Abstract

Ultramafic-mafic rocks from the Bo Ninh area (three blocks of Bo Ninh, Minh Sang and Coc Phat) were sampled and studied for their petrographic, mineralogical, and major and trace element geochemistry to assess the potential of Cu-Ni-PGE minerals. The petrographic composition of the intrusions ranges from ultramafic (lherzolite and plagio-lherzolite of Coc Phat block) through ultramafic-mafic (plagio-lherzolite and gabbro of Bo Ninh block) to mafic (olivine gabbro and gabbro of Minh Sang block) with the main mineral assemblage being olivine-pyroxene-plagioclase, which varies with each rock type. Sometimes phlogopite is also observed. Olivine is large in size and the forsterite content reaches 68-82%. Pyroxene is not zonated and its composition corresponds to augite, Mg-Ti-augite, and diopsite. Plagioclase is often sericitized and ranges in composition from anortite to andesine. Phlogopite is reddish brown and has high TiO2 and MgO and low Al2O3 compositions. Ultramafic rocks are larger grained and contain more sulfides than mafic rocks. Sulfide minerals include pyrotite, pentlandite, violarite, chalcopyrite, and sphalerite. Other ore oxide minerals such as ilmenite, Cr-spinel, and magnetite are also present. The ultramafic-mafic rocks of the Bo Ninh area have chemical compositions corresponding to the calcareous sub-alkaline to tholeite series, with medium to high MgO (18.7-28.6 %wt.) and low alkali (0.79-1.96 %wt.) and TiO2 (0.49-0.75 %wt.); high Cr (315-2.2024 ppm), low Co (54.4-125 ppm), Ni (216-1.598 ppm) and Cu (44.7-563 ppm) on average. The distribution characteristics of rare earths and trace elements in the ultramafic-mafic rocks of Bo Ninh area are quite similar. They are characterized by the enrichment of light rare earths compared to heavy rare earths and negative anomalies of Ti, Ni, Ta, and Eu. The total content of PGE elements reaches 117-1184 ppb in total rocks and 1.01-3.19 ppm in 100% sulfide phase. Based on the above results, the ultramafic-mafic rocks in the Bo Ninh area are believed to be differentiated crystallization products of magma with basalt composition rich in Ni and Mg formed from the lithospheric mantle with the participation of crustal material and bearing many mineralogy and geochemical indicators for the prospect of Cu-Ni-PGE sulfide ore.