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Regional geochemistry of tertiary igneous rocks in Central Chile: Implications for the geodynamic environment of giant porphyry copper and epithermal gold mineralization


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Hollings, P, Cooke, DR and Clark, AH 2005 , 'Regional geochemistry of tertiary igneous rocks in Central Chile: Implications for the geodynamic environment of giant porphyry copper and epithermal gold mineralization' , Economic Geology, vol. 100, no. 5 , pp. 887-904 , doi: 10.2113/100.5.887.

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

Item Type: Article
Authors/Creators:Hollings, P and Cooke, DR and Clark, AH
Keywords: ridge, subduction, crustal thickening, volcanism ,plutonism
Journal or Publication Title: Economic Geology
ISSN: 0361-0128
DOI / ID Number: 10.2113/100.5.887
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