Crystallisation conditions (T, P,fO_2) from mineral chemistry of Cu- and Au-mineralised alkaline intrusions in the Red River-Jinshajiang alkaline igneous belt, western Yunnan Province, China

摘要

The Oligocene Yao'an syenite porphyry, associated with gold mineralisation, and the Machangqing alkali granite porphyry-monzonite porphyry, associated with Cu mineralisation, belong to the Red River-Jinshajiang alkaline igneous belt that formed in a continental setting in southwestern China. A study of the mineral chemistry of major silicate minerals in these two mineralised intrusions provides insights into their overall crystallisation conditions. The temperature and pressure conditions, derived from amphibole-plagioclase and perthite-plagioclase geo-thermometry and Al-in-amphibole barometry, suggest that the Yao'an intrusion crystallised at around 820 + - 50 deg C and 0.9-1.3 kbar, whereas the Machangqing intrusion crystal-lised at around 730 + - 50 deg C and 2.2-2.8 kbar. The higher temperature and lower pressure of crystallisation for the Yao'an intrusion relative to the Machangqing intrusion indicates that it was emplaced at a shallower crustal level. Based on biotite composition, the two intrusions formed under imposed oxygen fugacities above the Ni-NiO buffer (NNO), and the Yao'an intrusion crystallised under more oxidising conditions than the Machangqing intrusion. The results show that the intrusions associated with Cu-Au mineralisation in the Red River-Jinshajiang alkaline igneous belt were emplaced at a relatively high fO_2, which, together with the weakly fractionated magma, favoured Au enrichment relative to Cu in the belt.