MINERALOGY AND CHEMISTRY OF THE RUGGED MOZJNTAIN PLUTCb~W: A MELANITE-BEARING ALKALINE INTRUSION

The Rugged Mountain intrusion is a small (14 km% zoned alkaline pluton located in northwestern British Columbia, 25 kilometres southwest of Telegraph Creek (Brown eta/., lYY2;r. h). The pluton is exposed on the north side of Shakes Creek, a southeasterly flowing tributary of the Stikine River and on the south flank of Rugged Mountain from which it takes its name. The Rugged Mountain pluton was first mapped by EA. Kerr (1948). In his exploration of the Stikine River, Kerr mapped a series of seven alkaline bodies stretching down river from Telegraph Creek to the Iskut River. The northernmost of these bodies, referred to in his paper as the Shakes Creek mass, is known as the Rugged Mountain pluton. Souther (1972) also studied the Rugged Mwntain intrusion while mapping the Telegraph Creek map sheet at I:250 000 scale. Porphyry copper-gold exploration of the intrusion was undertaken during the 1988 and 1989 field seasons by Homestake Mineral Development Company as a joint owner with Equity Silver Mines Limited. Several samples of magnetite-rich pyroxenite with malachite staining returned copper values as high as 2.32 per cent with 1.58 grams per tonne gold. Subsequent I50 000.scale mapping of the Chutine and Tahltan map sheets by the British Columbia Geological Survey Branch (Brown cf al.. 1992b) provided an opportunity for detailed mapping of the intrusion by the senior author (Figures I-IO-I and I-10-2). Rocks of the Rugged Mountain pluton have similar mineralogy throughout and, in paticular, have igneous melanitr garnet in all phases of the intrusion. Other common miner& include aegirineaugite, potassium feldspar, magnetite, apatite and titanits. The field petrographic and chemical observations described in this paper suggest that the suite is cogenrtic. Furthermore, these same field, mineralogical and chemical characteristics are found in other alkaline intrusions within the Cordillera of British Columbia, suggesting a common mechanism is dominant in the genesis of this type of alkaline pluton.