Petrogenesis and geodynamic evolution of Ordovician volcanics from the Baiyinchang volcanic-hosted massive sulphide district, Gansu Province, China

Abstract

The Baiyinchang (BYC) District contains several mid- to large-sized volcanic-hosted massive sulphide (VHMS) deposits and is located within the north Qilian Orogeny. The volcanic lithostratigraphic units throughout the BYC District formed via multi-stage and multi-cyclic volcanism, leading to the formation of volcanics with differing geochemical and petrographic characteristics, though all are subalkaline. The geochemical characteristics of the Zheyaoshan spilites are similar to those of ocean island basalts (OIB), displaying relatively enriched light rare earth element (LREE) and near-flat heavy rare earth element (HREE) patterns, the LILE (Rb, Sr, Ba, Th, U and K) are relatively enriched, whereas, HFSE and HREE are relatively depleted. The acid volcanics have geochemical characteristics of the FII rhyolite group. The geochemical characteristics of the Tongchanggou spilites are similar to those of N-MORB, displaying near-flat light rare earth element and near flat heavy rare earth element (HREE) patterns, and the acid volcanics have geochemical characteristics of the FIIIa rhyolite group. Based on previous research and this study, the Baiyinchang volcano-tectonic setting was transitional from an island-arc rift to a back-arc basin environment. The volcanic rocks near the Zheyaoshan and Huoyanshan deposits formed in an Ordovician island arc rift environment, and the volcanic rocks near the Tongchanggou deposit formed in an Ordovician back-arc basin environment. The Xiaotieshan volcanic association is located between the Zheyaoshan and Tongchanggou deposits and has geochemical characteristics of both MORB and OIB. The volcanics of the Tongchanggou deposit have different mantle sources, slab contributions, partial melting and fractional crystallisation pathways compared to the volcanics from the Zheyaoshan deposit. The Zheyaoshan and Huoyanshan CuZn deposits occur within quartz -keratophyre tuff of II-2 affinity and their sites of emplacement were controlled by the western volcanic eruption center in the district. The Xiaotieshan and Tongchanggou Pb-Zn-Cu deposits are located within the quartz-keratophyre tuff of II-1 affinity and their sites of emplacement controlled by the eastern volcanic eruption center in the district. Thus, the Baiyinchang Orefield volcanic association spans differening tectonic environments of formation, magmatic sources, and physio- chemical conditions in the two main volcanic eruption centers.