GEOCHEMICAL CHARACTERISTICS AND METALLOGENESIS OF THE LIMABE MAFIC-ULTRAMAFIC INTRUSION, SICHUAN

Abstract

The Limahe nickel deposit is one of the most important magma sulfide deposits in the Emeishan LIP (Large Igneous Province). The ore-bearing intrusion is a small mafic-ultramafic intrusive body which is fractionated well, consist of plag-bearing ultramafics and gabbros. The ores are rich in sulfide with high grade of nickel and copper but poor in PGE (platinum-group elements). It is a typical PGE-poor sulfide deposit in the Emeishan LIP. This paper will provide a basic study on major elements, trace elements and platinum-group elements for various rocks and ores in the intrusion. The major and trace elements variations of the rocks show tholeiitic evolution trend. The characteristics of the trace elements in ratio shows it is similar with high Ti Emeishan Continental Flood Basalt (ECFB), and different to Low-Ti ECFB. But the calculated concentration of strong incompatible trace elements in primary magma of the intrusion is much lower than High-Ti ECFB. So, the intrusion is not a differentiated body from a magma related with high-Ti or Low-Ti ECFB (not including picrites). The composition of PGE for the ultramafic rocks and sulfide ore are in a style of no Ru depletion, and the ratio of Pd/Ir is lower in about 5, that is quite different to ECFB, but same to the picrites in Emeishan LIP. By estmating, the composition of primary magma is picritic with about 17% in MgO and 48% in SiO2. The primary magma is originated from about 130 km depth in mantle, with degree of about 19% partial melting of the mantle which is analogous to the source mantle of OIB(Ocean Island Basalts). PGE analyses indicates that the sulfide ore are depleted in PGE with contents in ppb to 0.1 × 10-6. The ratio of Cu/Pd is higher over primitive mantle 10 to 100 times. Mantle-normalized Ni-PGE-Cu plots are distinguished in U shape. By model analyzing, it is concluded that the ores are formed by "the second sulfide segregation". The primary magma had sulfide segregation in the first, the removal of sulfide liquid from magma made the magma depleted in PGE more than Cu and Ni. The ore-sulfides that formed subsequently from this PGE depleted magma have very high Cu/Pd ratios.