Geochemical Characteristics And Ore-forming Potential Of The Hongliugou Copper-nickel Mine, Gansu Beishan

The geological structure of Beishan area is complex,which experienced many tectonicactivities and magmatism.The metallogenic conditions in Beishan area is well,where is theimportant metallogenic belt of copper-nickel sulphide deposit in the northwest ofChina.Hongliugou pluton is a newly discovered mineralized basic-ultrabasic complex atBeishan area in recent years,which is located in the west of Heishan copper-nickel depositabout40km.Metallogenic conditions of this pluton is still unclear.Through the studies ofgeochemical characteristics,mineralogy and petrology characteristics,properties of magmasource region, geochronology, diagenesis forming background and the metallogenic potentialof Hongliugou basic-ultrabasic complex pluton,we achieved the main research results asfollows:(1)Geochemical characteristics indicates that, Hongliugou pluton is iron-riched(m/faverage of3.42) ultrabasic rock, magma has calc-alkaline series evolution trend, with highernickel content in olivine, Fo is72.36~82.92, belonging to chrysolite. Olivine componentswere susceptibled by silicate and sulfide which were in fissure. Orthopyroxene is bronzite;clinopyroxene isdiopside;plagioclase is labradorite and bytownite.(2)Through the observation of microscope,the mineral crystallization sequence ofHongliugou pluton: Olivineâ†'orthopyroxeneâ†'clinopyroxeneâ†'plagioclaseâ†'hornblende,thecrystallization order of main metal sulfide: chalcopyriteâ†'pyrrhotite/pentlanditeâ†'magnetite.(3) Hongliugou pluton enrich large ion lithophile elements (Rb,Ba,Sr,U,K),butTh,P,Ti,and high field strength elements Nb,Zr,Hf arerelative poor.Nb/La value(between0.03and0.84) shows continent or island arc magmatism features related with the subduction zone.LREE relative to HREE is slightly richer, which consistent with the basaltic REE distributioncurve of intraplate extensional environment. After comprehensively analysised, the magmasource is mainly the rear rift environment of active continental edges impacted by subductionzone.(4)According to the formula:WMgO=0.56KdFo/(1-Fo)WFeO, MgO and FeO of the parentalmagma is7.11%and8.75%, respectively. And the Mg#is between0.72and0.95,average is 0.8.The Mg#is significantly higher than that of original magma balanced with the mantleperidotite, which further demonstrates that the Hongliugou pluton is low evolution.(5)The diagenesis and mineralization age of Hongliugou pluton is396.7±3.8Ma bymeasuring single zircon SHRIMP U-Pb ages in gabbro,consistent with the age of plutons inHeishan（377.0±5.6Ma～367.4±5.4Ma）and Guaishishan（353～405Ma）, belonging toDevonian.It reveals that there is an important copper-nickel sulphide metallogenic period inBeishan area.(6)The average crystallization temperature of olivine is similar with clinopyroxene andthe main minerals of magma crystallization temperature is1074.7~1114.5℃.(7)The metallogenic geological background,geochronology and geologicalcharacteristics of Hongliugou pluton is similar with Heishan plutons, which has the formationpotential of magmatic nickel-copper sulphide deposits.