THE POSSIBLE IMPORTANCE OF SULFIDE LIQUID DISSOLUTION IN THE GENESIS OF HIGH-GRADE MAGMATIC SULFIDE DEPOSITS

The formation of a magmatic sulfide deposit requires that several key processes occur sequentially. The most critical is the development of an immiscible sulfide liquid, which then extracts metals from associated silicate magma. The metal contents of sulfide liquids largely depend upon the amount of silicate magma that they are able to "process" . Some Ni-Cu deposits, and many PGE deposits, require high silicate liquid:sulfide magma ratios that are hard to visualize. Recent models invoke multistage "upgrading" processes in which an early-formed low-grade sulfide liquid is enriched by subsequent batches of fresh magma that are undepleted in metals. However, this continued magmatic flux is likely to be sulfur-undersaturated, and will thus be able to partly redissolve prexisting sulfide liquids. As metals will be concentrated in the residual sulfide liquid and extracted from the new magma at the same time, this combined process is very efficient. Such a process also explains local occurrences of high-grade sulfides in relatively evolved mafic rocks, from which they are unlikely to have been generated by single-stage processes.

If sulfide liquids can be partially redissolved by later magmas, it is also possible that they may be completely dissolved, and all metals returned to later magmas. Under such circumstances, the strong metal-depletion signatures characteristic of initial sulfide liquid segregation would still be preserved in the geological record, even though the sulfides no longer exist. It is unlikely that the subtle metal-enrichment patterns that record their dissolution would be detectable. Caution is therefore dictated in the use of such depletion signatures as exploration tools, to infer overall mineral potential, or to estimate the possible size of undiscovered magmatic sulfide deposits.