Genesis of the Bayan qagan dongshan Sn-Pb-Zn-Ag deposit, NE China, based on sphalerite chemistry and sulfur isotope

Abstract

The Bayan qagan dongshan Sn-Pb-Zn-Ag deposit (223,383 t Sn @ 0.755 %; 9,446 t Ag @ 187.46 g/t; 752,944 t Zn @ 1.60 %; 150,828 t Pb @ 0.521 %) is mainly hosted by the contact zones between early Cretaceous granite porphyry and Permian epimetamorphic rock in the southern part of the Great Khingan Range of northeastern China. Nevertheless, the migration and precipitation form of silver in the deposit is undefined. In this study, seven subtypes of sphalerite (SpⅠ, SpⅡ-1, SpⅡ-2a, SpⅡ-2b, SpⅡ-2c, SpⅡ-3a and SpⅡ-3b) were identified according to mineral assemblages in the quartz-tourmaline-sericite-sphalerite stage (Ⅰ) and cassiterite-sulfide stage (Ⅱ) of the deposit. SpⅠ is distributed in the granite porphyry as disseminated ores, accompanied by tourmalinization and fluoritization. SpⅡ-1 is predominantly associated with cassiterite, whereas SpⅡ-2 (including SpⅡ-2a, SpⅡ-2b and SpⅡ-2c) mainly coexisted with stannite, in which chalcopyrite exsolution is found. SpⅡ-3 (including SpⅡ-3a and SpⅡ-3b) is mainly characterized by its irregular shape, and intimately associated with galena and Sb-bearing mineral. Representative time-resolved depth profile and inter-element correlation analyses based on the LA-ICP-MS method illustrated that: 1) Fe, Cd, Mn, Co, Cu, In, Ag and Sb are present in solid solutions formed through direct cation substitution as Zn2+ ↔ (Fe2+, Cd2+, Mn2+, Co2+) and coupled substitution as Zn2+ ↔ Cu+ + In3+ and Zn2+ ↔ Ag+ + Sb3+; 2) Pb, Bi, Sn and Te may be intimately associated with galena and Sn-bearing telluride inclusions hosted by sphalerite. The characteristics of trace and minor elements, along with the results of sulfur isotope studies, suggest that: 1) the ore forming temperature of Bayan qagan dongshan deposit gradually decreases and the silver mineralization mainly occurred in the late stage; 2) the changes of physicochemical conditions (e.g., temperature and pressure) are likely to be an important depositional mechanism for Ag in the Bayan qagan dongshan deposit; 3) the Bayan qagan dongshan deposit may belong to magmatic-hydrothermal origin.