The Characteristics And Origin Of The Xiaonanshan Sulfide-bearing Gabbro,Inner Mongolia

The northern margin of the North China Craton(NCC)and tectonic evolution of the Central Asian Orogenic Belt have been hot topics since decades ago,and are also the focus of many researches.Copper-nickel sulphide deposits hosted in mafic-ultramafic rocks are distributed at the northern margin of the NCC and adjacent areas,but its spatial and temporal distributions are unclear and their petrogenesis are still in debate.Xiaonanshan copper-nickel sulfide deposit,in the middle of the northern margin of NCC,is part of metallogenic belt in the northern margin of the NCC.This study investigates gabbros of the copper-nickel sulfide deposit in Xiaonanshan,Siziwangqi,Inner Mongolia.We carried out petrographical,mineralogical and geochemical studies on the sulphide-bearing gabbros and discuss their origin and tectonic setting.The main conclusions are summarized below:(1)The sulfide-bearing gabbros in Xiaonanshan copper-nickel sulfide deposit occur as veins intruding along the intersection plane of north-east and north-west trending faults.They are composed of gabbro and altered gabbro.Major rock-forming minerals are pyroxene,plagioclase and hornblende.Pyroxenes are dominated by augite,with minor enstatite diopside.Feldspars include labradorite feldspar and culture,with minor albite.Amphibole belongs to tremolite-mafic amphibole series.It is thus considered that the Xiaonanshan intrusion is small and has undergone a certain degree of mineralization and alteration,indicating its mineralization potential.(2)Based on the petrographical observations and geochemical characteristics,Xiaonanshan intrusion is composed of gabbro instead of diorite as previously proposed.The sulfide-bearing gabbro generally has high Mg,Fe,low Na2O+K2O contents,LREE and LILE enrichment,HREE depletion,and positive Eu anomaly.These suggest that the parental magma has experienced fractional crystallization during diagenesis.(3)According to primitive mantle normalized trace element diagrams,it is believed that the ore-bearing rock has geochemical characteristics of oceanic island basalts.Combined with regional geological environment analysis,we propose that its tectonic setting is similar to inland basalt.(4)Trace elements and REE diagrams show that Xiaonanshan and Tunanbao intrusions have the same origin.PGE distribution patterns,Ni/Cu ratios and total PGE contents indicate that the Xiaonanshan deposit intruded after the Tunanbao deposit.Xiaonanshan orebody has differnent branches with different sizes.We thus speculate that Xiaonanshan and Tunaobao intrusions formed from the same magma conduit.According to Ni/Cu ratio and Pt and Pd contents,we propose that the magma flows from Xiaonanshan to Tunaobao.Thus,Tunaobao is among the first to crystallize.The adjacent and deep areas of the Xiaonanshan deposit still have great mining potential.