The metallogeny of the Devonian sediment-hosted sulfide deposits, South China: A case study of the Huodehong deposit

Abstract

The Devonian sedimentary rocks in South China host numerous sulfide deposits (e.g. the Siding and Beishan Pb-Zn sulfide deposits, and the Yingde iron sulfide deposits) that are distinctly characterized by a wide variation of δ34S values in sulfides (−28.7‰ to +42.8‰). Such signature, however, has not been investigated in depth and thus the metallogeny of these deposits remain poorly understood. This paper investigated the Huodehong deposit hosted by sedimentary rocks of the Middle Devonian in the western Yangtze Block, South China. A complex paragenesis has been defined: I) micro-grained pyrite (Py1) associated with organic materials; II) radial and crustiform pyrite (Py2) and marcasite (Mc1) aggregates; III) subhedral-anhedral pyrite (Py3), microcrystal colloform sphalerite (Sp1), and dendritic galena; and IV) coarse-grained euhedral sphalerite (Sp2) occurred as veinlets. The mineralogical evidence shows that the Py1 occurred as syn-sedimentary pyritic stromatolites, which is overprinted by cavity-filling hydrothermal Py2, Py3, Sp1, and Sp2. In situ S isotopes show that the δ34S values of Py1-Py3 and Sp1 range from –22.8‰ to −10.8‰. Such highly depleted heavy sulfur isotopes indicate that the reduced sulfur was mainly derived from bacterially reduced seawater sulfate in an open system. The Sp1 has similar δ34S values to its reworking Py1, implying that part of reduced sulfur was sourced from sedimentary pyrite. Such sedimentary pyrite might act as a geochemical barrier in accelerating sulfate reduction via the hydrolysis of Fe2+ resulting an increase in the H+ concentration and the formation of HSO4−. In addition, the Sp2 has slightly depleted heavy sulfur isotopes (δ34S = −9.3‰ to −1.1‰), implying the late input of heavy sulfur isotopes component. Hence, the highly heterogeneous sulfur isotopes at Huodehong likely resulted from bacterial sulfate reduction coupled with reworking sedimentary pyrite, and subordinate incorporation of 34S-rich sulfur. The reduced sulfur of the Huodehong deposit has multiple sources (i.e. sedimentary pyrite and sulfate-bearing evaporites), and the ore formation has experienced the epigenetic hydrothermal mineralization with mixture of metals from the metamorphic basement rocks. The outcomes will have a great implication for understanding the origin and exploration of the Devonian sulfide deposits in South China.