Wall-rock control on the mineral assemblage of hydrothermal veins: examples from the Mahuaping Be-W-F deposit, Yunnan, SW China

Abstract

The 32 Ma vein-type Mahuaping Be-W-F deposit is closely related to the folding and faulting of its host-rocks, Ordovician slates and Silurian limestones. We demonstrate that mineral assemblages in the veins are controlled by the wall-rocks and argue that fluid-rock interaction triggered mineral precipitation. Veins show two stages of mineralization. Stage I mica-quartz veins with beryl, scheelite and fluorite occur in limestone, slate, and their contact zone. Dissolution of limestone provided Ca to form scheelite and fluorite in veins in the limestones and in the contact zone. The precipitation of fluorite destabilized Be-fluoride complexes in the fluid and induced the precipitation of beryl. Alteration of slates provided Fe to produce wolframite in veins hosted in slates and in the contact zone. The composition of Stage I mica ranges from muscovite in veins in the limestones to phengite in veins in the slates. The trace element contents of Stage I mica depend on the nature of the wall rocks. Stage II mica-quartz veins are barren. Stage II veins are shielded from interaction with the wall-rocks by Stage I mineral assemblages. Therefore, the composition of Stage II muscovite and its trace-element signature are independent of the nature of the wall-rocks. The trace element inventory of Stage II mica indicates that the mineralizing fluid, which carried the ore elements Be, W, F, and Sn, is derived from low-grade sedimentary rocks rather than an unexposed intrusion.