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Mei Hou-chun  
The primary concern of this paper is to analyse the systematic differences in chemical composition between two suites of trap intrusives that bear a close genetic relationship to titaniferous magnetite and nickel sulfide mineralizations respectively. A discussion is also given to the possible mechanisms of differentiation for such differences.The "alkali-calcium index" ( Calk/m number ofatoms % ) and acidity number or atoms % ) were calculated and plotted for various facies belts of the trap intrusives.It is apparent from these diagrams that alkali-calcium indexes of the suites with strong titaniferous magnetite mineralization are systematically higher than that of the nickelsulfide-rich ones. The former is therefore referred to as "alkali-calcium trap" and the latter "partial-magnesium trap". The experimental results of D. H. Green and A. E. Ri.ngwood (1964) for samples of olivine thoteiite composition agree in many respects with the trend of compositional variation among the two trap groups in Southwestern China. In conjunction with the intimate tectonic relationship bet-ween the two suites, it can be inferred that they represent products of the upper and lower daughter magmas derived from a single parent trap magma by fractional crystallization at 12.5-20 kbar and 1200-1300℃.As is evidenced by their compositional features, the Permian Omeishan basalt floods are probably the eruptive facies of an alkali-calcium daughter magma, but, owing to the peculiarity of differentiation mechanism in the magmatic sources, they exhibit neither rockforming mineral assemblage characteristic of typical olivine tho-leiite nor can be regarded as alkali-olivine basalt.A review of available data on various types of magmatic rock formations through-out the world indicates that the chemistry of basic and ultrabasie intrusives related to titaniferous magnetite mineralization is approximately consistent with the charac-teristics of alkali-calcium trap series, i.e., being rich in calcium and aluminum, slightly rich in alkalis and relatively poor in magnesium. On the other hand, the general petrochemical character of intrusives related to large nickel sulfide deposits is the low contents of alkalis and titanium. It has also been noted that, among these intrusives, a chemical composition comparable to that of "partial-magnesium trap" and a TiO2 content of less than 1.6% may he much more favorable for nickel sulfide deposition. Furthermore, as an additional requirement, the ultrabasic facies belts at the lower parts of these intrusives should be ferro-ultrabasic rocks produced by differentiation from tholeiite magma. In many cases, however, little evidence of appreciable nickel sulfide mineralization has been observed in magnesium-ultrabasic rocks.
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