Ralph I. Thorpe

UPb zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Craton, Western Australia☆

Abstract Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, “granitic-gneissic” batholiths. A UPb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (ZnPbCuAu) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon UPb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of ∼20 Ma, from ∼3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471±5 Ma and 3465±3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454±1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458±1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than ∼3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments, is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449±3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465±3 Ma and 3449±2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T0=4560 Ma, A0=9.0818, B0=9.9002 and C0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).

Age, source and stratigraphic implications of Pb isotope data for conformable, sediment-hosted, base metal deposits in the Proterozoic Aravalli-Delhi orogenic belt, northwestern India

Abstract Stratiform, sediment-hosted, base metal deposits in the Aravalli-Delhi orogenic belt of northwestern India are hosted by the sediment-dominated Bhilwara, Aravalli and Delhi sequences of Proterozoic age. Pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information. Model Pb ages using a newly developed ‘Proterozoic model’ are ∼ 1800 Ma for the Rampura-Agucha, Rajpura-Dariba and Saladipura deposits, ∼ 1700 Ma for the Zawar deposits and ∼ 1100 Ma for the Ambaji and Deri deposits. Mineralization in this orogenic belt thus occurred at three stages in Middle to Upper Proterozoic times. The oldest of these, reflected by deposits in the Bhilwara sequence, was of broad regional extent. The Rampura-Agucha deposit is thus hosted by highly metamorphosed rocks that belong to the Bhilwara belt and not to an older supracrustal component of the Archean Banded Gneissic complex. The host rocks to the Saladipura deposit are Bhilwara-equivalent and are not part of the Delhi Supergroup, as previously mapped, and therefore stratigraphic assignments in the area must be re-examined. Model ages of ∼ 1100 Ma for the Ambaji-Deri deposits are the only evidence obtained in this study regarding the age of the Delhi Supergroup, and are in apparent conflict with much older ages indicated by previous studies. Rocks of such late Upper Proterozoic age may be restricted to the southwestern segment of the orogenic belt, or, alternatively, thrust stacking and/or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available. All the data can be readily explained by the extraction of Pb from sediments derived from an upper-crustal basement. The isotopic compositions of the Zawar deposits provide the clearest evidence for an ancient, U-enriched, upper-crustal source. The Ambaji-Deri Pb data suggest a mature status of the Delhi are when these deposits were formed.

Zircon U–Pb and galena Pb isotope evidence for an approximate 1.0 Ga terrane constituting the western margin of the Aravalli–Delhi orogenic belt, northwestern India

Abstract Zircon U–Pb ages of 987±6.4 and 986.3±2.4 Ma have been established for rhyolites in the southern and northern parts, respectively, of the Ambaji–Sendra arc terrane in the western fringe of the Aravalli–Delhi orogenic belt. Pb isotope data for volcanogenic massive sulphide deposits within this arc terrane define a linear trend, which is considered a useful reference palaeoisochron or mixing line at an age of about 990 Ma, and establish continuity of the terrane between Ambaji and Sendra. These results are in contrast to the traditional and continuing assignment of the Ambaji–Sendra terrane to the >1700 Ma Delhi Supergroup. The isotopic composition of galena from an apparently epigenetic occurrence at Punagarh Hill yields a model age of about 940 Ma, suggesting that the Punagarh Group could form part of the same arc sequence. The Phulad lineament, which has been considered by most workers to represent the western boundary of the Aravalli–Delhi orogenic belt, appears, in terms of this stratigraphic assignment, to represent an oblique structure, which has not greatly offset the Punagarh and Ambaji–Sendra domains within the arc terrane. The eastern boundary of the terrane is marked by the Sabarmati fault. A zircon U–Pb age of 836+7/−5 Ma for the Siwaya gneissic granite, in the southern part of the Ambaji–Sendra belt, is in accord with previous age data for felsic Erinpura plutons that have intruded the arc sequence. A monazite age of 826±5 Ma may reflect slow cooling of the Siwaya pluton or a younger thermal event. A model age of about 820 Ma for galena is obtained from the Tosham Sn–Cu mineralized zone, in Haryana state, about 280 km north–northeast of Ajmer, which is related to the felsic, anorogenic, Malani volcanism–plutonism. Hence, this widespread magmatism, found extensively west of the Ambaji–Sendra terrane, may have been coeval with Erinpura plutonism or followed it very closely. The present geochronological data warrant the recognition of the Ambaji–Sendra arc terrane, as defined here, as a distinct metallogenic province, which saw the formation of VMS-type deposits in the late Mesoproterozoic, around 1.0 Ga. The Pb-isotope compositions of the deposits however, do not clearly define the metal sources. The lead from Danva prospect is most primitive and must have the greatest component from a juvenile mantle source. The Birantiya Khurd deposit, on the other hand, contains a much greater component of lead from a significantly older crustal source.

