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

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 vit rinite and liptinite DOM xenoliths in the pipes. Percent reflectance in oil (%Ro) of wood-derived vitrinite, visible light region fluorescence propert ies 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 kimberiite pipes h ave been inferred by comparing vitrinite reflectance with data from rapid p yrolysis experiments conducted on low rank vitrinites. Crater facies DOM co nsists 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 degree s to ( 100 degrees C. Vitrinitcs from diatreme facies not associated with h ypabyssal 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 tem perature on the order of 150 degrees to 200 degrees C. Liptinite macerals f rom 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 sign ificantly 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 degrees C to a maximum of similar to 650 degrees C. The presence of petroleum-deriv ed cokes are consistent with estimates of 400-450 degrees C for samples rec overed close to, and within hypabyssal dykes and macrocrystic breccia and m assive hypabyssal facies. Below this zone, tuffisitic kimberlites are marke d by a reduction in vitrinite reflectance (0.5 to 1.8 %Ro) and a lower infe rred alteration temperature estimated at 200 degrees to 450 degrees C. Anom alous 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 bitu mens produced during pyrolysis of associated liptinite macerals. (C) 1999 E lsevier Science B.V. All rights reserved.