Sulfide paragenesis and sulfur mobility in Fe-Ni-Cu sulfide ores at Lunnon and Juan Main shoots, Kambalda; textural and sulfur isotopic evidence

Abstract

Sulfur isotope data from a range of ore types at Lunnon and Juan Main shoots, Kambalda, have been determined in an attempt to constrain models for the crystallization of the sulfide ores. The place of pyrite in the sequence of ore formation and modification is evaluated from data representing the widest possible range of pyrite types showing variable textural relations to coexisting pyrrhotite.The sulfur isotopic distribution (68 mineral analyses) yields a mean value of 2.4 per mil (range -0.5 to +3.8%; standard deviation + or - 0.8%). The distribution is characterized by a small spread in values and isotopic disequilibria between coexisting minerals. There is no statistical difference between pyrrhotites in massive ores and matrix ores at Lunnon shoot and no systematic variation in isotopic composition of pyrrhotite and sparse pyrite in a single ore section from the same deposit. These data are consistent with previous suggestions that massive and matrix ores crystallized independently at the magmatic stage. Variable but overall less positive delta 34 S values for sulfides from disseminated ores from Lunnon shoot are consistent with control of isotopic compositions by Fe-related redox mechanisms during alteration of the ultramafic hosts.The delta 34 S values of pyrite and pyrrhotite from massive ores at Lunnon and Juan Main shoots are indistinguishable and the origin of coarse-grained pyrite remains equivocal despite the isotopic data. The data are, however, consistent with derivation of fine-grained pyrite by late-stage, relatively low temperature, sulfurization processes as suggested by textural studies. The greater permeability of the massive ore, relative to more disseminated ores and barren ultramafics, due to thermal contraction following magmatic and/or metamorphic cooling, is considered the controlling factor.The secondary nature of a significant proportion of the pyrite in massive ores severely limits attempts to use present bulk compositions to predict magmatic crystallization sequences within ore and the use of sulfur/metal ratios to estimate parameters such as f (sub o 2 ) at the magmatic stage.