Distribution and characteristics of diamonds from Myanmar

Abstract

Diamonds occur in headless placers at several locations within Myanmar. Twenty-six stones from the Momeik area of northern Myanmar and 111 stones from the Theindaw area of southern Myanmar have been studied to characterise their morphology, crystal forms, colour, degree of resorption, surface features, internal structures, mineral inclusions, and nitrogen content and aggregation state. Most stones grew originally as octahedra, but now show very high degrees of resorption, and highly polished surfaces, reflecting transport in a magma. Etch features are abundant, and breakage and abrasion are common, due to alluvial transport. Brown radiation spots are common, suggesting that these diamonds have a long history in surface environments. Cathodoluminescence (CL) images of plates and whole stones commonly display marked oscillatory zoning of yellow and blue bands, outlining octahedral growth zones. Many other stones show uniform yellow CL. Syngenetic mineral inclusions identified thus far are mainly of peridotitic paragenesis and include olivine, chromite and native iron. Infrared spectroscopy studies show that ∼10% of the diamonds have very low-N contents (Type II diamonds). More N-rich diamonds show high degrees of aggregation (Type IaAB). Both types are consistent with derivation from the upper mantle, rather than from crustal metamorphic sources. The primary source of these diamonds is believed to be an alkaline igneous rock (lamproitic rather than kimberlitic) but they may have reached their present locations via a secondary collector such as a sedimentary rock.