Petrogenesis of the quartz diorite from the Lietinggang-Leqingla Pb-Zn-Fe-Cu- (Mo) deposit in southern Tibet: implications for the genesis of a skarn-type polymetallic deposit in the Tibetan-Himalayan collisional orogen

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Bobtraillite from Gejiu hyperagpaitic nepheline syenite, southwestern China: new occurrence and crystal structure

Abstract. A second occurrence of bobtraillite is described from the Gejiu nepheline syenite, southwestern China. The extremely rare and complex boron-bearing zirconium silicate is associated with albite, orthoclase, jadeite, fluorite, andradite, titanite, as well as other REE and zirconium-bearing minerals, catapleiite, moxuanxueite, låvenite, eudialyte, britholite-(Ce), and calcioancylite-(La). The EMP and LA-ICP-MS analyses of the studied material give an empirical formula: (Na9.70Li0.42K0.08□1.80)Σ12.00(Sr10.61Ca1.14Fe0.07□0.18)Σ12.00(Zr12.87Ti0.53Nb0.31REE0.08Y0.06U0.02Th0.01□0.12)Σ14.00(Si42.41B5.59Al0.02)Σ48.02O132(OH)12 ⚫ 12H2O. Bobtraillite is trigonal, with space group P3¯c1, a=19.6977(6), c=9.9770(3) Å, V=3352.4(2) Å3, Z=1. Single-crystal structure refinement revealed that all sodium occupies the Na(1) and Na(2) sites; the site occupancy of these two positions is 0.835(18) and 0.15(2), respectively, suggesting that Na(1) site is Na dominant, while Na(2) is a vacancy-dominant site. The [8]-coordinated site has been assigned to Sr and Ca, with free occupancy factors, 0.874(10) and 0.126(10), respectively. These new data indicate that the ideal formula of bobtraillite could be written as (Na,□)12(□,Na)12Sr12Zr14(Si3O9)10[Si2BO7(OH)2]6 ⚫ 12H2O

Formation and evolution of multistage ore-forming fluids in the Miocene Bangpu porphyry–skarn deposit, Southern Tibet: insights from LA–ICP–MS trace elements of quartz and fluid inclusions

The large Bangpu Mo–Cu–Pb–Zn deposit, is a Mo-rich (~0.089%), Cu-poor (~0.32%) porphyry–skarn deposit located in the northeastern Gangdese porphyry copper belt (GPCB), Southern Tibet. In this study, LA–ICP–MS data for fluid inclusions and quartz formed during different stages of the mineralization are used to (1) constrain changes in fluid composition during mineralization; (2) infer the origin, transport pathways, and conditions of ore precipitation; and (3) explore the influence of magma chamber on fluid exsolution and shallow mineralization. The Cu content of the vapor-rich (VL) inclusions is higher than that of the halite-bearing (LVH) inclusions, while the Mo, Pb, and Zn contents of the VL and LVH inclusions are similar, indicating that Mo, Cu, Pb, and Zn had a common source, which is inferred to be the intermediate-level magma chamber. The metal contents of fluid inclusions in the early stage potassic alteration-related vein (A vein) and later stage sericitic alteration-related vein (B vein) are similar but variable, implying the intermediate-level magma chamber was too small to form a large stable fluid system. The enrichment of Mo, Pb, and Zn in the Bangpu deposit relative to other porphyry deposits in the GPCB is related to the participation of ancient crustal material. A comparison of our results with those from super-large porphyry Cu deposits elsewhere indicates that metal-rich fluids are not the sole factor to form ore deposits, and the variability of metal contents in the early exsolved fluids is a better predictor of mineralization potential than absolute metal concentrations

Geology and origin of the post-collisional Narigongma porphyry Cu-Mo deposit, southern Qinghai, Tibet

