Cause Features and Prospecting Significance of Ultrabasic Rocks in Cebojiyi Area,Xainza County,Tibet

Cebojiyi area,belonging to southern middle Bangong Co-Nujiang River metallogenic belt,is located at the northeast of Xainza county about 40 km.There are produced with nickel sulfide minerals in the ultrabasic rocks of this deposit,also with chromium and cobalt mineralization been found.It plays an important role in resource potential evaluation and mineral exploration for Cebojiyi,even the whole Bangong Co-Nujiang River metallogenic belt that researching the geochemical characteristics and origin of the ultrabasic rocks in this area.The rocks containing nickel are mainly ultrabasic rocks such as harzburgite,most of which were strongly serpentinized.Nickel minerals are mainly alloy minerals such as inconel alloy,iron-nickel alloy,sulfide minerals such as millerite,iron millerite and heazlewoodite,silicate minerals such as nickel-containing olivine,nickel-containing serpentine and so on.The Mg#values of Cebojiyi harzburgite are in the range of 87.03~88.25,with an average of 87.82,close to the Mg#(89~91)of metaperidotite in ophiolite.The m/f values are in the range of 6.71~7.51,with an average of7.22,suggesting the harzburgite are magnesia.And the Cebojiyi harzburgite are alkaline series.In the rocks,large ion lithophile elements such as U,Th,Pb and high field strength elements such as Ta,P are enriched,while the elements such as Ba,K,Sr and Nb,Ti are poor,which roughly comply with the characteristics of island arc magmas.The total REE of the rocks are very low,and the∑REE values are in the range of 0.39×10-6~1.37×10-6,with an average of 0.94×10-6.It is obvious of light and heavy REE fractionation,with LREE/HREE ranging from 5.07 to 6.04and(La/Yb)Nfrom 2.98 to 6.08.The rocks have a weak positive Eu anomaly(δEu=1.15~1.54,with an average of 1.42)and no obvious Ce anomaly(δCe=0.93~1.15,with an average of 1.04).The 206Pb/204 Pb values are 18.428~18.554(average is 18.476),207Pb/204 Pb are 15.498~15.614(average is 15.575),and 208Pb/204 Pb are 37.712~38.730(average is 38.386).While the 87 Rb/86 Sr values are 0.8686~3.8985(average is 2.1145),87 Sr/86 Sr are0.709148~0.711524(average is 0.710257),147 Sm/144 Nd are 0.1090~0.1237(average is 0.1178),and143 Nd/144 Nd are 0.512028 ~ 0.512112(average is 0.512075).The results show that the tectonic environment of Cebojiyi harzburgite are island arc environment,and the magma are from the subduction zone mixed with upper crust and mantle,simultaneously,the magma source are spinel amphibole lherzolite enriched mantle source region.The average nickel content of harzburgite in Cebojiyi deposit is 2017.3 ×10-6,while it is 2429.2 × 10-6 in Cebojiyi serpentinized harzburgite.If the nickel mineral were sulfide minerals,then the content have been reached the cut-off grade both in Cebojiyi harzburgite and serpentinized harzburgite.At the same time,the nickel content of iron ore in Cebojiyi deposit has been reached the cut-off grade,with an average of 6323.1×10-6.What's more,the cobalt content of iron ore in Cebojiyi deposit has been reached the cut-off grade,with an average of 632.6×10-6.And the chromium content of iron ore in Darucuo deposit has been reached the cut-off grade,with an average of 52679.4×10-6.So attention should be paid that the nickel,cobalt and chromium prospecting work in the ultrabasic rocks associated with ophiolite,especially the iron ore in ultrabasic rocks in Bangong Co-Nujiang River metallogenic belt.