Explanatory for the map of geomagnetic interpretation of the Tanami and northern Arunta regions in Western Australia and Northern Territory

Abstract

This is a compilation and distillation of work presented at various conferences. The geology of the map area includes four major stages: 1) Archaean gneisses form the basement of, 2) Palaeoproterozoic orogenic provinces, 3) Mesoproterozoic to Neoproterozoic deformed basins and 4) Paleozoic platform cover sequences. The Archaean basement, which consists of banded granitic gneisses, paragneisses, amphibolites and possible granulites, has been observed in the southern and eastern areas of the Tanami region, and in cores of structural domes. The main blocks of Archaean gneisses appear to separate Tanami region from the Arunta region to the south and east. Archaean ages of -2500 and 2540 Ma have been obtained from two areas: the first was from banded granitic gneiss sample collected from one outcrop in the area to the southwest of Mount Davidson; the second was from banded grey gneiss in the basement core of the Mesoproterozoic Browns Range Dome. The Palaeoproterozoic orogenic provinces include ten Palaeoproterozoic tectonic cycles in the whole Tanami - Arunta region, but the map area includes only strata of the earlier five cycles. The strata of each tectonic cycles are the cycle 1 Tanami Group in the Tanami Orogen and cycle 1 Yundurbulu Group in the Arunta Orogen, the cycle 2 Pargee Sandstone in the Tanami region and the cycle 2 Woodforde River Group in the northern Arunta region, the cycle 3 Tanami Mine succession the Tanami region and cycle 3 Lander River Group in the Arunta region, the cycle 4 Nanny Goat Creek beds, and the cycle 5 Mount Winnecke Formation and Supplejack Downs Sandstone. The stratigraphic units of Meso proterozoic to Paleozoic are adopted from Survey maps. Each tectonic cycle includes deposition of strata, magmatism, deformation and mineralisation. Stratigraphic groups deposited during different cycles are separated by angular unconformities. Events of later cycles usually have given influences to the geological bodies formed in the earlier cycles. Strata formed in the later cycle obviously cannot record the earlier events. Each tectonic cycle started from deposition of sediments (may include volcanic rocks) within a subsiding basin which was largely caused by extension of the lower crust, and ended by compression which was caused by shortening of the lower crust. The cycle 1 Tanami Group recorded six ductile deformation events, which caused ductile deformation and thermodynamic metamorphism. In addition to Dl to D5, a pre-Dl deformational event was recognized by the author since 1990. D1 was the orogenic deformation of cycle 1 Tanami Group. D2 to D5 were the influence from later sequential orogenic events. Pre-orogenic deformation was labeled ED1, which was a long lived bedding parallel or subhorizontal shearing, which caused bedding parallel schistosity (ES I), recumbent folds (EF1), mineral and stretching lineation (ELI) and features caused by bedding parallel elongation (e.g. boudinage of competent layers or bands, sheathing of EFl folds). During prolonged ED1, multi phase hydrothermal activities, sequential intrusion of granite sills and dolerite, and a progressive development of ES1 within the pre-Dl granite and dolerite provide marks for further subdividing of the ED1 event into EDla, EDlb, EDlc and EDld. Dl was a NW-SE oriented compressional event, which caused NE trending (F1) fold belt, or orogenic belt. Thermodynamic metamorphism caused new mineral growth and formed pervasive S 1 schistosity and was probably responsible for the progressed metamorphism in one zone and retrogressed metamorphism in the next zone. Synorogenic magmatism occurred as syn-D 1 gneissic granite. Post-orogenic acid magma intruded into the region and formed large plutons and plugs. These granitoid plutons partially cratonised the crust of the region. D2 was a NE-SW oriented compressional event and caused refolding of F1 folds. Thermodynamic metamorphism also caused new mineral grow and formed pervasive S2 schistosity. Syn-D2 magma intruded into the strata along or cross cut S2 schistosity and formed foliated syn-tectonic tonalite dykes, which have been further metamorphosed and boudinaged during D2. D2 was the first