Genesis of some calcretes in the southern Yilgarn Craton, Western Australia: Implications for mineral exploration

Calcrete is extensively distributed in the semi-arid and arid regions of the southern Yilgarn Craton of Western Australia. Two general types are recognised, groundwater and pedogenic calcretes; a model for the genesis of the latter is proposed, based on detailed studies in two regions (Mt Gibson and Kalgoorlie) in the southern Yilgarn Craton. At Mt Gibson, calcrete is largely restricted to erosional regimes on greenstones and is absent or present in minor amounts in relict and depositional regimes. In granitic terrain, calcrete is absent in relict and erosional regimes, but it is present in minor amounts in depositional regimes. By contrast, it is present in all the geomorphic regimes in the Kalgoorlie region although it is more abundant in erosional regimes on mafic bedrock. In soils over Ca-poor ultramafic rocks, calcrete abundance is highly variable. Calcium and Mg in calcretes comes from two sources, in situ from weathering and from external sources (such as laterally moving soil water and erosion and deposition of calcrete fragments). The contribution from these sources varies according to region/area and geomorphic regime. In situ weathering is the dominant source in erosional regimes, whereas external sources have a larger input in depositional regimes. The differences in distribution of calcrete in relict and depositional regimes between the Mt Gibson and Kalgoorlie regions may be attributed to different conditions of weathering and landscape development. Modification of pre-existing regolith by carbonates is common. Nodular and pisolitic calcrete may result from replacement of clays and of nodular and pisolitic Fe oxides by calcite and dolomite. Calcite and dolomite are the main carbonates species in the soil profiles, dolomite being more abundant on Mg-rich mafic and ultramafic bedrocks. Dolomite crystals are, in places, coated with fibrous palygorskite. Calcium and Mg are present in both carbonate and non-carbonate forms, thus Ca/Mg ratios do not directly reflect carbonate chemistry. Two distinct carbonate micromorphologies, euhedral-rhombohedral and needle, suggest two modes of formation. Needle and micro-rod calcites probably resulted from replacement of fungi and bacteria. Euhedral-rhombohedral crystals of calcite and dolomite suggest inorganic precipitation from solution.