Evolution of Hydrothermal System at the Dizon Porphyry Cu-Au Deposit, Zambales, Philippines

摘要

Evolution of hydrothermal system from initial porphyry Cu mineralization to overlapping epithermal system at the Dizon porphyry Cu-Au deposit in western central Luzon, Zambales, Philippines, is documented in terms of mineral paragenesis, fluid inclusion petrography and microthermometry, and sulfur isotope systematics. The paragenetic stages throughout the deposit are summarized as follows; 1) stockwork amethystic quartz veinlets associated with chalcopyrite, bornite, magnetite and Au enveloped by chlorite alteration overprinting biotite alteration, 2) stockwork quartz veinlets with chalcopyrite and pyrite associated with Au and chalcopyrite and pyrite stringers in sericite alteration, 3) stringer quartz veinlets associated with molybdenite in sericite alteration, and 4) WNW-trending quartz veins associated with sphalerite and galena at deeper part, while enargite and stibnite at shallower levels associated with advanced argillic alteration. Chalcopyrite and bornite associated with magnetite in quartz veinlet stockwork (stage 1) have precipitated initially as intermediate solid solution (iss) and bomite solid solution (bnss), respectively. Fluid inclusions in the stockwork veinlet quartz consist of gas-rich inclusions and polyphase inclusions. Halite in polyphase inclusions dissolves at temperatures ranging from 360 deg C to >500 deg C but liquid (brine) and gas (vapor) do not homogenize at <500 deg C. The maximum pressure and minimum temperature during the deposition of iss and bnss with stockwork quartz veinlets are estimated to be 460 bars and 500 deg C. Fluid inclusions in veinlet stockwork quartz enveloped in sericite alteration (stage 2) consist mainly of gas-rich inclusions and polyphase inclusions. In addition to the possible presence of saturated NaCl crystals at the time of entrapment of fluid inclusions that exhibit the liquid-vapor homogenization temperatures lower than the halite dissolution temperatures in some samples, wide range of temperatures of halite dissolution and liquid-vapor homogenization of polyphase inclusions from 230 deg C to >500 deg C and from 270 deg C to >500 deg C, respectively, suggests heterogeneous entrapment of gaseous vapor and hypersaline brine. The minimum pressure and temperature are estimated to be about 25 bars and 245 deg C. Fluid inclusions in veinlet quartz associated with molybdenite (stage 3) are dominated by gas-rich inclusions accompanied with minor liquid-rich inclusions that homogenize at temperatures between 350 deg C and 490 deg C. Fluid inclusions in vuggy veinlet quartz associated with stibnite (stage 4) consist mainly of gas-rich inclusions with subordinate polyphase inclusions that do not homogenize below 500 deg C. Fluid inclusions in veinlet quartz associated with galena and sphalerite (stage 4) are composed of liquid-rich two-phase inclusions, and they homogenize into liquid phase at temperatures ranging widely from 190 deg C to 300 deg C (suggesting boiling) and the salinity ranges from 1.0 wt percent to 3.4 wt percent NaCl equivalent. A pressure of about 15 bars is estimated for the dilute aqueous solution of 190 deg C from which veinlet quartz associated with galena and sphalerite precipitated. In addition to a change in temperature-pressure regime from lithostatic pressure during the deposition of iss and bnss with stockwork quartz veinlets to hydrostatic pressure during fracture-controlled quartz veinlet associated with galena and sphalerite, a decrease in pressure is supposed to have occurred due to unroofing or removal of the overlying piles during the temperature decrease in the evolution of hydrothermal system. The majority of the sulfur isotopic composition of sulfides ranges from +-0 per thousand to +5 per thousand. Sulfur originated from an iso-topically uniform and homogeneous source, and the mineralization occurred in a single hydrothermal system.