Nd and Pb isotope ratios of the Abitibi greenstone belt: new evidence for very early differentiation of the Earth

Nd and Pb isotopic compositions were determined for 39 samples from mafic and felsic volcanic and subvolcanic units that underlie volcanogenic massive sulfide deposits in the Noranda and Matagami districts of the Abitibi greenstone belt. At Noranda, Pb and Nd isotopic data plot along a single linear array which implies derivation from an isotopically homogeneous source (average ϵNd = +2.5). At Matagami, in contrast, the Pb and Nd data define two linear arrays. The more compositionally evolved rhyolites have less radiogenic Pb isotopic compositions and higher average initial ϵNd values (ϵNd = +3.2) than the more primitive compositions (ϵNd = +2.4). We interpret these isotopic differences at Matagami, as well as those between Noranda and Matagami, as not due to assimilation of crustal components, but rather primary mantle heterogeneity in the Abitibi mantle at 2.7 Ga. This implies that, although the Abitibi mantle was broadly uniform in terms of initial ϵNd values (+2.5 ± 1), it was characterized by small heterogeneities on both local and regional scales in the Archean. On the basis of Pb and Nd isotopic data, we conclude that the Pb isotopic compositions of galena in Noranda and Matagami ores represent initial Pb isotopic compositions of the underlying volcanic rocks. Pb isotopic compositions of such massive sulfide ores throughout the Abitibi greenstone belt plot along a coherent linear PbPb array and Noranda and Matagami galenas are the most and least radiogenic examples, respectively. This array is nearly coincident with the geochron at 2.7 Ga, using an age for the Earth of 4.53 Ga, and indicates not only that the Abitibi mantle was isotopically heterogeneous at 2.7 Ga but also that these heterogeneities were developed within the first few hundred million years of Earths history. Available data on initial ϵNd and initial 207Pb204Pb for the Superior Province are negatively correlated, implying that this early UPb fractionation was accompanied by SmNd fractionation and involved incompatible element depletion of the mantle.

Hindoli Group of Rocks in the Eastern Fringe of the Aravalli-Delhi Orogenic Belt-Archean Secondary Greenstone Belt or Proterozoic Supracrus tals?

Abstract The very low-grade metamorphic sequence of volcano-sedimentary rocks, sandwiched between the platform sediments of the Vindhyan Supergroup to the east and the Banded Gneissic Complex (BGC) to the west, in the eastern fringe of the Aravalli-Delhi orogenic belt, has remained a stratigraphic enigma in the Precambrian geology of Rajasthan. This sequence known earlier as the Gwalior ‘series’ and in contemporary literature as the Hindoli Group, has been considered by several workers as a Proterozoic supracrustal unit and by some others, as an Archean secondary greenstone belt, based purely on geological considerations. U-Pb zircon geochronology was conducted to find an answer to this controversy on samples of felsic volcanics, conformably intercalated with the Hindoli sediments and hence, considered contemporaneous with them. Zircons from a sample of massive rhyodacite gave a concordia age of 1854k7 Ma though zircons from a sample of felsic tuff gave a wide range of ages between 3259-1877 Ma. Careful consideration of the nature of the samples and their constituent zircons suggests that the Hindoli Group rocks represent a low-grade Proterozoic supracrustal cover sequence in the eastern part of the Bhilwara belt, broadly synchronous to the Aravalli-Bhilwara sedimentation around 1.8 Ga.

