The role of greisenization in cassiterite precipitation at the East Kemptville tin deposit, Nova Scotia

Abstract

The role of fluid-rock (leucogranite) interaction in forming the greisen-hosted tin (Cu-Zn-Ag) deposit at East-Kemptville, Nova Scotia was investigated by evaluating compositional and mineralogical changes in a representative zoned greisen, and by using this information to estimate physicochemical changes in the fluid-rock system at each step of the alteration. A large sample comprising one side of a symmetrically zoned greisen was cut into 1.5-cm-wide slabs and each slab was analyzed for its major and trace element content. The results of these analyses were used to calculate the modal mineralogy of each slab which, in conjunction with petrographic observations, revealed two main alteration zones. First, or distal to the vein, the leucogranite was converted to a quartz-sericite greisen through the replacement of albite by muscovite. This alteration, specifically the dissolution of albite, created some porosity. Closer to the vein, the neoformed muscovite was converted to topaz and quartz, creating a quartz-topaz greisen; pyrite, sphalerite and cassiterite precipitated in the earlier created pore space.Mass balance calculations show that during alteration almost all major elements, including aluminum, were mobile. Only silicon was essentially immobile because the fluid was saturated with quartz due to earlier interaction with the leucogranite. The alteration of the leucogranite resulted in a quantitative removal of the alkalies, Mg and Mn, and an addition of Fe, S, F, Zn, and Sn. The difference in the modal mineralogy of each pair of adjacent slabs was used to quantify mineralogical changes in the two main alteration reactions responsible for the observed zonation. Variations in the molar abundances of species in solution were evaluated by balancing the alteration reactions and by estimating the water/rock ratios at each step of the alteration using oxygen isotope data. The study indicates that there was an increase in the pH of the fluid and a decrease in f (sub O 2 ) during alteration, and that these changes were most important close to the vein. Cassiterite deposition occurred contemporaneously with the alteration of muscovite to topaz and quartz and is interpreted to have been caused by the sharp increase in pH that accompanied this reaction. Changes in f (sub O 2 ) and Cl (super -) acted against cassiterite precipitation, but were apparently insufficient to prevent its saturation.