RE-OS SYSTEMATICS OF PYRITE FROM THE BOLCANA PORPHYRY COPPER DEPOSIT, APUSENI MOUNTAINS, ROMANIA

We report here the results of a Re-Os study of the Bolcana porphyry copper (Cu-Au) deposit, in the Apuseni Mountains of Romania. The age of the Deposit is known only from its spatial relationship with basement rocks of known age. The minieralization has not been directly dated. The Re-Os isotope system provides an excellent means of dating the mineralization itself and may additionally place constraints on the metal source. This methodology has been applied to pyrite, a ubiquitous mineral, because no molybdenite was found in our sampling level. The results yield an age of 11.8 +0.51/-2.8 Ma. This age is in good agreement with available geochronological information. The large dispersion around the age regression line, which exceeds that expected from identified analytical uncertainties, may results from diffusive redistribution of (187)Os. The low initial (187)Os/(188)Os ratio of the least radiogenic sample may suggest at mantle-dominated source. Common Os contents are very low in all of the samples (0.6 to 17.5 ppt). In contrast, Recontents vary considerably (from 3 to 867 ppb), depending on the sample location in the mineralized porphyry intrustion. Re is considerably enriched in the apex zone of the mineralized porphyry. This accumulation could be explained by intensive boiling at the top of porphyry, which concentrates chloride in the liquid phase, allowing Re to be retained as Re-rich chloride complexes. This Re may then have been incorporated into the lattice of the pyrite that crystallized from the brine in stockwork veins.