Browsing by Author "Osterberg, Steven Arvid"

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    Stratigraphy and Hydrothermal Alteration of Archean Volcanic Rocks at the Headway-Coulee Massive Sulfide Prospect, Northern Onaman Lake Area, Northwestern Ontario
    (1985-10) Osterberg, Steven Arvid
    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.
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    Stratigraphy, Physical Volcanology, and Hydrothermal Alteration of the Footwall Rocks to the Winston Lake Massive Sulfide Deposit, Northwestern Ontario
    (1993-09) Osterberg, Steven Arvid
    The Winston Lake Zn-Cu-Ag massive sulfide deposit is situated above a sequence of metamorphosed Archean calc-alkaline volcanic and volcaniclastic rocks. A detailed mapping, petrographic, and chemical study was undertaken to evaluate the stratigraphic and hydrothermal development of the footwall rocks with regard to depositional environment and spatial controls on metasomatism and mineralization. The footwall rocks are dominated by interlayered successions of metamorphosed volcaniclastic and volcanic rocks that have been extensively intruded and block faulted. Volcaniclastic-sediments were deposited at the base of the stratigraphy where they were interlayered with felsic pyroclastic deposits and/or their turbiditic equivalents. Locally massive sulfide and cherty exhalative beds were deposited. A relatively thick section of interlayered felsic and mafic lava flows were erupted and deposited above the basal volcaniclastic rocks; minor interflow elastic and base metal-poor exhalative sediments accumulated during pauses in mafic volcanism. An upper elastic succession accumulated above the lava flows; basinal volcaniclastic-sediments were deposited and were overlain in part by felsic pyroclastic material that was erupted from a distant, extraneous source. Interlayered mafic lava flows and volcaniclastic rocks cap the footwall stratigraphy and host the Winston Lake deposit and stratigraphically equivalent mineralized occurrences. Facies analysis of lava flows, along with the basinal distribution of volcaniclastic-sediments indicates the Winston Lake footwall stratigraphy developed in a subsiding, subaqueous rift environment. Subsidence was focussed in the rift axis; associated stresses resulted in development of synvolcanic faults within and distal to the rift axis. The dominance of passive eruption products indicates volcanism occurred in relatively deep water beneath the volatile fragmentation depth. Approximately 50% of the footwall stratigraphy has been hydrothermally altered in subconcordant to cross-stratal zones. Interaction of the rocks with metasomatic fluids, followed by isochemical metamorphism has resulted in unusual modal abundances of tremolite/actinolite, biotite, sillimanite, staurolite, anthophyllite/gedrite, chlorite, and quartz relative to metamorphosed primary compositions. Microprobe analyses indicate extreme Fe/Mg enrichment offerromagnesian silicates near the base of the stratigraphy. Mass balance analysis indicates variable enrichment of MgO, Fe2O3T, and K2O, and depletion of CaO and Na2O in altered rocks; TiO2 and Al2O3 were relatively immobile. Overall mass losses, indicative of metasomatic leaching, dominate alteration towards the base of the stratigraphy, whereas both gains and losses occurred in the upper portions of the section. Mg enrichment occurred in stratiform zones through shallow circulation of seawater-based hydrothermal fluids during progressive stratigraphic growth. Minor associated base metal-poor exhalites developed during intermittent pauses in volcanism and sedimentation. Substratiform zones of iron-aluminous-potassic alteration developed as chemically evolved fluids, which originated at depth, interacted with permeable lithologic units through which they buoyantly migrated. The distribution of alteration indicates that chemically-evolved fluids rarely reached the sea floor environment but were generally confined beneath impermeable stratigraphic units. Metalliferous fluids periodically passed through the footwall rocks to the sea floor; no distinct chemical or mineralogical fingerprint of their passage is evident in the rocks, suggesting the metalliferous fluids were similar to chemically-evolved fluids except in metal content. The metalliferous fluids reached the sea floor during at least two stages of stratigraphic growth in which metals were deposited as massive sulfides. The first stage was at the Pick Lake deposit, near the base of the stratigraphy and the second stage was at the Winston Lake deposit at the top of the section. The distribution and composition of alteration and associated base metal sulfide and cherty exhalative occurrences indicates the Winston Lake hydrothermal system was multistaged and involved multiple hydrothermal fluids. Stratigraphic development in a subsiding rift environment spatially controlled the movement of buoyant hydrothermal fluids through permeable lithologic units. Periodic synvolcanic faulting released metalliferous fluids to the sea floor where base metal sulfides were deposited.