Selective chemical extraction of Cu from selected mineral and soil samples: enhancement of Cu geochemical anomalies in Southern Portugal

The study described herein concerns the application of selective chemical extractions on metal-bearing minerals and soils to geochemical exploration. Specifically, the study aims at the detection of anomalous soils in the vicinity of two mineralized zones in Southern Portugal.A kinetic study of the selectivity of partial chemical extractions applied to Cu minerals has been carried out in order to establish a systematic procedure (reagents, extraction plateau, etc.) which could be recommended for soils of the two study zones. It is shown that: (1) NH₄ acetate dissolves malachite, azurite and cuprite completely and chrysocolla, conicalcite and atacamite only partially; (2) hydroxylamine hydrochloride dissolves chrysocolla and conicalcite only partially; (3) H₂O₂ dissolves chalcopyrite only partially; (4) NH₄ oxalate (U.V.) dissolves conicalcite and atacamite only partially; (5) mixed-acid attack dissolves residual chalcopyrite, chrysocolla, atacamite and conicalcite.A total of 740 soil samples were collected from the Salgadinho and Tinoca areas. The Cu mineralization in the Salgadinho area is situated in an Upper-Devonian Volcanic-Siliceous Complex. The weathering products of the mineralization are mainly amorphous iron oxides, goethite and hematite. The Cu mineralization in Tinoca area is located in the Arronches-Campo Maior belt, where stratiform Cu mineralization is found. The weathering products of the mineralization are mainly malachite, amorphous iron oxides, goethite and hematite.In order to identify the Cu-bearing phases and the extraction plateau, the −80 mesh fraction of two soil samples was submitted to an extraction procedure using the following reagents in sequence: NH₄ acetate, hydroxylamine hydrochloride, NH₄ oxalate (dark), NH₄ oxalate (U.V.) and finally strong acids.In soils from the Salgadinho and Tinoca areas, the use of NH₄ oxalate (U.V.) in single dissolution which would incorporate those phases dissolved by NH₄ acetate and NH₄ oxalate (dark), gives the broadest anomalous surface and higher contrast for Cu than acid digestion. The analysis of NH₄ oxalate (U.V.) extractions, instead of acid digestion, can thus be recommended for both areas.