The geology of the Decker Creek molybdenite deposit, British Columbia

Decker Creek molybdenite deposit is located in northwestern British Columbia, at latitude 57° 25' north and longitude 131° 59' west.The deposit is situated within the eastern part of the northern Coast Range Plutonic complex, adjacent to the southwest margin of the Chutine batholith. The batholith within the study area is divisible into three quartz monzonite units: - medium-grained mafic quartz monzonite, medium-grained leucocratic quartz monzonite, and fine-grained leucocratic quartz monzonite. These units form relatively homogeneous bodies with abrupt mutual contacts. Quartz monzonite everywhere contains pods, lenses and dykes of pegmatite, and is cut by fine-grained leucocratic quartz monzonite dykes.A lenticular border zone of quartz monzonite, up to 700 metres across, has been deformed by close-spaced fracturing, and is cut by numerous quartz feldspar porphyry dykes. A major proportion of quartz monzonite within the southwestern two thirds of this zone has been significantly affected by hydrothermal alteration. Silicified, albitised and sericitised quartz monzonite are the principal alteration rock types. Quartz monzonite within the zone of significant alteration is cut by a sheeting of quartz veins, veinlets and stringers. The common northnorthwest strike of quartz feldspar porphyry dykes, fracture planes and quartz veinlets is an important feature of the deposit. Hydrothermal alteration, fracturing and veining were followed by emplacement of 'late' granodiorite porphyry dykes and formation of north trending faults and shear zones. Pyrite and molybdenite are the only significant sulphide minerals which occur within the deposit. Molybdenite is almost entirely confined to the zone of significant hydrothermal alteration, where it occurs largely in quartz veinlets and stringers, and as fracture plane coatings.The results of a rubidium-strontium isochron study of the Chutine quartz monzonite indicate an age of emplacement of 41 m.y. and a Sr87/Sr86 initial ratio of 0.7044. The low Sr87/Sr86 initial ratio suggests that the quartz monzonite magma was generated from parent material with a similarly low Sr87/Sr86 ratio, quite likely basic material within the lower crust or upper mantle. The magma was probably generated under conditions of crystal-liquid equilibrium, and did not undergo significant differentiation prior to emplacement.The Decker Creek deposit shows a sufficient degree of similarity to the 'typical1' porphyry molybdenum deposit to merit inclusion in this deposit class. It does, however, differ from the majority of porphyry molybdenum deposits in being associated with a quartz monzonite batholith, and in containing a sheeting, rather than a stockwork, of molybdenite-bearing veinlets.