Solevantamiento andino, erosion y emplazamiento de brechas mineralizadas en el deposito de cobre porfidico Los Bronces, Chile central (33°S): aplicacion de geotermometria de inclusiones fluidas

Abstract

ABSTRACT. Andean uplift, erosion and emplacement of mineralized breccias in the Los Bronces porphyry copper deposit, central Chile (33°S): application of fluid inclusion geothermometry. Thermometric information from fluid inclusions indicates that during the last 4.9 my between 500 and 1,000 m of rock were removed from above the Donoso breccia of the Los Bronces copper deposit, central Chile (33°S). The average of these estimates suggests a rate of erosion of 150 m/my during the last 4.9 my. The 11.3 Ma quartz monzonite host of the Donoso breccia crystallized ca. 2,500 m beneath the paleosurface. This indicates between 1,500 and 2,000 m of erosion from above this intrusion occurred before the emplacement of the Donoso breccia, in the 6.4 my period between 11.3 and 4.9 Ma, at a rate, based on the average of all estimates, of 260 m/my. The data imply that erosion processes have been active in the Andes of central Chile since the late Miocene, at higher rates than in the north and lower rates than in the south of Chile. It is proposed that this erosion was due, in part, to tectonic uplift that has occurred in this part of the Andes since the middle Miocene as a consequence of the decreasing angle of subduction below this region. Decreasing subduction angle also caused the eastward migration of the volcanic front during the Pliocene. The migration of the arc and erosion speeded the cooling of a waning magmatic system that had been active at least since the early Miocene. Cooling of this system during the late Miocene, when erosion rates were the greatest, released large volumes of mineralizing magmatic fluids, which formed the Donoso breccia. Other mineralized breccias in central Chile, such as Rio Blanco, Los Pelambres, and El Teniente could have formed by similar mechanisms.