Physical Volcanology and Hydrothermal Alteration of the Footwall Rocks to the Archean Mattabi Massive Sulfide Deposit, Northwestern Ontario
1993-05
Title
Physical Volcanology and Hydrothermal Alteration of the Footwall Rocks to the Archean Mattabi Massive Sulfide Deposit, Northwestern Ontario
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1993-05
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Thesis or Dissertation
Abstract
The hostrocks to the Archean-aged Mattabi volcanogenic massive sulfide deposit consist of a steeply dipping 2 km homoclinal sequence of extensively altered dominantly pyroclastic rocks. These rocks form the central section of the larger sturgeon Lake Caldera Complex. Extensive diamond drilling around the Mattabi deposit creates an ideal situation for the study of the evolution and physical volcanology of' the caldera complex in three dimensions, from a pre-caldera subaerial mafic shield volcanic environment through subaqueous caldera collapse to infilling with coarse heterolithic breccias, felsic pyroclastic flow deposits, massive sulfide deposits, lava flows, sedimentary rocks and debris flow deposits. Rocks of this caldera sequence are divided into seven successions based on composition, physical volcanology and depositional environment. The lowermost Darkwater Succession consists of sub-aerial mafic lava flows and related scoria cones and epiclastic rocks which form a broad mafic shield volcano upon which the caldera is superimposed. The overlying High Level Lake Succession was deposited during initial caldera collapse, is up to 800 m thick, and consists of a lower thick mesobreccia unit overlain by quartz-rich and quartz-poor pumiceous pyroclastic flow deposits, that form the footwall to the Mattabi massive sulfide deposit. The Tailings Lake Succession represents a period of volcanic quiescence with deposition of heterolithic debris flow deposits shed from caldera walls. During deposition of dacitic ashes within the High Level Lake Succession, high temperature hydrothermal fluids exiting along steeply dipping syn-volcanic faults, produced the lower "E" ore zones of the Mattabi deposit. Ore forming hydrothermal activity continued throughout the explosive eruption and deposition of the Mattabi Succession quartz- and pumice-rich rhyolitic pyroclastic flow deposits which enclose the main lenses of the Mattabi ore body. The ore body is overlain by a complex succession of rocks which include quartz and plagioclase porphyritic pyroclastic flow deposits, ash tuffs and volcaniclastic sedimentary units. These hangingwall rocks have been named L Succession by previous workers and are divided into a lower, middle and upper succession based on physical volcanology and composition. Within the "L" are extensive intermediate lava flows of the Bell River Lake Succession. The entire caldera sequence is capped by the No Name Lake Succession of andesitic pillow flows and flow breccias. Footwall hydrothermal alteration associated with the Mattabi deposit is wide-spread, extending beyond the limits of the study area. The mineralogy and zonal distribution of the minerals led to recognition of nine alteration assemblages. The study of cross-cutting relationships, alteration intensity and mineralogical zonation reflect an approximate timing of the alteration phases. Least altered assemblage rocks are found only in the hangingwall and as scattered zones within the footwall. The large-scale lower-footwall alteration pattern is characterized by a broad semi-conformable zone containing anomalous amounts of iron-carbonate and chlorite. A zone of silicified rocks generally caps this iron-rich zone 500 m stratigraphically below the Mattabi ore deposit. A similar sized and shaped zone of chloritoid-rich alteration occurs through most of the host-rock stratigraphy. This alteration zone extends 100's of meters along strike and down-dip. Chloritoid is also associated with syn-volcanic faults deeper in the footwall. Synvolcanic faults are fringed by chloritoid-rich rocks and are usually cored by steeply-dipping tabular zones of rocks containing abundant aluminum-silicates and chloritoid. These zones extend up to 600 m into the footwall. Within the host rock stratigraphy, the zones of alumino-silicate + chloritoid altered rocks occur mostly within zones of chloritoid altered rocks, but also as cross-cutting zones. End product alteration are rocks with abundant alumino-silicate minerals. This alteration type is directly associated with the ore deposit and forms a envelope around the ore deposits and two small "keels" along syn-volcanic faults into the footwall. The last alteration to affect the wall rocks consists of an assemblage of sericitic alteration or Mg-chlorite ± carbonate. This alteration overprints much of the earlier alteration. The basic changes in the mineralogy of the altered rocks are reflected in the geochemistry. Quantitative methods to measure geochemical change are hampered by the effects of the primary geochemical variability within the rocks coupled with the overprinting nature of the hydrothermal alteration.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jamieson Scott Walker in partial fulfillment of the requirements for the degree of Master of Science, May 1993. Plates 1-4 referenced in the thesis are also attached to this record.
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