Territory Stories

Woodgreen, Northern Territory : explanatory notes



Woodgreen, Northern Territory : explanatory notes

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Northern Territory Geological Survey


E-Publications; E-Books; PublicationNT; Australia 1:100 000 Geological Map Series




Australia 1:100 000 Geological Map Wood 5458; Australia 1:250 000 Geological MapAlcoota SF5310; Australia 1:250 000 Geological MapAlcoota SF5310

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1:100 000


Made available via the Publications (Legal Deposit) Act 2004 (NT); Available from GEMIS - Geoscience Exploration and Mining Information System




Geology; Georgina Basin; Arunta Region

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Northern Territory Government; Northern Territory Government

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1st ed.


Australia 1:100 000 Geological Map Series

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Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government



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https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/81885 [GEMIS]

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11 in weathered outcrops. In many weathered outcrops, a biotiterich phase, which is sometimes porphyritic, shows a strong, northwesttrending steep foliation. Fresh outcrops show little or no foliation and it is unclear if the foliation has been strongly enhanced by weathering, or if there is a strongly foliated phase that is preferentially weathered. Pegmatites are abundant throughout the area, but most are too small to distinguish on the map. Those in the south are typically rich in tourmaline. The granites intrude amphibolite near the Utopia tantalite prospect, but otherwise the relationship with other units is unknown. The granites typically have a low to moderate (up to 1000 x 105 SI susceptibility) magnetic response. Tabular bodies of fresh layered calcsilicate rock are present within the granitoid about 4 km eastnortheast of New Bore. The calcsilicate rocks are compositionally variable and include felsic, epidoterich and amphibolerich varieties. Felsic rocks are rich in plagioclase and quartz, with lesser epidote, titanite, actinolite and clinopyroxene. These are interlayered with rocks that are rich in grossular garnet, together with very epidoterich rocks that contain the assemblage epidote, plagioclase, titanite, clinopyroxene, grossular and quartz. Hornblende amphibolite, with common titanite and coarse phenocrysts of plagioclase, may have a metaigneous origin. The two main outcrops (433400mE 7553763mN, 432300mE 7554000mN) both have a similar northwesterly trend and may be linked beneath cover. Given the calcsilicate composition of the rock, it is most likely a large raft of metasedimentary country rock. It has mineralogical similarities to the amphibolite near Utopia tantalite prospect at 444800mE 7541700mN and may have a similar origin. This is also indicated by similar Upredominant radiometric signatures. Preliminary SHRIMP UPb zircon geochronological data (J Claoue Long, Geocience Australia) indicates that the Mollie Granite Complex intruded during the Strangways Event. undivided granitoid rocks (LPg) Near the central northern border of Woodgreen, the Grant Bluff Formation overlies a mediumgrained, equigranular, leucocratic muscovite granite. This granite is effectively nonmagnetic. Most outcrops are strongly weathered. The granite is white in colour away from the unconformity, but is notably reddened within tens of metres of the unconformity surface. Similar granite on the southern margin of BARROW CREEK intrudes the Ledan Schist (Haines etal 1991). Several low outcrops of deeply weathered and ferruginised (but nonmagnetic) mediumgrained equigranular granite are found around 400800mE 7540800mN, south of Mount Harper. Around 444592mE 7532298mN, weakly foliated pink muscovitebearing leucocratic microgranite is overlain by Georgina Basin rocks. It is almost nonmagnetic (maximum susceptibility 66 x 105 SI), with a moderate radiometric response. At four localities (429772mE 7517459mN, 432000mE 7517000mN, 434900mE 7515400mN, 433600mE 7515200mN), small foliated granitoid bodies intrude the Delny Metamorphics. The easternmost of these outcrops may be slightly magnetic; the others are nonmagnetic. The rocks are strongly layered and, in places, migmatitic, with discrete biotiterich and leucocratic zones. Petrographic examination shows significant deformation, recrystallisation and epidote alteration. A number of outcrops of weathered granite are present to the west of Mount Skinner trigonometric station and near Kunoth Knob. Most are medium grained, leucocratic and nonmagnetic. A small, relatively fresh outcrop of fine to medium biotite granite is present at 441648mE 7533631mN. This rock is no more than weakly magnetic (maximum susceptibility 188 x 105 SI), although much of the unexposed basement in the vicinity (interpreted as LPgw3; see above) appears to be highly magnetic. It is possible that all these granites belong to the Woodgreen Granite Complex, but pending more detailed studies, they remain undivided. Granite basement (essentially nonmagnetic; see Geophysics) has also been intersected beneath Georgina Basin sedimentary rocks in diamond drillholes CMS2, CMS3 and CMS4. ultramafic intrusive rock (px) A 30 mdiameter circular body of coarsegrained pyroxenite is present within the Delny Metamorphics at 431178mE 7517459mN and is interpreted as a pipelike intrusion. The rock has largely recrystallised to secondary minerals, but appears to have comprised orthopyroxene, clinopyroxene, magnetite and possibly olivine. Abundant, late fibrous Figure11. Woodgreen Granite Complex (LPgw5). Typical exposure of dark porphyritic phase. 433994mE 7517894mN, 4 km westnorthwest of Ugnada Bore.