Potassic Magmas Derived from Metasomatized Lithospheric Mantle: Nomenclature and Relevance to Exploration for Diamond-Bearing Rocks

Kimberlites, orangeites and lamproites are rocks derived by the differentiation and crystallization of genetically diverse mantle-Derived magmas. Currently, nomenclatural distinctions between these rock types are made on the basis of mineralogical-Genetic classifications and not simple modal classification schemes. The principle followed in such classification schemes is that the groundmass minerals reflect the fundamental characteristics of the melt from which they crystallized. Classification of most of these rocks cannot be made using bulk rock geochemistry as many contain significant and variable amounts of xenocrystal and xenolith material. It is now well-Known that kimberlites are mineralogically and geochemically distinct from orangeites and lamproites and are probably derived from depleted astheriospheric mantle sources. In contrast, isotopic and experimental petrological data indicate clearly that orangeites, lamproites and other potassic rocks are probably derived from ancient veined metasomatized (enriched) lithospheric mantle. Petrological studies have shown that each lamproite province has unique mineralogical and geochemical characteristics. These differences result from the differing metasomatic histories of the source rocks within individual cratons and accreted mobile belts. Orangeites are considered to represent the expression of potassic magmatism in the Kaapvaal craton and to have similar sources and origins to lamproites. The unique cratonic metasomatic history, perhaps coupled with distinct asthenospheric contributions as the impetus for magma genesis, of each potassic province explains why it is difficult to classify some potassic rocks occurring in the Sao Francisco, Aldan and Dharwar cratons as lamproites, orangeites or karnafugites. As the magmas from whlch these rocks crystallized are the unique expression of potassic magmatism in a particular craton they cannot, by definition, be classified according to schemes devised for potassic magmas in other provinces. Inter-Provincial similarities arise because of the common physicochemical character of the metasomatic process in the lithospheric mantle. The differences reflect variations in the ages and modes of the metasomatic veins coupled with differing degrees of partial melting and/or asthenospheric contributions to the magmas. Any given craton can contain both kimberlites and potassic rocks. Rocks in each potassic province could in theory be given its own terminology, which taken to an extreme could lead to a proliferation of cratonic type-Locality names. Alternatively, all of these diverse magmas might be collectively termed the metasomatized lithospheric mantle magma group (or MLM magmas). The mineralogical-Genetic approach to classification implies that none of the MLM magmas are transitional to kimberlite magmas and that use of the portmanteau term "kimberlite clan rocks" for diverse diamondiferous rocks of different genesis is inappropriate for both scientific and economic purposes. The diamond potential of each province of potassic rocks must assessed independently as this will also reflect the unique geological history of the parental lithospheric mantle. Correct classification of potentially diamondiferouos rocks is essential as this has implications with respect to exploration for, and evaluation of, particular intrusions using heavy mineral indicator suites and/Or geophysical methods.