Abstract:
The Jinchuan ultramafic body on the southwestern margin of the North China craton hosts a giant Ni-Cu-(PGE) deposit. This dike-like intrusion consists mainly of lherzolite, dunite, and minor pyroxenite. Fortly-seven vol% of the intrusion is composed of disseminated sulfide ore and minor massive and net-textured ores. Ore bodies occur commonly in the lower part of the intrusion; the largest contains about 50% of the total metallic resources in Jinchuan, and has a "flame-like" shape in vertical cross-section. The ultramafic rocks have high MgO (19-45 wt%), low CaO (<6.5 wt%) and Al2O3 (<7 wt%), and right-inclined chondrite-normalized REE patterns. These rocks contain variable Ir (0.4-17 ppb), Ru (0.6-21 ppb), Rh (0.05-8.4 ppb), Pt (0.6-196 ppb), and Pd (1.2-135 ppb), in general lower than those in the sulfide ores (2.9-110 ppb Ir, 3.3-260 ppb Ru, 1.5-237 ppb Rh, 6.9-3972 ppb Pt, and 15-532 ppb Pd). They have high Th/Nb (0.15-0.5) ratios and show primitive mantle-normalized trace element patterns with Nb-Ta negative anomalies, consistent with derivation from mantle magmas variably contaminated by crustal materials. Compositional heterogeneity of the disseminated sulfides cannot be accounted for by in-situ fractional crystallization, but is consistent with differentiation and sulfide segregation in a staging magma chamber at depth. We propose a compressive tectonic model in which injection of sulfide-poor, crystal-rich magmas from the upper part of the staging magma chamber was followed by injection of a sulfide-rich crystal-mush from the lower part of the staging magma chamber to form the Jinchuan body. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.
