Metallogenesis Of The Dahongshan Fe-Cu-(Au)deposit In The Kangdian Region

The Kangdian region in the western margin of the Yangtze is an important Proterozoic iron oxide-copper-gold?IOCG?polymetallic metallogenic province in the world.Based on early exploration reports,there are total confirmed reserves of at least 1billion tonnes of iron-oxide and 6 million tonnes of copper metal.Since the discovery of several typical iron-copper deposits in the 1960s,the region has attracted the attention of a large number of scholars and geological professionals.Although after being mined and investigated for over fifty years,key scientific issues such as the ore-forming models?including the protolith components,hydrothermal alteration models,ore-controlling factors,etc.?,age of mineralization,corresponding tectonic settings,and ore-forming and later remolibization processes are still in uncertain,which has hinder the understanding of ore genesis and the establishment of regional exploration strategies.In this study,we focus on Dahongshan iron-copper-?gold?deposit,the most typical and the largest one in the region.Enlightening by the previous publications and our detailed field geological observations,the alteration phases vectoring to mineralization at the deposit scale have been systematically presented.Based on the detailed mineralogical research,we use combined geochronological methods?Sm-Nd;Re-Os;U-Pb?and in situ multi-isotopes?S-B-Nd?analyses to clarify the metal sources of the Dahongshan iron-copper deposit,ore-forming processes and remobilization histories,and the evolution of ore-forming fluid,aiming to refine the genesis of this deposit,and to provide an unambitious example of geologically complex IOCG deposit in the region.Here we outline the major findings below:The host rocks of the Dahongshan IOCG Deposit are the Paleoproterozoic Dahongshan Group,which is a set of volcano-sedimentary units.Sedimentary strata contain sedimentary facies transitioned from fluvial to shoreface environments to tidal carbonate platform facies.The main lithologies include pebbly sandstone-sandstone-siltstone-muddy siltstone to pelite to interbeded carbonaceous argillaceous siltstone and dolomite?IASD?,to sandy dolomite and finally to thick dolostone sequence.The volcanic rocks are bimodal but the exposed rocks are dominantly of mafic.The rocks of Dahongshan Group underwent extensive hydrothermal alteration,which largely erased the rock phases and mineral assemblage.Based on detailed field mapping and alteration facies work,we have reconstructed the protolith of major host units in Dahongshan:the800-meter thick“Hongshan Formation”which was previously considered as consist of Spilite-keratophyre sequence is actually intensely Na-Ca-Fe altered and partly brecciated sedimentary sequences transition from underlying Manganghe formation,and contain only minor volcanic rocks.The main ore bodies can be divided into two types according to different ore textures and mineral assemblages:the stratabound and banded iron-copper orebodies in the garnet-mica schist and the massive iron ore body hosted in the“Hongshan Formation”.Integrating detailed field observations,microscope,microscopic cathodeluminescence?Cold-CL?,and X-ray elemental mapping technologies,we found that different host lithologies have similar alteration facies in Dahongshan and have an evolution path along the T-decreasing trend as follows:Na–?Na?-HT Ca-Fe–HT K-Fe–LT K-Fe–LT Ca-Mg.The key host units for Fe-Cu orebodies,termed as Garnet-mica schist,are recovered as IASD.The proximal alteration for magnetite mineralization is HT Ca-Fe alteration,whereas the proximal alteration for sulfide mineralization is LT K-Fe alteration containing mineral assemblage of Ser-Chl-Ank.In order to trace the fluid sources and evolution in Dahonsghan,we systematically conducted the mineral chemistry of sulfides?Py+Ccp?and tourmaline.Three generations of sulfides have been identified:the PyI pyrite occurs as sparse inclusions within the coarse-grained magnetite in massive iron ore,representing the HT Fe stage;PyII,plus CcpII,represent sulfides precipitated in main Cu-mineralization stage of the low temperature K-Fe phase,and is further classified into two sub-type occurences according to host lithology:sulfide in sandstone or sandy dolomite?II-1?And IASD?II-2?;PyIII+CcpIII are products in later reworking veins crosscutting sedimentary bedding seams.Mineral textures and trace element compositions show that there are systematic differences among different sulfides occerences:Py I yield low?³⁴S values?-2.2‰to5.3‰?,low Se/S ratios,and low Co/Ni ratios,indicating a dominantly magmatic fluid in the early HT Ca-Fe stage.Then Se/S ratios of the fluid system increases as the temperature decreases,and mass sulfides precipitate in the coming K-Fe stages:magmatic fluid component dominates the ore-forming fluid in sandstone and sandy dolomite,evidenced by the conventional?