Economic Geology Vol.93, 1995, pp. 1026-1051 Origin and Evolution of theGreisenizing Fluid at theEast Kemptville Tin Deposit, Nova Scotia, Canada WERNER E. HALTER, •'*ANTHONY E. WILLIAMS-JONES, Department of Earth and Planetary Sciences, McGill University, 3450University Street, Montreal, Quebec, Canada H3A2A7 ANDDANIEL J. KONTAK Nova Scotia Department of Natural Resources, P.O.Box 698, Halifax, Nova Scotia, Canada B3] 2T9 Abstract The process responsible for greisen-hosted tin mineralization at East Kemptville was investigated using petrographic, chemical, and fluid inclusion analyses of samples froma single deep drillhole (90-1) in the western partof thedeposit (Baby zone). Based ontheir trace element chemistry, thegreisens intersected in the hole appear tohave allformed from a single leucogranite protolith. Alteration developed as symmetrically zoned halos around central fractures as a result of theinteraction of a fluoride-rich orthomagmatic fluid with evolved leucogranite. During alteration, K feldspar was first replaced byalbite. Withfurther alteration, albite wasreplaced by muscovite resulting in the formation of quartz-sericite greisen. Ore minerals are locally present in this zone. Closer to veins, muscovite is replaced by topaz andquartz, producing a quartz-topaz greisen with which the most intense mineralization is associated. This latter greisen is enriched in F, Fe, S, Zn,and Sn, suggesting that these elements were added bythe mineralizing fluid. Next tothe vein, dissolution ofpyrrhotite, sphalerite, and cassiterite characterizes quartz-greisen inwhich the concentration ofore-forming elements (Sn, Fe, S,F, Zn, Cu)islower than in thequartz-topaz greisen. Microthermometric measurements of fluid inclusions in quartz from thevarious alteration zones show that this alteration sequence was formed byonly one greisenizing event and that temperature was approximately constant (450øC). The fluid responsible for greisen formation was an NaCl-brine, containing subordinate andvariable concentrations of Fe, Mn, and K. Measured eutectic temperatures of fluidinclusions are lowest in quartz-topaz greisen asa result of an increase in the Fe concentration due to pyrrhotite dissolution. Oxygen fugacity, which was calculated from theCO•/CH4 ratio in gases released bycrushing fluid inclusion-rich samples, displays a corresponding mini- mum since pyrrhotite dissolution releases Fe '2+. Thedistribution of pyrrhotite and thefluid inclusion data indicates that pyrrhotite was precipitated close tothe vein during early stages ofthealteration and reprecipi- tated farther outas alteration progressed. This suggests thatalteration zones moved away from thevein and widened with time. Thesalinity of fluid inclusions varies between 27 and 41wt percent NaC1 equiv and increases linearly with increasing distance fromthe vein,even in the absence of Na-bearing phases. It, therefore, follows that sodium was transported toward the fracture (vein) down a chemical potential gradient. This occurred through compensated infiltration, i.e.,a regime in which flow was dominantly parallel to the fracture but individual aliquots of fluid followed complex paths back and forth between thefracture and the rock. Cassiterite precipitated in quartz-topaz greisen in response to a pH increase of the mineralizing fluid due to its interaction with thewallrock. Other components affecting cassiterite solubility wereeither constant (temperature) or acted against itsprecipitation (fo2and acl-). Introduction INVESTIGATIONS of greisen-hosted tin deposits (e.g., Duri- •ov5 et al., 1979; Burt,1981; Haapala andKinnunen, 1982; Norman and Trangcotchasan, 1982; Eadingto, n, 1983; Rankin and Alderton, 1985; Shepherd et al.,1985b; Stemprok, 1987; Taylor and Pollard, 1988; Schwartz and Askury, 1989; Schwartz andSurjono, 1990) have established thatthese de- posits are produced byacid orthomagmatic fluids ofinterme- diate salinity at temperatures between 300 ø and 500øC. Tem- porally, thehydrothermal systems evolve from anearly stage generally dominated by a high-temperature CO2-CH4-rich fluid, to a greisen-forming stage in which theprincipal fluid isanNaCl-rich brine, and to a later, lower temperature stage • Corresponding author: email, Whalter@cam.unil.ch * Present address: Universit• de Lausanne,Sciences de la Terre, CAM, BFSH 2 CH-1015 Lausanne, Switzerland. during which lower salinity fluid produces sulfide- and phos- phate-beating veins. Cassiterite precipitation is most fre- quently interpreted to result from a dropin temperature between early and late stages. Less commonly, it isattributed to changes in fluid chemistry arising from fluid-rock interac- tion (Heinrich, 1990), and this appears to have been thecase for theEast Kemptville tin deposit, Nova Scotia, where the available evidence suggests that temperature wasconstant during mineralization. The zonal distribution of alteration minerals and cassiterite around veins in this deposit provides a spatial context in which to reconstruct the temporal evolu- tion ofthe hydrothermal system and anexcellent opportunity to evaluate the roleof fluid-rock interaction in producing a tin greisen. Thepurpose of this study isto establish theorigin of the fluidandthe metals in the EastKemptville hydrothermal system and document thechanges in rock and fluid composi- tionduring greisenization in order to evaluate the possible 0361-0128/98/2012/1026-2656.00 1026