Mass,volume and chemical changes in the alteration zone at the Norbec mine,Noranda, Quebec

A funnel-shaped alteration pipe in Archean rhyolite and andesite below massive sulphide Zn-Cu ores at the Norbec mine in northwestern Quebec was outlined from drill core samples, geochemical parameters, and normative alteration mineralogy. The pipe has a mass of 37.9 million tonnes giving a volume of 13 × 10⁶ m³, which represents volume increase of 10% relative to the unaltered host volcanic rocks. A bulk chemical composition was calculated using weighting procedures for volumes of influence for the samples. Net mobile mass change, or chemical flux, for the alteration pipe was + 5.8 × 10⁶ tonnes; inclusion of the massive ore lens yields a flux of + 9.6 × 10⁶ tonnes for the whole hydrothermal system. The largest additions to the system in millions of tonnes were: FeO(+4.2), SiO₂(+3.8), S(+1.8), K₂O(+0.55), and MgO(+0.5); the only depletions were Na₂O (–1.4), and CaO(–0.44). Base and precious metals accounted for 3.4% of the total element flux.