Diamonds and their sources in the Venezuelan portion of the Guyana Shield

Abstract

The primary source of alluvial diamonds in the Guyana Shield of Venezuela, Guyana, and Brazil has been the subject of controversy from the time of their initial discovery in the late 1800s. Based on available evidence it is suggested that at least two source regions are currently present for diamonds in the Guyana Shield. One source, albeit secondary, is the thick (>2,000 m) sequence of Proterozoic clastic sediments, referred to as the Roraima Group. However, the primary source of the diamonds and the source of the clastic material constituting the Roraima Group is unknown. A West African source has been suggested by some authors but supporting evidence is lacking. A second source of diamonds in the Guyana Shield is responsible for the diamond-bearing alluvial deposits in the Guaniamo district of western Venezuela. For over a decade the kimberlite indicator minerals pyrope and garnet and Mg ilmenite have been known to occur in these alluvials and recently the primary Proterozoic kimberlite-type source rock(s) have been discovered. However, the characteristics of Guaniamo diamonds as a whole are quite different from Roraima diamonds and a common source is unlikely. It is highly probable that other Proterozoic kimberlite (or lamproite) primary sources remain to be discovered in the Guyana Shield area of the Amazonian craton.