Nickel laterites, origin and climate

Abstract

Nickel laterites account for ?40 % of global nickel production and contain 60 % of the world's total land based nickel resources. Despite the importance of these deposits published studies, detailing their morphology and formation processes are relatively few and the interaction of variables responsible for the formation of different nickel laterites are poorly understood.

To better understand the process of nickel laterite formation, the Caldag and Bitincke paleodeposits were studied and their geological history established. The Caldag laterite, western Turkey, formed by intense chemical weathering of a serpentinite protolith in a region with a high water table and relatively low topography, resulting in the formation of an oxide deposit. In addition silica precipitation is common in the upper horizons of the deposit, where it creates an indurated layer, protecting the deposit from erosion. The Bitincke Nickel laterite, Albania is composed of two distinct zones characterized by silicate nickel and iron oxide phases. At Bitincke laterite formation and variations in thickness were controlled by the interaction between topography, faulting and protolith fracture density. The morphological and geochemical study of the Caldag and Bitincke paleodeposits indicates that there is a complex interplay between structures, topography, water table height and climate. Therefore nickel laterite deposits developed on very similar protoliths can form deposits with distinct and different characteristics.

By comparing climatic data for regions where suitable ultramafic rocks are exposed and defining the climatic conditions favourable for the formation of nickel laterite deposits, the optimum temperatures and precipitation rates for nickel laterite development can be identified. A compilation of paleoclimatic data from western Turkey and Albania allows for the optimum periods of laterite formation within these regions to be established. Calculation of temperatures of formation from goethite oxygen and hydrogen isotopes could provide additional data on paleoclimaitc conditions. However due to the heterogeneity of laterite deposits and an extended weathering history, data gained from goethite appears not to provide a robust measure of paleotemperature.

The study of the Caldag and Bitincke deposits combined with the analysis of the optimum conditions for nickel laterite formation has shown that there are four main factors which effect laterite formation: 1) Exposure of a suitable protolith; 2) Optimum climatic condition; 3) Geological variables; 4) Environment of preservation. Knowledge of these variables will assist in future laterite studies and will improve predictability of the location of new deposits.

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