Structural geology and geostatistical parameters of the Afton copper-gold mine, Kamloops, B.C.

CIM Bulletin, Vol. 76, No. 856, 1983

ALAN REED, Head, Economic Geology Division, Piteau & Associates, Vancouver, B.C.

Afton Mines Ltd. (N.P.L.) operates an 8500-tpd open-pit copper-gold mine near Kamloops, B.C. The pit is currently 480 feet deep and will reach an ultimate depth of 800 feet. The east, south and west walls of the pit are composed of Triassic andesites (Nicola Volcanic Formation) and contemporaneous sub-volcanic intrusions of micro-syenite, micro-diorite, diorite and picrite-basait (Iron Mask Pluton). The north wall of the pit consists of Tertiary sandstones, shales and mudstones (Tranquille Formation) and dacitic dykes, sills, flows and tuffs (Kamloops Volcanic Formation). The Afton copper-gold orebody (30 million tons of open-pit ore with a grade of 1.0 per cent copper, 0.016 oz/ton gold and 0.12 oz/ton silver} occurs within the Triassic Iron Mask Pluton adjacent to its faulted contact with Tertiary rocks. The Triassic-Tertiary contact is a complex fault system dipping 65 degrees southward and causing the Triassic rocks to overlie the Tertiary rocks. The detailed trace of this contact is very intricate due to the mutual interference of three principal fault systems. The dominant geological structures in both Triassic and Tertiary rocks are faults striking WNW and dipping 65 degrees southward, i.e. sub-parallel to the orebody (strike faults). Cross faults and oblique faults are abundant and highly significant in some sectors of the pit. The Tertiary sedimentary rocks show an early series of northwesterly plunging folds deformed by a later series of northeasterly striking folds. Major lithological and structural boundaries divide the pit into 8 structural domains within which the pitwall slope stability is controlled by the local geological structures and lithology. Assay data from production blasthole samples and exploration diamond drillhole samples show a log-normal frequency distribution, with a fairly low relative variance. Variograms computed from production blasthole samples show good continuity with elliptical horizontal zones of influence which are truncated by the major faults that bound the ore zone. Deep diamond drillholes in the west part of the orebody show a rhythmically banded distribution of mineralization, with copper, gold and silver decreasing generally westward, while sulphur gradually increases westward. Experimental semivariograms computed from diamond drillhole samples show excellent continuity that can be modelled by nested spherical structures.

Keywords: Economic geology, Structural geology, Geostatistics, Afton Mine, Mineralization, Open-pit mining, Gold, Silver, Copper, Slope stability.