different sources with different B isotopic compositions or by remarkable B isotopic fractionation during magma differentiation. As indicated by mineral textures and compositions, the mixing of different magma batches can be ruled out as a major cause in the case of GMTL. Alternatively, the bimodal distribution of tourmaline δ11B most likely suggests the occurrence of multiple-stage B isotopic fractionation during the magma evolution. Our modeling based on Rayleigh fractionation shows that this fractionation may have been induced mainly by extraction of fluid at a late magmatic stage, but crystallization of tourmaline may have also played a minor role. Interestingly, a compilation of δ11B values (>260 data points) in magmatic tourmaline from eight Himalayan leucogranite bodies systematically shows a similar bimodal distribution (i.e., with peaks at −7‰ and − 13‰), implying that this significant B isotopic fractionation may be a common scenario during magma evolution of the Himalayan leucogranites. However, multiple magma sources with contrasting B isotopic compositions cannot be fully ruled out for explaining the bimodal distribution of B isotopic composition. Combining our results with SrNd isotopic and other geochemical characteristics of the Himalayan leucogranites, we propose that the granitic magmas with high δ11B originated either from fluid-present melting involving boron-rich fluids, or more probably from dehydration melting of the Greater Himalayan Sequence that had been metasomatized by boron-rich fluids. The geological context implies that these fluids were probably derived from dehydration of mica-rich rocks from the Lesser Himalayan Sequence underlying the Greater Himalayan Sequence.

Molybdenum isotope systematics of subduction-related magmas from the Zhongdian region: Assessing the Mo fractionation behavior in magmatic-hydrothermal processes

The behavior of molybdenum isotopes in magmatic–hydrothermal systems is poorly known. Late Triassic andesitic rocks from the Disuga and Lannitang areas, and contemporaneous porphyry–skarn Cu polymetallic deposits (e.g., Pulang, Xuejiping, Yaza, Hongshan, and Langdu), from the Zhongdian region, southeastern Tibetan Plateau, China, present an ideal opportunity to study Mo isotope fractionation in magmatic–hydrothermal systems. In this study, we report Mo isotopic compositions, analyzed by multi-collector–inductively coupled plasma–mass spectrometry, of ore-forming porphyries and hydrothermal chalcopyrite, pyrite, and molybdenite from the Pulang deposit, and coeval barren andesites from the Disuga and Lannitang areas. The δ98Mo values of the Pulang porphyries (−0.59‰ to −0.05‰) are lower than those of the Disuga and Lannitang andesites (−0.29‰ to −0.03‰). In the Pulang deposit, whole-rock δ98Mo values are lower (−0.59‰ to −0.05‰) than those of pyrite (+0.73‰ to +2.09‰). Chalcopyrite has lower δ98Mo values (−0.16‰ to +1.06‰), than pyrite, but similar δ98Mo values to molybdenite (−0.18‰ to +0.06‰). We infer, tentatively, that fractional crystallization was not a major control on the Mo isotopic composition of the Pulang magmatic–hydrothermal system. Heavy Mo isotopes partitioned into pyrite relative to molybdenite, and 98Mo is thought to partition into Mo6+-bearing species relative to Mo4+-bearing species. Therefore, if MoO42− was the predominant Mo species in exsolved hydrothermal fluids, then formation of pyrite from these fluids explains the high δ98Mo values of pyrite. We infer that the chalcopyrite from the Pulang deposit contains micron-scale molybdenite inclusions, which partially explains the comparable δ98Mo values of chalcopyrite and molybdenite. Pyrite has a strong influence on Mo isotope fractionation, at least in the magmatic–hydrothermal system described here.

Chemical and boron isotopic compositions of tourmaline at the Baiyanghe Be[sbnd]U deposit, northwest China: Implications for Be[sbnd]U mineralization

