Talk presented at Geol. Soc. Amer. -- New Orleans 1995

39Ar/40Ar Geochronology of Gold and Copper Mineralization in the Potrerillos District, Atacama region, Chile

MARSH, Timothy M., EINAUDI, Marco T., and McWILLIAMS, Michael O.
Dept. of Geological and Env. Sciences, Stanford University, Stanford, CA, 94305 (marsh@pangea.stanford.edu)

One popular ore deposit model casts porphyry Cu and proximal Au deposits as genetically linked entities forming spatially separated but temporally equivalent parts of a single hydrothermal system heated by porphyry intrusion. Deposits that exhibit both Cu and Au components of the model and that provide unambiguous evidence of their genetic link are scarce, but the Potrerillos district in the Chilean Andes has been cited as one example. At the Lepanto-FSE deposits in the Philippines, Arribas et al., 1995, used K/Ar ages of porphyry Cuand high sulfidation epithermal Cu-Au ores to establish contemporaneity of the ore types and to constrain the lifespan of the hydrothermal system to 300 ka. At Potrerillos the spatial evidence for a genetic link seems strong; 200 million tons of 1.1% Cu ore have been mined from the Cobre porphyry, located less than 2 kilometers from the 16 million ton, 1.6 gram Au/tonne El Hueso deposit. Relative temporal relations between the two ore types are lacking.

Ar/Ar dating of samples comprising fresh porphyry phenocrysts and alteration minerals from the two deposits shows that muscovite from Au-quartz veins cutting porphyry in the largely carbonate-hosted El Hueso deposit (40.25 +/- 0.05 Ma) predates hornblende, primary and secondary biotite, and sericite from the Cobre porphyry (35.65 +/- 0.04 Ma). Mineralization that accompanied cooling of the Cobre porphyry from hornblende Ar closure (550 C, 35.83 +/- 0.21Ma) to biotite closure (250 C, 35.65 +/- 0.04 Ma) was accomplished in less than 300 ka. Advanced argillic alteration (36.23 +/- 0.07 Ma, alunite, with diaspore, pyrophyllite, dickite, and zunyite) at El Hueso significantly postdates Au mineralization there, attesting to the reutilization of structural fluid conduits by a later hydrothermal system. Other porphyries in the district, including the Norte Cu porphyry (36.63 +/- 0.04 Ma) and the barren Bochinche porphyry (37.91 +/- 0.06 Ma), show no detectable differences in age between phenocrysts and alteration minerals having high or low closure temperature, indicating rapid cooling and mineralization following emplacement of the stocks. While the spatial association between Cu and Au at Potrerillos appears to fit a popular genetic model, the temporal relations would require the existence of additional, unexposed porphyry stocks and related mineralization.

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