Tin-bearing Pegmatites Research Articles

The Proterozoic Choma-Kalomo Block, SE Zambia: Exotic terrane or a reworked segment of the Zimbabwe Craton?

The Choma-Kalomo Block in south-eastern Zambia was hitherto considered as solely Mesoproterozoic in age based on granitoids which were previously dated at 1.37 and 1.18Ga respectively. The Choma-Kalomo Block was therefore thought to constitute an exotic terrane with respect to the neighbouring Palaeoproterozoic Magondi Belt and Archaean Zimbabwe Craton. This study of the Choma-Kalomo Block presents new U-Pb SIMS age data for zircons collected from previously undated metasedimentary rocks, revealing an abundant Palaeoproterozoic component (2040–1861Ma). Palaeoproterozoic (2040±5Ma) xenocrystic zircons in a Mesoproterozoic (1370±3Ma) leuconorite point to reworking of older crustal material, and suggest that the Choma-Kalomo Block is not a juvenile Mesoproterozoic terrane. Our U-Pb age data on columbite-tantalite fragments from tin-bearing pegmatites in both the Choma-Kalomo Block and the Dete-Kamativi Inlier (which is part of the Magondi Belt) indicates that Sn-(Ta-Nb-W-Li-Be) mineralisation within the two terranes occurred c. 1030 to 920Ma. New 40Ar-39Ar dating, confirms previous data, and indicates that a thermal event affected the region between 1020 and 980Ma. The similarities between the Choma-Kalomo Block and the Dete-Kamativi Inlier imply that these two terranes have a common history, at least as far back as the Palaeoproterozoic, but were certainly juxtaposed by the late Mesoproterozoic era. A large difference in lithospheric thickness between the Choma-Kalomo Block and the adjacent Zimbabwe Craton could explain the different rheological behaviour of the Choma-Kalomo and Dete-Kamativi areas, with the former being strongly deformed during the Neoproterozoic (Pan-African) orogeny. Our data indicate that the Choma-Kalomo Block is not an exotic terrane, rather it may represent a reworked portion of the Zimbabwe Craton, however what is unclear is whether the subcontinental lithosphere has been thinned, or the Choma-Kalomo Block represents a terrane with an originally thinner lithosphere.

40Ar/ 39Ar dating of micas from granites of NE Kibaran Belt (Karagwe-Ankolean), NW Tanzania

40Ar/ 39Ar total gas ages of muscovites and biotites from granites associated with NE Kibaran belt (Karagwe-Ankolean) in NW Tanzanian are in the range of about 945-700 Ma, much less than the estimated age of the granites. Age gradients in the muscovite spectra are indicative of partial gas loss as a result of thermal overprinting. Evidence for at least two tectonothermal events, at ca. 950 Ma and ca. 700 Ma, is noted. The older of these correlates with the formation of tin-bearing pegmatites and hydrothermal veins in the Kibaran belt; the younger with vein emplacements in the Burundian and/or a deformational episode. Correlation of 40Ar/ 39Ar age data with K-Ar and Rb-Sr data from other parts of the Kibaran belt in Burundi, Rwanda and Zaire indicates that the NE Kibaran belt, east of the Western Rift, experienced a tectonothermal history similar to that of the western part of the during the late-Proterozoic.

Trace elements in potassium-feldspar and muscovite as a guide in the prospecting for lithium- and tin-bearing pegmatites in Sweden

Trace elements of blocky K-feldspar and muscovite have been tested as a prospecting method for Sn- and Li-mineralized pegmatites in Sweden. There is a strong correlation between elevated amounts of Sn in muscovite, > 500–600 ppm, and the presence of cassiterite. However, a combination of high Sn and low Mg with moderate to low Fe tot, and especially negligible Fe 3+, in muscovite seems to be a more reliable indicator for the presence of cassiterite in a granitic pegmatite. In barren pegmatites, muscovite with elevated Fe 3+ (5–10% of Fe tot) can apparently be Sn-rich even in the absence of cassiterite. The method merely indicates the presence but not the amount of cassiterite. Elevated contents of Li (> 80 ppm) in K-feldspar are a more reliable indicator for spodumene-bearing pegmatites than Li in muscovite. In less fractionated pegmatites muscovite is probably the only Li-bearing mineral. Y is considerably higher in muscovites of the gadolinite-type pegmatites than in those of lithium pegmatites, and can be used as an indicator for gadolinite-type pegmatite fields.

