Geochemistry of black shale deposits - A summary report

Abstract

Various sedimentary provinces in the United States and Canada were examined for beds of metal-rich black shale similar to those described as ore deposits in other parts of the world. From the samples analyzed for minor elements, 20 sets of samples (comprising 779 individual samples) were selected as representative of a wide variety of geologic environments of black shale deposition. These samples include black shale and associated organic-rich rocks transitional with black shale. Statistical methods were used to determine the composition of the average black shale and the normal range in composition of black shale and to provide a definition of metal-rich black shale for any one of 21 minor elements. A black shale sample is defined as metal-rich if any minor element occurs in excess of the 90th percentlie as determined from the sum of the percent frequency distribution of elements in the 20 sets of black shale samples. A black shale deposit is classified as enriched if the percent of samples that are metal-rich with respect to each of 21 minor elements exceeds an arbitrary minimum. Principal component analyses of correlation data were used to determine the association of minor elements with major rock constituents. As interpreted from statistical analysis of chemical data, the detrital mineral fraction of most black shale deposits is characterized by the elements aluminum, titanium, gallium, zirconium, and scandium, and may also include any of the following elements: beryllium, boron, barium, sodium, potassium, magnesium, and iron. The carbonate fraction of black shale deposits commonly includes calcium plus magnesium, manganese, or strontium. These elements are readily available from solution and are regarded as mobile. The organic fractions of black shale deposits are locally enriched in other mobile elements including silver, molybdenum, zinc, nickel, copper, chromium, vanadium, and, less commonly, cobalt, lead, lanthanum, yttrium, selenium, uranium, and thallium. Anomalously high syngenetic concentrations of these elements with organic matter are best explained by higher than normal concentrations in sea water. The enrichment of minor elements with organic matter in black shales is commonly an accumulative process beginning with the living organism and continuing: throughout the periods of decay, burial, and exposure to ground waters of various compositions throughout the history of the deposit. Thin black shale beds may provide a local sink for minor elements after burial, whereas thick units that include much organic matter but are exposed to hypersaline solutions may provide a source of mobile elements that are expelled and transported to adjacent more permeable rock bodies. © 1970 Society of Economic Geologists, Inc.