Petrogenesis And Mineralization Of Heishan Intrusion In Beishan Area, Gansu, P.R. China

It develops a series of regional mafic-ultramafic rock at the northeastern of Tarim plate,which was highly concerned by geologists in recent years. Recent exploration result showed that the scale of deposit host in Heishan intrusion has reached large.which make the area become the next hot spot to finding the Ni-Cu sulfide deposit.Presently,the comprehensive study of Heishan is relatively weak. This paper select Heishan intrusion as the research object, through systematically research on petrography, crystal chemistry, rare earth elements, trace elements and isotope chronology of the intrusion, scientifically demonstrates its magmatic evolution process, primary magma and mantle source nature. Based on these researches we get some cognition as follows:1 Heishan intrusion is mainly composed by mafic-ultramafic rock.the main rock type is pyroxene peridotite and hornblende gabbro. The major rock-forming minerals are olivine,pyroxene,plagioclase,hornblende,the accessorymineral is chromite.2 The Fo value of olivine is 81～86.87,which belong peridot,no significant change based in the depeth.The composition of discrete olivine is uniform from core to edge.The trapped silicate and sulfur liquid had changed the origin composition of olivine.the species of Cpx and Opx are diopside and bronzite respectively,the plagioclase mainly belong labradorite and feldspar.3 The m/f ration of samples are 2.25-5.04,which belong to a series of ultramafic iron;Mg# is 0.80-0.84,content of TiO2 is less than 1%,REE content is low (12.73-38.48ppm).The REE pattern is lightly right,The fraction between LREE and HREE is slightly.Samples are enriched in LILE(Rb,Sr,U), with significant Nb,Ta,Ti negative anomlies.Geochemical characteristics show that the magma had assimilate a weak crust material.4 The MgO of the primary magma is 11.6%, which belong to high magnesium tholeiitic magma. During the magma evolution,olivine and orthopyroxene are the main fractional crystallition.5 The main ore structure is star-like and disseminated, which was probably formed by the "mush like"lean ore-bearing magma stated at the top of deep magma chamber.