Formation of Au-polymetallic ore deposits in alkaline porphyries at Beiya, Yunnan, Southwest China

Abstract

The Beiya gold deposit is located in the central part of the Jinshajiang–Honghe strike–slip belt, at the junction zone of the Tethys-Himalaya orogenic belt and the Yangtze plate in SW China. This large-scale (125.6millionmetrictonnes with a mean grade of 2.42g/t) Au-rich polymetallic deposit is related to alkaline porphyry intrusions. Previous studies show that the Beiya alkaline porphyry intrusions are the fractionation product of a mantle-derived magma emplaced in an extension environment at the post-collision stage of the India–Eurasian plate collision; the Beiya deposit is considered to be skarn-related. Based on detailed field studies and previous work, we propose that the Beiya porphyry and associated Au-rich polymetallic ores were formed by the emplacement of magmas within the Jinshajiang–Honghe strike–slip fault during the late stage of the India–Eurasian plate collision at 45–25Ma.At Beiya, the mineralized zones in the Cu–Au-rich porphyries are surrounded by Au–Cu–Fe skarns and Au (Cu) veins. Pb–Zn–Ag-rich mineralization was derived from the inner porphyry, and is widely developed outside the central alkaline-rich porphyry.The sulfur isotope signature of the sulfide mineralization in the Beiya and Qinhe deposits is −2.40‰–4.50‰ and 1.25–2.75‰, respectively. These values are close to 0‰, indicating that the sulfur may be mantle-derived. The δ18O compositions of the ore-forming fluids responsible for the formation of calcites at Qinhe are 8.10‰–9.61‰, which is lower than that of Beiya (δ18O=10.5‰) where the ores contain a larger contribution of oxygen from the mantle. The Beiya porphyry magmas provided fluids and the heat that drove the transport of the metals to the site of deposition.Alkaline porphyries are widely distributed throughout the Jinshajiang–Honghe strike–slip fault belt, and they are potential hosts to future discoveries of Beiya-style mineralization.