Metasomatic crystallization of muscovite in granite and tourmaline in schist related to pegmatite emplacement near Yellowknife, Canada

Abstract

Numerous pegmatite dikes occur in the Sparrow pluton (muscovite-biotite granite) and in the adjacent cordierite-zone schist-hornfels of the Yellowknife Supergroup. Where pegmatite dikes cut granite, the adjacent granite is enriched in muscovite and apatite, and depleted in K-feldspar. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that H, P, and locally B, Ti, Fe, and Ca were added, and K, Sr, Ba, and locally Na were removed (hydrogen metasomatism). In one alteration zone (8 cm wide) the calculated change (in terms of μ mols/gram of unaltered granite) is, 600 K-feldspar+24 biotite+190 plagioclase +[770 H+36 P+3 Ti+13 Fe+13 Ca] → 400 muscovite+1100 quartz +11 apatite+[240 Na+260 K]. Where pegmatite dikes cut schist-hornfels (biotite-plagioclase-quartz), the adjacent rock is, in places, enriched in tourmaline, apatite, and quartz, and depleted in biotite and plagioclase. These alteration zones are variable in width; most are less than 20 cm wide. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that B, P, Zn, and locally Ca, Fe, and Al were added, and that Na, K, Fe, Rb, Sr, Ba, and locally Mg and Si were removed (boron metasomatism). In one zone, 2 cm wide, the calculated reaction (in units of μ mols/gram of unaltered schist) is, 730 biotite+1530 plagioclase +[1080 B+600 H+430 P+360 Ca] → 480 tourmaline+480 quartz+115 apatite +[3630 Si+870 Na+590 K+110 Fe]. Changes in the volume fraction of muscovite, K-feldspar, tourmaline, and biotite, relative to distance from pegmatite, are progressive, and in most alteration zones may be expressed by use of an error-function equation. Some tourmaline zones are more complex. Zone formation is considered in terms of a steady-state reaction model in which grainboundary diffusion is the transport mechanism.