Tourmaline records the physical and chemical conditions of the crystallization environment and preserves the information through younger magmatic-hydrothermal processes. Therefore, tourmaline is a favorable geochemical tracer and can be used to reveal processes that have contributed to the metallogenesis of hydrothermal deposits. The Baiyanghe deposit, located in the Xiemisitai Mountains, Northwest China, is the largest volcanogenic BeU deposit in Asia. The deposit has three types of ores: U type, Be type, and BeU type. These ores either occur separately or co-exist but have cross-cutting relationships. Major ore bodies occur as fracture fillings along contact zones between the Yangzhuang granite porphyry and the underlying Devonian volcanic rocks. Three types of tourmaline (e.g., hosted in the granite porphyry, rhyolitic crystal tuff, and basalt) were identified based on their occurrences, and seven sub-type tourmalines were clarified according to their textures and chemical compositions. They all belong to the alkali schorl group and rich in F. The low Al concentration (< 6 apfu) and Fe + Mg > 3 apfu tourmaline hosted in the rhyolitic crystal tuff suggest the substitution at Z site by Fe3+. The tourmalines hosted in the granite porphyry are rich in Li, Be, U, Ca, Ti, Cr, Y, Zr, Nb, Mo, Sn, Sb, Ta, and Th, whereas the tourmalines hosted in the rhyolitic crystal tuff and the basalt are rich in Mg, Sc, V, Co, Cu, Ni, Ga, and Sr, respectively. The δ11B value ranges from −5.94‰ to −4.48‰, from −8.01‰ to −6.93‰, and from −8.31‰ to −4.94% in the tourmalines hosted in the granite porphyry, the rhyolitic crystal tuff and the basalt, respectively. These data indicate that the chemical components that have contributed to the growth of tourmaline were derived from granitic magmas essentially. The oscillating zonation and the apparent variations of trace element compositions (e.g., Fe/(Fe + Mg) and Na/(Na + Ca) ratios) and REE patterns of the tourmalines indicate the involvement of periodic fluids and significant fluid-rock interaction, which are responsible for the precipitation of sub-type tourmalines. It is proposed that the tourmalines at the Baiyanghe deposit are the products of fluids exsolved from either a deep-rooted crystallizing magma chamber or one of the younger intrusions of the hosting composite granitic pluton in addition to the crystallizing the Yangzhuang granitic magma. The chemical and boron isotopic composition of tourmaline can provide a fingerprint to decipher the magmatic-hydrothermal process during the cyclic UBe mineralization.

A genetic link between albitic magmas and IOCG mineralization in the Ossa Morena Zone (SW Iberia)

A detailed geological and isotopic study of the Colmenar deposit (Ossa Morena Zone, SW Iberia) shows that the magnetite-rich mineralization formed by complex magmatic-hydrothermal processes related to the crystallization of water-rich albite-magnetite igneous rocks derived from the crystallization of unusual melts formed during anatexis in a high temperature-low pressure metamorphic regime. The most likely protolith includes a sequence of iron-rich chemical sediments, amphibolite and possible meta-evaporites of early Cambrian age. The albite-magnetite rock occurs as up to 20 cm-thick dyke and breccia bodies and show complex immiscibility relationships with an albite-K-feldspar-quartz leucogranite. Iron-rich fluids exsolved during the crystallization of these melts are responsible of the formation of hydrothermal breccias and the widespread replacement of the hosting calc-silicate hornfels by a magnetite-ferroactinolite-albite assemblage along syn-mineralization shear zones. Geochronological data obtained for mineralization and related hydrothermal alteration points to a Variscan age (ca. 340 Ma), interpreted also as the age of the high-grade metamorphism driving anatexis at the Valuengo Metamorphic Complex. Despite the low Cu and Au contents, this mineralization shares features with the IOCG systems, which in other districts show a spatial relationship with albite-rich rocks, evaporites and pre-existing iron mineralization. The observations presented from Colmenar support an alternative genetic model with prospective implications for the Ossa Morena Zone that can be extrapolated to other IOCG belts worldwide.

Discriminate between magmatic- and magmatic-hydrothermal ore deposits using Fe isotopes

Although discriminating high temperature mineral systems, for example, magmatic- vs magmatic-hydrothermal origin Fe deposits, is challenging, the emerging Fe isotope systematics have shown good potentials to provide crucial information on resolving the problem. Here, Fe isotopes of multiple types of samples from one iron oxide-apatite (IOA) deposit and two skarn Fe deposits are measured, aiming to study their behaviors in high temperature magmatic and magmatic-hydrothermal environments. The primary observation of this study is that magnetite samples from high grade ores of magmatic origin IOA deposits has a heavier Fe isotopic signature relative to the associated igneous rocks, whereas a lighter Fe isotopic composition in magnetite samples from magmatic-hydrothermal origin skarn deposits relative to the causative intrusions. Another distinctive feature is that δ56Fe values of the causative intrusions in the magmatic origin IOA deposits are lighter than that of non-fertile intrusions, whereas the ore-associated intrusions of the magmatic-hydrothermal deposit are heavier in δ56Fe than that of non-fertile igneous rocks. The above two observed scenarios suggest that Fe isotope fractionation exists in both magmatic immiscibility and hydrothermal fluid exsolution processes. Magma immiscibility leads to heavy Fe isotope enrichment in immiscible Fe-rich melt phase, and resulting light Fe enrichment in Si-rich melt phase. Hydrothermal fluid exsolution results in lighter Fe isotope enrichment in the exsolved liquid phase, with a heavy Fe isotope enrichment in the residual magma. Fe isotopes behave in different ways in magmatic and magmatic-hydrothermal mineralization processes, which makes Fe isotopes an effective tool to discriminate mineral deposits of magmatic origin from those formed by magmatic-hydrothermal processes.