Study of pegmatite bodies and enclosing rocks, Yellowknife-Beaulieu region, District of Mackenzie

Pegmatite bodies are found in muscovite-biotite granite plutons, dated at 2550 my and in biotite-, cordierite-, and garnet-bearing schists that form broad aureoles about the plutons. Most of the pegmatite bodies consist of quartz, albite, potashfeldpar, and muscovite, and some contain notable amounts of beryllium, lithium, and niobium- tantalum mineral s. Most of the pegmatite dykes found in the Sparrow Lake granite pluton and in its contact aureole are several centimetres or a few metres thick and dip steeply in different directions. Da ta on the geometry of some of these dyke suggest that they are fracture-controlled dilation dykes. Tourmaline, beryl, and spodumene are found within the dykes at progressively greater distances from the granite pluton. The distribution of minerals within individual dykes is normally irregular and the composition of minerals is variable. In one large dyke, bite crystals near the centre contain less sodium and more potassium than those near the margin, whereas crystals of muscovite near the centre contain more sodium and less potassium than those near the margin. The granite and schist adjacent to some pegmatite dykes have been altered. In granite, muscovite, tourmaline, and apatite have crystallized a t the expense of potash feldspar and biotite, and in schist, tourmaline, quartz, and apatite have crystallized a t the expense of biotite and plagioclase. These mineral react ions were accompanied by an addition to the country rocks of boron and phosphorus, and a removal of potassium. The pegmatite and the muscovite- biotite granite were evidently derived from a common source. The pegmatite-forming matter was relatively mobile and capable of transport for great distances through the country rocks. At numerous structurally favourable sites, the pegmatite minerals proceeded to crystallize, ca us in g enlargement of the pegmatite bodies. The various chemical and physical processes that were involved are not clearly understood.