Metamorphosed Mesoarchean Cu-Mo-Ag mineralization: Evidence from the Callingiri deposits, southwest Yilgarn Craton

The southwest Yilgarn Craton has received little attention from mineral explorers, relative to the eastern part of the craton, due to limited bedrock exposure, private land ownership, and a perceived lack of prospectivity. In 2010, granite-gneiss-hosted, bulk-tonnage Cu-Mo-Ag mineralization was discovered at Calingiri, 120 km north-northwest of Perth, adjacent to a domain boundary in the southwest of the craton. The majority of mineralization is contained within three granite-gneiss-hosted deposits (Bindi, Dasher and Opie), which have a combined Indicated and Inferred Resource of 529 Mt grading 0.27% Cu (1.4 Mt contained Cu), 54 ppm Mo, 1.33 ppm Ag and 0.02 ppm Au. The smaller Ninan deposit is hosted in the ca. 3010 Ma Wongan Hills greenstone belt. The Dasher mineralization is hosted in a 3010 ± 4 Ma (SHRIMP U-Pb zircon) High-Ca biotite monzogranite that is compositionally banded (SBAN). An early foliation (S1) in the monzogranite is cut by discrete 3010 ± 4 Ma (SHRIMP U-Pb zircon) syenogranite intrusions that are also affected by S1. The resultant granite-gneiss (sensu lato) was intruded by a pair of ~150 m wide, unmineralized granite (sensu stricto) dikes at 2673 ± 5 Ma (SHRIMP U-Pb zircon). D2 ~east-west compression and ~synchronous upper amphibolite facies metamorphism occurred after ca. 2673 Ma as indicated by: (1) peak metamorphic sillimanite in the S2 foliation; and (2) trace sillimanite and garnet in the granite dikes. At Dasher, D2 resulted in west-verging F2 folds of SBAN, S1 and the granite dikes. The district-scale, granite-gneiss mineralized corridor (Opie through Dasher to Bindi) was also folded during D2. Early, Mo-dominant mineralization at Dasher was synchronous with D1, as evidenced by quartz-feldspar-molybdenite veins being affected by isoclinal F1 folds, and Re-Os geochronology on molybdenite in that setting giving an age of ca. 2997-2957 Ma. The main stage of copper-dominant mineralization occurred late- to post-D1, as evidenced by sulfide stringers and quartz-sulfide stockwork veins that are parallel to, or cross-cut S1. Mineralization is associated with D1 high-strain zones, at all scales. All mineralized veins are affected by F2 folds, confirming implications from geochronology that Cu-Mo-Ag mineralization at Dasher was pre-D2, and pre-peak metamorphic. Syn-mineralization hydrothermal alteration assemblages at Dasher were recrystallized during peak metamorphism. A widely developed biotite-garnet-sillimanite-magnetite assemblage represents a plagioclase-destructive hydrothermal alteration (including original sericite-chlorite) that depleted Na and Ca in the granite-gneiss, creating Al-rich to peraluminous bulk compositions. Metamorphic recrystallization produced the observed garnet- and sillimanite-bearing assemblages. A localized granoblastic quartz-microcline-plagioclase-sillimanite assemblage likely represents recrystallized hydrothermal K-feldspar alteration. Metamorphosed hydrothermal alteration assemblages suggest peak metamorphic P-T conditions of ~660°C to 800°C and <10 kbar at Dasher. Dehydration reactions during peak metamorphism may have initiated partial melting, producing pegmatoidal mineralized leucosome veins. The Calingiri Cu-Mo-Ag deposits have grade-tonnage profiles, metal distributions and hydrothermal alteration characteristics that are comparable to those of Phanerozoic porphyry Cu-Mo deposits. The discovery of the Calingiri mineralization reveals the latent mineralization potential of little-explored Archean granite-gneiss terranes.