High temperature, salinity and high concentrated ore metal magmatic fluids: an example from Grasberg Cu-Au porphyry deposit.

The Grasberg Cu-Au porphyry system is centered around multiple quartz monzo diorite composite stocks. There are several alterations including potassic alteration, sericitization and magnetitization. The mineralization could be divided into disseminated and network types. Field relationships show that the chalcopyrite quartz vein and veinlets are the major mineralization and cut K-silicate alteration. Four types of fluid inclusions: magmatic, daugh ter mineral bearing, gas-rich and liquid inclusions have been found, and th e daughter mineral bearing and gas-rich inclusions are found in quartz of c halcopyrite quartz veinlets. The temperature of homogenization of fluid inc lusions ranges from 400 to over 700 degreesC and with high salinity from 60 similar to 78 wt% NaCl equivalent in daughter minerals bearing ones. Besid es these, ore metals such as Cu, Pb, Zn and Mn have been found in fluid inc lusions by petrography and synchrotron X-ray fluorescence analyses. Boiling is an important factor during the mineralisation which is provided by the coexisting daughter mineral bearing and gas-rich inclusions, and their simi lar homogenization temperatures. The evidences from this study and the isot opic research suggest that the ore forming fluids of Grasberg Cu-Au porphyr y deposit are magmatic in origin.