Open File Report 2008-06
The unique occurrence of garnet-rich beach sands in east-central Alberta has generated interest for its industrial mineral potential, for its possible association with an undiscovered cluster of diamondiferous kimberlite, and as a curiosity regarding the genre and source of the 'purple beaches.'
This comprehensive report provides the first-known physical and chemical account of all garnet species present in selected east-central Alberta beaches, along with discussion on kimberlite, gold and metamorphic/magmatic massive sulphide-indicator minerals. Garnet reaches lithological proportions of up to 74 vol.% and consists overwhelmingly (up to 99%) of almandine followed by decreasing abundances of grossular, spessartine, lherzolitic and harzburgitic pyrope, megacrystic chromium pyrope, andradite and Cr-andradite. Other indicator-minerals of interest recovered include clinopyroxene, ilmenite, gahnite, corundum, low-chromium diopside and apatite.
Deterrents to industrial garnet production in east-central Alberta include scattered garnet distributions, small (~250-500 µm) and weathered (rounded) garnet morphologies, and the potential for environmental and public conflict. Garnet production as a co-product of sand and gravel, however, should warrant consideration with sand and gravel operators in the region. In addition, a small niche market should not be discounted, especially given the high concentrations of garnet and generally accessible infrastructure.
Observations presented in this study should be of particular interest to diamond explorers. Pyrope garnet was recovered from sample sites throughout east-central Alberta with distinct clusters in the Marie Lake-Cold Lake-Wolf Lake and Calling Lake areas. Results from this study confirm the presence of subcalcic (G10) garnet in east-central Alberta and a G10 to G9 ratio that is generally not present in other parts of Alberta. In addition, several lherzolitic garnets from this study plotted near the G10-G9 boundary line with high chromium and knorringite.
The overriding mechanism for deposition of surficial materials in east-central Alberta is glaciation, and garnet species studied in this report must have originated from the last glacial phase of ice flow during retreat of the Laurentide ice sheet, and therefore, were derived from areas north-northeast of the study area along the westernmost margin of the Canadian Shield. Selected garnet species (i.e. pyrope) could be derived from a fairly local, near-surface source because of their unique composition and texture (e.g. orange- peel texture and kelyphitic rims) compared to surficial pyropes reported in other parts of the province. This raises the potential for the discovery of an unknown cluster of kimberlites in east-central Alberta to northwestern Saskatchewan, possibly in the Cold Lake-St. Paul and Calling Lake areas, or directly north within, for example, the Cold Lake Air Weapons Range. Lastly, this study raises awareness of the potential for secondary deposits of diamond that might have been relocated and concentrated in the same way garnet has.
Eccles, D.R. (2008): Geological evaluation of garnet-rich beaches in east-central Alberta, with emphasis on industrial mineral and diamondiferous kimberlite potential; Energy Resources Conservation Board, ERCB/AGS Open File Report 2008-06, 57 p.