Mineralization Type And The Genesis Of Ore-Hosting Breccias In The World-Class Angouran Deposit, NW Iran

The world-class Angouran deposit is the second-largest Pb-Zn deposit which contains more than 7Mt metal resources.The orebodies,including sulfides and non-sulfides(mainly smithsonite)are hosted in the discordant breccia pipe within Neoproterozoic-Cambrian schist and marble units.However,it remains unclear whether the genesis is associated with coeval magmatic activity or not.Meanwhile,the basic features of the ore-hosting breccias and brecciation mechanism have been not well documented in previous research.In this study,we present a new detailed geological investigation by comprehensive petrology observation,integrated with the geochemical analysis data,to reveals the brecciation mechanism,mineralization process and genesis on this deposit.In the Angouran deposit,the marble and schist were thrusted over the Miocene volcanic rocks and terrestrial evaporitic sequence deposited in the Cenozoic foreland basin.Three different kinds of breccias have been distinguished by detail geological mapping: the sulfide-cement breccia(B1),the carbonate-cemented breccia(B2)and mud-cemented breccia(B3).The early sulfide-cemented breccias are restricted in the bottom in the breccia pipes.The carbonate-cemented breccias,dominated by smithsonite with minor calcite,resulted from the “oxidation” and overprinting of the preexisting sulfide breccias;and partly cemented the karst breccias within the marble wallrock.Mud-cemented breccia occurs along the contact zone between the marble and schist.In some cases,the carbonates replace the mud locally in the breccia matrix.Three mineralization events can be identified: pre-ore stage,sulfide stage,and non-sulfide stage.Mineral paragenetic association in the pre-ore stage are composed of the anhydrite and quartz,with minor marcasite.In the sulfide stage,sphalerite shows genetic association with galena.Two generations of sphalerite have been distinguished: early-stage(S1)red-brown sphalerite and late-stage(S2)honey-yellow sphalerite.Minerals in the non-sulfide stage are dominated by smithsonite with minor sulfide and oxides.There are two different kinds of smithsonite can be identified.Type I smithsonite are present as tabular morphology after anhydrite or colloform and commonly paragenetic with galena,arsenopyrite;sphalerite and greenockite are present occasionally.Type II smithsonite is paragenetic with hemimorphite,newly arsenate minerals,and Fe-Mn oxides,which are commonly present in an oxidation environment.The two former minerals association are present in sulfide-cemented breccia and the late minerals association are present through three kind breccias as we mentioned above.Primary sulfide-cemented breccias are restricted within steeply dipping,discordant breccia pipe.The breccia fragments are predominated by the irregular angular schist and marble,with minor but ubiquitous exotic greyish-green Miocene tuffaceous volcanic or volcaniclastic rocks.All breccia fragments are irregular angular,no boundary erosion.In some cases,breccias and matrix show flowage texture and the majority of primary breccias are matrix-supported chaotic breccia and show the float-stone structure and no visible alternation.All these features are consistent with salt dome-related breccia rather than dissolution-collapse breccia,cryptoexplosive breccia or hydraulic breccia.Meanwhile,the ubiquitous presence of anhydrite pseudocrystals and anhydrite inclusions,the double-terminated quartz crystals and their tabular or spherical carbonate mineral inclusions in the sulfide matrix provide the compelling evidence on evaporite-bearing matrix before sulfide precipitation.Apart from the exclusive morphology,trace elements of these euhedral quartz crystals are characterized by high Li,Na,K,Ca,Mg and elevated B concentration,which is consistent with quartz from the evaporitic environment.Integrated with the widespread occurrence of the salt diapirs in the foreland basin,we suggest the breccia in Angouran deposit can be classified as salt dome-related breccia.We speculate these salt minerals originated from the regional Upper Red Formation(URF)and underwent dissolution or replacement before/during the sulfide precipitation.Ore-forming fluid is characterized by low temperature and high salinity basinal brines suggested by the previous study.Trace element composition of sphalerite,the sulfur isotopic composition of sulfate and sulfides and lead isotopic composition of sulfide are present to constrain the mineralization type and process.Trace elements of two stages of sphalerite from the sulfide stage are characterized by high Ge(< 464.7ppm),Cd(< 8522ppm),low In(<20ppm),Mn(<100ppm)concentrations and low Zn/Cd ratios(<200),which is constant with sphalerite from MVT deposits and shows no obvious genetic relationship with coeval magmatic-hydrothermal activities.Geothermometer studies suggest that sphalerite in the Angouran deposit precipitated from a low-temperature(<200?)hydrothermal system.Sulfur isotopic composition of sphalerite are +5.1‰~+10.9‰,which is atypical magma-derived sulfur isotopic compositions and suggest the reduced sulfur may be mainly derived from the thermochemical reduction of sulfate.Lead isotopic compositions of sulfide are similar with these of regional metamorphic basement rocks,suggesting the metal may be derived from the regional basement rocks and have no genetic link with the coeval magmatic-hydrothermal activities.Based on our study,we suggest that the world-class Angouran deposit is a typical salt diapir-related deposit.This new observation suggests that evaporite diapir-related ores are more important than previously recognized and salt diapir-related deposits should be given more attention in future exploration.