Zircon geochemistry of Edong granitoids in the Middle-Lower Yangtze Metallogenic Belt (Eastern China): Constraints on W-Cu-Fe skarn mineralization

Abstract

It has long been established that skarn deposits of different metals are associated with intrusions of different compositions. Intermediate-felsic granitoids in the Edong district of the Middle-Lower Yangtze Metallogenic Belt host skarn deposits with varying W-Cu-Fe proportions, yet previous whole-rock geochemical studies could not distinguish between the magmatic suites that formed the different skarn deposits. In this study, LA-ICP-MS zircon geochemical data are presented for the Ruanjiawan W skarn (60300 t WO3), Tongshankou Cu skarn (0.6 Mt Cu), Tonglushan Cu-Fe skarn (1.1 Mt Cu and 56.8 Mt Fe) and Jinshandian Fe skarn (200 Mt Fe) deposits in the Edong district. Zircons from the ore-forming intrusions of these skarn deposits are all characterized by heavy rare earth element (HREE) enrichments and light rare earth element (LREE) depletions, with positive Ce anomalies and slightly negative Eu anomalies. Tungsten contents in zircons from the W ore-forming intrusion (0.02–3.16 ppm W; avg. 1.14 ppm) are generally higher than in zircons from the Cu ore-forming intrusion (0.00–0.24 ppm W; avg. 0.03 ppm). Copper content in zircons from the Cu ore-forming intrusion is higher than in those from the W ore-forming intrusion. Fertile intrusive rocks from the four skarn deposits contain zircons with high calculated Ce4+/Ce3+ ratios (Ruanjiawan: 71.7; Tongshankou: 207.5; Tonglushan: 263.6; Jinshandian: 189.0. Fluorine content of the Ruanjiawan intrusion is higher than the Tonglushan and Jinshandian intrusions, followed by the Tongshankou intrusion. Chlorine contents show the opposite trend. We propose that the different source regions for the ore-fertile intrusions is the critical factor that controls the metal content, oxygen fugacity and volatile components. Our studies suggest that zircon geochemistry is a good indicator to distinguish tungsten causative intrusions from copper- and iron ore related intrusions (i.e. W content ≧ 1.14 ppm and Ce4+/Ce3+ ratios ≦ 207.5).