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Characteristics and chronology of fracture — fluid infiltration in the Archean,Eye Dashwa Lakes pluton,Superior Province: evidence from H,C, O-isotopes and fluid inclusions
Authors:R Kerrich  D C Kamineni
Institution:(1) Department of Geological Sciences, University of Saskatchewan, S7N OWO Saskatoon, Saskatchewan, Canada;(2) Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Ltd., ROE 1LO Pinawa, Manitoba, Canada
Abstract:The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock delta 18O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3permil), quartz-biotite (5.2 to 5.4permil) and quartz-magnetite (+9.8permil) fractionations. Magmatic fluids had a calculated isotopic composition of delta 18O=7.9±0.5, and deltaD=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature lsquomagmaticrsquo fluids where delta 18O=8.0 to 10.3 and deltaD=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl2 brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin (delta 18O=+0.4 to +3.8, deltaD=–10 to –35) undergoing transient boiling. Upper plateau 40Ar/39Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by delta 18O=4.7 and delta 13C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl2-NaCl brines, where delta 18O=0 to –6.5 and deltaD=–10 to –30. Incremental 40Ar/39Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by delta 18O=–0.4 to –5.4 and deltaD=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl2-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay (delta 18O=12 to 20, deltaD=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature (delta 18O=23±0.5, delta 13C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of sim200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of sim15 km in the crust.
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