There are 200 basic-ultrabasic bodies of Yuanbao mountain. They are layered and divided into three zones: west, middle and east, which have been folded along with surrounding rocks. They are characterized by the rhythmic units consisting of basic-facies, ultrabasic-facies and mid-basic pyroclastic bed which retaines the pyroclastic textures and the cross bedding. The ultrabasic-facies, which constitutes a principal proportion of basic-ultr- abasic rocks, belongs to the harzburgite-websterite series. There are the cumulative textures and the chilled margin in their border facies. The petrochemical characteristics of the rocks are as follows: Na2O+K2O<0.5%, K2O<0.1%,TiO2<1%, CaO/Al2O3(most)=0.45-1.15. The rocks are roughly similar to the ultramafic komatiites.The middle zone contains a high content (70 ppm) of tin in the central facies of ultrabasic rock and local concentrates occur in the border facies forming the ore bodies. The mid-basic pyroclastic bed has strongly mineralized. The tin ore is as thick as 1-20m belonging to a type of cassiterite-sulfide. The tin deposits had been formed by following geological processes. The tin-bearing magma from the upper mantle is liquation-defferentiated first producing a tinbearing or tin-rich unltrabasic and basic magma. With the magma erupting crystallization-differentiation and hydrothermal activity mobilized the oringiual enrichment, as a result, the tin deposits formed. The ultrabasic rock of the east zone has a high content of platinum (up to 0.04 g/t) concentrated to the border facies to form ore. The platinum ore was formed by the magmatic crystallization and the mobilizing of the hydrothermal solution.