Geochemistry Of Ultramafic Rock And Associated Ni Mineralization In Jinchang,Mojiang County,Yunnan Province

The Jinchang ultramafic intrusion in Mojiang County,Yunnan Province,is tectonically located in the west part of the Ailaoshan fracture zone between the Yangtze Block and the Indian Block.It intruded into the Lower-Middle Silurian stratigraphic units and exposed on the east part of the Mojiang Au-Ni mining district.The Jinchang ultramafic rock mainly consists of peridotite and harzburgite with intensive alterations,including serpentinization,chloritization,talcization,limonitization,asbestosilization,and magnesitation.Nickel mineralization spatially occurs in the contact boundary between the ultramafic rocks and the country rocks.On the basis of systematic study of petrology,petrography,bulk rock geochemistry and isotope geochemistry of the Jinchang ultramafic rocks and Ni-bearing ores,some conclusions with respect to magma evolution and metal enrichment processes have been reached,and are listed as follows:1.The Jinchang mafic-ultramafic intrusion consists of peridotite and harzburgite with strong hydrothermal overprinting,which experienced the crystallization differentiation of orthopyroxene and plagioclase.The MgO/FeO^T ratios of the intrusion vary from 3.23 to 5.55,with m/f ratios between 2.86 and 5.91, indicating that Ni-bearing rocks are mafic-ultramafic rocks.The K₂O+Na₂O contents are low,showing the characteristics of the tholeiite series for the studied samples.2.The trace elements and rare earth elements?REEs?compositions of the Jinchang intrusion are very low compared to that of the primitive mantle.The REEs compositions of the ultramafic rocks are slightly higher than other trace elements, and have relatively elevated large ion lithophile elements but depleted high field strength elements.Fractionation between the light and heavy REE is weak, showing the flat distribution patterns for the chondrite-normalized samples.In addition,Eu/Eu*ratios show positive anomalies.In contrast to the ultramafic rocks,the chondrite-normalized Ni-bearing ores illustrate LREE enriched patterns,indicating that extra sources of metals may have been involved.3.The PGE compositions of the Jinchang ultramafic rocks and the Ni-bearing samples from the Sishibaliangshan orebody are similar,indicating the PGEs have a consistence source,i.e.,the PGEs in the Ni ores were originated from the Jinchang ultramafic rocks.The Jinchang ultramafic rock may be formed by partial melting of the depleted mantle.Sulfur may has been close to saturated in the primitive magma.Mantle-derived sulfur and metals precipitated to form sulfide when saturated condition formed at shallower depth.4.The trace element geochemistry and Sr-Nd isotope characteristics of the Jinchang ultramafic intrusion indicate that the magma may have initially been derived from mid-ocean ridge.The magma undergoes different degree of mixing with shallower mid-upper crust during the process of ascending.In combination with previous tectonic studies,this study infers that the Jinchang ultramafic intrusion may be related to the spreading of the Tethys Ocean that occurred during Late Devonian to the Early Carboniferous.