Dragan Krstic

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.

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.

Source of lead in Central American and Caribbean mineralization, II. Lead isotope provinces

Abstract In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher 206 Pb/ 204 Pb and 207 Pb/ 204 Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in 207 Pb/ 204 Pb and 208 Pb/ 204 Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280±310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.

Mississippi valley-type mineralization in the Silurian paleoaquifer, central appalachians

This study used lead and sulfur isotope tracers to determine the nature and geometry of MVT-related brine flow in the Silurian paleoaquifer of the central Appalachians. Although the stratigraphy and lithology of this sedimentary package is similar to that of the important Lower Cambrian MVT-hosting paleoaquifer of the southern Appalachians, it lacks important mineralization. Lead isotope compositions from MVT deposits in the Lockport Dolomite, in the upper part of the Silurian paleoaquifer, form a cluster at about 206Pb204Pb = 18.3, 207Pb204Pb = 15.6, and 208Pb204Pb = 38.2, which is similar in form but slightly different in composition from those seen in deposits in underlying clastic rocks. Sulfur isotope compositions for Lockport MVT sulfides range from −19 to −6‰, and have lower δ34S values than MVT sulfides hosted by underlying clastic rocks. These relations, in combination with NaClBr compositions of fluid inclusion leachates, suggest that MVT deposits in the lower part of the paleoaquifer formed when rising metal-bearing brines intersected reduced sulfur of probable thermogenic origin, whereas deposits in the upper part of the paleoaquifer (Lockport Dolomite) formed where metal-bearing brines intersected reduced sulfur of biogenic origin that was dissolved from sulfate evaporites in the overlying Salina Group and transported downward into the Lockport.

Further analyses of radiogenic minerals from the Bidjovagge gold-copper deposit, Finnmark, Northern Norway

SummaryWe recently reported U/Pb and Sm/Nd dates on davidite from albitic felsites hosting the gold-copper mineralization of the Bidjovagge gold-copper deposit of northern Norway (69°17′N, 22°29′E). Isotopic dating of the north-south trending Proterozoic Kautokeino greenstone belt host rocks is sparse and confirmation of the dates was desirable, since the U/Pb data in particular were highly discordant and therefore somewhat uncertain.We have now supplemented the earlier measurements with U/Pb dating of uraninite from the felsites. The new uraninite data yield a well defined upper intersection with the concordia at 1837 ± 8 Ma, somewhat younger than the age of 1885 ± 18 Ma obtained on the davidites. The fact that the two minerals are apparently different in age lends support to the suggestion of a metamorphic origin of this deposit since a rather long period of cooling subsequent to metamorphism is indicated.ZusammenfassungWir haben erst kürzlich über U/Pb and Sm/Nd Daten von Davidit aus Albit-reichen, felsischen Gesteinen, in denen die Gold-Kupfer Mineralisation der Bidjovagge GoldKupfer Lagerstätte in Nord-Norwegen (69°17′N, 22°29′E) aufsitzt, berichtet. Es gibt wenige Isotopen-Datierungen der Gesteine, des N-S streichenden, proterozoischen Kautokeino Greenstone Belt. Außerdem ist eine Bestätigung der existierenden Daten wünschenswert, denn die bisherigen U/Pb Daten sind sehr widersprüchlich und unklar.Nun konnten wir die früheren Messungen mit U/Pb Daten von Uraninit aus den felsischen Gesteinen ergänzen. Die neuen Daten ergeben einen gut definierten Schnitt mit der Konkordia bei 1837 ± 8 Millionen Jahre, und sind damit etwas jünger als das von den Daviditen stammende Alter von 1885 ± 18 Millionen Jahre. Die Tatsache, daß diese beiden Minerale ein offensichtlich unterschiedliches Alter zeigen, unterstützt die Ansicht, daß die Lagerstätte metamorphen Ursprungs ist, wobei eine lange Periode der Abkühlung nach der Metamorphose zu vermuten ist.