Genesis Of The World-class Daqiao Gold Deposit,West Qinling Orogen,China

The Daqiao gold deposit is hosted in organic-rich Triassic pumpellyite-actinolite facies metamorphosed turbidites in the West Qinling Orogen,central China.Gold mineralization is characterized by high-grade hydraulic breccias?B and C ores?that overprint an earlier tectonic breccia?A ore?.A complex paragenesis is defined by four sulfide stages:S1 diagenetic pre-ore pyrite?py?,S2 hydrothermal early-ore disseminated pyrite and marcasite?mc?,S3 main-ore pyrite and marcasite aggregates,and S4 late-ore coarse-grained marcasite with minor pyrite and stibnite.However,multiple generations of pyrite and marcasite may develop within one individual stage.Ore-related hydrothermal alteration is dominated by intensive silicification,sulfidation,sericitization,and generally distal minor carbonatization.Laser ablation-inductively coupled plasma-mass spectrometry?LA-ICP-MS?trace element analyses show that the py1 of stage S1 in the shale interlayers within turbidites contains low gold contents?mean of 0.05 ppm?and other trace elements?Mn,Co,Ni,Cu,Mo,Bi,and Pb?indicating an anoxic to euxinic sedimentary environment.Sulfide minerals formed in stage S2 contributed minimally to the gold endowment with relatively low gold in various sulfides including py2?mean of 0.09 ppm?,py3?0.84 ppm?to py4?0.70 ppm?,along with mc1?0.02 ppm?and mc2?0.14 ppm?.Most of the gold was deposited in multiple generations of various sulfides of stage S3,including rapidly crystallized,irregular?e.g.,framboids,colloform and cyclic zonation?cement-hosted py5a?mean of 27.35 ppm?,py5b?9.71 ppm?,and mc3?5.94 ppm?during repeated hydraulic fracturing.Other trace elements?e.g.,Ag,As,Sb,Hg,Tl,and W?are also significantly enriched in the main-ore stage pyrite and marcasite.Little or no gold is detected in the S4 py6 and mc4.LA-MC-ICP-MS in-situ sulfur isotope analysis of hydrothermal pyrite and marcasite from the Daqiao deposit provides important information on the source and evolution of sulfur and ore fluid components.These sulfide minerals haveb?³⁴S values ranging from-31.3 to+22.0‰but fall mostly between-10 to+10‰.The results show that ore fluids of stage S2(mean?³⁴S_sulfide=-0.8 to+5.2‰)were most likely derived from deep-seated Paleozoic carbonaceous sediments during regional metamorphism associated with the West Qinling orogenesis.Ore fluids of the main-ore stage S3(mean?³⁴S_sulfid=-9.7 to-6.0‰)are relatively depleted in ³⁴S relative to those of stage S2,presumably due to fluid oxidation associated with hydraulic fracturing caused by the over pressurized fluids.The textural,chemical,and isotopic data indicate two distinct gold-introducing episodes at Daqiao,forming sulfide disseminations during early-ore stage S2 and sulfide aggregates occurring as hydrothermal cements of the hydraulic fracturing during main-ore stage S3.The stage S2 mineralization took place in a tectonic breccia beneath low-permeability shale seals that capped the flow of deep-seated metamorphic fluids,facilitating reaction with pre-existing carbonaceous material and the host turbidites to form sulfide disseminations and pervasive silicification.In S3,continuous flux of hydrothermal fluids caused fluid overpressure and consequently hydraulic fracturing of the competent silicified rocks.Subsequent rapid fluid-pressure fluctuations led to phase separation and thus massive oxidation of ore fluids,which triggered fast precipitation of gold and other trace elements within the fine-grained irregular sulfides.We then use a novel combination of synchrotron X-ray fluorescence microscopy?SXRF?and nanoscale secondary ion mass spectrometry?NanoSIMS?to characterize trace element distribution and partitioning between OM and co-located pyrite in different processes related to a turbidite-hosted orogenic gold mineralization at Daqiao?China?.Four types of OM and pyrite are defined:sedimentary OM1 and pyrite polyframboids in black shale interlayer in host turbidites;indigenous OM2 and surrounding pyrite with minor marcasite in ores;OM3 and pyrite veinlets in ores;and OM4 graphite and pyrite porphyroblasts in underlying Silurian graphitic schists.Elemental mapping and quantitative calculations show that Au,As,Co,Pb,Y,Bi,U,and PGE are preferentially partitioned into OM1 relative to the pyrite framboids.These metals are largely remobilized and liberated during diagenetic recrystallization with homogenization of OM1 matrix.Raman