A total of 75 soil samples were collected from podzolic soils at eight sites in northwestern Quebec in order to compare results in contaminated locations near the Horne Copper smelter in Rouyn-Noranda with those near Hudson Bay, 800 km further north, and
a priori
devoid of pollution. Lead concentrations and Pb isotopic compositions were determined on soil samples leached by 0.25 M HCl.
Lead is enriched in the surface organic horizons at all sites in Rouyn-Noranda. Its concentration decreases abruptly from the surface organic horizons to the underlying mineral horizons. The
206
Pb/
207
Pb ratios are low in surface organic horizons, and they increase sharply in the mineral horizons. Along a NE–SW transect, the highest average Pb concentration (869 ppm) in organic horizons is found at the test site, 9 km from the smelter. The lowest average of 39 ppm Pb in surface organic layers is accompanied by the highest average
206
Pb/
207
Pb ratio (1.15) at site 5, 116 km NE of the Horne smelter. At the test site, the highest Pb concentration of 1414 ppm is also accompanied by the lowest
206
Pb/
207
Pb ratio (0.98), which is close to the isotopic composition of Noranda galena (
206
Pb/
207
Pb=0.92). Both Pb concentration and isotopic composition indicate that the Horne smelter is the main source of Pb contamination of soils in the Rouyn-Noranda region. The two soil profiles from near Hudson Bay display much lower Pb concentration and different trends of isotope ratios with depth.
Archean pyroclastic rocks at Reneault and D'Alembert in the Noranda region are composed of crystals and lithic, vitric and pumiceous fragments. The pyroclastic rocks contain two types of beds, whose primary structure sequences and other characteristics show that the deposits accumulated from different types of density flows. The first bed-type (A) is characterized by greater bed-thickness, reverse grading and the coarsest fragments in the deposit. Closest to source, the primary structure sequence in Type A beds is indicative of deposition from debris flows, but in more distal sections it suggests deposition from turbulent suspensions. The second bed-type (B) is characterized by a smaller fragment size, thinner beds, normal grading and the presence of parallel and (or) oblique stratifications. The primary structure sequence in Type B beds is similar to that observed in turbidites, which suggests deposition from turbulent suspensions of low density.Lateral and vertical variations ofgrain size, bed thickness and structure sequences indicate that the vent was south of Reneault and that the volcanic activity increased with time. Pillowed flows both underlie and overlie the pyroclastic units which suggests accumulation under water, but there is evidence that some eruptions were subaerial.Les pyroclastites, d'9ge Archeen, de Reneault et de D'Alembert de la region de Noranda sont composees de cristaux, de fragments lithiques et vitreux, ainsi que de pumices. On observe dans ces pyroclastites deux types de lits dont les skquences de structures primaires ainsi que d'autres caracteristiques indiquent que les dep6ts furent formes par differents types de coulee de densite. Le premier type de lits (A) est caracterise par une plus grande epaisseur des strates, la granulometrie la plus grossiere, ainsi que par un granoclassement inverse. Dans ces lits, les sequences de structures primaires suggerent un transport par debrisflow, pres de la source, et par suspension turbulente dans les sections plus distales. Le deuxieme type de lits (B) est caracttrise par des Cpaisseurs plus minces, des granulometries plus fines, et par la presence de granoclassement normal ainsi que de stratifications paralleles et obliques. Les sequences de structures observees dans ces lits sont semblables a celles observees dans les turbidites ce qui suggere un transport en suspension turbulente de faible densite.Les variations laterales et verticales de la granulometrie, de I'epaisseur des strates et des sequences de structures indiquent que lacheminee etait situee au sudde Reneault et que I'activite volcanique s'est accrue avec le temps. La presence de laves en coussinet au-dessus et en-dessous des pyroclastites suggere une accumulation sous I'eau. Certains faits supportent cependant des eruptions subaeriennes.Can. J. Earth Sci., 15,874-888 (1978)
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