Coarse-grained siderite in canga, Quadrilatero Ferrffero of Minas Gerais, Brazil: mineralogical evidence for the longevity of ferruginous duricrust

摘要

Abstract: The canga at Capitao do Mato, an iron-ore deposit in the Quadrila tero Ferrifero of Minas Gerais, Brazil, contains cavities filled with coarse-grained crystals of siderite, commonly exceeding 1 mm across. The siderite has δ~13C values between -16.7 and -11.5 %o. The siderite-bearing canga shows enrichments in some trace metals, particularly in mercury. These observations suggest that the siderite formed from solutions rich in dissolved organic matter derived from overlying organic material that accumulated in peat bogs or lakes. The former are known to have developed over the canga during the Tertiary, and they could rapidly have been removed by intense denudation as a re- sult of neotectonic activity. However, the longevity of canga in relation to erosion likely allowed the extremely slow precipitation rates of siderite to form coarse-grained crystals.