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CONTRIBUIÇÃO A METALOGÊNESE DO ESTANHO EM MEIO SUPÉRGENO - A JAZIDA DE SERRA DA ONÇA B EM RONDÔNIA

1993; Sociedade Brasileira de Geologia; Volume: 23; Issue: 4 Linguagem: Inglês

10.25249/0375-7536.1993234400407

2177-4382

Sônia Maria Barros de Oliveira, JÚLIO C.P. VALENTE,

Agricultural and Food Sciences

In the Serra da Onça B, State of Rondônia, Brazil, a supergene tin deposit has been derived by weathering of the Santa Barbara granite and greisenized gneissic country rock. Cassiterite is concentrated in the upper two horizons of a lateritic weathering profile comprised, from bottom to top, by a saprolitic level, a stone line of ferruginous nodules and loose-clay level. On the basis of geochemical and micromorphological evidence, this article suports the hypotesis of an autochthonous origin for the Serra da Onça B stone line lateritic profiles. Accordingly, they are thought to have resulted from secondary in situ transformation of an older cassiterite-rich ferricrete formed under dryer climatic conditions that supposedly prevailed in the region during the early Tertiary. Due to the northward drift of South America since then, the climate has become more homid, leading to the dismantling of the ferricrete. The remnants of this former pedological system are the ferruginous nodules that form the stone line. This hypothesis opposes the currente view that considers these profiles as the result of colluvial processes related to Quaternary climatic fluctuations.