Abstract

The Paleo- to Mesoproterozoic McArthur basin in the Northern Territory, Australia, is filled with mixed carbonate-siliciclastic successions and felsic and mafic volcanic units that contain authigenic minerals that formed during burial diagenesis. Petrographic investigations indicate that some of these lithologic units were cemented early by quartz overgrowths and became diagenetic aquitards, whereas secondary porosity was developed in others that became diagenetic aquifers for basinal brines. The authigenic minerals in both lithology types preserve geochemical information about the types of fluids that occurred in the basin, with those that resided in the diagenetic aquifers having similar geochemical composition to those indicated by alteration minerals in the McArthur River Pb-Zn deposit and the Westmoreland U deposits.

Quartz overgrowths formed early during shallow burial in all clastic lithologic units and are particularly common in the well-sorted, marine sediments; these became diagenetic aquitards due to the porosity-occluding cement. In the calcareous units, early dolomite replaced calcite and aragonite at shallow burial depths. Fluid inclusion microthermometry indicates that most quartz overgrowths formed between 80° and 125°C from a fluid with a salinity between 0 and 13.1 wt percent NaCl equiv. The isotopic composition of dolomite indicates δ18Ofluid of −4.5 ± 1.5 per mil and δ13Cfluid of −6.9 ± 1.0 per mil, suggesting formation from a mixed marine and meteoric fluid. Illite cemented by early quartz has a 40Ar/39Ar age of 1738 ± 10 Ma in one diagenetic aquitard, which reflects the age when the host sediment was deposited and confirms that quartz cementation occurred early during burial. With increased burial, the poorly sorted sandstones and conglomerates that were initially poor aquifers at or near the surface experienced framework grain dissolution and the creation of secondary porosity at depth. As a result, the sediments of the Westmoreland Conglomerate, the lower Yiyintyi Sandstone, significant proportions of the Warramana Sandstone, and most of the sandstone interbeds of the Gold Creek Volcanics became diagenetic aquifers and conduits for basinal brines. Illite, dolomite, and chlorite formed late in the paragenesis and filled the secondary porosity. Quartz veins formed last and primarily in faults and fractures. The dolomite and quartz veins contain fluid inclusions that have low eutectic temperatures indicative of Ca2+- and Na+-dominated brines and salinities between 18.5 and 30.9 wt percent NaCl equiv. Illite crystallinity data and chlorite chemical compositions indicate formation between 150° and 250°C. The isotopic compositions of illite and chlorite indicate δ18Ofluid values between 2.9 and 9.1 per mil and δDfluid values between −61 and −25 per mil, which suggest formation from a mixed meteoric and marine fluid. These δ18Ofluid values are indistinguishable from those determined from the isotopic composition of alteration minerals at the McArthur River Pb-Zn deposit (5 ± 5‰) or synore illite at the Westmoreland uranium deposit (4 ± 2‰), suggesting that the diagenetic aquifers and the deposits may be linked. Argon ages of illite from the diagenetic aquifers indicate that fluid migration began as early as 1680 Ma and continued to approximately 1541 Ma, coinciding with and extending past the time when the McArthur River Pb-Zn deposit (1640 Ma) and the Westmoreland U deposits (1655–1606 Ma) formed. The results presented in this study show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those that are reported from the Pb-Zn and U deposits of the southern McArthur basin.

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