Stratiform manganese mineralisation near Inverness,Scotland: A Devonian sublacustrine hot-spring deposit? |
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| Authors: | K Nicholson |
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| Institution: | (1) Geothermal Institute, University of Auckland, Private Bag, Auckland, New Zealand |
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| Abstract: | A stratiform manganiferous horizon, up to 4 m thick but localised in aerial extent, occurs 10 km south-east of Inverness in north-east Scotland. It overlies Precambrian Grampian Group schists and is itself overlain by Middle Old Red Sandstone (Devonian) conglomerate, which elsewhere rests unconformably on the schists. The deposit was mined briefly in the early 1920s and is composed principally of fine-grained braunite with minor bustamite. Lower sections of the deposit contain cavities, infilled by late baryte, calcite and rhodochrosite, which resemble stromatactis. This horizon is underlain by a hematite-quartz layer, thought to represent a laterite formed by pre-MORS weathering of the underlying schists. The mineralogy and geochemistry of the manganese ore (enriched in Ag, As, Ba, Bi, Pb, Sb, Sr and Zn) are characteristic of a hydrothermal origin. Such an origin, when considered in conjunction with the outcrop, textures, mineralogy and stratigraphy of the deposit together with the palaeogeography of the region, can only be explained by deposition from hot-springs in a sublacustrine environment. Nearby baryte mineralisation may also have been deposited from the same geothermal system, which had the Caledonian 407 Ma Moy granitoid as heat source. The managanese ore is enriched in gold pathfinder elements and minor detrital gold has been found in the area. Gold mineralisation may therefore have also been deposited in the upper sections of this Devonian geothermal system. |
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