Physical Volcanology, Hydrothermal Alteration and Massive ...

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This spectacular volcanic breccia is composed of 30-60% subangular to angular,1-35 cm diameter light grey felsic volcanic fragments which contain 1-5% 1 mm quartzphenocrysts. These fragments are alligned parellel to one another, and in many places,appear as if they can be placed back together with adjacent fragments. Rare sulfide-richfragments and iron-carbonate-rich fragments (pumice) up to 5 cm in diameter can also beobserved. The matrix of this rock is composed of fine grained quartz, iron-chlorite, ironcarbonate,and sericite.In thin section, the felsic lithic fragments from this outcrop are composed ofspherulitic quartz- and K-spar(?)-phyric lava. We believe that this breccia represents aproximal dome collapse breccia or block and ash flow deposit formed from a Middle "L"lava dome that may have been located to the east near the wall of the caldera. A late faultzone located along the northeastern margin of the Sturgeon Lake ore deposit precludesobservation of the "L" stratigraphy north and east of this fault.34
REFERENCESBusby - Spera, C., 1984, Large volume rhyolite ash flow tuff eruptions and submarinecaldera collapse in the lower Mesozoic Sierra Nevada, California: Journal of GeophysicalResearch, v. 89, p. 8417 - 8427.Cas, R. A. F., and Wright, J. V., 1987, Volcanic Successions, Modern and Ancient:London, Allen and Unwin Publishing, 528 p.Davis, D., Krogh, T. E., Hinzer, J., and Nakamura, E., 1985, Zircon dating of polycyclicvolcanism at Sturgeon Lake and implications for base metal mineralization: EconomicGeology, v. 80, p. 1942-1952.Franklin, J. M., Gibb, W., Poulsen, K. H., and Severin, P., 1977, Archean metallogeny andstratigraphy of the south Sturgeon Lake area: Mattabi Trip, 23rd Annual Institute on LakeSuperior Geology, 73 p.Groves, D. A., 1984, Stratigraphy and alteration of the footwall rocks beneath the ArcheanMattabi massive sulfide deposit, Sturgeon Lake, Ontario: Unpublished M. Sc. Thesis,University of Minnesota - Duluth, Duluth, MN, 115 p.Groves, D. A., Morton, R. L., and Franklin, J. M., 1988, Physical volcanology of thefootwall rocks near the Mattabi massive sulfide deposit, Sturgeon Lake, Ontario: CanadianJournal of Earth Sciences, v. 25, p. 280-291.Hudak, G. J., III, 1989, The physical volcanology and hydrothermal alteration associatedwith the F-Group Archean volcanogenic massive sulfide deposit, Sturgeon Lake,northwestern Ontario: Unpublished M. Sc. Thesis, University of Minnesota - Duluth,DUluth, MN, 172 p.Hudak, G. J., III, in progress, The physical volcanology and hydrothermal alterationassociated with late caldera volcanic and volcaniclastic rocks and volcanogenic massivesulfide deposits in the Sturgeon Lake Region of northwestern Ontario, Canada, Ph. D.Dissertation, University of Minnesota - Duluth, Duluth, MN.35
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