Geology, geochemistry, and genesis of orogenic gold–antimony mineralization in the Himalayan Orogen, South Tibet, China
Abstract The southern Tibet Au Sb metallogenic belt in the Himalayan orogen consists of more than 50 gold, gold–antimony and antimony lode deposits, and associated placer gold deposit. The deposits are hosted in a Mesozoic metamorphosed turbidite sequence of the Indian passive continental margin. The Zhemulang Au deposit, Mazhala Au Sb deposit, and Shalagang Sb deposit are three typical examples of such epizonal orogenic deposits. At Zhemulang, gold-bearing quartz veins occur in the Upper Triassic Songre Formation, consisting of carbonaceous phyllite and slate. Ore minerals are native gold, pyrite, galena, chalcopyrite, and limonite. At Mazhala, the gold- and stibnite-bearing quartz vein orebodies are hosted in Lower to Middle Jurassic slate, interlayered with metastandstone, metasiltstone, and limestone of the Lure Formation. Ore minerals are native gold, stibnite, pyrite, arsenopyrite, and trace amount of cinnabar. At Shalagang, the host rocks are Lower Cretaceous sandstone, siltstone, muddy limestone, and chert of the Duojiu Formation. Orebodies consist mainly of stibnite-bearing quartz veins and locally altered fault breccia. Ore minerals are stibnite, cinnabar, valentinite [Sb 2 O 3 ], limonite, and trace amount of pyrite, arsenopyrite, and realgar. For the three deposits, the wallrock alteration has produced the minerals silica, carbonates, white mica, sulfide and chlorite. The three deposits have a similar element associations, but with a few slight variations. The Zhemulang, Mazhala, and Shalagang deposits, in order of element enrichments relative to crustal abundance, are anomalous in Au, Sb, Te, Bi, As, Pb, Ag, and W; Sb, Au, Te, As, Pb, Bi, Ag and W, to Sb, Te, As, Au, Hg, W, Pb, and Ag, respectively, and all depleted in Cu, Zn, Sn, and Mo. Various aqueous, carbonic, and hydrocarbon fluid inclusions were recognized in quartz and/or stibnite at the three deposits. These include type 1a one-phase aqueous inclusions, type 1b two-phase aqueous inclusions, type 2a carbonic inclusions, type 2b aqueous-carbonic inclusion, and rare type 3 hydrocarbon inclusions that include two-phase hydrocarbon inclusions (type 3a) and dark one-phase hydrocarbon inclusions (type 3b). The three deposits have the similar low-salinity H 2 O CO 2 CH 4 N 2 ore fluids with trace amounts of hydrocarbons. For the Zhemulang, Mazhala, and Shalagang deposits, the salinities of aqueous inclusion range mainly between 3.3 and 6.4 wt.% NaCl equiv., 2.5 and 4.9 wt.% NaCl equiv. and 4.1 and 6.4 wt.% NaCl equiv., respectively. The ore-forming temperatures vary mainly from 180 to 320 °C, 160 to 300 °C and 140 to 240 °C, respectively. The estimated mineralization depths are 4 to 6 km, 3 to 5 km, and 1 to 4 km of the epizonal environment, respectively. The different mineralization temperatures and pressures led to the different element enrichments at the three deposits. For the Zhemulang, Mazhala and Shalagang deposits, ore fluid isotopic compositions are δD H2O − 107.5 to − 36.7‰ and δ 18 O fluid 2.8 to 8.2‰, δD H2O − 119.0 to − 72.7‰ and δ 18 O fluid 7.5 to 16.2‰, and δD H2O − 173.4 to − 139.2‰ and δ 18 O fluid 7.5 to 12.3‰, respectively; δ 13 C fluid values are − 11.7 to − 9.6‰, − 3.5 to − 2.5‰, and − 6.5 to − 5.1‰, respectively; and δ 34 S values are − 4.0 to − 1.1‰, − 0.8 to 2.3‰, and − 3.9 to 2.1‰, respectively. The ore-forming fluids were partly derived from metamorphic devolatilization of immediate or deeper level country rocks, with a deposit's corresponding metamorphic degree controlling the fluid PTX. The ore-forming fluid for Zhemulang, Mazhala, and Shalagang consisted of predominantly metamorphic water with minor involvement of meteoric water, a mixture of metamorphic fluid and meteoric water, and predominantly meteoric water, respectively. Ore metals were derived from country rocks, including synsedimentary Sedex-like sulfide layers in the Jurassic strata observed at the Mazhala Au Sb deposit. Among three deposits, the variation of δ 13 C fluid and δ 34 S reflects the fact that the Zhemulang Au deposit formed from a relatively high content of organic carbon and low ƒ O2 fluid, the Mazhala Au Sb deposit from a relatively low content of organic carbon and high ƒ O2 fluid, and the Shalagang Sb deposit from an intermediate content of organic carbon and ƒ O2 fluid. Fluid immiscibility was the main mechanism for ore metal precipitation at all three deposits. The vertical zonation of Au, Au Sb, and Sb mineralization suggests that additional gold resources may exist below the antimony or gold–antimony orebodies. Stream sediment and soil geochemical surveys and the occurrence of placer gold prospects are effective for identifying areas of orogenic gold and antimony deposits in the Himalayan and other orogens.