Systematic variation in the tin content of pegmatites in western central Namibia

Abstract More than 900 pegmatites were sampled during a tin exploration programme conducted along the Omaruru River in western central Namibia. The pegmatites are hosted by late Precambrian metasediments of the Damara Sequence and are probably of Cambrian or somewhat younger age. The pegmatites sampled occur in a belt 60 km long by 5 km wide which represents the bulk of what is known as the central tin belt, a sub-province within a greater tin province covering some 24,000 km2. Although there are S-type granites in the vicinity, there is a closer spacial association between tin-bearing pegmatites and a stratigraphic unit which will for convenience be referred to as the ‘tin schist’. This unit consists mainly of knotted, quartz-biotite schist which is in part tourmaline-bearing. The same rock-type is also present 40 km to the northwest at the Uis tin mine, the largest producer in the region. The tin schist has a very distinctive satellite imagery signature and this makes it possible to trace its regional distribution, which matches that of tin deposits to a remarkable degree. Richards (1986) stated that the tin schist contains anomalous concentrations of tin irrespective of whether any pegmatites or granites occur in close proximity or not. The possible genetic meaning of this association and the observed distribution pattern of tin deposits in the greater tin district are discussed.

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites in Swaziland, southern Africa

Late Archaean cratonization in the Kaapvaal craton of southern Africa was associated with voluminous granitoid emplacement, and we report on the Rb-Sr isotopic systematics of the Lochiel suite and associated tin-bearing pegmatites in northwestern Swaziland, one of the largest composite batholiths of this region. Thirty-nine samples of the various granitoid phases of the Lochiel batholith and related pegmatites were collected within the “tin-belt” of Swaziland and that extends in a SE direction from Makwane beacon (South African border) to Nyonyane hill some 20 km SW of Mbabane. Rb-Sr whole-rock dating yielded an isochron age of 2979 ± 33 M.a. for the Granodiorite phase (GD) with an initial 87Sr/ 86Sr ratio of 0.7017 ± 0.0005 while the Alkali-Feldspar Granite (AFG), that intrudes the GD, has an apparent age of 3000±125 M.a. and a 87Sr/ 86Sr initial ratio of 0.7047 ± 0.0103. Two pegmatite suites from widely separated localities have virtually identical isotopic systematics and, if regressed together, define an age of 2613 ± 35 M.a. The age of ca. 3000 M.a. for the GD and AFG is in good agreement with published ages for the Lochiel Batholith from other localities. The previously held contention that the various phases of the Lochiel Batholith are indistinguishable with respect to their Sr isotopes is supported by our new data. The low initial ratios for both granitoid suites makes it unlikely that the Lochiel batholith was produced by remelting of the 3.5-3.6 G.a. old Ancient Gneiss Complex, and we favour an underplating model with a significant juvenile magma component. The high Sr initial ratio of the pegmatite phase is in line with reported initial ratios of younger granitoids in Swaziland and suggests that these rocks are remelting products of older sialic crust.

Controls of Tin-bearing Pegmatites and Granites in the Precambrian of Broken Hill, Australia

Controls of Tin-bearing Pegmatites and Granites in the Precambrian of Broken Hill, Australia

The geology of the tin-bearing pegmatites in the eastern part of the Kamativi district, Rhodesia

In Kamativi, Rhodesia, large, flat dipping cassiterite pegmatites are emplaced as dome-like structures in Precambrian schists, gneisses and granites. In these complex, unzoned tin-bearing pegmatites, which were probably formed by intrusive silicate melts, three main mineral phases are superposed on each other: 1. alkali feldspar-quartz-spodumene; 2. quartz-albite; 3. muscovite-quartz. Petalite is sometimes developed at the expense of the first two mineral phases and is in its turn attacked by the muscovite-quartz assemblage.

