SUMARRY

Metallogeny potential of the ultramafic rocks in Phan Thanh area,

Hòa An, Cao Bằng province

Vũ Mạnh Hào, Ngô Xuân Thành, Trần Mỹ Dũng,

Trần Văn Miến, Vũ Quang Lân, Trần Hải Nam, Đặng Hương Giang

The mafic and ultramafic rocks in the Suối Củn and Phan Thanh areas were taken for study on their texture, petrography and geochemistry. The results is used to interprete the crystallization and differentiation process of magma melt, as well as its potential of Cu-Ni-PGE mineralization. Field structural studies suggest that the ulramfic rocks have mainly faulting and/or thrust fault boundaries with other rocks in the region. The rocks, therefore, had been displaced, turned and changed their original formation. Rock-forming minerals are euhedral to subeuhedral olivine, large and subhedral pyroxen, plagioclase, serpentine, biotite and opaque mineral. Geochemically, rocks show high MgO, Ni, Cu and Cr contents, low alkaline, Co and TiO₂ contents. The rocks also show an enrichment in LREE in comparision with HREE ((La/Sm)_N =1.5-4.0). Rb, Th, U elements are enriched and positive anomalies of Pb, while negative anomalies of Eu, Nb, Ti elements. All study samples show the enrichment in PPGEs (Rh, Pd and Pt) and depleted in IPGEs (Os, Ir, Ru). Geochemical data as well as petrography of the rocks suggest that the rocks experienced highly fractionation of olivine, together with pyroxene and plagioclase. The presence of plagioclase (5-10 wt.%) may indicates that the rock members are not belong to the lower/base of the ultramafic intrusion. Cu-Ni sulphide in the rocks may relate to crystallization and differentiation in the early stage of magma cooling. Estimation of mantle partial melting show that the rocks formed by a ca. 3-10% mantle melting, this low melting grade implies Ni metallogeny potetial of magma. Contamination with crustal material in the mantle source had played an important role for S-saturation in magmas. This could lead to form the Ni-Cu sulphide deposits.

Keywords: Cao Bằng, Ultramafic rocks, Cu-Ni-PGE, Suối Củn, Phan Thanh