The potential for diamonds in Britain

Most diamonds at the earth’s surface occur there because of the combination of two unusual processes, the lowering of the geothermal gradient deep in the mantle, which allows the diamond stability field to be entered, and the formation of a magma at or below these great depths within which diamondbearing xenoliths can be transported very rapidly to the earth’s surface. As most diamonds seem to have formed in sub-cratonic roots during the early history of the earth, the Lewisian terrane of the north-west Highlands of Scotland and the Hebrides, a fragment of the ancient Laurentian craton, represents the most favourable environment in Britain. No kimberlites are known in this terrane but, although it is probably the most intensely mapped cratonic segment in the world, exposure is very poor over large sectors of the Hebrides and existing aeromagnetic data is too widely spaced to give an adequate indication of whether any diatremes are present. The nature of the basement of the rest of the Scottish Highlands is less clear, but it could partly be of early Proterozoic age so that some potential for the occurrence of diamonds in lamproitic rocks exists. The centres of alkaline igneous activity in northern Scotland and major structures, such as the Great Glen Fault, which is rooted deep in the mantle, are possible environments of interest. Elsewhere in Britain the basement is uncertain but is probably mostly late Proterozoic in age and therefore too young to have sub-cratonic mantle roots, though in south-west England in particular, there could be segments of Archaean basement. The only British rock which fits clearly into the lamproite class occurs in south-west England, but there are several others in the same region and elsewhere, generally of Permian age, which approach lamproite in chemistry. Furthermore the aeromagnetic data from south-west England which is more closely spaced than for the rest of the country, suggests the presence of several unexposed pipe-like intrusions. In northern Britain the most potassic and alkaline rocks of similar age occur in Orkney. Other igneous rocks are capable of carrying diamonds to the surface from shallower depths in the mantle if the geothermal gradient had been depressed sufficiently to allow these shallower rocks to enter the diamond stability field. Recent thinking, emanating particularly from Australia, suggests that diamond could crystallise within a cold subducted slab which caused local depression of the geothermal gradient. Such a remnant cold slab could be left behind at the cessation of subduction and form a transient source of diamonds which could possibly be tapped by lamprophyres, alkali basalts and nephelinites. Recognition of such an environment is highly speculative but it could have been present in the Southern Uplands of Scotland at the end of the Silurian and early Devonian. The regional swarm of potassic minettes are a medium in which diamonds derived from such a source could have reached the surface. A subducted slab could also have been stagnant beneath south-west England and tapped by the Permian lamprophyric and lamproitic intrusions or alkaline lavas. Diamonds could occur in palaeoplacer deposits, particularly in the Proterozoic Torridonian sequence of north-west Scotland, since the source of these fluviatile rocks could have been an Archaean craton to the west of the Lewisian and the sequence has suffered little deformation and metamorphism. Further work is recommended, particularly in south-west England and north-west Scotland, to determine whether potentially favourable deepseated intrusions and palaeoplacers are diamondiferous and to investigate the source of aeromagnetic anomalies of potential interest.