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Laser-ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) is a rapid, accurate, and inexpensive technique for making in situ U-Pb isotopic measurements of uraninite and davidite. The advantages of this method include: (1) mineral separation and chemical digestion are not required; (2) measurements on complex samples are feasible because significant isobaric interferences can be resolved; and (3) accurate U-Pb and [sup.207]Pb/[sup.206]Pb dates on 10-25 [micro]m spots can be obtained rapidly. The LA-HR-ICP-MS method is applied to U oxide minerals from four deposits and prospects from northern Australia, and the new dates are compared to previously published conventional thermal ionization mass spectroscopy (TIMS) dates, and to known ages of geologically important events. These comparisons permit us to assess elemental fractionation of U and Pb for uraninite and davidite of the new method, relative to zircon, as well as its geochronological accuracy and precision. U-Pb apparent ages measured previously agree well with our measurements for El Sherena, Palette, and Mt. Isa. Additionally, the upper-intercept [sup.207]Pb/[sup.206]Pb dates for Adelaide River (701 [+ or -] 190 Ma, 1[sigma]) and Palette (841 [+ or -] 94 Ma, 1[sigma]) uraninite, measured here, are similar to those previously obtained for Palette (730 Ma), Nabarlek (920 Ma), and Koongarra (870 Ma), and the upper-intercept date for El Sherena uraninite (1573 [+ or -] 160 Ma) is within error of that previously determined for Ranger (1550 [+ or -] 15 and 1472 [+ or -] 40 Ma). Such apparent-age agreement for uraninite (and similarly for davidite) indicates that U and Pb fractionations are within error of that for zircon, whereas the inherent imprecision of our dates and their associated MSWD values greater than 2.5 probably indicate that multiple resetting events affected our samples. Analytically, these results demonstrate that the LA-HR-ICP-MS technique provides excellent spatial resolution while also removing argide, phosphide, sulfide, and halide interferences that can otherwise lead to erroneous data when using quadrupole-ICP-MS. Geologically, the individual [sup.207]Pb/[sup.206]Pb and upper-intercept U-Pb dates of uraninite from Adelaide River and Palette are ca. 800 Ma, possibly reflecting recrystallization of uraninite during the break-up of Rodinia. Keywords: U-Pb age, Pb-Pb age, laser ablation, HR-ICP-MS, uraninite, davidite, Australia, U-deposit

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at 1575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca2+- and Na+-bearing brine with a salinity of 21.6 wt percent NaCl equiv. δDfluid of the fluid inclusion water ranges from -89 to -83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74° and 125°C, which are lower than the 120° to 160°C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca2+, Na+, but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher δ18Dfluid values ranging from -89 to -61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has δ18Ofluid values of 1.6 and 2.4 per mil, indistinguishable from δ18Ofluid values between -0.3 to +7.4 per mil calculated from the isotopic composition of coexisting quartz, dolomite, and illite. The trend toward lower salinities and higher δDfluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na+-dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates δ18Ofluid from 3.3 to 10.7 per mil, which is larger than the range obtained from symmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and 40Ar/39Ar ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 ± 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin.… [ABSTRACT FROM AUTHOR]

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 elastic 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 Volcanies 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.… [ABSTRACT FROM AUTHOR]

The western succession of the Mount Isa basin in northwest Queensland hosts four supergiant Zn-Pb ± Cu deposits and numerous smaller Cu and Zn-Pb deposits. Mineralization is primarily hosted in carbonaceous and calcareous shales and siltstones belonging to the 1670 to 1575 Ma Isa superbasin, but little is known about the source of metals that formed these deposits. The underlying clastic and volcanic successions belonging to the 1800 to 1750 Ma Leichhardt superbasin and the 1735 to 1690 Ma Calvert superbasin are potential metal source rocks and host a variety of diagenetic minerals that preserve geochemical information about the evolution of brines in the basin. Quartz overgrowths and pressure solution features formed during shallow burial in all clastic lithologic units but are particularly common in the well-sorted, marine-dominated units that became aquitards due to the porosity-occluding diagenetic cement. Microthermometry on fluid inclusions in the quartz overgrowths indicates formation between 100° and 174°C from a low-salinity, 2.7 to 9.1 wt percent NaCl equiv fluid (fluid 1). These data together with sequence stratigraphic mapping, basin reconstruction, and stable isotope values from the quartz overgrowths show that the diagenetic aquitards formed at <5-kin depth. Illite and chlorite formed locally in some of the diagenetic aquitards and preserve δ18Ofluid and δDfluid values of 4.5 ± 4.2 and -34 ± 14 per mil, respectively, indicating evolution from a seawater-dominated source. Silicate dissolution and the widespread formation of diagenetic illite and chlorite occurred late, during deep burial diagenesis and primarily in the proximal fluvial lithologic units. These units are recognized as diagenetic aquifers and they occur adjacent to and within the Eastern Creek and Fiery Creek Volcanics where metals could have been sourced. Basin reconstruction shows that the diagenetic aquifers formed at depths between 5 and 10 km. Illite and chlorite extracted from the diagenetic aquifers have distinct δ18Ofluid and δDfluid values of 4.5 ± 2.8 and -63 ± 11 per mil, respectively, indicating evolution from a meteoric fluid with a variable marine contribution. These isotopic values cannot be differentiated from published isotopic values of fluid inclusion water in quartz-dolomite-chalcopyrite veins at Mount Isa or sphalerite and illite from the Century Zn deposit and the Zn lodes from the Burketown mineral field. This suggests that the diagenetic aquifers were likely source rocks for metals in the deposits in the Mount Isa basin. In contrast to the phyllosilicates in the diagenetic aquifers, regional dolomitic grainstones and dolomudstones in the Lawn Hill platform precipitated from fluids with δ18Ofluid between -2.6 and 1.1 per mil and δ13Cfluid between -8.6 and -3.9 per mil. This suggests that these units did not contribute to the ore-forming brines. Quartz veins formed during the later diagenetic history in the Mount Isa basin from a low-salinity brine, between 2.7 and 10.4 wt percent NaCl equiv (fluid 2). Oxygen isotope geothermometry on quartz-hematite pairs indicates that these veins formed at approximately 230°C. Crosscutting relationships reveal that a subsequent generation of quartz veins host fluid inclusions with a distinctly saline brine, with compositions between 11.9 and 23.2 wt percent NaCl equiv (fluid 3), indistinguishable from fluid inclusion compositions recorded in sphalerite from the Century and Walford Creek Zn deposits in the Lama Hill platform and the quartz-dolomite-chalcopyrite veins at Mount Isa.… [ABSTRACT FROM AUTHOR]

