Cassiterite at Kidd Creek

Abstract Cassiterite is a minor constituent of many volcanogenic and some sedimentary-exhalative massive sul-fide deposits. For some years, it was an important by-product of mining at Kidd Creek and is a minor constituent of the remaining ores. The mineralogy and chemistry of cassiterite from a suite of Kidd Creek ore was studied and compared with cassiterite from other massive sulfide deposits, as well as from granite-affiliated Sn deposits. At Kidd Creek, cassiterite grains are colorless, subhedral to euhedral, <0.5 mm in size, and commonly occur as isolated grains within a sulfide matrix. Cassiterite is most abundant in Pb-rich, massive sphalerite ore at the top of the North orebody, less abundant but widespread in Zn-rich pyritic ore, and least abundant in Cu-rich ore. It is most commonly associated with sphalerite, regardless of the abundance of sphalerite in the ore. Grains of cassiterite often contain single inclusions or clusters of small sphalerite grains at their cores. This association suggests that cassiterite coprecipitated with and possibly nucleated on the sphalerite grains. Cassiterite grains are also found locally within massive chalcopyrite and bornite, where these minerals have replaced earlier sphalerite at the base of the orebody. The cassiterite was preserved as relict grains and commonly contains inclusions of the original sphalerite. Some of the cassiterite has clearly broken down during high-temperature replacement, and the Sn was redeposited as various Sn sul-fominerals. The Zn-Pb ore at the stratigraphic top of the North orebody may have been enriched by partial remobilization of Sn from lower parts of the orebody. Concentrations of trace elements in cassiterite from Kidd Creek are generally low, with <500 ppm Fe, <50 ppm Nb, Zr, and Ta, and <20 ppm Cu, Zn, Pb, and W. This contrasts with cassiterite in granite-affiliated Sn deposits, which is usually strongly colored, Fe rich, and may contain high Nb, Zr, Ta, and especially W contents. The low trace metal contents in cassiterite from Kidd Creek and from other massive sulfide ores may reflect differences in the ionic charge and radii of the base metals and Sn4+, which might inhibit substitution in the cassiterite lattice. The reduced nature of the ore-forming fluids also favors the precipitation of Cu, Fe, and Zn in cogenetic sulfides and therefore was most likely an important factor controlling the abundance of trace metals in the cassiterite.