Is External Sulfur an Essential Prerequisite for Magmatic Ni-Cu-(PGE) Ore Genesis?
The importance of externally derived sulfur in the genesis of sulfide-rich,magmatic Ni-Cu-(PGE) deposits remains a key,yet unresolved,issue.Calculations utilizing a variety of magma types indicate that in general sulfide saturation by fractional crystallization occurs after Ni has been depleted due to olivine crystallization.Cu- and PGE-rich layers may form during later stages of closed system crystallization,but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient the mass of sulfide is too small to produce an economic deposit.Contamination of mantle-derived magmas by siliceous country rocks and partial melts will lower the sulfur content needed to induce sulfide liquid saturation,typically by amounts ranging from 200 to 700 ppm.The mixing of magmas,particularly if the result is to lower the liquidus of the mixed magma,may also lower the sulfur content needed to attain sulfide saturation by similar amounts.An increase in magma fO2 related to the addition of volatiles such as H2O and CO2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation.Although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur,Ni-rich sulfide deposits can form only from large volume,open systems,where the efficiency of sulfide collection is high.Sulfur isotope data clearly indicate that externally derived sulfur has been a requisite for the formation of many large deposits,and that such efficient collection of mantle-derived sulfide is a rare process.
E.M.Ripley
Department of Geological Sciences,Indiana University,Bloomington,IN 47405
国际会议
贵阳
英文
5-8
2012-06-16（万方平台首次上网日期，不代表论文的发表时间）