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Book Chapter

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

By
Jean M. Richardson
Jean M. Richardson
Ottawa-Carleton Centre for Geoscience Studies, Department of Earth Sciences, Ottawa, Ontario K1S 5B6, Canada
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Keith Bell
Keith Bell
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David H. Watkinson
David H. Watkinson
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John Blenkinsop
John Blenkinsop
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Published:
January 01, 1990

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-Cl complexing before the separation of F from the magma and the flat-lying granite-metawacke 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 Al 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 Cl, 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 contamination of the late-magmatic fluids by Sr derived from the metawacke and later thermal overprinting. Deformation, probably related to regional tectonothermal events, affected all rocks of the DLC, but postdates ore formation.

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Contents

GSA Special Papers

Ore-bearing Granite Systems; Petrogenesis and Mineralizing Processes

Holly J. Stein
Holly J. Stein
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Judith L. Hannah
Judith L. Hannah
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Geological Society of America
Volume
246
ISBN print:
9780813722467
Publication date:
January 01, 1990

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