Pediment landforms continue to be important mineral exploration targets in Nevada, and geochemical sampling of the alluvium that forms on pediments can provide useful information. However, to properly interpret assay results from such sampling, the geologic processes of pediment formation need to be understood. Pediments are gently sloping surfaces that occur between mountain fronts and valley basins. Historically, they were thought to be primarily erosional features, but research has shown that several geologic processes are necessary to form them. First, differential weathering erodes the bedrock and forms undulations and an uneven bedrock surface. Next, alluvium, derived from the bedrock, is deposited and fills these depressions. This forms a gently sloping surface. As this process of erosion and deposition continues, headward erosion of the mountain front contributes sediment to form a thin mantle of alluvium on the pediment, and thicker alluvial plain deposits accumulate further down slope. Utilizing these processes and other geologic factors, pediments can be categorized. This paper proposes to classify pediments as active, transitional, inactive, alluviated, and complex, based on the type of development. As mineralized rock is exposed on a pediment surface a weathered regolith forms, gets eroded, transported, and deposited. Gold and its pathfinder elements, eroded from this mineralization, may be transported and deposited> 300 m down gradient from the bedrock source. In thick pediments these anomalous elements may be detectable in alluvial samples 60 m, or more, vertically, but the results close to bedrock are most useful. Sampling of active and transitional pediments is relatively straightforward because the alluvial veneers are thin and bedrock may be exposed. However, inactive and alluviated pediments have thick accumulations of alluvial deposits so drilling is required, and obtaining quality samples of those sediments can be difficult. In order to correctly interpret the results of samples taken in pediment alluvium, sedimentary characteristics of the samples, such as lithologic composition of rock clasts, sorting, percentages of gravel, sand and clay, should be recorded. Typically drilling methods determine the amount and type of data that can be obtained in alluvium, but as much information should be gathered as is practical. It is also useful to determine the up-and down-gradient directions of the bedrock slope upon which the pediment formed so that the bedrock source of the alluvial geochemical anomalies may be identified. Sampling of pediment alluvium is a relatively small part of the total cost of drilling, and is a tool that can be used to obtain additional geologic data when exploring concealed lode targets. It is hoped that a better understanding of pediments will benefit future exploration in Nevada.
展开▼
