Geology, geochemistry, geochronology and genesis of granitoid clasts in breccia-conglomerates, MacLean Extension orebody, Buchans, Newfoundland

Stewart, Peter William (1985) Geology, geochemistry, geochronology and genesis of granitoid clasts in breccia-conglomerates, MacLean Extension orebody, Buchans, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Granitoid clasts found in association with the transported orebodies at Buchans are largest, and occupy the greatest volume of the host polylithic breccia-conglomerate subunit when proximal to the greatest sulphide accumulation in the sequence of debris flow deposits. The granitoid clasts are typically the most rounded clast lithology present in the debris flow deposits. They decrease in size and volume with increasing distance from the lowermost sulphide-rich sections of the debris flow sequence. They show igneous textures and compositions ranging from trondhjemite (rare) to quartz porphyritic microtrandhjemite to aplite to granophyric aplite. All granitoid clasts have been classified into six ‘types’ based on these textural and grain size differences. The five most abundant ‘types’ are reduced to ‘granitic’ and ‘aplitic’ groups based on textural similarities. -- Hydrothermal fluids deposited calcite, barite, and quartz sericitized plagioclase grains, and chloritized all mafic phases present. Despite this alteration, and variable alkalki metasomatism (loss of K) that is presumed du to late-stage volatile loss, all granitoid clasts appear to have a common magmatic source based on similar trace element abundances. -- The ‘granitic’ group clasts are typically larger, occupy more volume and are more altered than ‘aplitic’ group clasts. The proportion of ‘aplitic’ group clasts to ‘granitic’ group clasts increases with decreasing sulphide concentration in the debris flow sequence. Similarly, the average size and volume occupied by granitoid clasts in debris flow subunits decreases with increasing distance from the sulphide-rich sections of the debris flow deposits. -- All granitoid clasts appear to represent fragments of the same magma system that produced the felsic volcanic rocks of the Buchans Group. This conclusion is based on similar mineralogical and petrographic features, and similar major and trace element abundances (especially TiO2, Zr, Y, V, Nb, Ga and REE) between granitoid clast types and the Buchans Group felsic flows. U/Pb isotopic data for an ‘aplitic’ group clast, although imprecise, overlaps within analytical uncertainty the isochron produced from a Buchans River formation rhyolite. The calculated ages, 464 ± 40 Ma and 489 ± 20 Ma respectively, overlap within the estimated age uncertainties. -- The granitoid clasts were altered, rounded and transported to the surface in breccia pipes by explosive volatile activity that is probably due (at least in part) to the exsolution of an aqueous phase from the source magma chamber. The explosive hydrothermal events that transported the granitoid clasts (and fragments of previously deposited lithologies) to the surface may have initiated the movement of the debris flows when the breccia pipe breached the surface, disrupted the in sit sulphide mineralization process and resulted in the eventual cessation of massive sulphide deposition at Buchans. -- The change in character of the granitoid clasts during the period of production of the debris flows from highly altered ‘granitic’ group clasts to a finer grained, smaller, and volumetrically less abundant ‘aplitic’ group clasts suggests that the latter originated from shallower depths and had a more rapid transportation history (i.e. less exposure to the hydrothermal fluids) than the ‘granitic’ group clasts. -- The Feeder Grandiorite as exposed at Wiley’s River is considered a high level plutonic facies of the Buchans Group, and evidence of a slightly more differentiated magma than that which produced the Buchans Group felsic volcanic rocks and granitoid clasts. The textural, mineralogical and geochemical similarities between these populations suggested by Thurlow (1981 a, b) are verified. The Little Sandy Lake intrusion appears to be from the same magma system as the Wiley’s River intrusion, Buchans Group felsic flows and the granitoid clasts on the basis of textural and mineralogical similarities and major and trace element geochemistry. -- The geochronological study, although relatively imprecise, suggests that the Buchans Group is middle Ordovician (Llanvirn-Llandeilo?) in age rather than post-Caradocian as suggested by earlier workers. If true, this requires a reappraisal of the position of the Buchans Group in Central Newfoundland volcanic stratigraphy.

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