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Geology and petrology of the Troitsa Lake property, Whitesail Lake map area, B.C. Cawthorn, Nigel George


The Troitsa Lake Property is located at latitude 53º 32¹ north and longitude 127º 20¹ west in the Whitesail Lake Map area. Lower Jurassic andesitic flows, tuffs and breccias and intercalated argillite lenses of the Lower Volcanic Division of the Hazelton Group are intruded by a granodiorite stock. A younger sill-like rhyolite complex occurs to the northwest of the stock. A variety of northwesterly trending dykes, including feldspar porphyries of quartz latite composition, cut all other rocks. The stock is zoned from a coarse-grained quartz monzonite in the centre to a relatively fine-grained granodiorite at the margin.Calculated chemical compositions of the rocks show the stock has followed a calc-alkaline differentiation trend. The compositions of plagioclase, alkali feldspar and biotite systematically vary throughout the stock. The thermal effect of the stock is estimated to have produced hornblende hornfels facies conditions up to 400 feet from the contact. The stock was emplaced in the epizone at a probable depth of about four kilometres and was subject to a load pressure of a little over one kilobar. Compositions of coexisting feldspars allow only a crude estimate to be made of the crystallisation temperatures. This indicates a temperature of 720° to 770°C. Compositions of the biotites indicate the stock crystallised under conditions of constant or increasing f0₂ and the melt may have been water saturated. Biotite compositions and experimental data for the 'granite’ system indicate that the stock crystallised under a PH₂0 of about one kilobar and at temperatures ranging from 730° to 850° C. The PH₂0 must have approached, and perhaps equalled Pload. Potassium / Argon data yield an apparent age for the stock of 75.7 ± 2.3 million years. Several stocks in the Whitesail Lake Map area have closely similar ages. The feldspar porphyry dykes have been subject to hydrothermal alteration. In one major dyke this has a zonal distribution pattern; propylitic type alteration in the north passes southwards through quartz - sericite to biotite - ortho-clase types. Sulphide mineralisation, also showing a zonal pattern, is closely associated with the hydrothermal alteration. Weak propylitic alteration and fracture plane sulphide mineralisation affect the central part of the stock. The dykes appear to have acted as channelways for the hydrothermal ore-bearing solutions.

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