Narigongma is a poorly studied Mo-rich (similar to 0.06 wt.%) post-collisional porphyry Cu deposit located in southern Qinghai Province, Tibet, 400 km northwest of the Yulong porphyry Cu-Mo-Au belt. The Narigongma deposit has a similar age (43-40 Ma) to porphyry deposits in the Yulong belt, but different ore assemblages. The Narigongma deposit is associated with Eocene granodiorite and granite intrusions that were emplaced into a Permian volcanic-sedimentary rock sequence. An similar to 43.3 Ma biotite granite stock (P1 porphyry) is the earliest Eocene intrusion, and this was itself intruded by a number of smaller, similar to 43.6 Ma fine-grained granite porphyry stocks (P2 porphyry) and several post-ore quartz diorite porphyry dikes (similar to 41.7 Ma). The main Cu-Mo mineralization at Narigongma is associated with the P1 porphyry. Hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones, and an outer propylitic zone. Hypogene mineralization at Narigongrna was characterized by early-stage precipitation of molybdenite during potassic alteration and late-stage deposition of chalcopyrite during phyllic alteration. Deposition of both the Mo and Cu mineralization stages was caused by decreasing temperature. A high degree of crystallization of the P1 porphyry occurred prior to fluid saturation that produced Mo enrichment in the residual melt due to the incompatible behavior of Mo. However, compatible Cu was sequestered by the crystallizing phases and resulted in the generation of a high-Mo/Cu magmatic-hydrothermal fluid and the final Mo +/- Cu mineralization assemblage. Zircon epsilon(Hf)(t) values of +4.1 to +7.9 are indicative of magma derivation from a depleted source. These isotopic data, coupled with other geochemical characteristics of the Narigongma porphyry, such as high SiO2 and K2O contents, low MgO contents and compatible element abundances, and highly fractionated rare earth element patterns, indicate a mixing model for the origin of the porphyry bodies. Generation of the post-collisional ore-forming porphyries occurred in two stages: (1) partial melting of metasomatized phlogopite-bearing lithospheric mantle that generated potassic to ultra-potassic mafic melts, and (2) underplating of such melts beneath thickened juvenile lower crust, which triggered partial melting of the lower crust to produce the ore-forming, high-K adakitic magma. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved

Formation of the Dongmozhazhua Pb–Zn deposit in the thrust-fold setting of the Tibetan Plateau, China: evidence from fluid inclusion and stable isotope data

The Dongmozhazhua deposit, the largest Pb–Zn deposit in south Qinghai, China, is stratabound, carbonatehosted and associated with epigenetic dolomitization and silicification of Lower–Middle Permian—Upper Triassic limestones in the hanging walls of a Cenozoic thrust fault system. The mineralization is localized in a Cenozoic thrust-folded belt along the northeastern edge of the Tibetan plateau, which was formed due to the India–Asia plate collision during the early Tertiary. The deposit comprises 16 orebodies with variable thicknesses (1.5–26.3 m) and lengths (160–1820 m). The ores occur as dissemination, vein, and breccia cement. The main sulfide assemblage is sphalerite + galena + pyrite + marcasite ± chalcopyrite ± tetrahedrite, and gangue minerals consist mainly of calcite, dolomite, barite, and quartz. Samples of pre- to post-ore stages calcite yielded d13C and d18O values that are, respectively, similar to and lower than those yielded by the host limestones, suggesting that the calcite formed from fluids derived from carbonate dissolution. Fluid inclusions in calcite and sphalerite in the polymetallic sulfidization stage mostly comprise liquid and gas phases at room temperature, with moderate homogenization temperatures (100–140°C) and high salinities (21–28 wt% NaCl eq.). Micro-thermometric fluid inclusion data point to polysaline brines as ore-forming fluids. The dD and d18O values of ore fluids, cation compositions of fluid inclusions, and geological information suggest two main possible fluid sources, namely basinal brines and evaporated seawater. The fluid inclusion data and regional geology suggest that basinal brines derived from Tertiary basins located southeast of the Dongmozhazhua deposit migrated along deep detachment zones of the regional thrust system, leached substantial base metals from country rocks, and finally ascended along thrust faults at Dongmozhazhua. There, the base-metal-rich basinal brines mixed with bacterially-reduced H2S-bearing fluids derived from evaporated seawater preserved in the Permo–Triassic carbonate strata. The mixing of the two fluids resulted in Pb–Zn mineralization. The Dongmozhazhua Pb–Zn deposit has many characteristics that are similar to MVT Pb–Zn deposits worldwide