The nature and origin of pegmatites in a fluorine-rich leucogranite, East Kemptville Tin Deposit, Nova Scotia, Canada

Abstract

Pegmatites at the East Kemptville Sn-Zn-Cu-Ag deposit occur within a F- and P-rich, 370 Ma topaz-muscovite leucogranite, the most evolved phase of the chemically zoned Davis Lake Pluton. Structural observations and geochronology indicate that the leucogranite intruded into an active shear/fault zone environment. Pegmatites are preferentially located near the roof zone of the intrusion and include: (1) quartz-feldspar pods (≤ 1–2 m) aligned parallel to a foliation in the leucogranite. Such pegmatites have marginal aplites or may be cored by aplites; (2) aplite-pegmatite zones with well-developed crenulate and unidirectional solidification textures; and (3) muscovite-bordered miarolitic cavities lined with quartz-feldspar ± cassiterite. Associated with areas of pegmatites are irregular- to cuspate-shaped zones enriched in muscovite. Mineralogically the pegmatites are simple, consisting of clear to black quartz, albite, microcline perthite and muscovite; topaz may be enriched in aplites coring pegmatites and coarse euhedral cassiterite is rare.

Chemically the leucogranites marginal to pegmatites are similar to the host leucogranite, but some aphanitic felsic dykes indicate extreme differentiation, as reflected by REE depletion and P-enrichment. Enrichment of pegmatitic feldspar in P₂O₅ (to 1 wt. %) indicates that melts contained 2–3 wt. % P₂O₅, whereas muscovite chemistry (to 5 wt. % F) reflects the F-rich nature of the melt. Trace and REE contents of pegmatitic feldspars are consistent with local segregation of volatile-rich melts to form pegmatites rather than extreme crystal-chemical fractionation.

Fluid inclusion studies of pegmatitic quartz and cassiterite indicate the presence of a highly saline brine (40 wt. % eq. NaCl; Na > K > Fe > Mn > Ca > Sr) of magmatic origin. Isochores modelled for a 40 wt. % eq. NaCl fluid constrain pegmatite formation at 550–600°C, thus the depressed solidus is consistent with the volatile-rich (F, P, H₂O) nature of the melt. Stable isotopes demonstrate that a magmatic fluid (δ¹⁸O_H2O = +5·5 to +10‰, δD_H2O = −33 to −41‰ for 450–500°C) equilibrated with the pegmatites and the system cooled abruptly at c. 425°C.

The low volume of pegmatite at East Kemptville suggests that the melt was not near volatile saturation, instead pegmatite generation is interpreted to have resulted from rapid decompression related to the active shear zone setting of the granite. Although feldspar chemistry reflects a local segregation model, the chemistry of aphanitic dyke rocks indicates that the leucogranite did evolve into a more fractionated melt. The local presence of cassiterite in pegmatites and miarolitic cavities indicates that locally, saturation of Sn occurred, but not throughout the EKL where elevated Sn contents are attributed to infiltration of mineralising fluids (i.e. deuteric alteration).