FLUID INCLUSION MICROTHERMOMETRY OF EL TENIENTE SUPERGIANT CU-MO DEPOSIT OF CHILE

Fluid inclusion microthermometry was performed on four core samples from El Teniente supergiant Cu-Mo deposit of Chile. Each of the samples represented quartz-molybdenite veins emplaced into siliceous, porphyritic stocks of the ore complex. Quartz and molybdenite were sometimes accompanied by chalcopyrite and hydrothermal anhydrite. Multiple fluid inclusion assemblages (FIA) were investigated in vein and disseminated quartz, and in the anhydrite.

Petrographically, the quartz-inclusion assemblages suggest growth coeval with sulfide mineralization. However, entrapped fluids, even within single grains, span a range of conditions. Most vein quartz FIA appear to reflect primary fluid entrapment, a model intended to be evaluated by SEM-CL. Collectively, the inclusions reflect a limited T range but variable fluid chemistry. Some are free of daughter crystals, others bear halite and sylvite, and many show sulfide daughter crystals.

Minimum entrapment temperatures in quartz recorded by bubble homogenization temperatures (T_h), largely fall between 310^oC and 375^oC. No trend emerged between T_h and age or stage of mineralization indicating a complex history. However, the youngest sample does show a significant pulse of lower T fluid entrapment and quartz growth. As expected of true FIA, T_h was not continuous across the reported temperature interval, but clustered more closely between 350ºC and 370ºC. T_h in disseminated quartz was not markedly or systematically different from T_h in vein quartz. Pressure corrections are unknown. The fluid inclusions in anhydrite were free of daughter crystals and showed little uniformity of T_h with values ranging from 140ºC to 255^oC.

Freezing studies recorded a wide range of salinities in the entrapped fluids. Many were hypersaline, with total salinities in excess of 59%, based on the failure of halite daughter crystals to dissolve at 500ºC. However, freezing point depression was as low as 1.2^oC, indicating total salinities in some inclusions as low as 2.1 wt %, with intermediate salinity values recorded as well. Fluid inclusions showing such diverse characteristics were observed in close proximity, within single grains, and with no clearly definable parageneses evident in most cases.