Subaqueous Pyroclastic Volcanism in the Vicinity of the Helen Mine, Wawa, Ontario
1982-12
Title
Subaqueous Pyroclastic Volcanism in the Vicinity of the Helen Mine, Wawa, Ontario
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1982-12
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Thesis or Dissertation
Abstract
A superbly exposed and preserved succession of felsic subaqueous (partially subaerial) pyroclastic flow deposits and related hydroclastic rocks are exposed stratigraphically below the Helen Iron Formation near Wawa, Ontario. This succession is located in the Abitibi-Michipicoten greenstone belt and is regionally associated with volcanogenic iron formation, pillowed mafic flows, and volcaniclastic sedimentary rocks. The metamorphic grade is lower greenschist facies. Felsic volcanism began with the emplacement of finely bedded, poorly sorted, fragmented hydroclastic rhyolites produced by phreatomagmatic explosions. The hydroclastic rocks pass stratigraphically upward into bedded pyroclastic flow deposits which are overlain in turn by finely bedded ash deposits. This sequence is repeated at least twice and is then overlain by crystal tuffs and contemporaneous pumice-bearing tuffs. The pumice-bearing tuffs are thought to be segregated from the crystal tuffs by the different settling rates of crystals and pumice through a water column. The top of the pumice-bearing tuffs marks the end of the first cycle of felsic volcanism. Interbedded with the pumice-bearing and crystal tuffs are chlorite rich dacite and/or andesite pillow breccia, amygdaloidal flows, and/or subvolcanic intrusions, and accessory bearing, lithic rich pyroclastic flow deposits. Stratigraphically above the andesites occurs a second cycle of felsic volcanism marked by the sequence hydroclastic rocks, crystal tuffs, and pumice-bearing tuffs. The upper portion of the second cycle is capped and/or intruded by massive, flow laninated, and spherulitic rhyolitic domes and/or lava flows. The domes and/or lava flows are surrounded by rhyolitic, coarse, poorly sorted, chaotic block and ash size volcanic breccias that are thought to be related genetically to the domes and/or lava flows. The block and ash deposits are in turn surrounded by well sorted, lapilli and ash deposits. Iron formation consisting of an upper banded chert member, a middle massive pyrite member, and a lower massive siderite member is located stratigraphically above the felsic volcanics and below the pillowed mafic flows. It is associated with an alteration zone in the footwall rhyolites marked by the presence of chloritoid, epidote, chlorite, and stilpnomilane stringers. Four ages of mafic dikes at the footwall felsic volcanics, iron formation, and pillowed mafic volcanics. Whole rock and trace element chemical analyses of representative samples showed subalkaline trends and confirmed the rhyolitic and dacitic field identifications of rocks from the area. The raw chemical data was corrected for metasomatism after the method of Beswick and Soucie (1978). This procedure identified so2, FeO + MgO, CaO, Na2o, and K2o as mobile constituents during post volcanic modification of original compositions. The eruption cycle is thought to have begun with short lived, subaqueous, phreatomagmatic activity, changing with time to a sustained eruption column that may have been subaqueous and/or subaerial. After two such cycles, rhyolitic domes and/or lava flows with associated Merapi-type pyroclastic flows capped with pyroclastic deposits. The last vestiges of felsic volcanism in the area was the fumerolic and hot spring activity that resulted in the alteration of the footwall volcanics and in the deposition of the Helen Iron Formation.
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Mark Warren Osterberg in partial fulfillment of the requirements for the degree of Master of Science, December 1982. There is 1 supplementary file also attached to this record, which contains Figure 3 referenced in the thesis.
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