CROCIDOLITE AND MAGNESITE ASSOCIATED WITH LAKE SUPERIOR-TYPE BANDED IRON-FORMATION IN CHAPARE GROUP OF EASTERN ANDES, BOLIVIA

The Chapare Group inlier in the Interandean zone of the eastern Andes of Bolivia is described. It is interpreted as a sequence that includes Lake Superior-type banded iron formations with associated crocidolite and magnesite, which have been mined on a small scale. The stratigraphy of the Chapare Group conforms with the types of sequence that host Lake Superior-type banded iron formations. It comprises a starved-basin phase, which is marked by carbonates and chemical sediments, including banded iron formation, of the Cristal Mayu Formation and black shales of Member 1 of the Limbo Formation, followed by coarse flysch of Member 2 of the Limbo Formation. The Chapare Group is interpreted as belonging to the Upper Proterozoic to early Palaeozoic Brasiliano Cycle rather than as having a Lower Proterozoic age and is considered to have been deposited on a stable shelf on the southwestern side of the Amazonian Craton. Crocidolite occurs as restricted stockworks and formed from the iron formation at very low temperature and pressure in areas of pressure relief related to fracture formation. It is noteworthy that the deposition of this Lake Superior-type banded iron formation was apparently contemporaneous with the deposition of major Rapitan-type banded iron-manganese formations, such as Mutun and Urucum, further east on the same passive margin during the Brasiliano Cycle.