The Mineralogy and Genesis of Uranium in Rhyolitic Ignimbrites of Precambrian Age from Duobblon, Sweden
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MINERALOGICAL MAGAZINE, JUNE 1982, VOL. 46, PP. 187-199 The mineralogy and genesis of uranium in rhyolitic ignimbrites of Precambrian age from Duobblon, Sweden JOHN A. T. SMELLIE Geological Survey of Sweden, Box 801, Lule~t, Sweden ABSTRACT. The Duobblon rhyolitic ignimbrites, of the highest U concentrations tend to be associated middle Precambrian age are 60 m thick. They consist of with the basal member. From the literature, the at least three flows with varying degrees of welding and Buckshot Ignimbrite of late Eocene to early Oli- have undergone devitrification, producing lithophysal goncene age (Anderson, 1975), and the pyroclastic and spherulitic textures. They are overlain by thick acid tufts of Permian age at Novazza (Guarascio, fluviatile red-bed-type conglomerates and sandstones, which in turn are capped by terrestrial acid volcanics. 1976), are the only published occurrences contain- Uranium enrichments of up to 3000 ppm U occur ing such high levels of U. within two or three peneconcordant tabular horizons Acid volcanics, which contain on average 5 to which are mostly lithophysae-bearing. Fission-track 7 ppm U and 20 to 30 ppm Th, are considered by investigations of the ignimbrites and overlying con- many workers to be important source rocks of U, glomerates and sandstones, which supplement earlier and their investigations have tended to emphasize mineralogical studies, show that U occurs as fine pitch- the post-depositional mobilization of the U. In- blende disseminations; as complex uranotitanates asso- vestigations by Rosholt and co-workers (1969, ciated with Fe-Ti-Mn oxides; and as coatings associated 1971), Shatkov et al. (1970), and Zielinski and with matrix sericite. Small amounts of O are present in such primary accessory minerals as sphene, apatite, and co-workers (1977, 1978, 1980), indicate that crystal- zircon. line rhyolites (felsites) tend to be depleted in U in It is suggested that oxidizing U-bearing solutions, contrast to co-existing non-hydrated glasses generated partly during devitrification of the ignimbrites (obsidian) or hydrated glasses (perlite), and there and partly from the overlying volcano-sedimentary pile, are also indications that U depletion increases with produced the U enrichments with later sulphide deposi- age at a rate which depends on whether the tion, along the more permeable lithophysal horizons in rhyolites are per-alkaline (fastest) or calc-alkaline the ignimbrites. (slowest). The Duobblon ignimbrites, being Pre- cambrian in age and having undergone post- THE Precambrian rhyolitic ignimbrites of the depositional devitrification and recrystallization, Duobblon area (figs. 1 and 2), previously described are therefore somewhat exceptional in containing by Einarsson (1979) and Lindroos and Smellie such high levels of U. Lindroos and Smellie (1979) (1979), are composed of welded and partially have previously described aspects of the U distribu- welded rhyolites and tufts of ash-fall and ash-flow tion in the ignimbrites. The present paper presents origin. The ignimbrite unit is some 60 m thick and a detailed mineralogical and fission-track study of 4 to 5 km in strike length, comprising three main U distribution, together with additional chemical flows each separated by a thin conglomeratic bed data. The overlying tuffitic sandstone and con- which varies in thickness from 1 to 10 m. The basal glomeratic units which have probably contributed flow has the greatest areal extent and the whole to the total U content of the underlying volcanics unit thins out laterally eastwards. are also considered. The ignimbrites contain relatively high U con- centrations for such rocks with an average of Geological setting 200-300 ppm U and with localized concentrations reaching 3000 ppm U. They occur within two or The geology of the Duobblon area (figs. 1 and 2) three peneconcordant tabular horizons 5 to 25 m has been described in detail by Einarsson (1970, thick and more than 1000 m long. The mineraliza- 1979) and by Lindroos and Smellie (1979). Briefly, tion is not confined to any single flow, although the rocks are of middle Precambrian age situated Copyright the Mineralogical Society 188 JOHN A. T. SMELLIE REGIONAL GEOLOGICAL MAP OF THE DUOBBLON AREA il~ TERRESTRIALVOLCANICS ~- SKELLEFTE-SULPHIDE DISTRICT "~ ARJEPLOG-ARVIDSJAUR-SORSE LE ..._) URANIUM ORE PROVINCE ,'~ DUOBBLON URANIUM OCCURRENCE CALEDONIAN ROCKS ,.>I GENERAL STRATIFICATION ,.....