Thermal maturity evaluation of dispersed organic matter inclusions from kimberlite pipes, Lac de Gras, Northwest Territories, Canada

Abstract

Dispersed organic matter (DOM) inclusions have been recovered from crater, diatreme and diatreme xenoliths within hypabyssal facies of diamondiferous kimberlite pipes in the Lac de Gras region of Northwest Territories Canada. Incident white and fluorescent light microscopy has been used to study vitrinite and liptinite DOM xenoliths in the pipes. Percent reflectance in oil (%Ro) of wood-derived vitrinite, visible light region fluorescence properties of sporinite, cutinite and resinite, and changes in maceral morphology have been used to evaluate thermal alteration within the kimberlite pipes. Temperature estimates for different thermal zones in the kimberlite pipes have been inferred by comparing vitrinite reflectance with data from rapid pyrolysis experiments conducted on low rank vitrinites. Crater facies DOM consists of very low reflecting (<0.20 %Ro), cellulose-enriched vitrinites, and liptinites with short wavelength fluorescence properties. Temperatures in the crater facies are estimated to have been on the order of >20° to <100°C. Vitrinites from diatreme facies not associated with hypabyssal kimberlite facies (Group A diatreme) and shallow diatreme facies associated with hypabyssal kimberlites (Group B diatreme), have reflectance ranging from 0.22 to 0.47 %Ro, suggesting a maximum thermal alteration temperature on the order of 150° to 200°C. Liptinite macerals from these diatreme facies have red-shifted fluorescence relative to crater facies liptinites. With increasing proximity of Group B diatreme facies to hypabyssal kimberlite facies, devolatilization in vitrinites increases significantly and reflectance increases from 0.50 %Ro to a maximum of 3.3–4.6 %Ro. Inferred alteration temperatures in this zone range from >200°C to a maximum of ∼650°C. The presence of petroleum-derived cokes are consistent with estimates of 400–450°C for samples recovered close to, and within hypabyssal dykes and macrocrystic breccia and massive hypabyssal facies. Below this zone, tuffisitic kimberlites are marked by a reduction in vitrinite reflectance (0.5 to 1.8 %Ro) and a lower inferred alteration temperature estimated at 200° to 450°C. Anomalous populations of very low-reflecting, partially devolatilized vitrinite (0.50 to 0.84 %Ro) in this zone may have suppressed reflectance, possibly a consequence of liquefaction-like conditions and impregnation by oily bitumens produced during pyrolysis of associated liptinite macerals.