Gˇologie et gˇochimie du gisement de Norbec, rˇgion de Rouyn-Noranda.

Cattalani, S., MacLean, W.H., Shriver, N., Barrett, T.J., and Hoy, L., 1994. Minist¸re de l'Energie et des Ressources du Quˇbec, Rapport ET 92-01, pp. 47. The Norbec massive sulphide deposit was discovered in 1961 and went into production in 1964. The deposit occurs in the Central Mine Sequence of the Archean Blake River Group, at the contact between footwall Waite rhyolite and hangingwall Amulet andesite. The deposit consisted of a main stratiform lens (zone A), a strongly mineralized stockwork (zones B and C) and a small massive lens (zone D) located ł600m away along the same contact. The main lens extended almost 200m laterally, with a maximum thickness of 65m. Between 1964 and 1976, the Norbec deposit produced 4.6 million tonnes of ore grading 2.61% Cu, 3.88% Zn, 43.8 g/t Ag and 0.65 g/t Au. Chalcopyrite is concentrated towards the base of the orebody, with sphalerite, pyrrhotite and pyrite abundant in the upper part.

The Norbec massive sulphide deposit lies at the top of the Waite rhyolite and is overlain by the Amulet andesite. The Waite rhyolite, which ranges from aphyric to quartz and plagioclase porphyritic, is about 200 meters thick below the mine, and is underlain by the Waite andesite which is at least 500 m thick. The Waite rhyolite contains massive and breccia units, with minor tuff beds. In the footwall of the massive sulphide orebody, both the Waite rhyolite and Waite andesite are hydrothermally altered and variably mineralized into a pipe-shaped alteration zone. The Main Contact Tuff, which consists of fine-bedded cherty tuff with sulfide mineralization, occurs between the Waite rhyolite and the hangingwall Amulet andesite. The Amulet andesite is >800 m thick (inclusive of dykes), and displays few hydrothermal alteration effects. This unit ranges in composition from basalt to andesite.

The freshest Norbec volcanic rocks are low-K tholeiites on a K2O-SiO2 diagram. However, basaltic to andesitic compositions have higher Al2O3 contents (ł16%) than is normal for tholeiites (13-14%), and exhibit only moderate Fe-enrichment. Zr/Y ratios for the Norbec volcanic units are ł5.5, transitional between tholeiitic and calc-alkaline rocks, but typical of Central Mine Sequence volcanic rocks. REE profiles have LaN/YbN ratios that are transitional between tholeiitic (=2) and calc-alkaline (>4) rocks.

The mineralogy of altered Waite rhyolite is quartz-chlorite-biotite-muscovite-pinite-andalusite-garnet-anthophyllite-gahnite, which has resulted from contact metamorphism of hydrothermally altered rocks near the Lake Dufault granodiorite. Andalusite and pinite (finely intermixed chlorite-sericite) occur as radiating masses apparently as retrograde metamorphic replacements of cordierite, and give a distinctive 'dalmationite' texture. The Amulet andesite has a mineral assemblage of hornblende-plagioclase-epidote-quartz-leucoxene.

At Norbec, the elements Zr, Y, Nb, and the REE were immobile during the alteration. Immobile element plots display a single alteration line for the Waite rhyolite, indicating that the precursor composition was homogenous. Mass changes in the Waite rhyolite are dominated by large additions of Si and Fe, and loss of much Na. The core of the alteration zone is enriched in Mg and Fe and is poorer in K than the periphery, which is enriched in K and Si. The increase in Fe is mainly due to sulphide addition, but some results from formation of Fe-rich chlorite. Net silicification characterizes the alteration pipe, which is unusual relative to alteration pipes below massive sulfide deposits, where silica leaching normally occurs. About half the samples show mass additions of K, which originally would have formed sericite, but which has been largely converted to biotite during contact metamorphism. There is a small increase in Ca, which is unusual in alteration zones around massive sulfide deposits, and probably results from carbonate alteration.

Normative alteration minerals were computed from the chemical analyses in order to estimate the pre-metamorphic alteration assemblage. The most abundant silicate alteration minerals are quartz, albite, chlorite and sericite, the proportions of which reflect original chlorite±quartz and the quartz-sericite alteration zones. These phases were converted by contact metamorphism to biotite, chlorite, actinolite-hornblende, andalusite, garnet, sericite, carbonate, anthophyllite and minor gahnite. The majority of the mafic metamorphic minerals are Fe-rich and probably were converted from chlorite and sericite with similar Fe/Fe+Mg ratios. The presence of andradite garnet, actinolite and quartz in the sulphide-rich samples is probably a function of low bulk Al and high Ca contents. A primary alteration assemblage of quartz, calcite and sulphide is indicated for the mineralized alteration pipe. The assemblage containing Mn-rich almandine garnet, biotite, hornblende, chlorite and epidote is characteristic of the lower amphibolite hornfels facies, which is also found in the metamorphosed alteration zones at the nearby Millenbach and Amulet mines.

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