Kimberlite Terminology and Classification

Abstract

Description, classification and interpretation of kimberlites and related rocks, and communi-cation of that information, underpin the development of three-dimensional geological models used in generating reliable diamond resource estimates. A rationalisation of kimberlite terminology and classification is presented in a practical, systematic framework or scheme. The scheme has five stages and is based on progressively increasing levels of interpretation building upon a series of descriptors that are applied independently of, and prior to, genetic classifications. Stage 1 of the scheme is rock description (alteration, structure, texture, components) and involves only limited genetic interpretation. The components are ascribed to three classes: compound clasts (kimberlitic, mantle, crustal), crystals, in particular olivine, and interstitial matrix (groundmass, interclast cement or clastic matrix). Kimberlitic compound clasts include magmaclasts (e.g. solidified melt-bearing pyroclasts), lithic clasts (e.g. autoliths) and accretion-ary clasts. Where possible, subsequent stages involve classification and higher levels of interpretation, based on increasing degrees of genetic inference. Stage 2 is the petrogenetic classification into parental magma type and mineralogical type. Stage 3a is the broad textural-genetic classification into coherent kimberlite and volcaniclastic kimberlite. In Stage 3b, coherent kimberlite is further subdivided into intrusive kimberlite or extrusive kimberlite, and volcaniclastic kimberlite into pyroclastic kimberlite, resedimented volcaniclastic kimberlite and epiclastic volcanic kimberlite. Pyroclastic kimberlites can be assigned into two main classes: Kimberley type (formerly tuffisitic kimberlite) and Fort à la Corne-type (formerly pyroclastic kimberlite). Stage 4 incorporates an assessment of the spatial relationship to and the morphology of the kimberlite body from which the rocks under investigation derive. Stage 5 involves more detailed genetic interpretation with more specific classification based on the mode of formation.