Assessment of the ore-forming process of the Gejiu tin district (South China)

Abstract

The Gejiu tin district of southern China is the largest tin–polymetallic ore district in the world. Even though the mining operation has lasted more than 2000 years in this area, the metallogenic mechanism of this gigantic tin mineralization is still controversial. Based on petrographic, geochemical, and zirconological information from this study, we present a particular model of the magma evolution and tin mineralization for the Gejiu tin district. The ore-forming processes involved both a batholithic biotite monzogranite (BMG) emplaced at ∼94 Ma and a satellitic muscovite alkali-feldspar granite (MAG) emplaced at ∼72 Ma. The BMG and the MAG were produced from a long-lived magma chamber by two stages of magmatic activity during intracontinental orogeny in the Youjiang block. The hiatus between the two magmatic stages indicates that the deep-seated magma chamber persisted during the dominant phase of the orogeny. Progressive fractional crystallization of the magma caused the extreme enrichment of incompatible elements, which include ore-forming components such as tin, aqueous fluids, and other volatiles, in the residual magma. When the regional tectonic environment evolved into the extensional regime at ∼72 Ma, the highly-fractionated residual magma ascended and once emplaced it formed the highly-evolved MAG with a mean zircon crystallization age of 71.8 ± 1.2 Ma. The MAG is temporally and spatially associated with cassiterite-bearing quartz vein (CQV) with a mean zircon crystallization age of 72.6 ± 1.6 Ma. This metallogenic model of the BMG–MAG–CQV association is generally applicable to other granite-related metallic deposits in South China, Southeast Asia and elsewhere (Australia, Bolivia, Brazil, USA, etc.). The recognition of this BMG–MAG–CQV association in regional geological survey may facilitate the future exploration for this type of deposit.