Constraints on the development of orogenic style gold mineralisation at Mineral de Talca, Coastal Range, central Chile: evidence from a combined structural, mineralogical, S and Pb isotope and geochronology study

Mineral de Talca is a rare occurrence of Mesozoic, gold-bearing quartz vein mineralisation situated within the Coastal Range of northern Chile. Quartz veins are controlled by NNW-SSE trending faults are hosted by Devonian-Carboniferous metasediments of greenschist facies, and younger, undeformed granitoid and gabbro intrusions. The principle structural control in the area is the easterly dipping, NNW-SSE trending El Teniente Fault, which most likely developed as an extensional normal fault in the Triassic, but was later reactivated as a strike slip fault during subsequent compression. A dilational zone in the El Teniente Fault appears to have focussed fluid flow and an array of NW-SE-trending veins is present as splays off the El Teniente Fault. Mineralised quartz veins typically up to a metre thick occur in three main orientations: (1) parallel to and within NNW-SSE trending, E-dipping faults throughout the area; (2) along NW-SE trending, NE-dipping structures which may also host andesite dykes; (3) rarer E-W trending, subvertical veins. All mineralised quartz veins show evidence of multiple fluid events with anastomosing and cross cutting veins and veinlets, some of which contain up to 3.5 volume % base metal sulfides. Mineralogically, Au is present in three textural occurrences: (1) with arsenopyrite and pyrite in altered wall rock and along the margins of some of the veins; (2) with Cu-Pb-Zn sulfides within quartz veins; and (3) as nuggets and clusters of native Au within quartz. Fluid inclusion work indicates the presence of CO2-CH4-bearing fluids with homogenisation temperatures of ~350°C and aqueous fluids with low-moderate salinities (0.4-15.5 wt% NaCl eq) with homogenisation temperatures in the range 161- 321°C. The presence of Au with arsenopyrite and pyrite in structurally controlled quartz veins, in greenschist facies rocks with evidence of CO2-bearing fluids is consistent with an orogenic style classification for the mineralisation. However, the significant amounts of base metals, the moderate salinity of some of the fluids and the proximity to felsic granitoid intrusions have raised the possibility of an intrusion-related origin for the mineralisation. Vein sulphides display S isotope signatures (δ34S +2.1 to +4.3 ‰) that are intermediate between the host rock meta-sediments (δ34 S +5.3 to +7.5 ‰) and the local granitoids (δ34S +1.3 to +1.4 ‰), indicating a distinct crustal source of some of the S in the veins, and possibly a mixed magmatic-crustal S source. The local granite and granodiorite give U-Pb zircon ages of 219.6 ± 1 and 221.3 ± 2.8 Ma, respectively. Lead isotopic compositions of galena in the veins are consistent, suggesting derivation from a homogeonous source. Differences, however, between the isotopic signatures of the veins and igneous feldspars from nearby intrusions implies that these bodies were not the source of the metals though an igneous source from depth cannot be 3 discounted. The Triassic age of the granitoids is consistent with emplacement during regional crustal extension, with the El Teniente Fault formed as an easterly dipping normal fault. The change to a compressional regime in the mid Jurassic caused reactivation of the El Teniente Fault as a strike slip fault and provided a structural setting suitable for orogenic style mineralisation. The intrusions may, however, have provided a structural competency contrast that focused the mineralising fluids in a dilational jog along the El Teniente Fault to form WNW-trending veins. As such, the mineralisation is classed as orogenic style, and the identification of the key mineralogical, isotopic and structural features have implications for exploration and the development of similar deposits along the Coastal Range.