Abstract:

The Jinchuan intrusion in northwestern China hosts the third-largest Ni-Cu-(PGE) deposit in the world. It is distinguished from other deposits of this type by a very large amount of mineralization (at least 500 Mt at ~1.2 wt% Ni) compared to the relatively small volume of the intrusion (6000 x 300m) and the predominance of net-textured ore over other textures.<br/>Most of the intrusion is composed of ultramafic rocks whose compositions include harzburgites, lherzolites, and plagioclase lherzolites. Rare wehrlites and gabbros along the margins of the intrusion represent the most evolved rock types and have been previously considered to be less olivine-rich equivalents of the lherzolite and to be representative of the parental magma composition. The intrusion contains three main Ore Bodies numbered 2, 1, and 24 from SE to NW. The Ni-Cu-(PGE) mineralization occurs as net-textured and disseminated sulfides, but the textures vary between the different ore bodies. The central part of Jinchuan (Ore Body 1) hosts -60% of the known mineralization and is described as concentrically zoned, with net-textured ore surrounded by patchy disseminated mineralization, barren peridotite, and marginal wehrlites with significantly lower olivine content. However, other features, such as olivine grain size, interstitial mineralogy and Ni/Cu ratio in the ore zone, appear to be asymmetric, consistent with previous interpretations that Jinchuan was emplaced as a sub-horizontal sill. The southeastern and northwestern parts are more asymmetrically zoned, with patchy net-textured and localized massive ore overlain by disseminated mineralization and barren lherzolite. The ores in Jinchuan generally contain moderate amounts of sulfide (rarely exceeding 30%) but are rich in Ni and Cu and depleted in PGE relative to these elements. Jinchuan rock types are very Mg and Fe-rich (reflecting their high olivine and orthopyroxene contents). They are enriched in LREE and their isotope and trace element geochemistry reflects contamination with lower crustal granitoids.<br/>The Jinchuan intrusion had been interpreted to have crystallized from a high Mg-basalt with 11.5% MgO, but such a composition is inconsistent with the essentially ultramafic composition of the intrusion, and the high Ni and Cu and only modest PGE contents of the ores. In previous studies it was proposed that Jinchuan formed by the injection of an olivine- and sulfide-rich mush that had accumulated from a PGE depleted magma in a much larger, deeper crustal chamber.<br/>In this study, we show that the marginal wehrlites have contrasting major, trace element, and isotope compositions and are not simply related to other rock types by differing degrees of olivine accumulation and fractional crystallization. As a consequence, the previous calculations had underestimated the Mg and Fe content of the parental magma. We applied a new method to calculate the composition of the Jinchuan parental magma taking into account the degree of olivine accumulation, the amount of in situ olivine crystallization, and the mineralogical variations between each rock types. We propose that the Jinchuan parental magma had a ferropicritic composition similar to the magma associated with the Pechanga deposits. A more Mg- and Fe-rich magma better explains the mineralogy of the rock types and the composition of the ores. It also implies that the amount of olivine and sulfide transported by the magma was much less than previously thought (<40% rather than 50-70%) and that the amount of in situ crystallization and accumulation was much greater.<br/>The compositions of the sulfides are consistent with in situ deposition of sulfide liquid from multiple pulses of magma into a magma conduit. The great abundance of net-textured mineralization in Ore Body 1 is interpreted to result from strong wetting of olivine by sulfide caused by highyC^ (up to AQFM +1.5) in the magma that formed the central part of the intrusion. Finally, the variation in interstitial mineralogy (non-olivine portion) from lherzolite above the mineralization to harzburgite below indicates that the concentric portion of Ore Body 1 formed at a late stage when the intrusion had already started to solidify and does not necessarily result from early injection of an olivine-sulfide mush.<br/>The Jinchuan intrusion, which has often been difficult to classify because of its small size of the intrusion, the range of ultramafic rock types, and high Ni-Cu and low PGE tenors of the mineralization, is best placed in the class of ore deposits associated with ferropicritic magmas (e.g., Pechanga, Kabanga) with which it shares similar lithologies and ore compositions.