Geochronology and geochemistry of granites from the Longhua Ni-Co deposit, South China: Implication for the formation of Ni-Co mineralization

Mineral exploration activities since 2010 in a previously poorly explored region of northeastern Guangxi province, South China, confirmed the existence of a potentially large hydrothermal Ni-Co deposit. As a newly discovered deposit, many aspects of this deposit are still unclear, including in which tectonic setting the deposit formed and how the high-grade mineralization developed. To address these questions, in this study, we conduct a comprehensively geochronological and geochemical study on the Dajin pluton (consisting of biotite monzogranite and sericitized monzogranite), which is the only pluton occurring in the district. LA-ICP-MS zircon UPb dating shows that two types of granites were emplaced nearly at the same time, with biotite monzogranite and sericitized monzogranite being 449 +/- 15 Ma and 448.4 +/- 8.8 Ma, respectively. This, combined with previously published studies, confirms that the Dajin pluton was broadly emplaced during the formation of the Longhua deposit. These granites are characterized by high K 2 O, and noticeable depletion of Eu, Sr, P and Ti, with wholerock epsilon Nd ( t ) values ranging from-1.99 to-4.26 and zircon epsilon Hf ( t ) values ranging from-0.9 to 4.1. These features support that the studied granites are typical of A2-type granites, which experienced strong fractional crystallization and formed in a post-collisional environment. Although Co and Ni might not have been originally derived from granitic magmas, we suggest that the emplacement of the Dajin pluton facilitated the formation of highgrade Ni-Co mineralization in two ways: (1) melting the Co - and Ni-enriched mafic basement probably existing in the deep crust, and (2) remobilizing and concentrating metals from wall rocks.