Mass Balance of Metals in Dynamic Ore-Forming Magmatic Ni-Cu-(PGE) Sulphide Systems

Burnham, O.M. and C.M. Lesher

Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E2C6 Canada

 

Most magmatic Ni-Cu-(PGE) sulphide deposits have been interpreted to have formed by incorporation of crustal S. As many potential contaminants contain significant amounts of metals, their contributions must be considered when calculating derivative magma compositions, ore compositions, and silicate:sulphide mass ratios (R factors). However, current mass balance expressions do not incorporate terms for externally-derived metals. Mass balance requires that the masses of element i distributed between a final silicate magma () and a final sulphide melt () must be equal to the masses of element i distributed between the initial silicate magma () and an initial sulphide melt (), such that: = , where A is the mass fraction of silicate magma and B is the mass fraction of sulphide melt. Defining and R = A/B, and rearranging yields: . If is small (</10), if D is small (<R/10), and/or if R is very large (>10D), then the effect of the additional metal is insignificant. However, if is significant, if D is relatively large, and/or if R is relatively small, then this equation must be used to evaluate the mass balances of metals in dynamic ore-forming magmatic Ni-Cu-(PGE) sulphide systems.