Physical Volcanology, Hydrothermal Alteration and Massive ...

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are found throughout the lateral and vertical extent of this unit, making it a primeexploration target.The bedded quartz-crystal deposits are overlain by massive, very fine-grained ashflow tuff deposits which vary from 20 to 150 m in thickness. These units are composed ofzero to 2% small « 1 mm), broken and sliver-like quartz crystals in a quartz-rich matrix;it is not uncommon for these units to be intensely silicified and iron metasomatized. Thismassive unit is overlain by 5-35 m of bedded, fine-grained ash material which iscompositionally and chemically similar to the underlying massive ash flow tuffs. Themassive unit contains no known massive sulphide mineralization and lacks the pronouncedgeochemical zoning exhibited by the bedded deposits. In morphology these units aresimilar to those described by Busby-Spera (1984) from the Mineral King roof pendant andby Morton and Nebel (1983) from the Wawa area, and represent flow units deposited ina relatively shallow subaqueous environment.L SuccessionThis is a complex succession of rocks which range from quartz- and quartzplagioclase-bearingpyroclastic flow deposits through plagioclase-phyric lava flows anddomes to sedimentary sequences. The entire succession ranges from 250 to 1200 m thickwith the domes, lava flows and dome related sediments and debris flow deposits makingup the bulk of the stratigraphy. The pyroclastic flow deposits form two distinct units, eachof which may represent a major pyroclastic eruption (caldera forming ?). The distributionof these pyroclastic rocks and their physical volcanology and alteration is currently thesubject of a doctoral thesis (Hudak, in progress). Work completed so far shows that theash flows associated with the first eruptive event are quartz- and plagioclase-crystal richwhereas the ash flows associated with the second eruptive period were dominantly quartzcrystal-rich and contain traces to 5% of K-spar crystals with associated K-spar in thegroundmass. These quartz-rich pyroclastic flows are the host rocks for the Sturgeon LakeMine, Lyon Lake and Creek Zone massive sulphide deposits. Feldspar-bearing lava flowsand domes (up to 400 m thick and 3 km long), along with a variety of dome-derivedvolcaniclastic rocks and debris flow deposits, overlie the pyroclastic flow units. The domesoccur close to caldera margins and are believed to represent the last felsic eruptiveproducts associated with caldera formation. Locally iron formation (banded magnetite-12
chert and iron carbonate) and graphitic sedimentary rocks are interlayered with domederivedvolcaniclastic rocks and represent relatively quiet sedimentation and lowtemperature hydrothermal activity in basins that formed between the growing domes andcaldera walls.The L succession is overlain by extensive basaltic-andesitic lava flows which havebeen termed the No Name Lake Andesite (at Mattabi) and the Lyon Lake Andesite (at LyonLake). Previous workers believed that the No Name Lake Andesite separated the Mattabiand L pyroclastic rocks from what had been termed the NBU rhyolites. It was believed thatthe NBU rhyolites were the host rocks for the Lyon Lake and Creek Zone ore bodies, andthat the Lyon Lake andesite stratigraphically overlaid these ore deposits (Mumin, 1988,Franklin et aI., 1977). Detailed volcanological facies mapping and core relogging, alongwith geochemical studies, now shows that No Name Lake and Lyon Lake Andesites arelaterally correlative, and the so called NBU rhyolite can be correlated with rocks formedby the second L pyroclastic eruptive event. The No Name Lake-Lyon Lake basalts andandesites form amygdaloidal to massive flows, pillow lavas, hyaloclastites and scoriaceousrocks and bring an end to caldera pyroclastic activity.SUMMARY OF THE VOLCANOLOGICAL HISTORY OF THE STURGEON LAKECALDERA COMPLEX1) Subaerial basaltic shield volcanism.2) Subaerial-shallow subaqueous scoria and tuff cone formation as an end stage inthe development of the shield volcano.3) Formation of the ancestral Beidelman Bay silicic, high level magma chamber andinitial eruption of the Jackpot Lake pyroclastic rocks along incipient ring(?)fractures.4) Major eruption of the Jackpot Lake pyroclastic flow and fall deposits, triggeringinstability and initiating the first caldera collapse event.5) Eruption of quartz crystal-rich ash flow tuffs along major ring(?) faults, coupledwith the avalancl"ling and sliding of precaldera material from growing calderawalls, to form the mega- and mesobreccia deposits and the mixed breccia andpyroclastic units.6) Continued eruption of pyroclastic material from a zoned magma chamber with a13
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Page 39 and 40: REFERENCESBusby - Spera, C., 1984,
Page 41: associated with the Lyon Lake massi

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