-.''" MAJOR FAULT SORSELE GRANITE (1.63Go) URANIUM OCCURRENCES ",7<-A ~'. i l 9a'/,-~,, GtPPERVARE FORMATION(1.73Go)]> DUOBBLON BR~NABERGET BJORNKNOSEN FORMATION ] GROUP DUOBBLON GIPPERVARE @ GIPPERTRASKET OLDER (REVSUND) GRANITE (1.79Go) RABNATRASKET STABURTRASKET DUOBBLONBACKEN PAKSJO GROUP HARDY LINDROOS _ SO.U1978 FIG. 1. Generalized map of the Duobblon area, northern Sweden. Location map (top) shows position of the Duobblon U occurrence within the Arjeplog-Arvidsjaur-Sorsele U ore province (after Lindroos and Smellie, 1979). U IN SWEDISH RHYOLITIC IGNIMBRITES 189 near an important structural boundary of the Baltic Major features of the distribution of U Shield which separates a northern continent from a southern oceanic region in Svecofennian times The Duobblon U occurrence (fig. 1) is located (2.00-1.75 Ga). The geology comprises the Paksjo at the periphery of the Arjeplog-Arvidsjaur- and Duobblon groups together with the Revsund Sorsele U province (Adamek and Wilson, 1979). granite and the younger Sorsele granite masses. The most important U occurrences in the province The Duobblon group consists of rhyolitic ignim- are believed to be epigenetic, in contrast to brites and an underlying basal breccia, which Duobblon which is later and considered to be westwards lie unconformably on a deeply weathered syngenetic and possibly also related to devitrifica- granite basement (correlated with the Revsund tion (Lindroos and Smellie, 1979). Within the granite) of about 1.75 Ga. To the east, graphitic Duobblon group (fig. 2), U mineralizationhas been schists of the Paksjo group form the basement found associated with all members of the group to the ignimbrite flows which themselves thin and also with schists of the Paksjo group. The most out eastwards, suggesting a volcanic source to important concentrations in the Duobblon group the north-west concealed by the Caledonian occur in the lithophysae-bearing rhyolitic ignim- allochthon. brites of the Bj6rnkn6sen Formation, the highest The ignimbrites are overlain by thick fluviatile concentrations occurring in an area between a deposits of red-bed-type conglomerates and sand- basement high and the intersection of two erosion stones, which are in turn capped by acid to surfaces which form the base of the overlying intermediate terrestrial volcanics. The younger sandstone-conglomerate units. The interbedded granite, known as the Sorsele granite and dated at and overlying polymict conglomerates contain 1.59 Ga (Welin et al., 1971; Welin, 1980), occurs minor local enrichments (100 to 1000 ppm U) in to the north of the Duobblon area and is intrusive contrast to the underlying basal breccia and the into the other rock units. schists of the Paksjo group. The U content DUOBBLON _ PROFILE SSE NNW 76002 76001 77012 78002 77019 + + + + + + ~ ~ ~~?~'~-~ ~ + + + + + ~ ~ ~ c~c~c~c~ , ~,~ + §+++++§247 + + + + + + + + § :~4"~::..'."..'."' ~..'.~.!.- - :..' .: ~1-'~ §247247247247247247247 ' ~:." ....... + + + .. ,~.~:.{.:{: + ~5".":~-~.- ' OOLERITE DYKES + + + ~ %~ qUARTZ PORPHYRY J FORMATION (1.73~) ~++ "~'~~X~ POLYMICT CONGLOMERATE 77012 + § +a TUFFITICSANOSTONE / DUOBBLON DRILLHOLE + CLAYEY SANDSTONE BJORNKNOSEN GROUP 7- * ~ RHYOLITIC IGNIMBRITES FORMATION 0,16 VARIATIONAND AVERAGE POLYMICT CONGLOMERATE CONTENT OF URANIUM (m %) ~ BASAL BRECCIA SANDYMUDSTONE LOCATIONOF SECTION IN Fig2. GRANITE OLDER (REVSUND) ~0 1Kin GRANITE (1.79Go) I i I HARDYLINDROOS. SGU 19~ FIG. 2. Vertical cross-section through the Duobblon U occurrence. Studied samples were collected frem drill-cores 76001, 76002, 77012, and 78013 (after Lindroos and Smellie, 1979). 190 JOHN A. T. SMELLIE decreases markedly towards the erosion surface TABLE I. Modal analyses of welded (A), non-welded which truncates the lithophysal unit. (B), and partially welded (C) porphyritic tufts from the Duobblon ignimbrites Petrology of the ignimbrites A B C General aspect. Some aspects of the mineralogy have been described by Einarsson (1979) and in Felsite matrix 80.7 80.3 79.0 more detail by Lindroos and Smellie (1979). The Quartz phenocrysts 6.0 2.8 2.0 ignimbrites, consisting of ash-fall and ash-flow tufts Albite phenocrysts 9.2 8.0 6.3 with rhyolitic lavas, have been partly or completely Fe-Ti-Mn-oxides 3.6 4.8 3.6 welded and subsequently devitrified. Modal Fragments (pumice etc.) 0.5 4.1 9.1 analyses for welded, partly welded, and non-welded porphyritic tufts are compared in Table I. Changes 100.0 100.0 lO0.O in welding through the ignimbrite unit probably No. of samples 5 5 6 reflect different flows. No. of points counted/ The welded portions of the ignimbrites vary in sample 1500 1500 1500 colour from dark olive-green through several shades of grey to pink and reddish brown cor- responding to a sericite-rich and felsitic matrix, and a matrix with disseminated hematite respectively. Feldspar and quartz phenocrysts are common Microscopically, the partly welded tuff horizons throughout the ignimbrites and eutaxitic porphyry preserve many primary pyroclastic features includ- structures are abundant (fig. 5c). The feldspar ing partly consolidated pumice fragments, volcanic