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 geochronological study

• Published in: Mineralium deposita 2015-08, Vol.50 (6), p.675-696
• Main Authors: Firth, Emily A., Holwell, David A., Oliver, Nicholas H. S., Mortensen, James K., Rovardi, Matthew P., Boyce, Adrian J.
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carboniferous; Devonian; Earth and Environmental Science; Earth Sciences; Fluid flow; Geology; Gold; Homogenization; Isotopes; Jurassic; Lead; Mesozoic; Mineral Resources; Mineralization; Mineralogy; Minerals; Pyrite; Quartz; Rocks; Sodium chloride; Triassic; Veins (geology)
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-014-0568-6

Mineral de Talca is a rare occurrence of Mesozoic, gold-bearing quartz vein mineralisation situated within the Coastal Range of northern Chile. Quartz veins controlled by NNW–SSE-trending faults are hosted by Devonian-Carboniferous metasediments of greenschist facies and younger, undeformed granitoid and gabbro intrusions. The principal 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; and (3) rarer E–W-trending, subvertical veins. All mineralised quartz veins show evidence of multiple fluid events with anastomosing and crosscutting veins and veinlets, some of which contain up to 3.5 vol.% base metal sulphides. Mineralogically, Au is present in three textural occurrences, identified by 3D CT scanning: (1) with arsenopyrite and pyrite in altered wall rock and along the margins of some of the veins; (2) with Cu-Pb-Zn sulphides within quartz veins; and (3) as nuggets and clusters of native Au within quartz. Fluid inclusion work indicates the presence of CO 2 –CH 4 -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 of 161–321 °C. The presence of Au with arsenopyrite and pyrite in structurally controlled quartz veins and in greenschist facies rocks with evidence of CO 2 -bearing fluids is consistent with an orogenic style classification for the mineralisation. However, the significant amounts of base metals and 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 (δ 34 S +2.1 to +4.3 ‰) that are intermediate between the host rock metasediments (δ 34 S +5.3 to +7.5 ‰) and the local granitoids (δ 34 S +1.3 to +1.4 ‰), indicating a distinct crustal source of some of the