Discussion on Au transportation mechanism in melt-magma-fluid in porphyry Cu deposit - A case study from Jinchang porphyry Au (Cu) deposit, Heilongjiang province, China

摘要

At present, there are Au in most porphyry-type copper deposits, and there are also many reserves of Au resources. There are 11 porphyry-type Cu deposits with Au resources greater than 500 t (as of 2010). Previous studies on the source, migration, and mineralization of Cu, S, and Cl in porphyry-type Cu deposits were relatively thorough, but Au was rarely involved, especially since geologists recognized the genetic relationship of hydrothermal gold deposits and porphyry copper deposits. Here we proposed the mechanism of the Au source, transport, and storage in Jinchang porphyry Au (Cu) deposit, located in the easternmost portion of the Central Asian Orogenic Belt in China. The gold-bearing characteristics and Au chemical properties of siegenite show that in the initial magmatic environment with high oxygen fugacity, Au enters the melt phase from the solid phase with Au-0 oxidized to Au3+. The Ni3+ and Co3+ ions have similar chemical properties, thus migrate together in the melt phase. The gold-bearing siegenite provides strong evidence for the affinity and elemental replacement of Au3+ and Ni3+. The symbiosis and differentiation mechanisms of rare earth elements also provide evidence for the affinity and differentiation mechanism between Au3+ and Ni3+. After the Au-3(+) melt undergoes the initial magma evolution to the magma chamber, the oxygen fugacity of the melt phase is reduced to a certain degree during the crystallization and separation of the magma, which causes the Au-3(+) in the ore-forming elements to be reduced to Au1+ and enter the fluid phase. Au1+ is dissolved in the fluid phase and enriched gradually as the temperature, pressure, and oxygen fugacity further decrease, then Au1+ is reduced to Au-0 and precipitates to form native gold in porphyry or quartz veins.