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Basic characteristics of gold-rich porphyry copper deposits and their ore sources and evolving processes of high oxidation magma and ore-forming fluid

【摘要】:正Gold-rich porphyry copper deposits of Cenozoic are dominantly generated in subduction-related volcanoplutonic arcs and continental margin arc at convergent plate boundaries. Most of those older deposits appear to occur in island-arc terranes accreted to continental margins. The porphyry stocks span a range of compositions,from low K calc-alkaline diorite,quartz diorite and tonalite,through high K calc-alkaline quartz monzonite,to alkaline monzonite and syenite; and commonly emplaced at shallow crustal level(1~2km) and,hence,are likely to be associated with coeval volcanic rocks.And hydrothermal breccias are commonly associated with gold-rich porphyry deposits.Six broad alteration types are identified in these deposits,from early to late in time,including Ca-Na silicate alteration,potassic silicate alteration,propylitic alteration, intermediate argillic alteration,sericitic(phyllic) alteration and advanced argillic alteration,and advanced argillic alteration occurred in the same time as K-feldsparization silicification.Most of gold is introduced with copper during potassic silicate alteration stage,and gold contents also correlate well with the intensity of A type quartz veinlets.Many gold-rich porphyry copper deposits are deficient in molybdenum,rich in platinum group elements,especially rich in palladium and platinum.The key controlling processes on gold-rich porphyry copper deposits include:(1) source rich in ore-forming elements,(2) mechanism for Cu,Au and other mineralization elements efficient move into magma melt,(3) Cu,Au and other ore-forming elements have not been lost in the processes of magma evolution,(4)magmatic volatile efficiently escapes from magma,and the earlier,the more favorable for mineralization,(5) Cu,Au mineralized elements efficiently move into magmatic volatile,(6) developing relatively closed system at the top of ore-forming porphyries to inhibit the abscondence of magma volatile,(7) Cu and Au-bearing ore-forming fluid efficiently deposit,and(8) a magma chamber exists in the upper crust,which can provide ore-forming materials and heat energy driving hydrothermal recycle.And the formation of a giant deposit often have several periods of magma pulsative emplacement and hydrothermal events.However,the source of its ore-forming materials is still in debate.Currently,most researchers consider that metasomatized mantle wedge probably is the main source.These magmas favorable for the formation of gold-rich porphyry copper deposits consist of high-K calc-alkaline magma,alkaline magma,and adakitic magma,all of which have oxidation characteristics.The fluid and melt generated by dehydration and melting of subducted slab provide the high oxidation station for the mantle wedge,which is one of the main factors of mineralized elements moving into magma.The latest research indicates that fluid cooling may be the main factor of Cu and Au deposition.This article summarized the basic characteristics and the latest research approaches on the genesis and process of gold-rich porphyry copper deposits, discuss the existed problems and suggest some developing orientations.

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