In-situ trace element and S isotope compositions of sulfides from the Tonggou Cu deposit, Eastern Tianshan Orogen: Implication for ore-forming process

Abstract

The Tonggou deposit is located in the western Bogda Orogen, the northern margin of Eastern Tianshan. Porphyry Cu and vein-type Cu-Zn mineralization are both developed in the Tonggou orefield. This well-preserved porphyry-vein system, therefore offers an excellent opportunity to evaluate the genetic relationship and ore-forming process between porphyry Cu and vein-type Cu-Zn mineralization. Porphyry mineralization consists of magnetite-quartz (P1) and chalcopyrite-quartz (P2) stages, whilst vein-type mineralization includes pyrite-quartz (V1), polymetallic sulfide-quartz (V2), and pyrite-calcite (V3) stages. Four pyrite generations are identified, including porphyry Cu ore-related (Py1) and vein-type Cu-Zn ore-related (Py2, Py3, and Py4) ones. In most cases, Py1 (stage P2) occurs as anhedral grains, Py2 (stage V1) as fine anhedral grains, Py3 (stage V2) as euhedral-subhedral grains, and Py4 (stage V3) as anhedral aggregates. The Sb content first decreases from Py1 to Py2, then increases from Py2 to Py3, and finally decreases from Py3 to Py4. Cobalt and Ni contents show opposite trend to that of Sb. The relatively high Sb but low Co-Ni contents in Py1 and Py3 indicate that the main ore stage (P2 and V2) environment was high oxygen fugacity and low sulfur fugacity. Moreover, the contents of Cu (Py1: avg. 4108 ppm, Py2: avg. 735 ppm, Py3: avg. 284 ppm, Py4: avg. 149 ppm) and Ag (Py1: avg. 204 ppm, Py2: avg. 11.1 ppm, Py3: avg. 6.29 ppm, Py4: avg. 2.98 ppm) gradually decrease from Py1 to Py4, indicating that the initial ore fluid was Cu-Ag rich. The δ34S values obtained are 4.6–6.8 ‰ (avg. 5.9 ‰; Py2), 5.7–7.4 ‰ (avg. 6.6 ‰; Py3), and 7.3–8.8 ‰ (avg. 8.1 ‰; Py4), which indicate that the sulfur of vein-type mineralization was initially derived from felsic magma, and subsequently mixed with strata sulfur. Furthermore, the similar δ34S range of Ccp1 (4.7–5.3 ‰) from porphyry ore and Ccp2 (3.3–4.8 ‰) from vein-type ore indicates that they have a similar felsic-magma-derived sulfur source. Combining with principal component analysis (PCA) of pyrite trace elements and REE patterns, the different pyrite generations can be divided into porphyry pyrite and vein-type pyrite, albeit with similar fluid source.Consequently, we suggest that the Tonggou porphyry and vein-type ores have similar ore-fluid source, and may belong to the same porphyry mineralization system with different ore-forming processes. Redox change may have played an important role in the metal enrichment and precipitation at Tonggou.