Genetic Mechanism Of Danba-mianning Orogenic Gold Belt On Western Marigin Of Yangtze Craton

Orogenic gold deposits have been the focus of ore deposit research for many years.The controversy over its genesis mainly focuses on the formation mechanism of hypozonal orogenic gold deposits.There are two main viewpoints on fluid source of orogenic gold.One is metamorphic dehydration from greenschist facies to amphibolite facies of middle and upper crust rocks with mineralization preceding peak of metamorphism.Another is devolatilization of subduction oceanic crust and overlying sediments or metasomatic lithospheric mantle.Aiming at this scientific issue,systematic regional geology,deposit geology and geochemical studies of the Danba-Mianning orogenic gold belt along the>1000 km Mesozoic dome belt on the western margin of the Yangtze craton have been conducted.The Danba gold deposit is sited in Devonian sequences in a high-grade metamorphic terrane that includes a metamorphic core complex.Around the deposit,peak metamorphic conditions of 6±0.5 kbar and 650±50°C at ca.193 Ma were followed by retrograde sillimanite grade conditions of 4.5±0.5 kbar and 550±50°C.The deposit is hosted in a broadly strata-bound ductile-brittle shear zone with high-T proximal alteration assemblages of biotite-amphibole-plagioclase and ore assemblages dominated by pyrrhotite,but with a strong association between gold and bismuth tellurides.Alteration mineral thermobarometers,together with heating/freezing studies of low-salinity H₂O–CO₂–CH₄ fluid inclusions,indicate P-T conditions of early ore deposition of approximately 4⁵ kbar and 500⁶50°C at around 185±9 Ma indicated by Re-Os geochronology on ore-related molybdenite.In conjunction,all data demonstrate that Danba represents a Lower Jurassic hypozonal orogenic gold deposit that formed during post-peak metamorphic retrogression.The primary high P-T nature of the deposit,combined with its late-metamorphic timing,negate that the ore fluid was sourced via devolatilization of the hosting supracrustal sequences.A deep externally-derived ore-fluid source is required.The?¹⁸O_fluid values for auriferous quartz and biotite from alteration zones fall within the same range of 10 to 12‰as unaltered biotite and amphibole in the host rocks at the ore-formation temperature of about 520°C.This indicates that ore and host rocks reached isotopic equilibrium during ore formation,affirming the hypozonal nature of the Danba deposit.The?³⁴S values of ore-related pyrrhotite range from+3.1 to+9.9‰,similar to those of sulfides from metasomatized mantle lithosphere,but in sharp contrast to those from pyrrhotite in unaltered host rocks which are between–6.8 and–9.5‰.The ore-related pyrrhotite have less radiogenic and more variable Pb isotopic compositions,with ²⁰⁶Pb/²⁰⁴Pb=17.85-18.25 and ^207Pb/²⁰⁴Pb=15.48-15.67,than pyrrhotites from unaltered host rocks which have ²⁰⁶Pb/²⁰⁴Pb=18.28-18.47 and²⁰⁷Pb/²⁰⁴Pb=15.62-15.65.Importantly,ore-related pyrrhotites have high ³He/³⁶Ar ratios close to 10^-3 and ⁴⁰Ar*/³He values from 1.2 to 5.6×10⁵,which are similar to those ratios of metasomatized mantle lithosphere.All isotopic data affirm a model involving deep derivation of ore fluid rather than from a crustal source involving the Devonian wallrocks.The ore-related pyrrhotites have similar PGE patterns to sulfides in metasomatized mantle wedges in terms of marked Pt depletion and slight Ru enrichment.They differ significantly from PGE patterns for sulfides from porphyry Cu-Au deposits and lamprophyre dykes,which are considered to relate to partial melting of enriched mantle.This indicates that the ore fluid,with moderate fO₂ and moderate-low fS2,was directly from devolatilization of mantle lithosphere,which was previously metasomatized by slab-derived fluids during earlier oceanic subduction.The Yanzigou gold deposit is sited in Devonian greenschist facies metamorphic rocks at the edge of a dome,with orebodies controlled by extensional slip-shear joint structure,alteration dominated by pyrite sericitization and silicification,and pyrite as the main metal mineral.The?¹⁸O values of ore fluid?10.2-11.3‰?and?³⁴S values of ore pyrite?8.5-10.7‰?are very close to those of adjacent Danba gold deposit,indicating that they may have a same source of ore fluid.Similarly,the Zhangjiapingzi gold deposit is located in Triassic sub-greenschist to greenschist facies metamorphic rocks at the edge of a dome,with orebodies controlled by brittle-ductile shear zone,alteration dominated by pyrite sericitization,silicification and iron dolomitization,and early-stage pyrite and late-stage chalcocite as main metal minerals.The late ore-stage?¹⁸O_fluid?4.4⁵.2‰?and?³⁴S?3.6⁵.1‰?are close to mantle values.The three orogenic gold geposits in the Danba-Mianning orogenic gold belt form a"Crustal Continuum Mineralization Model"which favor an external deep source of ore fluid for orogenic gold deposits.During the Paleo-Tethyan ocean closure and post-collision extension in Late Triassic,large-scale orogenic gold production formed in West Qinling and East Kunlun orogens on the northern margin of the Paleo-Tethyan Ocean.The Danba deposit and West Qinling deposits formed post regional peak metamorphism,while surrounding rock strata of some deposits in East Kunlun had been metamorphosed and devolatilized thousands of million years ago.Therefore,traditional metamorphic fluid model cannot explain these orogenic gold deposit.The gold metallogenic epoch in all the three regions coincides with the emplacement epoch of post-collision A-type granite.Orogenic gold mineralization in West Qinling and East Kunlun evolves from 235-210Ma at north to 218-200 Ma at south.Meanwhile,the?³⁴S of gold deposits in both areas transformed from 3¹5‰in back-arc basin to–5⁺5‰in suture/fore-arc accretionary prism,the latter close to mantle values.Metallogenic tectonic setting and isotope characteristics support that the ore fluids of these two gold fields originated from the mantle.The model conforming to the orogenic gold fluids in West Qinling and East Kunlun is return of mantle fluid from subduction zone.About 15 million years after the late Triassic orogenic gold mineralization in West Qinling-East Kunlun,the¹85 Ma Danba gold deposit in Songpan-Garze fold belt formed via devolatilization of metasomatic enriched mantle.The most likely source is the K–H₂O–CO₂ and ore-metal fertilized lithospheric mantle that was metasomatized during Neoproterozoic subduction.It is proposed that transition from lithospheric transpression to extension in the Jurassic triggered asthenosphere upwelling and subsequent devolatilization of this metasomatized lithosphere to cause the formation of this rare Phanerozoic amphibolite-hosted gold deposit at Danba.Apart from French Massif Central and some Canada Meguma depsoits,the Danba gold deposit may be the third Phanerozoic hypozonal orogenic gold deposit around the world,which provides key evidence for the genetic model of mantle fluid for orogenic gold deposits.