The selective extraction of Pb and Zn in selected mineral and soil samples,application in geochemical exploration (Portugal)

The study described herein concerns the application of selective chemical extractions on metal-bearing minerals and soils to geochemical exploration. Specifically, the study addresses itself to the detection of anomalous soils in the vicinity of a mineralized zone.A kinetic study of selectivity of partial chemical extractions-applied to minerals of Pb and Zn was undertaken in order to establish a systematic procedure (reagents, time of extraction, etc.) which could be recommended for the soils of the study area in Portugal. It is shown that: (1) NH₄ acetate dissolves smithsonite, willemite, cerussite, anglesite, pyromorphite completely and coronadite, galena, sphalerite only partially; (2) hydroxylamine hydrochloride dissolves coronadite only partially; (3) H₂O₂ dissolves sphalerite and galena only partially; (4) hot mixed acids dissolve residual galena, sphalerite and coronadite.About 850 soil and gossan samples were collected from two mineralized districts in Portugal, one in a carbonate environment (Portel) and the other in an aluminosilicate zone (Sanguinheiro).For the kinetic study in soil samples from Portel and Sanguinheiro zones, the minus 80 mesh fraction (−180 μm) was submitted to an extraction procedure using the following reagents, in sequence: NH₄ acetate, hydroxylamine hydrochloride, NH₄ oxalate in darkness, NH₄ oxalate under U.V. irradiation and finally strong acids.The mineralogy of the soils from the two areas is different. In the aluminosilicate zone, the products of weathering of sulphide mineralization are mainly anglesite, plumbogummite, amorphous iron oxides, goethite and hematite rich in Pb and Zn. In the carbonate area, the Pb- and Zn-bearing phases generally found in the soil and gossan are Mn oxides, lepidocrocite and hematite. The gossan, in this case, also contains smithsonite and cerussite.In soils from the aluminosilicate zone the use of NH₄ oxalate (dark) given a better geochemical response for Pb than NH₄ acetate, NH₄ oxalate (U.V.) and conventional acid digestion. On the other hand, NH₄ oxalate (U.V.) gives better results for Pb and Zn than conventional acid digestion in samples from carbonate area. The analysis of NH₄ oxalate (U.V.) extractions, instead of conventional acid digestion, can nevertheless be recommended for both environments.