Stratigraphy and Hydrothermal Alteration of Archean Volcanic Rocks at the Headway-Coulee Massive Sulfide Prospect, Northern Onaman Lake Area, Northwestern Ontario

Abstract

The Headway-Coulee massive sulfide prospect of northwestern Ontario is situated within the Superior Province of the Canadian Shield. Rocks at the prospect form part of the Archean Wabigoon greenstone belt and consist of an intensely hydrothermally altered succession of mafic and felsic volcanic and intrusive rocks. Subaqueously deposited pillowed and amygdaloidal to massive and autobrecciated mafic lava flows form a 1-2 km thick succession which is locally interlayered with, and overlies a thin sequence of felsic volcanic rocks. The felsic volcanic rocks are laterally limited (2 km) and are composed dominantly of bedded ash tuffs capped by massive to brecciated and flow-banded lavas. The tuffs are fine-grained, generally fragment-poor, and vary from laminated to thickly-bedded. An extensive polymictic diamictite deposit, which contains clasts of granite, mafic and felsic volcanic rocks, and iron formation, is interlayered with the felsic 1olcanic rocks and is believed to represent a debris flow deposit which had its source to the southwest of the study area. Based on their fine-grain size, limited lateral extent, and thin to thickly-bedded nature, the felsic tuffs are interpreted to be products of hydrovolcanic eruptions. Based on stratigraphic relationships the deposits are believed to have formed on the submerged flanks of two adjacent tuff cones. It is envisioned that the capping felsic lavas formed either under low water/magma ratio conditions as access of water to the erupting magma was restricted, and/or under high water/magma ratio conditions within a water flooded vent or on the submerged flanks of the cones. The majority of the volcanic rocks were intensely altered by hydrothermal solutions during the waning stages of felsic volcanism. Alteration in the rocks is relatively widespread and is subconcordant to stratigraphically conformable in distribution. The altered rocks have been subdivided into four distinct mineral zones. The zones, in order of formation and increasing alteration intensity, are: (1) least altered, (2) quartz-sericite, (3) iron chlorite, and (4) chloritoid. The progressive alteration of the rocks was studied by mass balance comparisons of the altered rocks and their less intensely altered, stratigraphic equivalents. These comparisons indicate that Al was generally immobile, and that volume losses during alteration range from 0 to approximately 50%; the largest volume losses occurred during alteration of the felsic ash tuffs. Major chemical trends involved in alteration of the rocks include large gains in K and loss of Na during sericitization, and generally addition of Fe, and loss of Ca and Na during formation of iron chlorite and subsequent development of the chloritoid alteration type. Based on the distribution of the alteration types as well as the alteration mineralogy and chemistry it is proposed that, by shallow circulation through porous volcanic rocks, an acidic, K-rich fluid evolved and caused widespread sericitization within the study area. Deeper circulation evolved an Fe-rich fluid which was discharged along synvolcanic faults from a pressurized reservoir at depth. The solution chemically reacted with the sericitized rocks to produce the iron chlorite assemblage, and the pre-metamorphic equivalent of the chloritoid assemblage. The chloritoid assemblage developed as pre-metamorphic, coexisting iron chlorite + hydrous Al--silicate became unstable and reacted to form chloritoid during regional greenschist facies metamorphism.