Abstract
While gold partitioning into hydrothermal fluids responsible for the formation of porphyry and epithermal deposits is currently well understood, its behavior during the differentiation of metal-rich silicate melts is still subject of an intense scientific debate. Typically, gold is scavenged into sulfides during crustal fractionation of sulfur-rich mafic to intermediate magmas and development of native forms and alloys of this important precious metal in igneous rocks and associated ores are still poorly documented. We present new data on gold (Cu-Ag-Au, Ni-Cu-Zn-Ag-Au, Ti-Cu-Ag-Au, Ag-Au) alloys from iron oxide deposits in the Lesser Khingan Range (LKR) of the Russian Far East. Gold alloy particles are from 10 to 100 µm in size and irregular to spherical in shape. Gold spherules were formed through silicate-metal liquid immiscibility and then injected into fissures surrounding the ascending melt column, or emplaced through a volcanic eruption. Presence of globular (occasionally with meniscus-like textures) Cu-O micro-inclusions in Cu-Ag-Au spherules confirms their crystallization from a metal melt via extremely fast cooling. Irregularly shaped Cu-Ag-Au particles were formed through hydrothermal alteration of gold-bearing volcanic rocks and ores. Association of primarily liquid Cu-Ag-Au spherules with iron-oxide mineralization in the LKR indicates possible involvement of silicate-metallic immiscibility and explosive volcanism in the formation of the Andean-type iron oxide gold-copper (IOCG) and related copper-gold porphyry deposits in the deeper parts of sub-volcanic epithermal systems. Thus, formation of gold alloys in deep roots of arc volcanoes may serve as a precursor and an exploration guide for high-grade epithermal gold mineralization at shallow structural levels of hydrothermal-volcanic environments in subduction zones.
Highlights
Previous studies have established a firm link between gold mineralization and magmatic-hydrothermal processes and have shown close genetic and paragenetic relationship between gold and intrusive igneous activity [1,2,3,4,5,6,7,8,9,10,11,12]
We propose that the Lesser Khingan Range (LKR) volcanic rocks and iron-oxide ores contain at least two generations of gold mineralization: 1) the early magmatic Cu-Ag-Au spherules and 2) the later-stage hydrothermal Ag-Au grains
Textural and compositional features of gold (Cu-Ag-Au, Ni-Cu-Zn-Ag-Au, Ti-Cu-AgAu, Ag-Au) alloys from iron oxide deposits in the Lesser Khingan Range (LKR) of the Russian Far East suggest their precipitation from evolving metal-rich silicate melts within the roots of volcanic mineralized systems
Summary
Previous studies have established a firm link between gold mineralization and magmatic-hydrothermal processes and have shown close genetic and paragenetic relationship between gold and intrusive igneous activity [1,2,3,4,5,6,7,8,9,10,11,12]. The presence of native gold in ultramafic rocks [28,29,30] along with bulk-rock gold enrichments in some mantle peridotites [31,32,33,34] and crustal plutonic complexes [35,36,37], indicate that gold is transported from lithospheric mantle sources through magmatic plumbing conduits and concentrated at economic levels in epithermal environment [16,19,38,39,40,41,42,43,44]. ATlhlye,fothrmeadtiaotnaocfonnatcievrengionlgd atnedxtguorleds-baenardincgoamllopyossinitimonagsmoafs ganodld and goldbveoalrcainngic asyllsoteymssinis sthtilel aeaproloyr-lystdaogceummeangtemdaatnidc apmlubmiguboinugslysyunstdeemrsstoaordepchoenospmiecnuoonu, sly lacking aws hmicohsrteaqvuairielsabfulrethinerfocromnsatrtaiionntsibsortehlafrtoemd troeptrheeselnattaetirv-estsaugiteesepofitnhaeturmralaslaemnpvliersoansments. This owbevliloaussexgpaeprimineontuarl sktnudoiwesl.eSdpgeecifoicfaglloyl,dthbeedhaatavicoorncinernfliungidt-esxatuturersataendd icgonmepoousistisoynsstems can be eoxfpgloalidneadndbgyohldig-bheamrinogbialliltoyyos fingothledeaanrldy,-sctaognesemqaugemnattliyc, pblyumalbminogsstycsotemmpslaerteecroenp-lacement of msepnaivgciumrooanutmsilcyenglatosc.lkdTinhwgisiatoshbmvsieoocusotsnagvdaaapirliaynbltoeeuxirntkufonrroamwl aaltenidodgneciosofmreglpoalotdesdibteitoohantvhaielofrlaoitnremrf-lusstiaddg-uesraeitnpuirgtahteeevrdmoilgau-ltion of gold mneinoeursaslyizstaetmiosnciann hbyederxoptlhaienremd ablysehtigtihngmso.bTilhiteyreoffogroel,dthaendb,ecsotncsoenqudeintitolyn,sbfyoraltmhoespt reservation ocfommpalgetme raetpiclagceomldenftoorfmmsagcmanatibcegoelxdpweictthesdecionndiganryetoeuxtsurraolcaknsdtchoamtpeoxspitieorniaelnfcoermdsfast cooling odruqriunegnecvholiuntgiown oitfhgoouldtmsuinbesrtaalinzatitaiolnlaintehry-dstraogtheerhmyadlrsoettthinegrsm. Tahleirmefporaec, t.heTbheesstecorno-cks include, fidristtioannsdfofrotrheemproesste,rvvatriioonuosfvmoalcgamnaitcicagnodldsfuobrm-vsoclacnanbiecefxopremctaedtioinnisgsnueocuhsarsocekxsptlhoastive breccias, igexnpimeribenricteeds faanstdcvooalriinoguosrtquufefsn.ching without substantial later-stage hydrothermal impact.WTehepserersoecnkts iinnctlhuidse,pfairpset ranredsufolrtesmoofsst,tuvdaryiooufs gvoollcdanmicicarnod-psaurbt-ivcolelcsaninic ifroorn- oxide ores amndatiaoWsnsesospcuricaehsteeandstevixnopltlchoaissinvpiecabprreoercckrceisassuf,rlitogsmnoifmPsboturpidteeyrseoacfnhgdnovlodayrmieo,iucKsrootu-spftfaesrn.tigclaesanindirKonayolxaindeiroornesoxide (with manadngasasnoceisaete)ddveoplcoasniitcsroocfktshfreomLePsospeerreKchhninogyea,nKoRsatennggea a(LndKKRa)yolarneirpornoovxiindcee(winiththe Russian FmaranEgaasnte.seP)rdeevpioosuitss sotfuthdeieLsesessetraKbhliisnhgeadn Rsiagnngeifi(LcaKnRt) rooreleporofvminacefiicnttohefeRlussicsiavnoFlcaarnism in the foEramst.aPtiroevnioaunsdsetuvdoileusteiostnabolifsthheedsseigmnifnicearnatl rdoelepofsimtsa[fi5c9t–o6f1e]l.siWc veoalclasnoisdmiscinusthsepfoosrs- ible models fomratoiornigainndaenvdoluetvioonluotfiothnesoefmginoeldralindedpoeseipts-s[5e9a–t6e1d]. cWrue satlsaol dmisacgumss aptoiscsisbylestmemodselws ith special efmorpohraigsiins aonndfoevromluattiioonnoof fggooldldinddeepeops-sitesatiendvcorulcsatanlimc eangmviartoicnsmysetnemtss. with special emphasis on formation of gold deposits in volcanic environments
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