Geology and Sulfide Mineralization of the Skeleton Lake Prospect, St. Louis County, Minnesota


Geology and Sulfide Mineralization of the Skeleton Lake Prospect, St. Louis County, Minnesota

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Banded, semi-massive, and disseminated sulfide mineralization was intersected in three drill holes near Skeleton Lake, northern St. Louis County, Minnesota. Pyrrhotite and pyrite are the dominant sulfides, commonly associated with magnetite and minor amounts of chalcopyrite. The sulfides occur within a sequence of Archean metavolcanic rocks that were deposited subaqueously, and consist primarily of basaltic and andesitic flows, mafic-intermediate tuffs, and associated diabasic rocks. Algoman-type iron formation and porphyritic intrusive rocks of varying composition are also present in lesser amounts. The sulfides were deposited: 1) in a restricted horizon within a fine-grained, bedded quartz host rock; and more generally, 2) as hydrothermal stringers in favorable sites below the sulfide horizon. The restricted sulfide zone is interpreted as being the product of volcanic-exhalative processes occurring at the sea floor, whereas the hydrothermal veinlets are considered to be epigenetic infillings concentrated in fractures below the seawater-rock interface. The circulating hydrothermal fluids that were responsible for the transport of metals through the volcanic pile have pervasively altered the rocks below the sulfide horizon in an irregular alteration "pipe". Two pyroelastic sequences, originally more permeable than the massive flow units, have been more profoundly affected by the hydrothermal fluids, resulting in stratiform alteration zones extending from the alteration pipe. A third discordant, pipe-like alteration area occurs stratigraphically above the sulfide horizon. The stratigraphic position of this zone, and the observation of hydrothermal veins cross-cutting the sulfide horizon, indicate a second period of hydrothermal activity at Skeleton Lake. These altered rocks are distinctive in their unusually high amounts (> 50%) of quartz, carbonate, epidote, chlorite and other hydrothermal minerals, that occur in veins and in envelopes around amygdules and fractures. These altered rocks are further distinguishable by anomalous depletion of MgO, and enrichment of K2O, SiO2 , and Na2O. The exhalative sulfide horizon and altered pyroclastic sequences were observed to be spatially related to andesitic rocks and iron formation. Although a geologic environment that was conducive to massive sulfide deposition existed at Skeleton Lake, base and precious metal contents are not sufficient to make the prospect economically exploitable at this time.


A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Peter Anthony Giangrande in partial fulfillment of the requirements for the degree of Master of Science, October 1981. There are 3 supplementary files also attached to this record, which contain Plates I, II, and III referenced in the thesis.

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Giangrande, Peter Anthony. (1981). Geology and Sulfide Mineralization of the Skeleton Lake Prospect, St. Louis County, Minnesota. Retrieved from the University Digital Conservancy,

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