The Mineralization And Mechanism Of The Iron Skarn Deposits In Laiwu District,Shandong Province

The Laiwu district is located in the eastern part of the North China Craton that formed an important high-grade iron metallogenic district in China.The skarn-type iron deposits in the Laiwu district are mainly hosted in the contact zone between the Mesozoic intrusive rocks and the Middle Ordovician carbonate rocks.The iron ore deposits controlled by the Kuangshan pluton are the biggest iron deposits in this region.The contact zone of Kuangshan pluton contains more than ten iron skarn deposits,including seven large to medium size deposits,which combined have proven reserves of about 500 Mt,accounting for more than 95%of the total iron ore in this district.Previous studies focus only on issues of the geological characteristics,ore-controlling structures,mineralization characteristics and alteration characteristics of ore-forming rocks in this district.However,the petrogenesis of the ore-related igneous rocks,the composition and evolution of iron ore-forming fluids and the high precious ore-forming ages are still needed to be severely constrained.Thus,the metallogenic pattern and model of iron skarn ores remains poorly understood.In addition,the skarn type iron deposits in North China are closely related to the Ordovician gypsum-bearing strata.Many scholars have suggested that the participation of the gypsum rock layer plays a key role in the skarn-type iron mineralization,but the role of evaporates in the formation of iron skarn ores are not clear.In this paper,we present a study of the Mesozoic intrusive rocks and the most typical Zhangjiawa iron skarn deposit in the Laiwu district.Based on detailed field geological surveys,petrological and mineralogical observations,we further use geochemistry of the igneous rock and geochronology of the ore-forming minerals to provide tight constraints on the genesis of ore-related intrusive rocks,geodynamic setting,and the timing,mechanism,and role of evaporate-assimilation in the ore fluids,understanding key control factors on the formation of high grade iron ores.Zircon U-Pb dating indicates that the Kuangshan,Jiaoyu,Jinniushan and Tietonggou intrusions in Laiwu were emplaced at the early Cretaceous?¹30 Ma?,coeval with the peak thinning of the lithosphere in the eastern NCC.These intrusions are characterized by the arc-like patterns of trace element distribution such as enrichment of light rare earth elements?LREE?and large ion lithophile elements?LILE?,depletion of high field strength elements?HFSE?,also with enrichment of Sr-Nd isotope compositions,indicating that they were derived from partial melting of enriched lithospheric mantle,which was between EMI-and EMII-domain in Luxi district.In the Laiwu area,lower crustal materials were variably involved into these intrusions through magma mixing at the base of the lower crust.In addition,the magmatic source of Tietonggou might involve a larger contribution from the upwelling asthenosphere.The ithospheric mantle in Laiwu might be a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust and hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic.The Zhangjiawa iron skarn deposits are mainly located along the contacts between the dioritie and Middle Ordovician carbonates and hosted in fracture zones developed along the unconformity between the Carboniferous Benxi Formation.Field and textural relationships indicate five hydrothermal paragenetic stages,which are represented by albitization?albite,scapolite?,prograde skarn alteration?diopside,forsterite,spinel?,retrograde skarn alteration?phlogopite,magnetite,serpentine,with minor epidote,titanite and apatite?,sulfide and calcite deposition?pyrite,calcite?,respectively.In Zhangjiawa deposit,the hydrothermal titanite and phlogopite,coexisting with magnetite,yield a weighted mean ²⁰⁶Pb/²³⁸U age of 131±4 Ma,and an ⁴⁰Ar/³⁹Ar plateau age of130±1 Ma,respectively,which is in excellent agreement with a zircon U-Pb age?130±1 Ma?of the ore-related Kuangshan diorite.Furthermore,we also applied garnet LA-ICP-MS U-Pb dating to constrain the mineralization age of Yinan Cu-Au-Fe skarn deposit in southeastern Luxi district and Zibo Fe skarn deposit in north Luxi,which yield the weighted mean ²⁰⁶Pb/²³⁸U age of 126±7–127±3 Ma and 128±3 Ma.Results from this study,when combined with existing isotopic age data,suggest that iron skarn mineralization and associated magmatism throughout both the eastern and western belts took place coevally between 135 and 125 Ma,with a peak at ca.130 Ma.As such,those deposits may represent the world's only major Phanerozoic iron skarn concentration hosted in Precambrian cratons.The magmatism and associated iron skarn mineralization coincide temporally with the culmination of lithospheric thinning and destruction of the North China craton,implying a causal link between the two.In this contribution,the author conducted abundant mineralogical and geochemical studies on the sulfides and apatite from causative intrusion,as well as the hydrothermal minerals in the different mineralization stages of the skarn iron deposit?including the scapolite in the albitization-prograde skarn stage,apatite and magnetite in the retrograde stage,and pyrite in the sulfide-calcite stage?.The above results detailedly trace the hydrothermal fluids evolution of the skarn iron mineralization and provide additional information on the timing and way of evaporate involving in the mineralization.The apatite from the causative intrusion?Kuangshan pluton?has significantly high Cl content,up to 7 wt.%,indicating the hydrothermal fluid exsolved from the pluton is high salinity and Cl-rich.The primary sulfides from the pyroxene cumulate and unmineralized intrusions have?³⁴S near 0‰,representing the primary sulfur isotope composition of the magma in the Laiwu district.The scapolite from the albitization-prograde skarn stage has molar Cl/Br ratios in a range of 565–1094,indicating the hydrothermal fluid in this stage is dominant by magmatic fluid.The hydrothermal apatite from the retrograde stage,which is coexisting with magnetite,has higher molar Cl/Br ratios?685–8875?,suggesting the halite-bearing evaporate was involved in the mineralization.This conclusion is further confirmed by the Sr isotopes of the hydrothermal apatite,which yields ⁸⁷Sr/⁸⁶Sr ratios?0.70765–0.70903?evidently higher than those of the causative intrusions?0.70645–0.70792?,but consistent with the Sr isotope compositions of Ordovician evaporate.The magnetite from the deposit is characterized by high Mg content,with MgO contents most above 1 wt.%,indicating mass of Mg-rich wall rocks were involved in the Fe mineralization.The sulfides from the sulfide-calcite stage have?³⁴S values mostly higher than 10‰,showing the sulfate in the Ordovician evaporate provide abundant sulfur for the hydrothermal fluid.In summary,this contribution proposes that the Ordovician evaporate was continuously involved in the Laiwu skarn iron mineralization from the retrograde stage by a means of hydrothermal fluid metasomatism and leaching.The Cl-rich fluid exsolved from the causative pluton facilitates the Fe exsolution and transportation,which is the key factor in Fe mineralization.