Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization

The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and , or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, lower epsilon Nd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernandez Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and acting as a source of metals.