The Physical Volcanology and Hydrothermal Alteration Associated with the F-Group Archean Volcanogenic Massive Sulfide Deposit Sturgeon Lake, Northwestern Ontario


The Physical Volcanology and Hydrothermal Alteration Associated with the F-Group Archean Volcanogenic Massive Sulfide Deposit Sturgeon Lake, Northwestern Ontario

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The F-Group volcanogenic massive sulfide deposit, situated 5 km west of the Mattabi massive sulfide deposit in the Archean Wabigoon greenstone belt, is located within an intensely altered succession of mafic to felsic metavolcanic rocks. Detailed mapping and petrographic studies allow the volcanic succession to be divided into several distinct lithological units. The base of the stratigraphic succession consists of bedded, scoria-rich volcaniclastic rocks which are overlain by heterolithic, massive, poorly sorted volcanic breccia deposits. The breccia is locally interfingered with, and grades up into, a series of quartz-phyric, pumice-rich pyroclastic flow deposits, which make up the immediate footwall rocks to the F-Group deposit. The hangingwall to this deposit consists of a 250 meter thick series of well bedded ash and quartz-phyric, punice-rich pyroclastic flow deposits which, to the east, host the Mattabi massive sulfide deposit. The uppermost 100-300 meters of the volcanic succession consists of a series of massive pyroclastic flow deposits which contain quartz and plagioclase phenocrysts. Across the central portion of the study area, the volcanic stratigraphy is dilated by a massive to amygdaloidal, intermediate composition, sill-like intrusion. The scoria-rich volcaniclastic rocks are interpreted to be the reworked equivalents of a scoria cone deposit located immediately east of the field area. The volcanic breccia and quartz-phyric, pumice-rich pyroclastic flow deposits are believed to have formed by the large scale collapse of a caldera with simultaneous felsic volcanism. The F-Group massive sulfide orebody was formed prior to a second cycle of volcanism that deposited the bedded, quartz-phyric, pumice-rich pyroclastic flows which host the Mattabi massive sulfide orebodies. The uppermost quartz- and plagioclase-phyric pyroclastic flow deposits are the products of the last explosive volcanic event. Regional carbonatization and silicification occurred prior to the development of the hydrothermal systems which formed the orebodies in the south Sturgeon Lake area. In the F-Group area, five mineralogically distinct, mappable alteration zones, which cross-cut or overprint the earlier regional alteration, have been recognized. They vary in geometry from semiconfonnable to locally cross-cutting and pipe-like, and are referred to as: a) least altered; b) iron carbonate / iron chlorite; c) chloritoid; d) aluminum silicate; and e) cordierite. The isocon method (Grant, 1986) has been used to determine geochemical trends associated with hydrothermal alteration. Relative gains and losses of components were measured from a constant aluminum isocon. Mass gains are associated with silicification, iron carbonate / iron chlorite, and aluminum silicate alteration, whereas mass losses are associated with cordierite alteration. No consistent mass changes are exhibited in comparisons between least altered rocks and the other alteration assemblages. Major elemental trends displayed include elemental gains in Fe, Mn, and OOi in iron carbonate / iron chlorite and chloritoid-bearing rocks, increases in Mg and Fe in cordierite-bearing rocks, and increases in Si within silicified and aluminum silicate-bearing rocks. It is envisioned that chemical reactions involving OOi-rich seawater and porous, volcaniclastic deposits led to the development of widespread carbonate across the area. Seawater percolating deep into the hydrothermal system was heated by a subvolcanic intrusion. This hot seawater reacted with the volcanic rocks, forming Fe-Mn-Ca-base metal-rich hydrothermal fluids. These fluids rose toward the seafloor in synvolcanic structures. During their ascent, reactions between the hot fluids and the rocks led to the formation of aluminum silicate minerals, iron-bearing clays, and iron-bearing carbonates. On or near the seafloor, metallic sulfide minerals precipitated from the hydrothermal solutions and formed the F-Group orebody. As the hydrothermal system developed, K and Mg-rich solutions were formed. These solutions reacted with the rocks to make sericite and Mg-bearing clay minerals. The widespread alteration present in the hangingwall to the F-Group deposit resulted when glassy, permeable pyroclastic rocks were laid down on the hot, active hydrothermal system. Later, greenschist grade metamorphism formed the alteration mineral assemblages present in the area today.


A thesis submitted to the faculty of the Graduate School of the University of Minnesota by George J. Hudak III in partial fulfillment of the requirements for the degree of Master of Science, May 1989. Plates 1-4 referenced in the thesis are also attached to this record.

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Hudak III, George J. (1989). The Physical Volcanology and Hydrothermal Alteration Associated with the F-Group Archean Volcanogenic Massive Sulfide Deposit Sturgeon Lake, Northwestern Ontario. Retrieved from the University Digital Conservancy,

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