The geochemistry of selected base-metal gossans, southern Africa

A comparative study of the gossans from ten base-metal deposits in southern Africa included the establishment of geochemical criteria for regional evaluation of gossans and ironstones. Factors such as ore composition, nature of gangue, element mobility, depth of weathering, degree of erosion and groundwater chemistry are discussed with respect to gossan geochemistry.Areal geochemical zonation, relative concentration ranges, element correlations for gossan versus sulphide and scattergrams comparing different gossans were all utilized to examine geochemical criteria for gossan classification. Of the elements studied, Ba (as barite) and Pb are the least mobile, Cu and Ag are variously retained in gossan, whereas Zn and Cd are generally dispersed.Gossans derived from such a wide variety of ore types contribute several multivariate populations to the total data set. Principal components analysis was consequently of little value in separating gossan suites from barren ironstones. Stepwise discriminant analysis successfully distinguished base-metal- from pyrite-derived gossans and ferricretes, discriminated among gossans from different ore provinces and classified individual gossans within base-metal provinces. Discriminant functions commonly comprised only 2 to 6 elements. Characteristic multi-element signatures for the various gossans were subsequently applied to the regional evaluation of ironstone in southern African exploration.