Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide-Cu (-Au) deposit in the Eastern Tianshan, NW China

The Heijianshan Fe-Cu (-Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic-intermediate volcanic and mafic-felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic-hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (-Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote-calcite-tourmaline-sericite alteration assemblage indicates a pre-mineralization Ca-Mg alteration event. Stage II Fe and Stage IV Cu (-Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore forming fluids. Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189-370 degrees C), II (301-536 degrees C), III (119-262 degrees C) and V (46-198 degrees C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic-hydrothermal-derived Fe mineralization fluids were characterized by high temperature (> 300 degrees C), medium-high salinity (21.2-56.0 wt% NaCl equiv.) and being Na-Ca-Mg-Fe-dominated. These fluids were overprinted by the external low temperature (< 300 degrees C), medium-high salinity (19.0-34.7 wt% NaCl equiv.) and Ca-Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (-Au) mineralization, as supported by quartz CL images and H-O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic-hydrothermal), Ill (basinal brine-related) and IV (magmatic-hydrothermal). Stage II disseminated pyrite has delta S-34(fluid) values of 1.7-4.3 parts per thousand, comparable with sulfur from magmatic reservoirs. delta S-34(fluid) values (24.3-29.3 parts per thousand) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite-chalcopyrite-pyrrhotite assemblage (with low delta S-34(fluid) values of 4.6-10.0 parts per thousand) may have formed at low fO(2) and temperature (119-262 degrees C). The continuous basinal brine-volcanic/volcaniclastic rock interactions during the basin inversion (similar to 325-300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like delta S-34(fluid) values (1.5-4.1 parts per thousand). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (similar to 160 degrees C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins. The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.