Remnants of (Possibly Diamondiferous) Ultramafic Igneous Rocks in Bedrock of the Kakwa/Wapiti area, West Central Alberta

Bedrock sampling at 23 sites, heavy mineral separation and microprobing confirmed that indicator minerals of ultramafic igneous rocks are present in Cretaceous bedrock of the Wapiti area (NTS 83L). Indicator minerals among 74 heavy mineral grains obtained from these samples include: eclogitic garnet, picro chromite (chromite geochemically similar to diamond inclusion chromite), chrome diopside and picro ilmenite. In addition, some of the non-diamond inclusion chromites are similar in composition to chromites found in diamondiferous lamproites from Argyle in Australia. Para-magnetic heavy minerals (>3.3 SG) from several of these samples were mounted on polished thin sections and microprobed, resulting in an additional 140 chromite and 50 garnet analyses. These analyses confirmed the presence of diamond inclusion chromites and eclogitic garnet (Group 5). In three of these thin sections kyanite was found. Indicator minerals can be easier described petrographically in thin section than as rough grains under the binocular microscope.

A supplementary study of thin sections from fist size samples at the same sample sites show that tiny chromite and garnet grains (generally smaller than 0.2 mm) can be recognized in thin section. A total of 90 chromite and garnet grains from 7 sample sites were microprobed and can be compared to the analyses from grains separated after crushing a 25 kg sample. The mineralogy of the heavies is similar to the ones separated from the rock and include one diamond inclusion chromite, chromites similar in composition to those found in the lamproites of Argyle (Australia) and Group 5 eclogitic garnets.

The indicator minerals were generally from the Upper Cretaceous Brazeau Formation, but they also include one diamond inclusion chromite from the lower Cretaceous Gates Formation. All of these minerals indicate possible local ultramafic diatremes (which could include diamondiferous ones), because the general provenance of the clastics in these sediments is from the west. The Buffalo Head Hills area or the NWT can not have been a source, because these are situated in the east. However, finding these ultramafic diatremes is a continuing challenge for explorationists in west-central Alberta.

Another conclusion is that 25 kg samples, used in this study, will procure many heavies, which for economic reason could not all be micro-probed.