Preliminary results on the inorganic geochemistry of metalliferous shale and mudstone in Alberta
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Organic‐rich shale is recognized as an unconventional reservoir for hydrocarbons in Alberta; however it could also be considered as a reservoir for polymetallic mineralization. Polymetallic occurrences or highly anomalous concentrations of various metals in black shales have been locally reported either by government or industry studies throughout the province since the mid‐1960s. These occurrences or anomalies include sulphide mineralization in the basal Exshaw Formation at Jura Creek, high metal contents in the Crowsnest Pass area in the Fernie and Exshaw shales, and also in Shaftesbury shale at Buffalo Head Hills. The most explored occurrence is a polymetallic low grade deposit hosted by Upper Cretaceous black shale of the Second White Specks Formation in the Birch Mountains of northeastern Alberta. That occurrence has a current indicated resource in the Buckton project area of 271 million tonnes of mineralized material containing recoverable Ni, U, Zn, Cu, Co, REE, Sc, Mo, Li, V and Th.
Recent investigation of the inorganic geochemistry of sedimentary units in Alberta’s basin by the Alberta Geological Survey (AGS) shows that organic‐rich shale, mudstone and lime mudstone are frequently repositories for metal enrichment including Zn, Ni, Mo, V, Co, Cu, and occasionally, U and REE. The stratigraphic units showing considerable metal enrichment include the Exshaw, Banff, Nordegg and Loon River formations, with concentrations up to 5991 ppm Zn, 2187 ppm Ni, 962 ppm Mo, 3939 ppm V, 1080 ppm Y and 2153 ppm REE. These metal maximums are higher than
those reported for the Second White Specks Formation in the Buckton polymetallic deposit. The Exshaw and Nordegg show the highest levels of enrichment followed by the Banff and then the Loon River. Total organic carbon concentrations for these units range from <0.01 to 26.27%, whereas total sulfur ranges from 0.02 to 19.83%. Although metal concentration shows a moderate relation to total organic carbon and total sulphur contents, there are several examples of high metal content which show no correlation to TOC or sulphur. Relationships among Ni/Co, V/Cr and V/(V+Ni) for metalliferous samples indicate anoxic accumulation conditions for Exshaw shale, dysoxic to anoxic for Nordegg, but a more widespread range from oxic to anoxic conditions for the Banff and Loon River. The C‐S‐Fe relationships also suggest dominantly anoxic environment of sedimentation for most of the metal enriched Exshaw, whereas the Banff and Loon River metalliferous samples show more variability from oxic to anoxic conditions, with the highest concentrations of metal for those samples indicating anoxic conditions.
Current research on metalliferous shale and mudstone by the AGS includes mineralogical studies, whereas future research will aim to constrain the regional extent and geological setting of these occurrences.
Gloria Lopez is an exploration geologist from Chile with experience in porphyry, epithermal, skarn and IOCG deposits. She currently works at the Alberta Geological Survey as a Minerals specialist. Her formal training and experience included mineral deposit studies in Chile, Peru, Mexico, Australia and the US. In Chile, she spent several years exploring the Andes and Atacama desert for the government-owned Codelco corporation. She holds a Ph.D. in Economic Geology from Colorado School of Mines, a M.S. in Structural Geology and B.S. in Geology from the University of Chile. Before joining the Alberta Geological Survey in 2015, Gloria was a post-doctoral research fellow in economic geology at the University of Alberta.