PRESSURE FLUCTUATIONS AT TACA TACA PORPHYRY COPPER, NW ARGENTINA

Taca-Taca Lower is a porphyry copper prospect located in the Puna plateau in Argentina. It is composed of a series of Oligocene granitic porphyry bodies that crosscut regional Ordovician monzogranite. Potassic alteration is developed below 250m and is overprinted by later alteration assemblages. Phyllic alteration and dense qtz (sulfides) stockwork are widespread. Local spotty argillic alteration is superimposed on earlier alteration stages. Pyrite, chalcopyrite, bornite, chalcocite and covellite comprise the dominant mineralization. Vein quartz from 15 drill holes were studied, establishing four main fluid inclusions types. I, liquid rich inclusions with a bimodal distribution with modes at 300 and 400°C and salinities between 2-12 eqwt%NaCl. II, gas rich with Th over 400°C. III, brine rich inclusions that could be subdivided by their homogenization behavior in IIIa, which comprise most inclusions, homogenize to liquid and have salinities between 30-44 eqwt%NaCl and Th with bimodal distribution, over and under 400°C. IIIb homogenize by halite disappearance at Th below 400°C with salinities between 35-45 eqwt%NaCl. Type IV are near critical inclusions with salinities between 8-20 eqwt%NaCl and Th between 420-580°C. These widely varying fluid inclusions can be reconciled by changes during cyclic fracturing and sealing of the system. A primary magmatic fluid with low to moderate salinity (5-10 eqwt%NaCl) was trapped under lithostatic conditions between 1-2 kb at over 550 °C. Fracturing cause a P drop and fluids reached critical condition. As fractures continue to open, boiling began and the fluid unmixes into low density (type II) and brine rich fluids (type IIIa). When hydrostatic pressure drops below 1/3 of the lithostatic pressure (in this case 250 bars), fractures seal, increasing pressure, though type IIIb inclusions are trapped at a minimun lithostatic pressure (900 bars). This cycle is repeated several times at decreasing P-T conditions producing the wide temperature-salinity distribution of the studied fluid inclusions.