Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 127-141).

Introduction: The object of mineral exploration is to locate economic deposits with minimum expenditure in time and money . In order to achieve this goal it is essential to select the most favourable environment and to design the search procedures for the environment in question. Duricrusts are a striking feature of tropical and subtropical landscapes where they occur as extensive, resistant horizons which form a surface or near-surface caprock and are best seen in incised landscapes. Since they cover many potentially rich mineral , provinces of the world, it is important to question what effect they have on mineral exploration. Duricrusts are defined as specific soil types formed by near-surface processes associated with weathering . In spite of the sheer aerial extent of soils over the landsurfaces of the earth, Levinson (1974) expressed surprise at how little exploration geologists know about this surficial environment . Weathering and pedogenetic processes therefore, require discussion in this text. However, in order to adhere to the scope of this dissertation only a brief description of the factors and characteristics of soil formation are given in the first chapter. In the second part, duricrusts are differentiated on the basis of composition into laterite, silcrete and calcrete. Beginning with a concise description of both natural and experimental investigations concerned with mineral decomposition and neomineralisation during weathering, the morphological, physical and chemical characters of each type are described. Environmental factors are then discussed in an attempt to identify specific duricrust types in terms of climate, geographic location, age and formation process . Once duricrust structure and texture are related to topography, specific types may be used to assess the significance of suspected markers in the landscape and permit identification of individual weathering surfaces on which mineralisation may give rise to gossans of widely different physical and chemical characteristics. The interpretation of photogeological, geochemical and geophysical data acquired during mineral exploration in duricrust terrains is problematical and some of the limitations to these search procedures are discussed in the final .chapter

Silica cemented pedogenic horizons known as duripans occur on relict landforms in climates having limited leaching potential. Under the influence of tephra, incipient silica cementation may also occur in late Quaternary soils. The source of silica for cementation in Holocene soils is rapidly weatherable volcanic glass. In response to wetting and drying cycles volcanic glass weathers to form sand-size composite particles composed of silt, clay and redeposited silica. A portion of the hydrolyzed silica is eluviated as monosilicic acid which accumulates as the wetting front evaporates. Illuvial silica augments existing composite particles eventually forming a continuously cemented duripan. In actively forming late Quaternary soils, the cementation process is a complex interaction between illuvial silica, clay and calcium carbonate and the soil matrix. Surface reactions between monosilicic acid and illuvial clay or soil matrix particles provide nucleation sites for polymerization of silica concentrated by evaporation. The resulting opaline silica bonds adjacent soil grains without necessarily plugging intervening pore spaces. In contrast, calcium carbonate preferentially precipitates in large pores and interped voids. Cementation occurs by the plugging of progressively smaller pores with relatively pure calcite rather than by heterogeneous bonding of mineral grains. In illuvial zones containing both silica and calcium carbonate, cementation may occur rapidly because the former holds small soil particles in place while the latter plugs large pores. The mineralogy of silica cement is determined by identification of varying amounts of crystal order using X-ray diffraction. Opal-A is recently polymerized, noncrystalline, highly hydrated silica gel. The more prevalent, partly crystalline opal-CT forms where surface reaction with clays create crystal orientation, where silica gel dehydrates or when silica precipitates from soil solutions having high concentrations.

This work takes a look at some of the literature on calcretes and especially the problem of geochemical exploration in calcrete terrains. The conclusion that will be reached is that exploration in calcrete terrains is not futile and that provided the explorationist is aware of the types of calcrete and their genetic implications calcrete can be used as a sampling medium and anomalies can be detected through calcrete (p.1.) / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in

Thesis(M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references.