Geology and genesis of the giant Beiya porphyry-skarn gold deposit, northwestern Yangtze Block, China

The Beiya ore deposit is located in the northwestern Yangtze Block, to the southeast of the Tibetan Plateau, SW China. The deposit is hosted by a porphyritic monzogranitic stock that is cross-cut by a porphyritic granite and later lamprophyre dikes. The whole-rock geochemistry of the porphyritic monzogranite and granite intrusions is both potassic and adakite-like, as evidenced by high K2O/Na2O (2.2 to 24.8), Sr/Y (53.2 to 143.2), and (La/Yb)(N) (4.9 to 28.9) ratios. Both intrusions have comparable zircon U-Pb ages of ca. 36 Ma and epsilon Hf(t) values of -6.8 to +2.7. Zircons within these intrusions have Hf isotope crustal model ages with a prominent peak at ca. 840 Ma, and both of the intrusions have similar Sr-Nd-Pb isotopic compositions that are comparable to the compositions of amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The contemporaneous lamprophyre dikes show Nb-Ta depletion, enriched (Sr-87/Sr-86)(i) and epsilon Nd(t), and extremely low Nb/U ratios (1.6-3.6), suggesting that these dikes were formed from magmas generated by partial melting of a metasomatized subcontinental lithospheric mantle (SCLM). The geochemistry of the porphyritic intrusions and the lamprophyre dikes suggests that the Beiya porphyries formed as a result of partial melting of a thickened and K-rich region of the lower crust, triggered by melting of metasomatized SCLM. The ca. 840 Ma U-Pb ages and epsilon Hf(t) values (-6.8 to +2.7) of xenocrystic zircons within the porphyritic intrusions suggest that these zircons were produced in a continental arc setting at ca. 840 Ma. The peak Hf model age of the zircons crystallized from the intrusions and the U-Pb ages of the xenocrystic zircons within the intrusions suggest that these porphyritic intrusions formed from magmas sourced from a juvenile crust that formed at ca. 840 Ma. This juvenile crust is most likely the source for the metals within the porphyry-skarn deposits in the study area, as the SCLM-derived lamprophyre dikes in this area are barren. Massive Fe-Au orebodies (similar to 99 million metric tons at an average grade of 2.61 g/t Au) within the study area are generally located within the skarn-altered boundary of the porphyritic monzogranite stock and along the faults in the surrounding Triassic carbonates. The Fe-Au orebodies are spatially and genetically associated with skarn comprising garnet and diopside. Petrographic observations show that the massive Fe-Au orebodies mainly consist of hematite and magnetite with disseminated pyrite that hosts native gold and electrum. The porphyritic granite contains porphyry-style mineralization in the form of disseminated and veinlet-hosted pyrite and chalcopyrite. Pyrite-hosted lattice-bound gold is present within both the massive Fe-Au and the porphyry-type mineralization in the study area, and is present at concentrations up to 10 ppm Au (as determined by in situ LA-ICP-MS analysis). Subsequent weathering altered the primary magnetite-hematite-sulfide assemblage in the Fe-Au orebody into a magnetite-limonite assemblage, and generated laterite-type mineralization in which gold is hosted by limonite. (C) 2015 Elsevier B.V. All rights reserved.