Geology of the Middlemarch area 1:50 000 (digital map)

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Martin, A.P.; Cox, S.C.; Smith Lyttle, B. 2016 Geology of the Middlemarch area 1:50 000 : digital vector data 2016. Lower Hutt, N.Z.: GNS Science. GNS Science geological map 5. 1 map

Abstract: The ‘Geology of the Middlemarch area’ is a 1:50 000 scale map and text covering 864 km2 (24 km by 36 km) of East Otago around Middlemarch township. The availability of regional airborne geophysics data (magnetics and electromagnetics) was a key factor in deciding to update the existing geology for this sheet, and is a point of difference between this and earlier geological maps. New geological interpretations presented here are particularly relevant to understanding the distribution of groundwater, metals and other natural resources, and this resource focus is evident in the updated localities of resource occurrences on the map and in detailed overviews of Barewood and Nenthorn goldfields. The most common rock type is greyschist of the Permian-Triassic Rakaia Terrane, derived from a sedimentary protolith, which has been overprinted by, and forms the protolith to, the Otago Schist. The greyschist in the map area is within the most highly deformed, central core of the schist belt identified as Textural Zone IV and equivalent to greenschist facies garnet-biotite-albite zone. Gold, antimony and tungsten mineralisation occurs within greyschist and faults in greyschist, within the map area and continues to be targeted by exploration. Greenschist, derived from a mafic igneous protolith, is interlayered with the metasedimentary greyschist. Porphyroblasts of magnetite in the greenschist result in a high magnetic susceptibility for the rock. Outcrop of greenschist corresponds with strong highs in the aeromagnetic data, allowing these units to be traced in the sub-surface. Greenschist forms important marker horizons for understanding the structural history of the region including several phases of folding. The schist is intruded and overlain by Late Oligocene – Miocene, alkalic Dunedin Volcanic Group, basaltic rocks that are an important source of aggregate. Eocene – Miocene sedimentary rocks of the Haerenga and Māui supergroups also overlie the schist, and although a volumetrically minor part of the geology of the Middlemarch sheet area, they are an important source of diatomite. Immediately to the north, around Hyde township and elsewhere, the Haerenga and Māui supergroups are also sources of coal, silica and ceramic clay. Quaternary sediments of the Pākihi Supergroup are mainly alluvial fan deposits that have formed on the flanks of topographic highs or fluvial deposits from river systems. The Quaternary sediments contain groundwater resources, are a source of aggregate, and have been worked historically for minor quantities of alluvial gold. This map data package includes: processed geophysical datasets (magnetic, electromagnetic, gravity), geophysical interpretations and models, updated resource locations, a detailed review of mineral, aggregate and groundwater resources, structural measurements and interpretations, cultural features, a digital elevation model, lithology, updated geological interpretations (e.g. a surface geology map, cross section), a schist outcrop map showing 250 000 outcrops, a summary of historical mineral production figures, geology, mineralisation and modern exploration history at Barewood and Nenthorn gold deposits, magnetic susceptibility and whole-rock chemistry measurements, sub-division of the Dunedin Volcanic Group and classification using the new high level stratigraphic nomenclature for New Zealand. (auth)