Jarosita e plumbojarosita nos gossans do distrito mineiro de Canoas (PR)

Jarosites and plumbojarosites in gossans related to Pb-Zn mineralization in the Canoas District are described. They are always associated with goethite and in minor extent with hematite. Some crystals show zoning with a core of jarosite and a rim of plumjarosite. Others are rather homogeneous with intermediate composition. In the general formula AB3(SO4)2(OH)6, A sites are occupied by K and Pb and B sites by Fe, Al and Zn. A and B sites are often incompletely filled. A sites generally show excess of charge. B sites show deficiency. Jarosites and plumbojarosites are secondary minerals formed by sulfur, iron and lead derived from the weathering of sulphides, and potassium from the gangue mineral orthoclase. The jarosite-goethite association in gossans percolated by meteoric waters is thermodynamically unstable. It persists due to the sluggishness of the transformation of the jarosite into goethite. Thus, the Canoas jarosites are transitory Pb bearing phases. In the mature gossans Pb is associated only with the oxyhydroxides. (Résumé d'auteur