John Carranza opined that the old adage ‘mineral exploration is like finding a needle in a haystack’ recaps the difficulty of finding economic mineral deposits near or beneath the surface. In general, two main approaches can be used to detect the presence of mineral deposits near or beneath the ground surface. One employs a mineral deposit model and the other aims at the identification of a primary halo about a mineral deposit. A mineral deposit model provides systematic empirical (descriptive) and/or theoretical (genetic) descriptions of the essential geological, geochemical and geophysical features of a type of mineral deposits (cf. Cox and Singer, 1986; Roberts et al., 1988). The primary halo of a mineral deposit, as defined originally by Safronov (1936), is ‘an area including rock, surrounding mineral deposit (ore bodies) and enriched elements that make up that deposit’. Thus, descriptions of primary haloes may form part of a mineral deposit model, but both deposit model and primary halo approaches to mineral exploration are based on studies of primary geochemical characteristics of mineral deposits because chemical processes during mineralization are the ones that ultimately bring about metal precipitation or mineral formation.
However, a deposit model approach to mineral exploration is apt for identifying ‘which haystacks contain a needle’ whereas a primary halo approach to mineral exploration is proper for deducing ‘where in a needle-bearing haystack is the needle located’. Because mineral deposit models provide only empirical and/or theoretical guidelines but not diagnostic criteria for exploration, the identification of ‘which haystacks contain a needle’ requires lithogeochemical studies using major/trace element data to distinguish between mineralized and barren rocks of certain types (e.g., Makkonen et al., 2008; Zhao et al., 2011). However, deposit model descriptions of regional geochemical features of a mineral deposit type have been used traditionally as reference ‘expert knowledge’ for preparing geochemical evidence maps, which are integrated with other thematic evidence maps in a geographic information system (GIS) for creating regional-scale mineral prospectivity maps.
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