spectroscopic analysis suggests that OM2 in the ores is poorly crystallized with low maturity,and the estimated temperatures of 283–355? are much higher than that of the ore fluids?100–240??.This temperature difference indicates an in-situ sedimentary origin modified by regional metamorphism for OM2,rather than a hydrothermal origin.During fluid-OM2 interaction,Au,As,Zn,Hg,Tl,and U were incorporated into the rim of OM2,with liberation of V and Ni from OM2structure into proximal ore fluids and newly formed pyrite.These results show that indigenous OM2 grains in host rocks were likely an important reductant of Au complexes in ore fluids.OM3 and Au-rich pyrite likely formed as a consequence of local dissolution of OM2triggered by fluid oxidation associated with hydraulic fracturing,with dissolved organic acids assisting in the local metal transport.Only As and Hg are detectable in OM3 veinlets,indicating the strong partitioning of most trace metals into associated Py5.OM4 graphite is interpreted to form as a result of remobilization of sedimentary OM hosted in protolith shales during regional metamorphism.Carbonic metamorphic fluids that deposited OM4have caused the hydrothermal replacement of crystallized Py7 porphyroblasts by secondary siderite and ankerite in the graphitic schists.Both OM4 and Py7 are depleted in Au and other ore-related trace elements such as As,Hg,Sb,and Tl,indicating that these metals hosted in carbonaceous sediments were largely liberated via either metamorphic recrystallization or subsequent hydrothermal replacement facilitated by the deposition of OM4 graphite,rather than via the commonly-proposed pyrite-pyrrhotite transition.The recognition of trace element distribution and partitioning between OM and associated pyrite in a variety of geological environments provide valuable information that advances our understanding in the complex ore-forming processes of sediment-hosted Au and other ore deposits worldwide.To provide tighter constraints on the geotectonic setting and its relationship with orogenic evolution of Daqiao gold deposit,systematic U-Pb and ⁴⁰Ar/³⁹At geochronology study on the ore-related sericite and various granitic dikes in and around the mine were conducted.Six granodiorite dykes have similar zircon U-Pb ages of 215.0±1.1 to 211.5±1.5 Ma?1??,whereas one diorite porphyry has a zircon U-Pb age of 187.5±2.1 Ma?1??.The age of gold mineralization is constrained by two types of sericite:sericite aggregates coexisting with disseminated auriferous pyrite in relatively high-grade breccia ores and sericite coexisting with auriferous pyrite in weakly mineralized granodiorite dykes.Sericite aggregates from the breccia ores have ⁴⁰Ar/³⁹Ar plateau ages of 150.7±3.1 to 142.3±2.5 Ma?2??,whereas grains from the altered granodiorite dykes and low-grade breccia ore have 40Ar/39Ar plateau ages of 130.8±3.1 to 127.2±0.6 Ma?2??.The ⁴⁰Ar/³⁹Ar ages thus suggest two episodes of gold mineralization in the latest Jurassic and Early Jurassic that are likely associated with repeated brecciation at Daqiao.This Jurassic-Cretaceous mineralization ages coincide with discounted ages from several other gold deposits in the region and suggest that there is an underappreciated gold event in the West Qinling Orogen.Results presented here,in combination with geological evidences,suggest that Daqiao can be best classified as epizonal orogenic gold deposits,and is genetically associated with orogenic deformation and regional metamorphism.Diverse types of organic matters have played a critical role in accumulating gold by sorption or reduction,mobilizing gold via chelation or complexation,and affecting hydrothermal fluid conditions during the prolonged sedimentary,diagenetic,metamorphic,and hydrothermal processes that are related to the formation of sediment-hosted orogenic gold deposits.This Late Jurassic to Early Cretaceous mineralization event may not have been associated with the orogenic deformation,rather it is more likely to be genetically related to the far-field effects of plate reorganization during paleo-Pacific subduction beneath the eastern Eurasian continent.Preliminary results suggest that future gold exploration should pay more attention to gold deposits formed in the previously discounted Late Jurassic to Early Cretaceous mineralizing episode,and focus on the favourable structural locations in the Triassic and Devonian metasediments where regional deep faults are developed.