Geochemically measured half-lives of 82Se and 130Te

We have repeated the measurement of radiogenic 82Kr and 130Xe in the mineral kitkaite, NiTeSe, and obtained half-life values of (1.2 ± 0.1) × 1020y for 82Se and (7.5 ± 0.3) × 1020y for 130Te based on the parent/daughter ratios in the kitkaite and the xenon retention age of associated uraninite. The mineralization age of the kitkaite is used to set upper limits of T1282 ⩽ 2 × 1020y and T12130 ⩽ 12.5 × 1020y.

Double beta-decay of 82Se and 130Te

Abstract The isotopic compositions of xenon and krypton in the mineral kitkaite, NiTeSe, were measured to determine the amounts of radiogenic 82Kr and 130Xe produced by the double-beta decay of 82Se and 130Te, respectively. From the ratio of radiogenic 82Kr to radiogenic 130Xe in this mineral, it is concluded that the half-life of 130Te is larger than that of 82Se by a factor of 7.3 ± 0.9. If the age of the host rock, 2.00 × 109 y, is taken as an upper limit on the gas-retention age of the mineral, then values of (1.25 ± 0.08) × 1021y and (1.72 ± 0.19) × 1020y are obtained as upper limits on the half-lives of 130Te and 82Se respectively. A more realistic upper limit of T 82 1 2 ⩽ (1.4 ± 0.2) × 10 20 y is obtained from these results and those of a recent measurement here indicating T 8130 1 2 ⩽ 1 × 10 21 y .

Double Beta-Decay of Tellurium-128 and Tellurium-130

Abstract The isotopic composition of xenon has been determined in the gas released by stepwise heating of two, geologically-old tellurium minerals — melonite, NiTe2, from the Robb-Montbray property in Quebec and altaite, PbTe, from the Mattagami Lake area of Quebec. We calculate values of 2540 ± 680 and 2550 ± 1300 for the ratio of the ββ-decay half-lives, T 1 2 128 /T 1 2 130 , from the amounts of radiogenic 128Xe and 130Xe in the melonite and the altaite, respectively, and a value of T 1 2 128 = (1.8 ± 0.7) × 10 24 y . Lepton number violation is not required by these results.

Ratio of double beta-decay rates of 128,130Te

Abstract We have measured the amounts of radiogenic 128 Xe and 130 Xe in two, old telluride minerals-krennerite (AuTe 2 ) from Western Australia and altaite (PbTe) from Quebec. We calculated values of (4.2±0.8) × 10 −4 and (4.4±0.8) × 10 −4 for the ratio of the total ββ-decay half-lives, 130 T 1 2 / 128 T 1 2 , from the amounts of radiogenic 130 Xe and 128 Xe in the krennerite and the altaite, respectively. These values are in good agreement with the ratio of half-lives calculated by the quasi-particle random phase approximation for 2 v ββ-decay.

Double beta decay of tellurium-130

Abstract The isotopic composition of xenon is reported in four, neutron-irradiated tellurium minerals — tellurobismuthite from Boliden, Sweden, native tellurium from the Good Hope Mine of Gunnison County, Colorado, altaite from the Kirkland Lake area, Ontario, and altaite from the Mattagami Lake area, Quebec. From the amount of radiogenic 130Xe and pile-produced 131Xe in these samples, it is concluded that the half-life of 130Te for ββ-decay is ≲ 1 × 1021 y based on measured values of (1.0 ± 0.3) × 1021 y and higher. Our results demonstrate that there has been no significant partial leakage of radiogenic 130Xe from these minerals over geologic time. Larger values of T 1 2 as indicated from some of the analysis reported here and in other studies, are attributed to recrystallization of the soft telluride minerals and complete resetting of the TeXe system after mineralization. The value obtained here for the half-life of 130Te is substantiated by recent measurements on xenon in tellurides from Kalgoorlie, Western Australia.