Geochemical and biogeochemical exploration research near early precambrian porphyry-type molybdenum-copper mineralization, northwestern Ontario, Canada

Abstract

An extensive zone of molybdenite concentration occurs in the north part of a small (5.7 km2) epizonal pluton of equigranular to porphyritic granodiorite-quartz monzonite that intrudes Early Precambrian mafic metavolcanics and metasediments at Setting Net Lake in northwestern Ontario. Within an east-trending mineralized zone up to 460 m wide and 2500 m long, widespread molybdenite occurs along the margins of narrow, variably spaced quartz veins filling joints in the stock, and as minor disseminated mineralization throughout the granodiorite-quartz monzonite. The zone also contains minor chalcopyrite. The study area is typical of extensive poorly-drained, low relief regions of northwestern Quebec, northern Ontario and central Manitoba that are covered by transported Quaternary deposits of glacial till and calcareous lake clay. Average metal abundances in plutonic rocks of the Setting Net Lake stock are (in p.p.m.): Cu - 8, Mo - 1.7, Zn - 30, Mn - 277 in the barren south half of the stock, and Cu - 61, Mo - 26, Zn - 30, Mn - 240 in the mineralized north half of the stock. Molybdenum concentrations range from 1 p.p.m. to 80 p.p.m. in soils and from 2 p.p.m. to 71 p.p.m. in the ash of second year black spruce needles. Anomaly patterns can be explained in terms of pH-solubility relationships in a wide range of Eh-pH environments and it is apparent that Mo uptake by coniferous vegetation is controlled mainly by Mo concentration and mobility in supporting soils. Copper concentrations range from 3 p.p.m. to 311 p.p.m. in soils and from 32 p.p.m. to 322 p.p.m. in the ash of second year black spruce needles. Anomalous copper levels in soil extend into areas of high pH soils developed on calcareous parent material. Copper anomaly contrast is limited to 3/1 in vegetation growing in soil containing as much as 30 times background levels of copper and for this reason, it is suggested that Cu uptake by coniferous vegetation is controlled mainly by the specific biochemical requirement of the plant for this metal.