In-situ element geochemical and sulfur isotope signature of pyrite and chalcopyrite: Constraints on ore-forming processes of the Laoshankou iron oxide-copper (-gold) deposit, northern East Junggar

Abstract

The Laoshankou deposit is an iron oxide-copper–gold (IOCG)-like deposit in the northern margin of East Junggar, Central Asian Orogenic Belt (CAOB), in which pyrite is ubiquitous and present from stage II magnetite mineralization (Py1a and Py1b), stage III-A pyrite mineralization (Py2a and Py2b), to s tage III-B chalcopyrite mineralization (Py3a and Py3b). Despite of many previous studies, the source(s) of ore-forming fluids in the Laoshankou deposit are still controversial. Whether non-magmatic fluids were involved in the IOCG systems has also been highly debated in recent years. To further characterize the nature of ore-forming fluids in Paleozoic IOCG deposits in basin inversion settings, we conducted in-situ secondary ion mass spectrometry (SIMS) sulfur isotope and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis in a set of pyrite/chalcopyrite-bearing samples in the Laoshankou deposit. Sulfur isotope results of the sulfides in all stages (−3.5 to 2.6‰) fit in the magmatic sulfur range, indicating the contribution of magmatic sulfur. However, the weakly negative δ34S values of − 3.5‰ to − 1.5‰ (Py3), possibly indicating the addition of organic sulfur, which can also be proved by organic-rich fluid inclusions and strata. The low Se/S (<5 × 10−5), the volcanogenic-like Co/Ni ratios (3.9–28, average 16), and the varying Ni contents (81–978 ppm, average 315 ppm) of Py1 indicate that its magmatic sulfur was leached from mafic volcanic rocks by heated seawater in a reducing condition at stage II. The elevated Ni (433–3736 ppm, average 1401 ppm) and Se (39–143 ppm, average 80 ppm) contents, and the Se/S (∼1.5 × 10−4) ratios of Py2 indicate a more oxidizing condition. We suggest that the magmatic sulfur of Py2 in stage III-A was also leached from mafic rocks by seawater, but magmatic hydrothermal fluid input cannot be excluded. The high Co/Ni (7.7–415, average 142) and low Ni/Se (0.43–3.4, average 1.7) ratios, and the low Ni contents (31–381 ppm, average 133 ppm) of Py3 show that such organic sulfur was likely leached from organic-rich sequence by relatively low-temperature and reduced fluid, with significant contribution from the felsic rocks in stage III-B. Combined with previous halogen and C–H-O isotopes results, external non-magmatic fluids can be proved in the Laoshankou deposit. Stage III-B chalcopyrite grains replaced early-stage pyrites, during which some alleged less-mobile elements (e.g., Co, and As) in the replaced pyrites were released into the Cu ore-forming fluids and formed extremely low-Co, porous/fractured, and inclusion-rich Py1b and Py2b, together with high Co-As contents in the Py3 and the Py3-chalcopyrite boundary. We propose that external seawater and organic bittern brine may have an effect on the mineralization at Laoshankou. Combined with previous researches in the Andean and East Tianshan, external fluids may play a key role for the IOCG deposits in arc-related basin inversion setting.