Geology and geochemistry of molybdenite showings of the Ackley City batholith, Fortune Bay, Newfoundland

Whalen, Joseph Bruce (1976) Geology and geochemistry of molybdenite showings of the Ackley City batholith, Fortune Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Devonian (345±10 my) Ackley City batholith is an oval, approximately 5,400 km² granitoid which intrudes Ordovician and Precambrian rocks of the Gander and Avalon zones, respectively, in southeast Newfoundland. It is a composite intrusion consisting of K-feldspar mega-crystic granite in the north, east and west, intruded in the southeast by genetically related, more differentiated, alaskitic granite. Spatially related to the southeast contact of the latter are younger, fine grained and pegmatitic intrusive phases, within which are six separate Mo showings. Contacts of the batholith are sharp and characterized by low grade contact metamorphism. -- The granitoid phases are composed essentially of quartz, perthite, plagioclase (andesine to albite) and minor biotite, magnetite, sphene, zircon, fluorite, and tourmaline. The pegmatite is composed of large quartz crystals (up to 50 cm diameter) and orthoclase. Myrmekitic texture is common in the megacrystic granite while granophyric texture s rare in the alaskitic granite but common in the aplitic and pegmatic phases. -- The Mo host intrusives are all extreme differentiates and consequently show little variation in major elements. However, fractionation trends of trace elements and elemental ratios indicate they are more highly differentiated than the alaskitic granite. The latter shows a regional variation in geochemistry, indicating that its degree of differentiation increases gradationally toward the intrusive contact. -- The alaskitic and fine grained intrusive phases crystallized between .5 and 1 Kb PH₂₀ and resemble a hypersolvus granite. The fine grained and pegmatite phases are explicable as erosional remnants of a once extensive roof zone complex possibly formed by in situ differentiation resulting in concentration of volatile and Mo rich, highly fractionated magma in the roof of the alaskitic granite. -- The different showings exhibit variation in lithology of host rocks, mode of mineralization and associated trace metals, although Mo is the only economically important element. They represent a gradation between magmatic (pegmatite) and hydrothermal (porphyry affinity) deposits, resulting essentially from retention and release of volatiles, respectively. The roof zone localization of the granitoid mineralization has important ramifications for exploration on a local and regional scale. The porphyry affinities of some showings indicates that models of absence of porphyry deposits in pre-Mesozoic-Cenozoic orogenic belts due to erosional level may not be valid.

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