³⁴S range and relatively high Se/S ratios;whereas mixing of magmatic fluid and basinal brine is advocated on the basis of bimodal distribution of?³⁴S values?1.0‰to 5.1‰vs.13.5‰to 15.8‰?.Negative linear correlation between?³⁴S and Se/S ratios is evident in this stage.Abnormally high Co-Ni contents and Co/Ni ratios in all sulfides in Dahongshan also suggest a magmatic-hydrothermal origin of mafic rock affinity.Sulfides in later reworking veins show inherited?³⁴S range and trace element composition from the primary ores.Tourmaline is a useful phase to trace fluid evolution because of its refractory nature and chemical diversity.It occurs widely in the five stages of mineralization and alteration of the Dahongshan deposit,including pre-mineralization sodic alteration?Tur I?,HT Ca-Fe stage with magnetite mineralization?Tur II?,HT K-Fe stage?Tur III?.LT K-Fe stage with sulfide mineralization?Tur IV?,and late quartz-calcite veining?Tur V?.The temporal association between tourmaline and hydrothermal alteration provides an excellent opportunity to investigate the fluid evolution,especially the early stages that do not contain sulfides.Tourmaline from all four stages belongs to the alkali group and ranges from dravite to schorl in composition.The compositions were mainly controlled by the fluid composition,host lithology,and,combined,the fluid/rock ratios.Tur I inclusions in albite phenocryst in intensely metasomatized sandy dolomite has?¹¹B values ranging from-14.7‰to-7.0‰,very similar to Tur II from HT Ca-Fe alteration with?¹¹B values of-12.3‰to-5.7‰.Tur III and Tur IV from K-Fe alterations with sulfide mineralization has much higher?¹¹B values from-10.7‰to-0.5‰and from-10.7‰to-2.2‰,respectively.Tur V from distal quartz veins yields the highest range from+2.9‰to+5.9‰.The variation of B isotopes in Dahongshan is too large to have been induced by fractionation alone,and indicates mixing of fluids from magmatic and marine sources,which is also supported by O-S isotopes.The ore-fluids responsible for albitization and Fe-oxide mineralization in this deposit were dominantly of magmatic origin,whereas fluids of the Cu-sulfide stage show significant mixing.This study highlights that ore fluids of IOCG deposits may be initially derived from a magmatic source,but that non-magmatic fluids or fluid mixing may be vital for triggering economic sulfide mineralization.Numerous isotopic data enable us to propose an integrated geochronologcal framework for Dahongshan deposit.Hydrothermal zircon linked to sulfide mineralization yield a U-Pb age of 1653±18 Ma,indistinguishable with Sm-Nd isochrone age of1654±55 Ma defined by REE minerals within errors.These ages are consistent with the bracketed age interval?1665±13 Ma–1661±7 Ma?between the intrusion and host volcanic rocks.Besides,numerous geochronological attempts have been performed on various accessory minerals,namely,Molybdenite+Pyrite+Chalcopyrite Re-Os,Allanite+Garnet+Rutile+Monazite U-Pb,plus whole rock and Apatite+Monazite+Allanite Sm-Nd,in Dahongshan by various analytical techniques?LA-MC-ICP-MS,ID-TIMS,LA-ICP-MS.etc?.Overall five temporally discrete hydrothermal events have been recognized in time evolved tectonic regimes:?a?1441±58 Ma with magmatic fluid affinity;?2?1026±15 Ma with magmatic fluid affinity;?3?910±23 Ma⁹40±12 Ma associated with cryptic tectonothermal events;?4?872±12 Ma⁸76±2 Ma with magmatic fluid affinity;?5?799±13 Ma⁸30±5 Ma related with regional magmatism and metamorphism.The broadly similar age of host rocks and bimodal magmatism suggest that the¹.7 Ga Fe-Cu mineralization in Kangdian region formed in a rift-related tectonic setting.The obtained younger ages are associated with multiple regional tectonothermal-magmatic events.To further investigate the source of metals and to assess whether there are metal contributions in later fluid activities,we conducted systematic analyses of the Sm-Nd isotopic compositions of whole ore and major REE minerals?e.g.apatite,monazite and allanite?.Samarium-Nd isotopes of REE-bearing minerals,in combination with in situ U-Th-Pb age constraints,can provide unique insights into the REE sources and the evolution history,allowing for time-resolved evaluation of potential REE input in the complex system.Our results show that there is only a small amount of new contribution during the¹.45 Ga activities.REE minerals?allanite+monazite?with different U-Pb dates have REE components eventually deriving from the 1.65 Ga ores,suggestive of an internal redistribution process of REE at the deposit scale and negligible REE input in later hydrothermal activities,so it is better to interpret the younger U-Pb isotopic records as remobilization,rather than independent mineralization events.