George L. Cumming

PbNdSr isotopic constraints on the origin of the Caledonian Bindal Batholith, central Norway

Abstract The Bindal Batholith is the largest granitoid batholith in the Scandinavian Caledonides, emplaced prior to or during the Scandian collision in a complex scenario of Ordovician to Middle Silurian nappe assembly. The Bindal Batholith ranges in compositon from mafic gabbro to leucogranite, but granites and granodiorites are by far the most abundant rock types. PbPb, SmNd and RbSr isotopic results from plutons of the batholith constrain the origin of the Bindal Batholith magmas. The isotope results suggest the presence of several source reservoirs, giving rise to the granitoid magmas. Both a source relatively depleted in U, Th and Rb and enriched in Sm, a source enriched in U and Rb and depleted in Sm, a source enriched in Th and Rb, but depleted in Sm, and, finally, a source enriched in Th and Sm, but depleted in Rb, is indicated by the initial compositions of the radiogenic isotope ratios. It is suggested that the depleted source reservoirs were contemporaneous depleted mantle and mantle derived rocks in the nappe sequences, that the enriched source reservoir was sediments derived from Proterozoic upper crust of Baltic Shield affinity and that the Th-enriched source reservoir was various Proterozoic rocks, in a lower crustal position, of either Baltic or Laurentian affinity.

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.

Source of lead in Central American and Caribbean mineralization

Abstract Laramide and younger mineral deposits in the Central America-Caribbean area exhibit a regional compositional zonation in which Pb-Zn deposits (low in Ag) are confined to northernmost Central America and Au and/or Cu deposits are found throughout the rest of the area. A zone of Ag-rich deposits separates these two regions and marks the boundary between the Paleozoic-Precambrian(?) cratonic environment of the Pb-Zn deposits and the Tertiary island arc environment of the Au and Cu deposits. Intrusive igneous rocks (quartz diorite to quartz monzonite) in the area reflect this zonation in that they are Pb-rich (about 10–25 ppm) in the Pb-Zn deposit zone and Cu-rich (about 80–200 ppm) in the Cu-Au zone. Pb isotope data for galenas from 15 deposits in the Central America-Caribbean area form a linear array with slope 0.065 in which the two least radiogenic samples are from the Au-Cu zone and the two most radiogenic samples are from the Pb-Zn zone. A simple two-stage model with the end of the second stage in the Tertiary suggests an initial source of age about 750 m.y. implying a late Precambrian to early Paleozoic component in the lead if the regression is not a mixing phenomenon. These data suggest that the Pb isotope composition of Central American and Caribbean mineral deposits is affected by the type and composition of crustal material underlying each zone.

Fractional removal of lead from rocks by volatilization

Abstract Incremental heating of a rock sample yields lead of varying isotopic composition depending on the heating temperature. Most of the data fit on a single straight line analogous to a mineral isochron and indicate that the rock sample became a closed system 2040 ± 135 my ago, a time considerably before the last metamorphism indicated by K-Ar dating in the area (approximately 1750 my). The results indicate that the volatilization method must be used with care when total lead composition is required and that important information regarding the history of the rock may be determined in a simple fashion utilizing the inhomogeneous distribution of lead in the rocks.

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.