Abstract

Most known primary Au deposits are produced by hydrothermal processes. In this paper, we report Au mineralization of magmatic origin in the Bilihe deposit, China (8.5 Mt averaging 2.9 g/t Au). At Bilihe, most (>70%) native Au (no detectable Ag) grains are euhedral or subspherical in shape, and occur as trails in dendritic quartz phenocrysts and comb-layered quartz in a moderately reduced, highly fractionated diorite-granite intrusion. The hosting quartz typically has a dendritic core (Q1) and a rim (Q2), with Q1 having concentric zoning and sector zoning in cathodoluminescence (CL) images. Cathodoluminescence petrography and crystallographic modeling reveal that most of the Au trails occur along intersecting crystallographic planes of the host quartz, indicating simultaneous precipitation of both Au and quartz. Abundant melt inclusions are present in Q2 with >950°C homogenization temperatures. Minor Au grains also occur in melt inclusions in quartz. In rare cases, necking of Au melt inclusions is present. Neither Q1 nor Q2 contain primary fluid inclusions; only secondary fluid inclusions were found in healed cracks. The above observations indicate a direct magmatic (quartz phenocryst phase) origin for the Au. This defines a new type of Au deposit, and thereby opens new potential for Au exploration. The magmatic origin of Au at Bilihe also implies that enrichment of Au may occur in a source melt prior to volatile escape, which would enhance the possibility of forming a magmatic-hydrothermal Au deposit.

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