Abstract

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 mineralization 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 result from diffusive redistribution of 187Os. The low inital 187Os/188Os ratio of the least radiogenic sample may suggest a mantle-dominated source.

Common Os contents are very low in all of the samples (0.6 to 17.5 ppt). In contrast, Re contents vary considerably (from 3 to 867 ppb), depending on the sample location in the mineralized porphyry intrusion. Re is considerably enriched in the apex zone of the mineralized porphyry. This accumulation could be explained by intensive boiling at the top of the 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.

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