DATING OF ALTERATION AT THE RADOMIRO TOMIC PORPHYRY COPPER DEPOSIT, NORTHERN CHILE BY THE HIGH PRECISION 40Ar/39 Ar METHOD.

Abstract

The Radomiro Tomic (RT) porphyry copper deposit is located approximately 8-lO km north of the Chuquicamata mine and 245 km northeast of the port city of Antofagasta. Estimates place the deposit at over 800 million tonnes of ore with an average grade of 0.59% Cu (mostly supergene ore) and a mine life of 22 years. RT does not outcrop as the entire deposit is buried beneath Miocene piedmont gravels up to 200m in thickness and has been known only from drillcore and limited underground workings. Stripping is now underway and the mine is due to start operating in 1997 as a new separate division of CO DELCO, the Chilean national copper mining company. The deposit is hosted within an intrusive body of quartz monzodiorite composition that is approximately lower to middle Oligocene in age and was emplaced within the regional N-S trending Domeyko Fault system. This system is spatially associated with all the major porphyry deposits in northern Chile. High precision, stepwise degassing 40 ArP9 Ar dating was conducted at Dalhousie University to determine the ages of the potassic, argillic and quartz-sericite alteration assemblages at Radomiro Tomic. K-feldspar, biotite and sericite grains were hand picked from six drillcore samples representing the three hydrothermal alteration zones. Results are suggestive of potassic and argillic alteration assemblages having an average age of 32.6±0.3Ma as part of an initial hydrothermal event. As well, a younger hydrothermal event was dated to 31.8±0.3Ma from quartz-sericitic alteration minerals. Age differences among the potassic and argillic alteration minerals (K-feldspar and biotite) having high or low closure temperatures are not detectable, are indicative of rapid cooling following emplacement of the host porphyry. 39 Ar degassing plateau patterns combined with X-ray diffraction analyses on the K-feldspar samples also support the scenario of a rapid decrease in temperature to less than 150°C in the order of hundreds of thousands of years, rather than millions, following each hydrothermal event. A full suite of ancillary data was generated on the dated samples establishing the close similarity of the petrology, mineralogy and chemistry to samples of similar alteration types from Chuquicamata.

Key Words and Phrases: Radomiro Tomic, porphyry copper deposit, Chile, argon dating, geochronology, hydrothermal, alteration, potassic, argillic, quartz-sericitic, Chuquicamata