IDENTIFICATION OF MINING RESIDUAL POLLUTANTS IN THE SONORA RIVER BASIN (NORTHWESTERN MEXICO) USING THERMOMAGNETIC MEASUREMENTS

In Cananea, Sonora (northwestern Mexico), occurred a spilling of 40 000 m³ of an acid solution coming from the Buenavista del Cobre mine, which reached some tributaries and the main water stream of the Sonora River basin. This study aims to evaluate magnetic properties as proxies to assess residual pollution in soils and sediments affected by this kind of events. Particularly, the study focused on the temperature dependence of magnetic susceptibility. Measurements of volume magnetic susceptibility vs temperatures were performed, heating samples to 700 ºC and cooling them back to room temperature. A strong increase (from three to six times the initial value) of magnetic susceptibility during heating was observed on precipitates of the acid solution dam, and on affected soils and sediments, starting at ~330 ºC, and reaching a maximum at ~430 ºC. An even stronger increase was observed during cooling. These increases are due to the transformation of iron bearing minerals (as jarosite, goethite and ferrihydrite) to ferrimagnetic iron oxides (as magnetite). Samples of non-affected soils and sediments did not exhibit this increase during heating, but a decrease at ~550 ºC related to (titano)magnetite Curie temperature, and they presented lower κ values during cooling. The transformation of these iron-bearing minerals (commonly occurring in mine wastes and acid drainage) marked by a strong κ increase during heating, can be used as an indicator of the presence of residual pollution.