THE DISTRIBUTION OF TIN-ORE DEPOSITS WITHIN FOLDED ZONES

The author makes an attempt to establish the general laws governing distribution in time and position of tin-bearing centers and areas in the course of the geological history of the earth; on the basis of methods of regional metallogenic analysis, developed in the U. S. S. R. by a group of scientists under the guidance of G. A. Bilibin. The author has considered the principal geotectonic and structural geological features and factors controlling the regular occurrence during a certain stage of development of a folded zone, of a certain type of tin-bearing intrusions and, hence, of a certain mineralogical-geochemical type of tin deposits. Considerable tin concentration evidently did not take place during the initial and early stages in development of folded zones. The middle stages of development, corresponding to the period of transformation of a geosyncline into a folded belt, were highly productive with regard to tin. Tin accumulations are associated with syntectonic batholithic intrusions of acid and ultra-acid “potassium” granites. Tin-bearing batholiths and their linear belts are controlled by regional ancient deepseated fractures, confined either to the points of anticlinorium contact (central upheavals) with the adjacent inner synclinoria or to the contact points of downwarpings with consolidated geoanticlinal zones. The late stages of development of geosynclinal regions corresponding to the period of consolidation of the folded territory also produced considerable tin concentrations. According to the depth of the controlling intrusions, the showings of tin occurrences belonging to the late stages of development are subdivided into two natural groups: the hypabyssal and the subvolcanic. Tin ore-bearing areas belonging to the late stages of development, are confined to the following two types of geotectonic environments: 1) the mobilized marginal parts of geosynclines in places of their contact with projections of platforms of intermediate massifs (blocks) and consolidated geoanticlinal structures. A number of large tin ore-bearing areas is confined to structures of late superimposed graben-like downwarpings (complicated by fractures) with a powerful development of terrestrial volcanism; 2) the inner portions of geosynclines, where the tinore-bearing areas are confined to structural facial zones of inner synclinoria, more frequently in places of contact of the latter with zones of central uplifts. The distribution of the world tin resources in the course of geological times is as follows: The Precambrian: 3. 3 percent (Nigeria, Rhodesia, Congo, Union of South Africa); The Caledonian folding: 6. 6 percent (Africa and Australia); The Variscian folding: 18.1 percent (Cornwall, Erzgebirge, the Pyrenees, Australia, Tasmania); The Kimmeridgian folding: 63.1 percent (South- Eastern Asia). Simultaneously with the growth of commercial tin concentrations from the Pre-Cambrian to the Mesozoic, complication of the nature of tin mineralization is observed; from tin-bearing pegmatites in the Pre-Cambrian to complex multicomponent deposits in the Mesozoic and the Cenozoic. Author

The occurrence of nigerite, a new tin mineral in quartz-sillimanite-rocks from Nigeria

A new tin mineral, which has been given the name ‘nigerite’, was discovered in June 1944 during au investigation of the tin-bearing pegmatites of Kabba Province, central Nigeria. The chemical and physical properties of nigerite have been determined by F. A. Bannister, M. H. Hey, and H. P. Stadler, and their results will be published in the next number of this Magazine. Dr. Hey's analysis shows SnO2 25·33, Al2O3 50·91, Fe2O3 11·90, ZnO 4·51 %, &c., and the unit-cell contents are given by the formula (Zn,Mg,Fe″)(Sn,Zn)2(Al,Fe‴)12O22(OH)2.

Tin-bearing pegmatites in eastern Manitoba

Tin-bearing pegmatites in eastern Manitoba