Ultra-long magma residence time leading to a new model for the tungsten mineralization in the Nanling Range (South China)

Abstract

Granites generated in massive volume during an orogeny frequently concentrate abundant trace metals over the requirement of commercial mining. Tungsten mineralization usually develops in massive granite generated during orogenic activities. However, there is no general metallogenic model capable of explaining metal enrichment. In specific, the source of the metals and criteria to distinguish barren from mineralized granites have yet to be clearly identified. In this paper, we present that the muscovite alkali-feldspar granites, as the parental rocks to the tungsten mineralization in the Nanling Range (South China), may result from extremely fractionated granitic magma from the deep-seated magma chamber. Further, the geochemical and geochronological evidence on hydrothermal rims and magmatic cores of zircon grains from these granites reveals two stages of magmatic activities, i.e. the syn-orogenic biotite monzogranites at 155.0 ± 1.0 Ma and the post-orogenic muscovite alkali-feldspar granites at 133.4 ± 1.0 Ma respectively. Since the two granites with the similar zircon εHf(t) values and in the same location should develop from the same magma chamber during the Yanshanian movement, we interpreted that the deep-seated magma chamber survived longer than 20 Myr, within which a continuously fractional crystallization led to unusual concentration of incompatible elements (tungsten, volatile, and aqueous fluid) in the residual magma. When the regional tectonic environment entered the extensional regime at ~133 Ma, the highly fractionated residual magma ascended rapidly from the magma chamber, generated the muscovite alkali-feldspar granite, and developed the accompanying giant tungsten deposit. Our thermal modeling supports the possibility of an ultra-long magma residence time at deep-seated chamber.