Abstract
The Yuanjiang Ni deposit in southwestern margin of the Yunnan Plateau is the only economically important lateritic Ni deposit in China. It contains 21.2 Mt ore with an average grade of 1.05 wt % Ni and has been recognized as the second largest Ni producer in China following the Jinchuan super-large magmatic Ni–Cu deposit. This Ni deposit is hosted within the lateritic regolith derived from serpentinite within the regional Paleo-Tethyan Ophiolite remnants. Local landscape controls the distribution of the Ni mineralized regolith, and spatially it is characterized by developing on several stepped planation surfaces. Three types of lateritic Ni ores are identified based on Ni-hosting minerals, namely oxide ore, oxide-silicate mixed ore and silicate ore. In the dominant silicate ore, two phyllosilicate minerals (serpentine and talc) are the Ni-host minerals. Their Ni compositions, however, are remarkably different. Serpentine (0.34–1.2 wt % Ni) has a higher Ni concentration than talc (0.18–0.26 wt % Ni), indicating that the serpentine is more significantly enriched in Ni during weathering process compared to talc. This explains why talc veining reduces Ni grade. The geochemical index (S/SAF value = 0.33–0.81, UMIA values = 17–60) indicates that the serpentinite-derived regolith has experienced, at least, weak to moderate lateritization. Based on several lines of paleoclimate evidence, the history of lateritization at Yuanjiang area probably dates to the Oligocene-Miocene boundary and has extended to the present. With a hydrology-controlled lateritization process ongoing, continuous operation of Ni migration from the serpentinite-forming minerals to weathered minerals (goethite and serpentine) gave rise to the development of three types of Ni ore in the regolith. Notably, the formation and preservation of the Yuanjiang lateritic Ni deposit has been strongly impacted by regional multi-staged tectonic uplift during the development of Yunnan Plateau. This active tectonic setting has promoted weathering of serpentinite and supergene Ni enrichment, but is also responsible for its partial erosion.
Highlights
Chemical weathering of ultramafic rocks has produced numerous lateritic profiles and associatedNi deposits, as best illustrated by those in New Caledonia, Cuba, Philippines, Indonesia, Colombia, Australia, and Brazil [1,2]
Bulk density is an important physical parameter of the lateritic Ni ore as it is required for industrial utilization and ore genesis study
Petrographic observations show that the parent rock is mineralogically dominated by serpentine, which is characterized by a fibrous or laminar crystal shape with pale to white color under transmitted-light microscopy (Figure 5a,b)
Summary
Chemical weathering of ultramafic rocks has produced numerous lateritic profiles and associated. 60%–70% of the world’s nickel resource and nearly 60% of the annual global nickel production [3] They are mostly situated in equatorial latitudes characterized by warm and humid tropical climate at present or ancient time. It is associated with obducted ophiolite complexes in accretionary belts or komatiites and layered ultramafic rocks in stable cratonic blocks [4,5]. Previous studies have significantly advanced our understanding in the nature and origin of lateritic Ni deposits: (1) Based on the dominant Ni-hosting minerals, lateritic Ni deposits can be divided into the oxides, hydrous Mg silicates and clay silicates subtypes Such a subdivision is of economic importance in terms of resource evaluation and mineral processing [3,6]. The results are used to provide insights into the formation and evolution of the Yuanjiang deposit, which in turn help build a genetic model in the context of regional geological evolution
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