Northwest Territories Geological Survey

Studies into the relationship between oxygen fugacity of mantle fluids/melts and etch features on diamond surfaces have shown specific fluid/melt compositions correspond to associated etch features. A classification scheme has been proposed to determine the fluid composition within a kimberlite by examining etch features associated with diamond surfaces as a proxy for fluid composition in an ascending diamondiferous kimberlite. A suite of 388 microdiamonds(defined as diamonds which pass through a 0.5mm square mesh screen) and 88 macrodiamonds taken from various drill hole depths in the Hearne kimberlite and 88 inclusion-bearing macrodiamonds from the Gahcho Kué mine (NWT) were viewed under a secondary electron microscope for their surface features in accordance with this scheme. Two hundred and thirty specimens show shallow-depth etch features that can be easily classified: the main features observed were trigons and truncated trigons on the {111} faces and/or tetragons on the {100} faces (indicating etching by fluids of variable CO2:H2O ratios). Thirty-four specimens show deeper etched features that represent either extreme degrees of regular etching (such as deeply-etched tetragons), or corrosion type etching, wherein the diamond lattice is etched in a fluid-free melt. Variability between crystal habits exists between the size fractions studied, with cubic habits only being observed in the microdiamond population. This implies variable formation conditions for the two different diamond size fractions studied from Gahcho Kué. Among microdiamonds, surface textures associated with fluid-related etching are markedly more variable, with truncated trigons, tetragons, and both positive and negative trigons being observed. However, these often occur in combination with features showing a large variability in their depth to size ratio between samples, which is typically caused by mantle-related etching. These observations suggest repeated interaction of fluids/melts with the Gahcho Kué diamond population, with at least some of the fluids affecting the microdiamonds being more CO2-rich than those that etched the macrodiamond fraction.