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TEACHING EARTH SCIENCES ● Volume 26 ● Number 3, 2001 Iron Ore Formation: A Laboratory Model JOHN MOSELEY Geochemical processes are not easy to replicate. Actual time, temperature, pressure and pH-Eh levels can be difficult to reproduce accurately in a school laboratory. However, an interest in small-scale hematite deposits in Charnwood Forest (Leicestershire), South Shropshire, and the well-known iron ore deposits of the Forest of Dean, Gloucestershire has stimulated the development of some simple laboratory models. This investigation can be used to develop interesting and perhaps contentious discussion on geochemical processes as well as providing material for ‘A’ level geology course work. Figure 1 The Precambrian geology of Charnwood Forest Geological Background The very late Precambrian to early Cambrian Charnian Supergroup of Charnwood Forest is overlain unconformably by red Triassic breccias, sandstones and mudstones. Field evidence and geochemical analyses show that more silicic rocks, silicified rhyolitic volcanics and quartz arenite sandstones are all preferentially hematised (Moseley 1994). The strong, slaty cleavage that penetrates all Charnian rocks provides structural planes that are stained, or carry thin deposits of hematite. Pale grey rhyolitic dust tuffs low in the succession carry a reddish hematite signature while quartz-rich sandstones from the Brand display thin hematite deposits along joint and cleavage planes. Cracked quartz phenocrysts in some of the porphyries (Whitwick) Complex of northwest Charnwood are an attractive red colour where hematite fills fractures (see Figure 1). Figure 2: Geology of South Shropshire Localities where rocks have been hematized 1. Manstone Rock; 2. The Knolls; 3. Myndtown; 4. Wart hill In South Shropshire the Stiperstones Quartzite of the Ordovician Shelve inlier and Precambrian Uriconian Volcanic Group of The Knolls and Wart Hill inliers display some small scale hematisation, as do Longmyndian sandstones near Myndtown and Caradocian sandstones near Wart Hill (Moseley 1994). There is no red bed cover here, but red Triassic strata crops out 20 km to the north. These strata might once have extended further south but have since been removed by erosion (see Figure 2). Sparse hematite deposits near Manstone Rock on the Stiperstones have some malachite and azurite associated with them and the possibility of derivation from weathering of traces of chalcopyrite associated with the sulphide-barite deposits of the overlying Mytton Flags Formation should not be overlooked. www.esta-uk.org 98
TEACHING EARTH SCIENCES ● Volume 26 ● Number 3, 2001 Figure 3: The Geology of the Forest of Dean The Carboniferous Crease Limestone and Coal Measure sandstones of the Forest of Dean (see Figure 3) display some intense hematisation (Lowe 1993, King 1998, Moseley 2000). These deposits were once extensively worked, and views differ on their origin. Weathering of either red Triassic strata or pyritous Coal Measure shales and coal probably sourced the iron. Speliogenesis might have accompanied hematisation as acidic, iron-bearing solutions descended and circulated. A pleasant walk of 500 metres westwards from the south end of Cannop Ponds along a forest track brings one to the memorial for miners lost in the Union Colliery disaster of 1904 (see Figure 4). Close by is a small privately owned drift mine. Pyritous shale waste can be seen here, already limonite stained due to pyrite decomposition. At the entrance to the nearby Slade Hill quarry, large joint-bounded sandstone blocks carry hematite deposits up to 2 cm thick. N O m 200 Drift Mine Memorial Quarry Slade Hill To Lydney Forestry Track B4234 Cannop Ponds Works Figure 4: Slade Hill Area, Forest of Dean Below: Titration A Laboratory Model The interpretation of British hematite deposits is that iron-bearing solutions derived from weathering of a red bed cover, or pyrite rich rocks (King 1998, Lowe 1993), or as in the Furness deposits (Rose and Dunham 1977), from a volcanic source, have circulated laterally and/or downwards, and deposited iron oxides in silicic or carbonate rocks. Unweathered source rock material is unproductive for generating solutions for laboratory 99 www.esta-uk.org
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