Enrichment Mechanism Of Gold And Silver In Hydrothermal Sulfides From The Okinawa Trough

Since first discovery of seafloor hydrothermal activities in late 1970s,metal incorporation and enrichment mechanism in mineralization processes of seafloor hydrothermal sulfides has been a hotspot and forefront in worldwide research.Hydrothermal sulfide deposits in immature back-arc basins,exhibits unique hydrothermal mineralization relative to their counterparts on mid-ocean ridges and in intra-oceanic back-arc basins,e.g.,coexisting occurrence of zinc-and lead-enriched polymetallic sulfides,significant enrichment in precious metals.Based on national 973 project 'hydrothermal activities and mineralization mechanism in typical back-arc basin',this study investigates incorporation and enrichment mechanism of gold and silver in various hydrothermal sulfides from middle and southern Okinawa Trough(a typical back-arc basin),via in situ micro-analytical techniques like EMPA,SEM/EDS and LA-ICP-MS.Through a detailed mineralogical and mineral geochemical study on sulfide samples via microscopic observation and in-situ micro-analyses,in combination with thermo-dynamical calculation,this study reveals incorporation mechanism and differentiated enrichment characteristics of Au and Ag,providing scientific basis for better understanding hydrothermal activities and sulfides in Okinawa Trough.1.A systematic mineralogical and geochemical study has been conducted on typical hydrothermal sulfides from lheya North Knoll and Yonaguni Knoll IV hydrothermal field in middle and southern Okinawa Trough,respectively,and hydrothermal sulfides are classified as Zn-Pb-Ba-Cu-rich,Zn-Fe-Cu-rich and Si-Ba-Fe-rich.Dominant sphalerite and common chalcopyrite,and the euhedral to subhedral morphology of pyrite in Zn-Fe-Cu-rich sulfide sample are indicative of high-medium temperature ore formation.Fine-grain sphalerite-tetrahedrite-galena intergrowths,and common barite and amorphous silica in Zn-Pb-Ba-Cu-rich sulfide sample,indicate medium-low temperature ore formation.Dominance of amorphous silica and barite,and common occurrence of colloform sulfides in Si-Ba-Fe-rich sulfide sample indicate low-temperature hydrothermal mineralization.2.A set of host minerals for Au has been identified in hydrothermal sulfides from the Okinawa Trough(arranged from high to low):colloform pyrite,orpiment,colloform sphalerite,tetrahedrite;as well as a series of Ag host minerals(arranged from high to low):Ag-sulfosalts,tetrahedrite,covellite,galena,colloform pyrite,colloform sphalerite,orpiment.Budget estimation of Au and Ag,based on Au and Ag contents and modal abundance of certain host mineral,gives the contribution of each Au host mineral to total Au budget(from high to low):colloform pyrite>colloform sphalerite>orpiment>tetrahedrite>massive chalcopyrite,sphalerite,pyrite;and contribution of each Ag host mineral to total Ag budget(from high to low):tetrahedrite>galena>covellite?colloform pyrite,colloform sphalerite>chalcopyrite>orpiment.3.Gold occurs as 'invisible' gold in host minerals,as both lattice-bounded and sub-micron inclusions in colloform pyrite,as homogeneously distributed lattice-bounded or nano-inclusions in colloform sphalerite,and generally lattice-bounded in orpiment and tetrahedrite.Silver is incorporated into major host minerals generally via lattice substitution:Ag is incorporated into tetrahedrite by substitution into the Cu site,and into galena via coupled substitution of(Ag++Tl++Cu+)and(Sb3++Bi3+)into the Pb site.Both in tetrahedrite and galena,higher Sb contents are favorable for more Ag substitution.4.Based on distribution pattern of Au and Ag,as well as mineralogical composition of sulfide sample,morphology and trace chemistry of sulfide minerals,three sets of Au host minerals and four sets of Ag host minerals are summarized with corresponding ore formation temperatures.The most important Au host minerals,colloform sulfides and orpiment,concentrated the majority of the Au at low temperatures(<150?);whereas tetrahedrite at medium temperatures(300-150?)and massive sulfides at high temperatures(>300?)concentrated only minor or trace amounts of Au.As for Ag,tetrahedrite and galena concentrated the majority of Ag at medium temperatures(300-150?).Other Ag host minerals,including massive sphalerite,chalcopyrite,and pyrite at high temperatures(>300?),colloform pyrite and sphalerite at low temperatures(<150?),and Ag-sulfosalts at even lower temperatures(<100?),concentrated only minor or trace amounts of Ag.In general,gold was concentrated into uppermost chert-barite-rich zone in low-temperature late-stage mineralization,whereas silver was enriched at medium temperatures into Kuroko type sulfide ores.5.Thermodynamic modelling calculations are conducted to constrain physicochemical controls of hydrothermal fluids on transport form and subsequent precipitation mechanism of silver and gold in hydrothermal sulfides from the Okinawa Trough.Silver(as well as base metals like Zn,Pb,Cu)transport is dominated by chloride complexes,and cooling of hydrothermal fluids primarily controls deposition of silver and other base metal sulfides,inducing co-precipitation of silver and Kuroko type ores at medium temperatures.Near-neutral pH in hydrothermal fluids(due to interaction with organics in thick sediment)results in transport of gold dominantly as Au-HS complexes rather than Au-C1 complexes.Solubility of Au-HS complexes is insensitive to temperature and pH changes,probably explains rare gold deposition under high-medium temperature together with Zn-rich and Cu-rich hydrothermal sulfides;whereas Au-HS complexes are sensitive to a(H2S)decrease,hence induces abundant deposition of gold together with low-temperature minerals after main ore-formation stage consuming and extensive seawater influx diluting reduced sulfur.