CRC LEME OPEN FILE REPORT 187
MINERALOGY AND GEOCHEMISTRY OF CALCRETES OVERLYING SOME KIMBERLITES IN INDIA AND SOUTH AFRICA
B Singh and M Cornelius
The Astro Yilgarn Regolith Project - Diamond exploration in regolith-dominated terrain, Yilgarn Craton , Western Australia - was sponsored solely by Astro Mining N.L- and was completed in January 2000. The principal objective of the project was to develop concepts, methods and technologies for locating diamond-bearing pipes (and associated regolith deposits) in the Yilgarn Craton, with the approach emphasizing regolith-landscape evolution. The specific objectives were (i) to establish a regional overview and framework of the geomorphic history and landform evolution of the Yilgarn Craton relevant to the project objectives, (ii) to establish district-scale frameworks of regolith relationship (mapping, stratigraphy and characteristics) for key tenements of Astro Mining, (iii) to establish through orientation, modeling and deduction, geochemical and mineralogical exploration methods optimized to the chosen regolith-landform regimes, and (iv) to translate research findings into practical exploration techniques to be applied by the Astro Mining exploration team.
During the project, Astro Mining collected calcretes overlying a number of kimberlites in India and South Africa . These were made available to CRC LEME for a detailed study. The principal objectives of the study were to characterise these calcretes, understand mechanism of their formation, and identify any characteristics that can be attributed to the parent kimberlitic material. This report summarises characteristics of the kimberlitic calcretes and discusses relevance of calcretes in general for kimberlite exploration.
1.1 Relevance of calcrete to diamond exploration
Calcretes occur extensively in the arid parts of the Yilgarn craton of Western Australia and in other parts of the world with similar climatic conditions (Milnes and Hutton, 1983). Two general types of calcretes have been recognised in the Yilgarn: groundwater and pedogenic calcretes (Mann and Horwitz, 1979; Anand et al., 1997). The groundwater calcretes are massive, tabular carbonate bodies that form in arid areas with periodic recharge of the groundwater system (Mann and Horwitz, 1979). Consequently, they are generally associated with present drainages and paleodrainages, and are common in the Yilgarn, particularly about 30° north of the Menzies line (Butt et al., 1977). Geomorphic setting and climatic conditions, rather than bedrock geology, appear to play a dominant role in the development of groundwater calcretes.
In contrast, pedogenic calcretes are carbonate accumulations that form in soil profiles as nodules, cobbles, pods and sheets. They occur in a variety of geomorphic settings, including relict, erosional and depositional areas on various bedrock types. However, they are generally more abundant in erosional regimes on mafic and ultramafic rocks (Anand et al., 1997), indicating that pedogenic calcretes receive a significant proportion of cations, and possibly anions, from weathering of underlying bedrock. It is, therefore, likely that pedogenic calcretes, developed from a weathered alkaline ultramafic, may have unique mineralogical and/or geochemical characteristics inherited from the parent body.
Calcretes also have the capacity to indurate and trap components of the soil matrix, fresh rock fragments and/or discrete minerals, and protect them from weathering. Bulk geochemistry and petrographic characteristics of inclusions can readily indicate the bedrock type. Thus, calcretes may serve as a potential sampling medium for kimberlite exploration.
1.2 Objectives of the study
The objectives of this study were to (i) investigate the geochemistry, mineralogy and fabric of calcretes developed on weathered kimberlites in India and South Africa , (ii) understand the mechanism of their formation and (iii) identify any characteristics that can be attributed to the kimberlitic nature of their parent material.