Big Onion: A supergene-altered porphyry copper-gold deposit

Special Volume, Vol. SV 46, No. 1995, 1995

Big Onion is an inactive porphyry copper prospect located 16 km east of Smithers, British Columbia. It was explored during the 1960s and 1970s by a total of 16 708 m of core and rotary drilling. Recognition of supergene chalcocite in unsampled drill core and subsequent geological re-interpretation were instrumental in discovery of the Big Onion deposit. The Cu-Mo deposit is related to a multiphase intrusive suite emplaced into mafic volcanic and sedimentary rocks of the Hazelton Group. The core of a north to northeasterly trending quartz feldspar porphyry rhyolite dike was intruded by dikes of quartz diorite porphyry. The quartz diorite porphyry is considered responsible for mineralization but is relatively fresh compared to the quartz-sericite altered quartz feldspar porphyry. Principal hypogene ore minerals are chalcopyrite and molybdenite which occur within northeast trending veinlets, parallel to the fault-controlled intrusions. The original shape of the mineralized zone is a classic hood draped over the quartz diorite porphyry. A fresh, postmineral dike dated at 49.5 ± 1.9 Ma provides a minimum age of mineralization. Initiation of Basin-and-Range tectonism resulted in segmentation of the Cu-Mo deposit with different erosional levels being preserved in each block. Big Onion has a well-developed pyrite halo. Recent weathering of the deposit produced a supergene zone up to 100m thick, characterized primarily by chalcocite and covellite coating chalcopyrite. The supergene zone has potential for development employing solvent extraction followed by electrowinning of copper.
Keywords: Porphyry, Copper, Big Onion, Core drilling, Rotary Drilling, Exploration, Pyrite halo