Geology, alteration, mineralization and magmatic evolution of the Southeast Zone (Cu-Mo) and Deerhorn (Cu-Au) porphyry deposits, Woodjam, central British Columbia, Canada

The Woodjam district is part of the porphyry deposits of Late Triassic to Middle Jurassic age (216–183 Ma) located in the Quesnel terrane in central British Columbia. Porphyry centers include the large Cu-Mo Southeast Zone (SEZ), hosted in the calc-alkalic Takomkane batholith, and the Au-Cu Deerhorn, Megabuck and Takom deposits. The latter are peripheral to the margin of the batholith and are associated with smaller monzonitic bodies of more mafic affinity that intrude and mineralize Nicola Group strata. The Takomkane batholith, host to the SEZ, intrudes and underplates the Nicola Group strata. The main phases include coarse-quartz-monzonite (CQM; 197.48±0.44 Ma) and a later, fine-grained-quartz-monzonite (FQM). Alteration is zoned from intense K-silicate in the center, weaker towards the margins, surrounded by albite alteration at the margins. Mineralization is zoned from chalcopyrite to pyrite-dominated at the margins. The Deerhorn deposit is centered on narrow monzonite bodies with “pencil” geometry and intrudes the Nicola Group volcanic sequence. Main stage of mineralization is hosted in Monzonite-A and is cut by Monzonite-D. Alteration is characterized by intense magnetite and K-silicate in Monzonite-A and adjacent volcaniclastics. Monzonite D displays moderate to weak K-silicate alteration. Mineralization is hosted in banded quartz-magnetite-hematite-chalcopyrite veins in Monzonite A and adjacent volcanic host-rocks. SEZ and Deerhorn intrusive units share geochemical characteristics. The early Takomkane units have higher SiO₂ and incompatible element concentrations, less-fractionated HREEs, negative Eu anomalies, and more radiogenic Nd isotope ratios than the Deerhorn monzonites. The latter are themselves similar to the late SEZ FQM which suggests that the latter and the Deerhorn monzonites are co-magmatic. Differences between alteration and mineralization of both deposits are partially attributed to the depth of emplacement; Cu-Au mineralization at Deerhorn represents a shallower expression of the magmatic hydrothermal system and Cu-Mo mineralization in SEZ represents the deeper segment of the partially contemporaneous system. Geochemical and isotopical affinities between the FQM/Monzonite-A/Monzonite-D intrusions and the plagioclase-phyric andesite of the Nicola Group indicate that these intrusions were partially coeval, and that mineralization in the Woodjam area is tightly connected to the Nicola Group volcanism, as age and chemistry of the rocks are similar.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat