Mineralization of an intra-oceanic arc in an accretionary orogen: Insights from the Early Silurian Honghai volcanogenic massive sulfide Cu-Zn deposit and associated adakites of the Eastern Tianshan (NW China)

Abstract

The Honghai volcanogenic massive sulfide (VMS) deposit occurs in the Kalatage inlier of the Dananhu-Haerlik arc, Eastern Tianshan, in the southern Altaids. The deformed deposit, hosted in a suite of early Paleozoic basic to intermediate volcanic, volcaniclastic, and sedimentary rocks, consists of lenticular massive to semimassive, and sulfide-silicate stockwork orebodies. The orebodies are characterized by zoning of metallogenic elements and sulfide minerals outward from a central stockwork zone as follows: (Fe +/- Cu), (Fe + Cu + Zn + Au + Ag; Cu > Zn), (Fe + Zn + Cu + Au + Ag; Zn > Cu), and (Fe +/- [Cu/Zn]). A typical VMS hydrothermal alteration zone that formed around the foot of the massive orebodies is several times larger than the lenticular massive orebody. The hydrothermally altered rocks contain sericite, chlorite, and epidote zones from the core outward. Re-Os and argon dating of pyrite and sericite shows that the Honghai VMS deposit formed in the Early Silurian (436 +/- 2 Ma) and was deformed in the Early Devonian (410 +/- 4 Ma). This is the first documented early Paleozoic VMS deposit in the Eastern Tianshan. Our multidisciplinary data indicate that the Honghai Cu-Zn deposit formed in an immature/nascent island arc, where early tholeiitic lavas evolved into transitional basic-andesitic volcanic rocks and calc-alkaline intrusions with relatively high epsilon(Nd)(t) values (+6.2-+8.4) and low (Sr-87/Sr-86)(i) ratios (0.70412-0.70541). The Sr-Nd-Pb-S isotopic and geochemical data suggest that the ore-forming melts started in mantle-derived volcanic rocks and deep subvolcanic intrusions, and some alteration was influenced by seawater. The deposit formed in a high-level subvolcanic intrusion driven by sub-seafloor CO2-rich, NaCl-CaCl2-H2O hydrothermal fluids (temperatures: 99-377 degrees C, salinities: 6%-18% NaCl equivalent) circulating in active extensional faults in a submarine caldera. Extension of the immature (nascent) intra-oceanic arc was one of the important factors that controlled the development and preservation of the VMS deposit. Future work will benefit from this improved understanding of the VMS deposits formed in immature (nascent) intra-oceanic arcs, especially in early Paleozoic island arcs in the Eastern Tianshan in the southern Altaids.