Geology, geochronology and sulphur isotope geochemistry of the black schist-hosted Haoyaoerhudong gold deposit of Inner Mongolia, China: Implications for ore genesis

The Haoyaoerhudong gold deposit is located in the Urad Middle Banner area of Inner Mongolia in China, and is hosted by black schist of the Bilute Formation in the Mesoproterozoic Bayan Obo Group. The gold mineralization is within a 4500 m long and 100 m wide zone associated with sulphide(-quartz) veins following the schistosity and crosscutting fractures in the host-rocks; locally as disseminated sulphide in altered black schist. The sulphide mineral assemblage is pyrrhotite-pyrite(-arsenopyrite-chalcopyrite-sphalerite) and is associated with a weak hydrothermal alteration characterized by a silica halo with variable amounts of biotite, sericite and carbonate. The Bilute Formation was intruded by multi-stage dykes; these include metamorphosed porphyritic diorite that pre-dates mineralization, unmetamorphosed amphibole olivine gabbro that pre-dates mineralization, and tourmaline-bearing pegmatite that post-dates mineralization. In order to constrain the timing of the gold mineralization and host rocks, zircons from the porphyritic diorite, amphibole olivine gabbro and tourmaline-bearing pegmatite were extracted for U-Pb dating. Gold-bearing pyrite samples were analysed for delta S-34 to help determine the possible source of S in the mineral zones. A weighted mean Pb-206/U-238 age date of 288 +/- 3 Ma (MSWD = 2.4) was obtained for the porphyritic diorite, a weighted mean Pb-206/U-238 age date of 282 +/- 1 Ma (MSWD = 0.58) was obtained from the amphibole olivine gabbro, and a weighted mean Pb-206/U-238 age date of 182 +/- 1 Ma (MSWD = 1.04) was obtained from the tourmaline-bearing pegmatite. These dates constrain the timing of gold mineralization to between 282 1 and 182 1 Ma, and are consistent with the hydrothermal alteration ages of ca. 267-246 Ma reported from the literature. These new zircon dates indicate that the regional metamorphism of the Bayan Obo Group ceased by the time the Haoyaoerhudong gold mineralization was deposited and the genesis of mineralization is not related to the regional metamorphic event The gold-bearing pyrites gave positive delta S-34(V-CDT) values between 6.8 and 13.4 parts per thousand, and are different from the delta S-34 widely distributed values for the typical black rocks, indicating that the sulphur is not derived from the black Bilute Formation, but sourced from some other relatively uniform reservoir. Widespread contemporaneous granitic magmatism is developed in the Haoyaoerhudong area and we suggest that the sulphur forming the gold-bearing sulphides is probably sourced from a carbonate-contaminated granitic magma reservoir. The new zircon dates for the dykes and the delta S-34 data for gold-bearing sulphides indicate that the gold mineralization did not form from metamorphosed black schist in the Bayan Obo Group, but is associated with magmatic-hydrothermal activity. (C) 2014 Elsevier B.V. All rights reserved.