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El Mutun |
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Bolivia |
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Fe Mn
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The El Mutún iron resource is located well to the east of the Andes in Santa Cruz department of far eastern Bolivia, bordering Brazil.
The El Mutún mineralisation in Bolivia is distributed over an area of 60 sq. km and is beleived to total of the order of 40 Gt @ >50% Fe and 16 Mt @ 35% Mn.
This mineralisation is a continuation of the similar ores in the Corumba-Urucum district western Mato Grosso do Sul, Brazil - see the separate Corumba-Urucum record. The combined El Mutún-Corumba-Urucum district is some 170 km in length, the greater part of which is in Bolivia. The distribution of the host sequence in Brazil represents a very shallowly dipping shield like antiform, the core of which is occupied by a graben. The eastern, east dipping limb is in Brazil, the west dipping limb in Bolivia. The almost flat lying limbs are segmented by steeply dipping antithetic faults.
In Brazil, the host Jacadigo Group is preserved in a series of 8 tabular mountains that rise up to 1000 m out of the swampy lowlands of the Pantanal. Basement is represented by gneisses and amphibolites, with lesser schists and quartzites, cut by 889 to 1200 Ma granitoids. These are unconformably overlain by the Neoproterozoic Jacadigo Group, which comprises a lower 200 m thick Urucum Formation composed of conglomerate and siltstone, followed by mainly black shale and siltstone, passing up into mainly sandstones. All of these rocks have been cemented by carbonates.
The Urucum Formation is followed by the 300 m thick Santa Cruz Formation. The base of this unit is marked by dropstones and the change from carbonate cement to manganese and iron oxides in the host cross bedded sandstone of the 80 m thick lower member. This member carries a basal 0 to 7 m thick manganese rich horizon (Mn1) with concretionary/nodular, detritus-rich and layered massive ores. The upper member is almost exclusively composed of chemical sediments. It commences with a widespread band of layered massive ore that is 0 to 3.5 m thick (Mn2). This is followed by a 70 to 270 m thick monotonous sequence of hematite-jaspilite which covers an area of 120 sq. km. A third 0-2.35 m thick manganese horizon (Mn3) is intercalated with the hematite-jaspilite 40 to 45 m above Mn2, while a fourth such band, Mn4 which is 0-1.4 m thick, is found a further 45 m higher. Mn3 and Mn4 are again layered massive manganese oxide beds. Erratic dropstone from 0.05 to 1 m across are found sporadically throughout the chemical sediments. The exposed Jacadigo Group is surrounded by the unconformably overlying limestones and dolomites of the late Neoproterozoic Corumbá Group.
The fresh jaspilitic ores of the lower Santa Cruz Formation average 50% Fe, while the supergene enriched sections carry up to 67% Fe.
The higher grade, enriched ores occur in two forms, namely:
i). canga - Fe-hydroxide-cemented breccias of hematite-jaspilite - occurs as a sub-recent weathering crust in places, and
ii). 'colluvial' ore in the form of weathered 0.01 to 10 m blocks of hematite-jaspilite which form debris cones and fans on the flanks of the tabular mountains.
The un-enriched hematite-jaspilites are predominantly composed of micro-crystalline hematite and crypto-crystalline red jasper forming alternating bands on a micro- and macro-scopic scale. In finely laminated ores, spherical and concentric aggregates of hematite and jasper are preserved.
The most recent source geological information used to prepare this decription was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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