The Ackley high-silica magmatic/metallogenic system and associated post-tectonic granites, southeast Newfoundland

Tuach, John (1987) The Ackley high-silica magmatic/metallogenic system and associated post-tectonic granites, southeast Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Ackley Granite Suite occupies 2,415 km² in southeastern Newfoundland, and intruded across the boundary between the Avalon and Gander tectonostratigraphic terranes. An Rb-Sr whole rock age of 355 ± 9 Ma is younger than ⁴⁰Ar/³⁹Ar ages around 370 ± 5 Ma. Two calc-alkaline granodiorite-granite plutons to the west and northeast of the Ackley Granite Suite provided an Rb-Sr whole rock age of 427 ± 12 Ma, in agreement with an ⁴⁰Ar/³⁹Ar biotite age of 410 ± 4 Ma. -- Much of the Ackley Granite Suite exhibits geochemical signatures of A-type granites, and a muscovite-bearing phase occurs in the north. Geochemical enrichment/depletion trends culminate at significant, endocontact, spatially separate, aplite-pegmatite Mo deposits at Recontre Lake, and quartz-topaz-Sn greisen deposits at Sage Pond. Hydrothermal muscovites from the deposits provide almost identical ⁴⁰Ar/³⁹Ar ages, indistinguishable from ⁴⁰Ar/³⁹Ar ages of biotite in granites. The granites and the mineralized areas have high-temperature, magmatic, oxygen isotope signatures, with minor evidence for high-temperature meteoric hydrothermal activity at the prospects, and evidence for low temperature subsolidus exchange with meteoric fluids through much of the Ackley Granite Suite. The data confirm previous interpretations that large areas of the Ackley Granite Suite were rapidly cooled, and together with fluid inclusion data, provide evidence for the contemporaneity of magmatic and mineralizing processes. -- The southern Ackley Granite Suite is the frozen equivalent of high-silica, chemically zoned, magma chambers responsible for the formation of large ash flows. Convective fractionation may have operated to produce the observed chemical zonations. An emplacement model based on available geophysical data indicates stoping of large (20 km x 10 km) blocks from the upper to middle crust in response to regional crustal extension and lower crustal melting. -- The H₂O content and the degree of trace element enrichment in the magma appear to have been important controls on mineralization. The Mo deposits formed from fluids evolved from a relatively H₂O-rich magma while Sn mineralization formed from F-rich fluids evolved from a more specialized, relatively anhydrous magma. A genetic trend from Mo-mineralization to Sn-mineralization during development of the high-silica upper magma layer is suggested. -- Keywords: Ackley Granite Suite; granite; high-silica; post-tectonic; tin; molybdenum; chemical zonation; metallogenesis; isotopes; geochemistry; pluton emplacement.

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