Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
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Australia 1:100 000 Geological Map Series
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Northern Territory Government
22 the sheet area, is interpreted to indicate substantial volumes of concealed granite (not Woodgreen Granite Complex, as outcrops of these units are not associated with Bouguer anomaly lows, thus they are not as felsic as the concealed body). The highest Bouguer gravity values are associated with outcropping and interpreted Delny Metamorphics and undivided metamorphic rocks in southern Woodgreen. gEologIcAlhIStory The oldest basement in Woodgreen comprises metasedimentary and metaigneous rocks of the Arunta Region. The oldest rocks are likely to be the precursor metasediments to the Anira Metamorphics, Delny Metamorphics and Mapata Gneiss. These are likely to be equivalent to either the 18101800 Ma Bonya Schist and Deep Bore Metamorphics (Ongeva package of Scrimgeour 2003) or the 18501820 Ma Lander package. A period of granite intrusion, probably accompanied by metamorphism, is inferred to have occurred during the Yambah Event at 17801770 Ma, and included the intrusion of the Crooked Hole Granite. Exhumation and erosion followed the Yambah Event, with deposition of quartz sand of the Utopia Quartzite on the exhumed basement. The 730700 Ma Strangways Orogeny led to regional amphibolite to granulitefacies metamorphism, deformation and intrusion of granitoids. Hightemperature, lowpressure metamorphism in the northeastern Arunta Region at this time has been attributed to magmatic advection of heat associated with a mantlederived thermal perturbation, resulting in regional aureole metamorphism accompanying compressional deformation (Scrimgeour etal 200). The Woodgreen Granite Complex is likely to have been generated by extensive crustal melting during this event. No record of the 15901570 Ma Chewings Orogeny has yet been identified in Woodgreen, although studies in surrounding regions suggest the likely existence of a thermal and/or uplift event in the area at this time. Local ultramafic intrusion occurred at some time after regional metamorphism, but the age of this event is poorly constrained. Deposition in the Georgina Basin was initiated at around 800 Ma as part of a then more widespread entity, the Centralian Superbasin. However, the earliest history of the basin is not preserved in Woodgreen. In this latter area, the oldest element of the Georgina depositional succession is the Boko Formation, deposited during a regional glacial episode, inferred to have occurred around 600 Ma. During and after ice sheet retreat, glacial debris was reworked by fluvial action and deposited as the Oorabra Arkose, overlying local relicts of proximal till (Boko Formation) and glacially eroded basement. A subsequent marine transgression led to the deposition of the Elyuah, Grant Bluff and Central Mount Stuart formations under shallowmarine and deltaic conditions. During deposition of the Central Mount Stuart Formation, abundant detritus was carried by rivers from uplifted regions to the northwest synchronously with tectonic control of accommodation space in faultcontrolled troughs. This tectonic event is broadly coeval with the Petermann Orogeny. At least two marine transgressions are recorded within the Woodgreen area during the Cambrian. It is likely that deposition continued episodically through to the midOrdovician, as in adjacent areas, but the record is not preserved. Compressional deformation associated with the Alice Springs Orogeny commenced at around 450440 Ma (Scrimgeour and Raith 200, Haines etal 200). Deformation during the Alice Springs Orogeny was dominated by westnorthwesttrending, southdipping, highangle reverse faults and shear zones that uplifted basement to the south and locally juxtaposed basement against Georgina Basin sedimentary rocks. Folding of the Georgina Basin occurred during this event, with monoclines developed, associated with southsideup reverse faults. It was at this time that the Centralian Superbasin was segmented and the Georgina Basin became a separate entity. Foreland deposition continued synchronously with tectonism, but the record of such synorogenic sedimentation is not preserved in Woodgreen. Ductile shearing of the basement may have been completed by the end of the Devonian, and all deformation in the Alice Springs Orogeny ceased by the end of the Carboniferous. No record of events between the late Palaeozoic and Cenozoic is preserved in Woodgreen; although the area is likely to have undergone slow erosion with sediments transported elsewhere. During the Cenozoic, local subsidence led to deposition within the lacustrine/fluvial TiTree Basin and within broad tributary channels that drained northwest into the basin from relatively elevated areas to the south and east. Deep weathering profiles and silcrete developed over flat and gently domed areas of basement. Alluvial fans developed around major ranges composed of Georgina Basin sedimentary rocks. A renewed phase of erosion up to the present time has led to partial dissection of earlier Cenozoic deposits and landforms. EconoMIcgEology Copper, lead, zinc In 1965, prospectors discovered widespread malachite and azuritestained, greygreen siltstone and loose nodules containing chalcocite between 9.5 km and 3 km north of Mount Skinner trigonometric station in what is now recognised as the Tops Member of the Central Mount Stuart Formation (Figure21). Youles (1965) visited the area on behalf of the Northern Territory Government and collected a grab sample that assayed 2.5% Cu. He optimistically extrapolated the gently dipping cupriferous interval over an area of at least 00 square miles. An orientation study by Kennecott Exploration established that soil sampling was unreliable in locating surface Cu mineralisation. Work then concentrated on subsurface exploration and seven costeans were bulldozed across a strike length of 9.5 km. Host rocks were identified as reduced siltstone within a thick interval of redbrown arkosic sandstone and siltstone. Minor lenses of chalcopyrite in the shallow subsurface correlated to the mineralised outcrop and visible mineralisation was generally confined to within 5 m of the surface. The highest value obtained from trench floor sampling was 7200 ppm Cu over 45 cm in Trench C. Three percussion holes (BH1 to 3) totalling 600 m were drilled, but the first two failed to reach their target depths because of water inflow. The highest metal intercept reported was
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