Geochemistry of Hydrothermally Altered Rocks at the Horne Mine, Noranda, Quebec*

Low K rhyolites of the Archcan Blake River Group that host the Horne volcanogenie massive sulfide deposit are altered to quartz-sericite-chlorite-albite (quartz-sericite zone) and chlorite q- quartz (chlorite zone) assemblages. The quartz-sericite zone accounts for •90 percent of the 5 km a of the altered hanging-wall and footwall rocks. Chlorite zone alteration is limited to portions of the footwall and flanks of the Cu-rich H orebodies. Clusters of gold-bearing chlorite-quartz veinlets crosscut large volumes of the quartz-sericite zone rocks. Alteration has added an average of 13 wt percent SiO2, 2.3 wt percent K20, 430 ppm Ba, and 46 ppm Rb to the system and leached 3.6 wt percent NaaO, 2.2 wt percent CaO, and 41 ppm Sr. Middle and light REE were increasingly mobile from Yb to La; they were leached from the chlorite zone rocks and added to quartz-sericite zone rocks. Chlorite was more Fe rich (Fe/Fe q- Mg-- 0.65-0.85) in the chlorite zone rocks than in other altered lithologies (Fe/Fe + Mg = 0.2-0.5). Sulfur isotope values for massive ores are in the range ba4S---2.1 to 1.9 per mil. Mineral separates from altered rocks yielded blSO values of 10.2 to 12.1 per mil for quartz, 7.2 to 9.