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 Prich, 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 solidi®cation 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 diVerentiation, as re¯ected by REE depletion and Penrichment. Enrichment of pegmatitic feldspar in P 2 O 5 (to 1 wt. %) indicates that melts contained 2±3 wt. % P 2 O 5 , whereas muscovite chemistry (to 5 wt. % F) re¯ects the F-rich nature of the melt. Trace and REE contents of pegmatitic feldspars are consistent with local segregation of volatilerich 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¯uid constrain pegmatite formation at 550±600 C, thus the depressed solidus is consistent with the volatile-rich (F, P, H 2 O) nature of the melt. Stable isotopes demonstrate that a magmatic¯uid (¯1 8 O H 2 Oˆ ‡ 5 : 5 to ‡10 ,¯D H 2 Oˆ¡ 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 re¯ects 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 in®ltration of mineralising¯uids (i.e. deuteric alteration).

U-Pb dating of the Musquodoboit Batholith, southern Nova Scotia: evidence for a protracted magmatic-hydrothermal eventin a Devonian intrusion

“…Equivalent to slightly deeper depths of emplacement have been estimated for the Port Mouton and Barrington-Shelburne plutons (Currie et al 1998). Using fl uid inclusion thermometry, Kontak et al (1999a) estimated emplacement pressures of 3-3.5 kbars during vein mineralization at the Dunbrack occurrence in the MB, which is similar to pressures suggested for greisen and vein mineralization at the East Kemptville tin deposit in the western part of the South Mountain Batholith (Halter and Williams-Jones 1999;Kontak et al 2001). Thus, a pressure equivalent to ca.…”

Nature and Origin of an LCT-Suite Pegmatite With Late-Stage Sodium Enrichment, Brazil Lake, Yarmouth County, Nova Scotia. Ii. Implications of Stable Isotopes ( 18o, d) for Magma Source, Internal Crystallization and Nature of Sodium Metasomatism

“…As noted above, fluid-inclusion petrography indicates there is a dominance of mixed H 2 O-CO 2 inclusions of secondary and pseudosecondary origin which, we infer, reflects incursion of a fluid that equilibrated with the surrounding pelite lithology in the wallrock. Thus, the muscovite and tourmaline appear to have retained the primary magmatic signature, but the fluid inclusions reflect latestage incursion of wallrock-buffered fluid, which was also suggested for the trend of the data at the other two pegmatite localities (Kontak et al 2001(Kontak et al , 2002.…”

“…This fractionation contrasts with the partitioning of 18 O among mineral phases in other pegmatites (e.g., Taylor et al 1979, Longstaffe et al 1981, Walker et al 1986, Kontak et al 1988, Laurs et al 1996, Kontak et al 2001, Matthews et al 2003 or granites in general (Taylor & Sheppard 1986). In the aforementioned cases, there is concordance of 18 O exchange among several mineral pairs (e.g., Qtz-Ms, Qtz-Kfs, Ms-Kfs), which reflects isotopic equilibrium, and similar estimates of temperature were determined on the basis of other thermometers.…”

“…Also noted in this work is the association of secondary muscovite with zones of albite, which indicates a local sink for the excess K produced during K-feldspar replacement. Laurs et al Table 2) compared to pegmatites hosted within the interior of the South Mountain Batholith, Nova Scotia (SMB; Kontak et al 1988, 1991, Carruzzo et al 2002, at Peggy's Cove, Nova Scotia (PC; Kontak et al 2002), and East Kemptville, Nova Scotia (EK; Kontak et al 2001 Tables 1 and 2). …”

Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: P, T and X constraints of fluids using fluid inclusion thermometry and decrepitate analysis

“…The bulk composition of the SMB falls largely in the system NaAlSiO 4 -KAlSiO 4 -SiO 2 , providing the sodium and potassium needed for the (Na, K)-rich fluids. At the East Kemptville and Dunbrack deposits, Kontak et al (1999a) and Kontak et al (2001) interpret (Na, K)-rich fluids as also being of magmatic origin. According to Orville (1963), fractionation of alkalis between a fluid phase and a crystalline phase (i.e.…”