Spatially and Temporally Associated Porphyry Deposits with Distinct Cu/Au/Mo Ratios, Woodjam District, Central British Columbia

Abstract

Abstract The Woodjam district is a cluster of porphyry Cu-Au deposits of Early Jurassic age (~196 Ma) and is located in the Quesnel terrane in central British Columbia. Porphyry centers include the Southeast zone Cu-Mo porphyry, the Deerhorn and Megabuck Au-Cu porphyries, and the Takom and Three Firs Cu-Au porphyries. The Takomkane batholith, which intruded strata of the Nicola Group and is host to the Southeast zone, has characteristics of a calc-alkalic Cu-Mo porphyry. The Deerhorn, Megabuck, and Takom deposits are centered on narrow monzonite bodies with pencil-like geometries that intruded the Nicola Group volcanic sequence. These small volume intrusions have characteristics of high K calc-alkalic intrusions. The Southeast zone, Deerhorn, Megabuck, and Takom deposits have similar ages and their intrusive units can be divided into two groups with distinct geochemical characteristics. Differences between alteration and mineralization of the deposits are attributed to the magmatic evolution of the system as well as differences in the depth of emplacement and preservation. Based on stratigraphic relationships, the Deerhorn and Megabuck deposits are interpreted to be emplaced at the shallowest structural level in the district and have the highest Au/Cu ratios and the lowest temperature alteration assemblages. The Southeast zone is hosted by the felsic units in the Takomkane batholith and represents the deepest parts of the hydrothermal system. Cu-Au mineralization in the Takom deposit shows characteristics intermediate between the Southeast zone and the Deerhorn deposit. Based on stratigraphic interpretations the difference in depth of emplacement between the Southeast zone and Megabuck is about 1,400 m, with Takom being emplaced about 400 m above the Southeast zone. The Woodjam district illustrates the variety of styles of porphyry mineralization that can form over a restricted time interval (<1 m.y.) within a single district.