Weathering of Ophiolite Remnant and Formation of Ni Laterite in a Strong Uplifted Tectonic Region (Yuanjiang, Southwest China)

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.

Genetic classification, distribution and ore genesis of major PGE, Co and Cr deposits in China: A critical review

“…按照富Ni矿物的不同, 红土型镍钴矿可 进一步区分为富水镁硅酸盐型、黏土型、和氧化物型 3种, 其中, 黏土型及氧化物型的Co品位可达0.3%, 而 富水镁硅酸盐型的Co品位相对较低 [56] . 值得注意的是, 红土型镍钴矿的Co品位还与超镁铁质岩的类型 [57] 、 地下水及水位 [56,58] 、断裂和剪切构造作用 [57] 以及地形 有关 [24] . , 玻 安质岩浆含Cr约1200 ppm [60] , 科马提质和苦橄质岩浆 含Cr可高达1800~3000 ppm [61] .…”

Weathered ultrabasic rocks in the Lapaopao, an implication for development of nickel laterite

“…Crucially, the presence of serpentine minerals within these ultramafic rocks plays a pivotal role in shaping the distinctive characteristics of laterite soils, particularly in the enrichment of Ni metals. The serpentinization process, triggered by rainwater addition (at temperatures around 25°C) or hydrothermal IOP Publishing doi:10.1088/1755-1315/1272/1/012028 2 alteration in seawater (> 350°C) [9], induces profound mineralogical and compositional changes in olivine and reduced pyroxene minerals, thereby altering the elemental composition of Mg, Ni, and Fe [10], [11]. In this research, we initiate an investigation to unravel the intricate origins and enrichment mechanisms of lateritic soils, shedding light on the fascinating interplay between ultramafic rocks, serpentinization, and the formation of these exceptional soil formations.…”