Genesis and fluid evolution of the East Kemptville greisen-hosted tin mine, southwestern Nova Scotia, Canada

Abstract

The East Kemptville greisen-hosted tin deposit (58 million tonnes of 0.165 percent Sn), southwestern Nova Scotia, Canada, occurs beneath undulations in the contact between granitic rocks of the Davis Lake complex (DLC) and Meguma Group metawacke. Cassiterite-topaz ore precipitated from a F-and Sn-rich fluid derived from the East Kemptville leucomonzogranite. Controls on tin mineralization include the unusual primary Sn and F abundances of the DLC magma, the vertical chemical zonation of the magma prior to crystallization, generation of an aqueous phase, and associated Sn-CI complexing before the separation of F from the magma and the flat-lying granitemetawacke contact.The Carboniferous DLC is composed of biotite-bearing monzogranites, leucomonzogranite, and high-F, low-B topaz greisen. The chemical and isotopic signatures of the monzogranites reflect the unusual source of this highly evolved pluton. Magmatic evolution was focused toward the granite-metawacke contact and culminated, at the current erosional level, in the East Kemptville deposit. Chemical variation is attributed to the vertical zonation of the magma prior to the fractionation of biotite, K-feldspar, ilmenite, and REE-bearing phosphate minerals. The remarkably high F and P contents resulted in unusual A1 and P distributions and concentration of metals beneath the contact as a result of increased magmatic depolymerization and diffusion.Pegmatitic segregations record the evolution of a F-and Sn-rich aqueous fluid. This aqueous phase scavenged CI, alkali elements, P, Sn, and other metals from the magma, concentrating them beneath the contact. In contrast to porphyry-style deposits, this fluid was not expelled from the granite. When crystallization was complete, more than 10,000 tonnes of F partitioned into this aqueous fluid, forming a hydrofluoric fluid that was neutralized by reaction with the leucomonzogranite. Quartz-topaz rock is either a direct precipitate from the F-rich fluid or extensively replaced leucomonzogranite. Quartz-mica greisen and incomplete greisen formed as the F/OH ratio in the fluid decreased. Veins and fractures emanating from massive greisen zones have alteration envelopes, indicating these structures were conduits for the F-rich fluid. Their orthogonal orientation suggests these veins are related to cooling, not hydraulic fracturing.Whole-rock Rb-Sr isotopic data from leucomonzogranite and quartz-topaz rocks yield dates and initial ratios statistically identical to the biotite monzogranite. High mean square of weighted deviations (MSWDs) resulting from the former data reflect contami-*Present address: , 1990, Genesis and fluid evolution of the East Kemptville greisen-hosted tin mine, southwestern Nova Scotia, Canada, in Stein, H. J., and Hannah, J. L., eds., Ore-bearing granite systems; Pedogenesis and mineralizing processes: Geological Society of America Special Paper 246. J. M. Richardson and Othersnation of the late-magmatic fluids by Sr derived from the metawacke and later thermal overprinting. Deformation, probably relat...

Oscillatory Epitactic-Growth Zoning in Biotite and Muscovite From the Lake Lewis Leucogranite, South Mountain Batholith, Nova Scotia, Canada

“…Webster (1990) argued that F-rich fluids could transport granophile elements (Sn, W, U, Mo) typical of those occurring in mineral deposits in the SMB. The East Kemptville topaz-cassiterite greisen at the southwestern end of the SMB is a clear example of the ability of F-rich fluids to move granophile elements (Richardson et al 1990, Halter et al 1998a. Furthermore, the clear separation of the two geochemically similar elements, Nb and Ta, between the muscovite and IMP zones has implications for the wide Nb/Ta compositional variation in granite-related Nb-Ta mineral deposits (Horng et al 1999, Pelrine 2003.…”

The Paleo‐Proterozoic High Heat Production Richardson Granite, Great Bear Magmatic Zone, Northwest Territories, Canada: Source of U for Port Radium?

“…The pattern of the LHP and HHP granite data of the area studied in the Qz–Or–Ab diagram is very similar to that of East Kemptville, Nova Scotia, in which data points plot around the minimum in the F‐poor system suggesting F concentration of less than 1% in the melt, while the effects of F in various geochemical trends and greisen formation are clear (Richardson et al ., 1990). The presence of primary magmatic fluorite in the studied HHP granite indicates the presence of F in the magma and its possible effects (see below) on the magmatic differentiation.…”

Petrography, Geochemistry and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia

“…However, in a calcium-poor granite system, like GEG, with Ab/An >15, as little as 0.5% fluorine in the melt could significantly affect the crystallization processes, since crystallization of fluorite will not occur until the late stages. The pattern of data in the Q-Or-Ab system in the GEG is very similar to that of East Kemptville, Nova Scotia, in which data points plot around the minimum in the F-poor system, suggesting F concentration of less than 1% in the melt, while the effects of F in the various geochemical trends and greisen formation are clear (Richardson et al, 1990). The large amount of F as topaz and fluorite which accompany all the alteration assemblages in the Glen Eden Mo-W-Sn deposit and the presence of primary magmatic fluorite and F-rich biotite in this granite indicate the presence of F in the magma and its effects on the magmatic differentiation of GEG.…”