THE PHYSICOCHEMICAL CONDITIONS OF HYDROTHERMAL ALTERATION OF MALASONGDUO PORPHYRY COPPER DEPOSIT, EASTERN XIZANG, CHINA, WITH SPECIAL REFERENCE TO MINERALIZATION
By the field investigation on Malason gduo porphyry copper deposit, three alteration zones have been determined, i.e. the K-f eldspar - bio-tite, quartz - sericite and chlorite - epidote - carbonate zones outwards. By means of determination and measurement of the composition, homogeneous temperature and salinity of fluid inclusions, the analyses of copper contents of wall rocks altered and unaltered respectively and the thermodynamic calculation of the replacement reactions, the physicochemical conditions of hydrothermal alteration and the relation to mineralization have been discussed. It is suggested that magmatism, hydrothermal alteration and mineralization had taken place in a continued and interconnected physico - geochemical process. The porphyry copper mineralization may have been the result of magmatic evolution at a certain stage. The fluid phase may have mainly been separated from a copper bearing magma during late - and post - magmatism, and its concentration and heat diffusion started from the centre of host rock and ended in the wall rocks. Consequently, it is impossible that the copper might have mainly been released from the wall rocks to cause copper mineralization. A great amount of copper may have been transported in melt phase to the upper levels of the crust during emplacement of the magmatic plutons. The activity of the postmagmatic fluid controlled the hydrothermal alteration. The latter changed the physicochemical properties of the fluid phase which facilitated the decomposition of the complex compounds of copper and precipitation of chalcopyrite. The extent of hydrothermal alteration, especially the intensity of potassium replacement, may be used as key in exploration of porphyry copper deposits, for the content of copper in a porphyry pluton is strikingly affected by potassium replacement.