Porphyries lure explorationists to Quesnel trough

A resurgence of exploration activity focused on alkalic porphyry copper deposits in the Quesnel Trough of British Columbia is taking place. But this time the lure is for gold as well as copper.

During the late 1960s and 1970s, alkalic porphyries were discovered and mined for their copper content, with gold recovered as a by-product. Such operating mines include Copper Mountain near Princeton, and Afton near Kamloops.

The gold potential of alkalic porphyry deposits is now being widely recognized in light of the success of the Mount Milligan gold-copper deposit, now owned by Placer Dome (TSE).

The deposit — containing 385 million tons grading 0.22% copper and 0.016 oz. gold per ton — is at the feasibility stage. Other mine prospects in the area are the Mount Polley copper-gold deposit of Imperial Metals (TSE), and the QR deposit.

Rea Gold (TSE) is currently in the process of obtaining ownership of the QR gold property from QPX Minerals (TSE) and Placer Dome.

Exploration for other copper-gold porphyry deposits in the Quesnel Trough area is very active. Major drilling programs were conducted in the summer of 1990 on the Cat property owned by BP Resources Canada (TSE) and Lysander Gold (VSE), on Cathedral Gold’s (TSE) and Eastfield Resources’ (VSE) Takla-Rainbow property, and on the BP Resources-Digger Resources (TSE) Chuchi Lake property. The Lorraine property was also investigated by a unit of Kennecott. Alkalic porphyry copper-gold deposits were first defined as a distinct type of deposit in 1975. Peter Fox, a consulting geologist, recognized that these deposits had different characteristics, including low silica content, compared to the calc-alkalic porphyry copper deposits found in southern B.C. With the use of his model in 1976, Peter Fox discovered the gold-rich alkalic deposit, the QR, near Kamloops. His model was also applied several years later in defining the Mount Milligan copper-gold deposit.

Although the gold content has always been acknowledged in these alkalic porphyry deposits, the yellow metal was not of interest due to low gold prices and high geochemical exploration costs.

The alkalic porphyry copper-gold deposits of British Columbia occur in a very specific geologic setting, the Quesnellia Terrane. The area is commonly called the Quesnel Trough a long, narrow belt of volcanic and intrusive rocks running north of Kamloops up to Quesnel.

The Quesnel Trough has become known in the industry as extending northward to the Galore Creek area and southward to Princeton. Near Princeton, the Quesnel Trough may host calc-alkalic deposits as well.

The alkalic porphyry copper-gold deposits have very specific characteristics which provide good exploration targets. They are associated with volcanic intrusions within the volcanic rocks of the Quesnel Trough.

The intrusive rocks have a distinct magnetic signature due to the secondary magnetite. This makes airborne and ground magnetic surveys an important exploration tool. The deposits also respond well to IP surveys.