ANDEAN UPLIFT, EROSION AND EMPLACEMENT OF MINERALIZED BRECCIAS IN THELOS-BRONCES PORPHYRY COPPER-DEPOSIT, CENTRAL CHILE (33-DEGREES-S) - APPLICATION OF FLUID INCLUSION GEOTHERMOMETRY

Thermometric information from fluid inclusions indicates that during t he 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-degrees-S). The average of these estimates suggests a rate of erosi on of 150 m/my during the last 4.9 my. The 11.3 Ma quartz monzonite ho st of the Donoso breccia crystallized ca. 2,500 m beneath the paleosur face. This indicates between 1,500 and 2,000 m of erosion from above t his intrusion occurred before the emplacement of the Donoso breccia, i n the 6.4 my period between 11.3 and 4.9 Ma, at a rate, based on the a verage of all estimates, of 260 m/my. The data imply that erosion proc esses have been active in the Andes of central Chile since the late Mi ocene, at higher rates than in the north and lower rates than in the s outh of Chile. It is proposed that this erosion was due, in part, to t ectonic uplift that has occurred in this part of the Andes since the m iddle Miocene as a consequence of the decreasing angle of subduction b elow 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 t hat 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 t he Donoso breccia. Other mineralized breccias in central Chile, such a s Rio Blanco, Los Pelambres, and El Teniente could have formed by simi lar mechanisms.