Rondi M. Davies

DIAMONDS FROM WELLINGTON, NSW : INSIGHTS INTO THE ORIGIN OF EASTERN AUSTRALIAN DIAMONDS

Abstract Diamonds from alluvial deposits near Wellington, New South Wales, have been characterized on the basis of morphological features, mineral inclusions, C isotope signatures, N content and aggregation state and internal structure. The diamonds are of two types. The larger group (Group A) is indistinguishable from diamonds found worldwide from kimberlitic and lamproitic host rocks. This group is inferred to have formed in a peridotitic mantle source in Pre-Cambrian subcratonic lithosphere. The second group (Group B) is unique in its internal structures (which show evidence of growth in a stress field and non-planar facets), has unusually heavy C isotopic compositions and contains Ca-rich eclogitic inclusions. This group is inferred to have formed in a subducting slab. Diamonds of both groups have external features (corrosion structures and polish) indicating transport to the surface by lamproitic-like magmas. The diamonds show evidence of long residence at the earths surface and significant alluvial reworking: they are not accompanied by typical diamond indicator minerals.

Unusual mineral inclusions and carbon isotopes of alluvial diamonds from Bingara, eastern Australia

Abstract A suite of diamonds from eastern Australia, “Group B”, are essentially eclogitic. However, mineral inclusion compositions are unlike eclogitic diamonds from other occurrences. Garnets are very Ca-rich (Gr 61–84 ) and clinopyroxenes are mostly diopsidic (16±10 mol% Jd). Other inclusions in these diamonds are SiO 2 , titanite, olivine, molybdenite and melilite—a truly unique suite! Garnet and clinopyroxene are not accompanied by indications of fertile mantle conditions. Rather, we find depletions in Fe, Mn, Na, K, coupled with high Ni and Cr in some clinopyroxenes. The chemistry of the garnets and clinopyroxenes suggests affinities with rodingites—metasomatised mafic rocks that occur in ophiolites. “Group B” diamonds are 13 C-enriched ( δ 13 C=+1.1±1.3‰ (1 S.D.)). Internal structures include evidence of brittle failure and of rapid growth. The diamonds have high nitrogen contents and positive nitrogen isotopes ( δ 15 N=+10.1±4.7‰). This may indicate that the diamonds formed from 13 C-enriched carbon (+3‰), such as that derived from marine carbonates within the mafic rocks. The many unusual features of these diamonds can be interpreted to indicate that the diamonds formed in a subduction environment. However, resorption on the surfaces of the diamonds indicates that they were brought to the upper crust by magmatic activity.

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.