Open Collections

Volcanic stratigraphy and epithermal mineralization of the La Coipa district, Maricunga belt, Chile

University of British Columbia

The La Coipa district is located in the northern part of the Oligocene-Miocene Maricunga belt, northern Chile. It consists of a cluster of high- to intermediate-sulfidation Ag-Au epithermal deposits that sustained one of the major producing mines within the region, otherwise dominated by gold-rich porphyry systems. The depletion of reserves led to a temporary mine closure in 2013, which highlights the necessity of a renewed effort to improve the district exploration model. Following this objective, this study is focused on a detailed district-scale geological characterization with emphasis on Tertiary magmatic rocks. Epithermal mineralization in La Coipa is hosted in an intra-arc Oligocene-Miocene basin that overlies rocks of Permian to Eocene age. The Oligocene-Miocene magmatic rocks have been divided into two groups: La Coipa Group (24-21 Ma) and Codocedo Group (21-16 Ma). A regional unconformity separates these groups. With the exception of the intermediate-sulfidation Purén deposit, the high-sulfidation systems are always associated with concentrically zoned advanced argillic alteration where economic grades are mainly associated with vuggy quartz and quartz+alunite core zones. Outer alteration zones are constituted by kaolinite ± dickite ± pyrophyllite, and illite ± kaolinite ± smectite. The Purén deposit is characterized by silicification and illite + kaolinite alteration. CA-ID-TIMS U-Pb (zircon) and ⁴⁰Ar/³⁹Ar (biotite) ages determined that Tertiary magmatic rocks range in age between ca. 37 and 16 Ma. ⁴⁰Ar/³⁹Ar alunite ages reveal multiple episodes of hydrothermal alteration and mineralization that range between 30 and 14 Ma, but with a main episode bracketed between 19 and 14 Ma. The prolonged history of epithermal mineralization at La Coipa requires a revised model for fertile metallogenic episodes for the Maricunga belt. These alunite ages indicate a more continuous distribution of mineralizing events, which contrasts previous models that indicated two fertile episodes limited to the 24-20 Ma and 14-13 Ma periods. Trace element ratios in Oligocene and Miocene igneous rocks commonly used as tectonic and porphyry fertility indicators (eg., La/Yb, Sr/Y) suggest that La Coipa is situated at the southern end of a domain characterized by a thickened crust that acted as a favorable place for emplacement of magmatic and hydrothermal fluids.

Text

2015-08-20

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/54563

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/