deformation affected the Pargee Sandstone and caused NW trending tight folds and low grade metamorphism. D3 was a NW-SE oriented compressional event, similar to that of D1, and caused refolding or kinking of F3 folds. S3 is mainly a crenulations of earlier formed schistosity. A possible example of syn-D3 magmatism is a NE trending foliated andesite dyke which cross cut S2. D3 was the first deformation affected the Tanami Volcanics and caused NE trending folds without metamorphism. D4 was a N-S oriented compressional event and caused large-scale E-W trending shear zones, faults and small-scale F4 folds. Syn-D4 magma intruded into the region and formed E-W trending foliation, which parallel to S4 in country rocks. No correlation has been made between the D4 and stratigraphic group in our ELs, but it is possible that an unrecognised stratigraphic group was deposited after D3 and deformed by D4. D5 was an ENE-WSW oriented compressional event and caused large-scale NNW trending faults and small scale F5 folds in the Tanami Group. D5 was the first deformation affected the Mt Winnecke Formation and caused northerly trending folds without metamorphism. Post-D5 extensive acid magmatism formed many plutons and largely cratonised the region. Mesoproterozoic strata include the Mesoproterozoic (Carpentarian) Birrindudu Group ~d) in Birrindudu Basin; the Carpentarian Pindar Beds ~c), the Carpentarian Baines Beds ~up) and the Carpentarian Ima Ima Beds (~um) in western Birrindudu Basin; the Carpentarian Bungle Bungle Dolomite (~s-~a) to Albert Edward Group in unnamed basin in northwest. They are all platform-like cover type strata. However deformation events occurred before the deposition of Neoproterozoic strata. The structural domes and basins within the Birrindudu Group are the results of interference between at least two events of deformation. Neoproterozoic strata include the Neoproterozoic (Adelaidean) Red Cliff Pound Group (~r, including Lewis Range Sandstone, Muriel Range Sandstone etc.), the (Adelaidean) Hidden Basin Beds (~ui), the (Adelaidean) Boee Beds, Jawilga Beds and Denison Beds ~u). They are all platform cover type strata, but all deformed and developed folds and faults. This region was finally cratonised before Cambrian and overlain by flat lying Antrim Plateau Volcanic sequence, which signaled the very end of multi phase tectonic cycles. The structures of the Paleozoic sequences in the Wiso Basin and Canning Basin have not been actually studied. Gold mineralisation occurred during ED1, Dl, D2, D3, D4 and post D5. The ED1 mineralisation caused banded metasomatic lithologies, known as host units within the Davidson Beds. They are thin banded lithologies contain various amphibole, biotite, garnet, quartz and sulphides in amphibolite facies area or contain various chlorite, cummingtonite, actinolite, biotite, quartz and sulphides in greenschist facies area. Both the schists and quartz veins can carry economic grade of gold The Granites deposit and the Dead Bullock Soak deposit are mainly contributed by the ED1 mineralisation. The D 1 mineralisation is mainly quartz and calc-silicate quartz vein type controlled by F1 folds and S1 schistosity. The pre-D1 dolerite was not mineralised during ED1, but can be the host rock of D1 mineralisation. The D2 mineralisation is mainly quartz -calcite vein type controlled by D2 shear zones and fault zones. The D 1 and D2 mineralisations also occurred within the Madigan Beds. The D2 and D3 mineralisation also made contribution to the Granites deposit and the Dead Bullock Soak deposit. The D3 mineralisation is quartz -chlorite vein type controlled by D3 faults and fractures. Its best known example is the mineralisation within the Tanami Volcanics at the Tanami Gold Mine. The D4 mineralisation is quartz -chlorite -calcite vein type controlled by D4 structures, i.e. large D4 fault zone, F4 folds, S4 schistosity and fractures. The Callie deposit and Titania deposit were mainly contributed by D4 mineralisation. Post-D5 mineralisation is found in the 1785 Ma small intrusive bodies, e.g. the Twin Bonanza prospect. Later mineralisations could be certainly hosted by older lithologies, but the early mineralisations cannot be found within the younger lithologies. There could be more unknown types of mineralisation waiting for us to discover.