The article offers information on the formation of basin system and fluid-flow models of the mineral deposits in Northern Australia. The origin and evolution of gold, zinc and lead deposits in the McArthur-Mount Isa-Cloncurry mineral belt is discussed. The formation of four types of mineral deposits, including the Mount Isa type, the Broken Hill type, volcanic hosted massive sulfide deposits and the Mississippi Valley type deposits, are discussed.

The Proterozoic sandstone-hosted Redtree and Junnagunna uranium deposits in the Westmoreland uranium field occur in the southern McArthur basin, Australia, at the contact between the Westmoreland Conglomerate and the Seigal Volcanics. Uranium mineralization consists of uraninite with hematite and illite and occurs within a zone of chlorite alteration that formed prior to the uraninite during peak diagenesis. Oxygen and hydrogen isotope ratios on synmineralization illite show that uranium was transported to the site of deposition by a basinal brine with δ18Ofluid and δDfluid values of 4 ± 3 and -33 ± 10 per mil. respectively. These values are consistent with evolved evaporated seawater but not with hot, oxidized fluids derived from underlying uraniferous granites or volcanic rocks as previously suggested. Illite crystallinity indicates that the uraninite-illite-hematite assemblage formed at 200° ± 50°C. The 40Ar/39Ar ages of illite and 207Pb/206Pb ages of uraninite indicate that mineralization occurred between 1655 ± 83 and 1606 ± 80 Ma, coincident with major tectonic events in northern Australia, and was later remobilized between ca. 1150 and 850 Ma. A U-Pb concordia age of 878 Ma is identical to the 870 Ma age recorded by previous workers on Westmoreland uranium deposits. The mineralogy, paragenesis, and geochemistry of the Redtree and Junnagunna uranium deposits are largely indistinguishable from the basement-hosted Nabarlek and Jabiluka uranium deposits in the northern McArthur basin, suggesting that the Westmoreland uranium field might also be prospective for basement-hosted uranium deposits where suitable structural and chemical traps can be found. [ABSTRACT FROM AUTHOR]

Comments on a Nabarlek uranium deposit in Northern Territory. Basis for the addition of hematite to the outer alteration assemblage; Information on quartz silification; Argument on the volume of sulfide minerals at Nabarlek.

Abstract: Quartz cement microstratigraphy and high precision in situ δ 18O values obtained by secondary ion mass spectrometry (SIMS) from μm-size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant Paleoproterozoic basins, the Athabasca and Thelon basins, Canada, and the McArthur Basin, Australia. In these examples, the earliest quartz cements have the highest δ 18O values (up to 33‰; mean=26.0‰, V-SMOW) indicative of low temperature precipitation at depths of 0 to 1.1 km. Some paragenetically early cement phases, however, have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures. Subsequent generations of quartz cements have progressively lower δ 18O values (mean=+16.1±3‰) consistent with precipitation at higher temperatures. These data combined with petrographic observations indicate quartz cementation began in near-surface vadose and phreatic zones; the next stage of cementation is characterized by syntaxial burial cement overgrowths. Regionally and stratigraphically, well-sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation, which resulted in cement-bridged pore throats at relatively shallow depths of 0–2 km and very early in their burial history transforming these units into diagenetic aquitards. As a result, these units did not experience mineralization. Compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement, but are instead marked by minor syntaxial overgrowth cement that make up <5% of the intergranular volume, formed at depths of 3–5 km, and were variably replaced by illite and chlorite during burial. These lithologies were still open to fluid movement at critical times in the basin when U and Pb–Zn–Ag deposits formed, and as a result experienced mineralization. Because of these relationships, the hydraulic properties of these Proterozoic sandstones would be difficult to predict based simply on their original texture. [Copyright &y& Elsevier]