ORIGIN AND EVOLUTION OF THE GREISENIZING FLUID AT THE EAST KEMPTVILLETIN DEPOSIT, NOVA-SCOTIA, CANADA

The process responsible for greisen-hosted tin mineralization at East Kemptville was investigated using petrographic, chemical, and fluid in clusion analyses of samples from a single deep drill hole (90-1) in th e western part of the deposit (Baby zone). Based on their trace elemen t chemistry, the greisens intersected in the hole appear to have all f ormed from a single leucogranite protolith. Alteration developed as sy mmetrically zoned halos around central fractures as a result of the in teraction of a fluoride-rich orthomagmatic fluid with evolved leucogra nite. During alteration, K feldspar was first replaced by albite. With further alteration, albite was replaced by muscovite resulting in the formation of quartz-sericite greisen. Ore minerals are locally presen t in this zone. Closer to veins, muscovite is replaced by topaz and qu artz, producing a quartz-topaz greisen with which the most intense min eralization is associated. This latter greisen is enriched in F, Fe, S , Zn, and Sn, suggesting that these elements were added by the mineral izing fluid. Next to the vein, dissolution of pyrrhotite, sphalerite, and cassiterite characterizes quartz-greisen in which the concentratio n of ore-forming elements (Sn, Fe, S, F, Zn, Cu) is lower than in the quartz-topaz greisen. Microthermometric measurements of fluid inclusio ns in quartz from the various alteration zones show that this alterati on sequence was formed by only one greisenizing event and that tempera ture was approximately constant (450 degrees C). The fluid responsible for greisen formation was an NaCl-brine, containing subordinate and v ariable concentrations of Fe, Mn, and K. Measured eutectic temperature s of fluid inclusions are lowest in quartz-topaz greisen as a result o f an increase in the Fe concentration due to pyrrhotite dissolution. O xygen fugacity, which was calculated from the CO2/CH4 ratio in gases r eleased by crushing fluid inclusion-rich samples, displays a correspon ding minimum since pyrrhotite dissolution releases Fe The distribution of pyrrhotite and the fluid inclusion data indicates that pyrrhotite was precipitated close to the vein during early stages of the alterati on and reprecipitated farther out as alteration progressed. This sugge sts that alteration zones moved away from the vein and widened with ti me. The salinity of fluid inclusions varies between 27 and 41 wt perce nt NaCl equiv and increases linearly with increasing distance from the vein, even in the absence of Na-bearing phases. It, therefore, follow s that sodium was transported toward the fracture (vein) down a chemic al 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 the fracture and the rock. Cassiterite precipitated in quartz -topaz greisen in response to a PPI increase of the mineralizing fluid due to its interaction with the wall rock. Other components affecting cassiterite solubility were either constant (temperature) or acted ag ainst its precipitation (f(O2) and a(CL)-).