Abstract:

It is widely accepted that porphyry type deposits were formed from some oxidized ore fluids. But recently, some typical ilmenitebearing, reduced TypeI granite related porphyry CuMoAu deposits with relatively reduced oreforming fluids were found, in which primary hematite, magnetite and sulfate minerals are absent. Some classification schemes of porphyryrelated granitoids are discussed in this paper, and the porphyry related deposits are subdivided into the oxidized (OPD) and reduced (RPD) types accordingly. According to related research results, the CH4 contained in RPD fluid system is proposed to be derived from the contamination of TypeI granite with Stype granite, but it is also possible that CH4 comes from the degassing process of the Earth interior. A chemical model is set in order to interpret the role of reduced fluids in porphyry metallogenesis. According to the model, H2S and CO2 are derived from the reaction between CH4 and SO2, both of which come from the same evolved igneous system, which inhabits the formation of sulfate. The characteristics of oreforming fluid sources, metalsolubility, transportation, precipitation, enrichment and depletion are obviously different in OPD and RPD systems. The low oxygen fugacity is not propitious for transportation and enrichment of Cu and Mo, which causes the poor prospective of RPD system, whereas the Au will be transported under reducing conditions and deposit in some sites far away from the granite system, forming a cogenetic subclass porphyry Au deposits. An “end member metallogenic model”, with sulfide ores in the bottom and porphyry Au ores on the top, is speculated if the contents of metals and reduced S in primary magma are appreciably enriched.

Key words: oxidized porphyry deposits, reduced porphyry deposits, TypeI granite, porphyry CuAu(Mo) deposits, oreforming fluid, CH4, metallogenic model