THE CANDELARIA CU-AU DEPOSIT, III REGION, CHILE: PRODUCT OF LONG-TERM MIXING OF MAGMATIC-HYDROTHERMAL AND EVAPORITE-SOURCED FLUIDS

Andesite-hosted mineralization in the Lower Cretaceous Candelaria deposit (Lat. 27°30'53"S, Long. 70°17'22" W; 470 Mt @ 0.95% Cu, 0.22 g/t Au) was largely associated with Ca-amphibole (⁴⁰Ar-³⁹Ar plateau: 111.7±0.8 (2ó) Ma), albite, microcline, magnetite and green biotite (112.8±0.7 Ma), followed by subordinate chalcopyrite-epidote-chlorite and, finally, chalcopyrite-hematite-calcite-chlorite stages. The hydrothermal system, hypersaline and CO₂-rich throughout, cooled from over 400°C to below 275°C. Ore deposition and Ca-metasomatism, which simultaneously generated Cu-poor andradite-diopside-scapolite “skarn” in the andesitic hanging-wall of the orebody, were preceded at 114.1±0.7 Ma by large-scale and locally intense Fe- and K-metasomatism, generating, respectively, magnetite and brown biotite, a process contemporaneous with ductile deformation along major faults. This barren precursor alteration was itself superimposed on a wider zone of quasi-pervasive albitization of the andesites and, locally, adjacent granitoids. O and S isotopic compositions of ore and alteration minerals predicate progressive changes in fluid chemistry during hydrothermal activity. Thus, d¹⁸O_fluid evolved from ca. +7.0‰ for pre-ore K-metasomatism, similar to values for contiguous granitoids (+5.4 to +9.6‰), to ca. –0.2‰ in the late “Olympic Dam-like” stage. d³⁴S_fluid simultaneously increased from “magmatic” values of £+5.7‰ in the earliest Cu(-Mo, Zn, Au, Ag) ore sub-stage to +12.0<x<+20.2‰ during precipitation of late-stage chalcopyrite-hematite and terminal-stage anhydrite. Hydrothermal activity was focused at the W margin of a defunct Valanginian-to-Barremian (ca. 138-119 Ma) back-arc, shallow-marine basin which incorporated evaporites. The isotopic data record a progressive mixing of magmatic-hydrothermal with non-magmatic, plausibly evaporite-sourced waters. Mineralization overlapped temporally with the intrusion of the medium-K calc-alkaline Ojancos Plutonic Complex from 117.2±1.0 to 110.5±1.7 Ma (new ⁴⁰Ar-³⁹Ar dates), a probable source of ore-metals and juvenile fluids, and also with the uplift of the nearby evaporites.