Descriptive Models

Mineral deposit models play a central role in an information system that will help the policy makers to make their decisions. Ideally, the different kinds of deposit models would provide the necessary and sufficient information to discriminate (1) possible mineralized environments from barren environments, (2) types of known deposits from each other, and (3) mineral deposits from mineral occurrences. Probably the most important part of creating mineral deposit models is the planning stage in which consideration of the purpose and possible uses of the models should determine the character of the models. The way to describe a model is first by thinking about what it is for, about its function, not the list of items that make up its structure (Churchman, 1968). Although there are many fine compendiums of mineral deposit models (Australian Geological Survey Organisation, 1998; Eckstrand, Sinclair, and Thorpe, 1995; Kirkham et al., 1993; Lefebure and Hoy, 1996; Lefebure and Ray, 1995; Roberts and Sheahan, 1988; Rongfu, 1995; Sheahan and Cherry, 1993), the focus in this book is on deposit models applied to quantitative resource assessment. The focus of this chapter is the descriptive aspects of the deposits because the goal is to provide a basis for interpreting geologic observations rather than to provide interpretations in search of examples (Cox, Barton, and Singer, 1986). Thus, the discussion herein is limited to mineral deposit models specifically designed for quantitative assessments such as those in Cox and Singer (1986), Bliss (1992a), Orris and Bliss (1991, 1992), and Rogers et al. (1995). Mineral deposits modeled for three-part assessments are defined as mineral occurrences of sufficient size and grade that they might, under favorable circumstances, be economic. Although history suggests that we can expect discoveries of as-yet-unrecognized deposit types, the three-part assessments discussed here do not include resources from these deposits simply because they cannot be modeled. Most published quantitative mineral resource assessments that have used models have relied upon descriptive and grade-and-tonnage models (chapter 6), which are also the foundations of other kinds of models such as deposit-density models (chapter 4) and economic cost models (chapter 5).