The geology and mineral deposits in the vicinity of the Mitchell and Sulphurets glaciers, northwest British Columbia.

The Mitchell-Sulphurets region is in the heart of the Coast Mountains of northwest British Columbia. Geologically it is situated on the western edge of the Bowser basin approximately 12 miles east of the main Coast Mountains plutonic complex. The map-area is underlain by partly or wholly metasomatized sedimentary, volcanic, and intrusive rocks. The volcanic and sedimentary rocks are tentatively dated as Lower Jurassic. They probably belong to Lower Hazelton and/or possibly Upper Takla group. The sediments are typical of a greywacke, turbidite suite. The volcanics belong to a marine sequence chiefly comprising pyroclastic members. Possibly in Jurassic time, the sedimentary and volcanic rocks of the area were invaded by the Mitchell Intrusions. The earlier members of the Mitchell Intrusions were injected as sills and dykes predominantly into the well-bedded sediments. The later members formed larger, more irregular bodies. There are marked mineralogical changes within the intrusions. Differentiation by fractional crystallization and composite intrusion account for original variations in mineral composition, but post-crystallization changes are the cause of unusual rock types. Spilitized diabase, syenodiorite, albite syenite, syenite, quartz syenite, and some granite have resulted from the "deliming" or albitization of the original plagioclases. Immense quantities of trapped volatiles, which were concentrated by differentiation processes, resulted in phenomenal amounts of rock alteration during the dying stages of the magmatic period. They have had a profound effect on an area of rock about three times that of the intrusions. During the period of rock alteration the area approached an equilibrium environment probably somewhere below 400° C. and probably at moderate pressures. Throughout the area altering fluids probably contained moderate concentrations of Na, K, SiO₂, and H₂S, high concentrations of CO₂ and H₂0, and in the Mitchell Valley trace amounts of HF. The end of the alteration period was sharp, possibly being terminated by the development of major faults which could have permitted the release of the fluids. The chief types of alteration - albitization, carbonatization, sericitization, silicification, chloritization, and pyritization - have affected the rocks in a similar manner throughout most of the area. In most areas secondary minerals in highly altered rocks are the same as those in the slightly altered rocks. The mineral deposits, which are of the "Porphyry Copper" type, were formed during the alteration period. The presence of large volumes of volatiles at an elevated temperature allowed extensive migration of the metal-bearing solutions from their magmatic source. Disseminated dopper and molybdenum mineralization is found in porphyritic, granitic intrusions and in altered volcanic, sedimentary, and intrusive rocks. The large quantities of fluids have also resulted in the separation of the copper and molybdenum sulphides into distinct deposits. The formation of distinct deposits was probably dependent upon the physical-chemical properties of the environment at the time of alteration. Major faulting occurred late in the alteration period. This marked the end of the Mitchell epoch of magmatic activity. Somewhat later in the history of the area, possibly in Tertiary time, a few keratophyre (basaltic (?)) dykes were emplaced. Extensive erosion by glaciers in Pleistocene and Recent times has sculptured the landforms into their present shapes.

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