Mineralogy and origin of the distal volcanosedimentary deposit at the Haellefors silver mine, Bergslagen, central Sweden
In the Zn-Pb-Ag deposit of Hiillefors silver mine, four types of sulfide ores are recognized: l) stratabound ore associated with manganiferous iton ores, 2) arsenopyritebearing stratiform (stratabound ?) ore, 3) galena-rich fissures, 4) galena-arsenopyrite veins. Type 3 and 4 are thought to originate by remobilization from types I and 2, respectively. The antipathetic behavior of arsenopyrite with magnetite in the stratabound ores probably reflects environmental conditions during the deposition and early diagenesis. Metamorphic, hydrothermal remobilization led to the formation of many Ag, Pb and Sb minerals. Results of electron-microprobe analyses indicate that the principal sources of silver are argentian galena, freibergite and pyrargyrite. The silver content of galena (up to 0.17 wt.9o) has a linear relation with its antimony content. Tetrahedritegroup minerals contain ll.3 to 31.6 fi.qo Ag. An extremely silver-rich freibergite (48,2 wt.9o Ag) exceeds the previously reported maximum (42,5 wt.Vo).
The tetrahedrite-freibergite series, with reference to the Mount Isa Pb-Zn-Ag orebody
Twenty eight electron microprobe analyses of freibergite from the Mount Isa (Queensland) Pb-Zn-Ag stratiform orebody, range in silver content from 18.4 to 42.5 wt. % Ag. These values significantly extend the tetrahedrite-freibergite series. The compositional range based on twenty-one complete analyses is indicated by the formula (Ag,Cu)9.21–11.44(Fe,Zn)1.59–2.31(Sb,As)3.87–4.43S13.0. As far as could be determined, Mount Isa freibergite is homogeneous and no marked compositional changes were detected either across individual grains, or in different grains of the same electron microprobe sample. The linear, atom for atom, replacement of copper by silver reported for lower silver bearing tetrahedrites continues in Mount Isa freibergite. A maximum silver content of about 51 wt. % Ag is predicted. X-ray investigations indicate however that in contrast to the structural expansion with increasing silver content reported for argentian tetrahedrite, Mount Isa freibergite contracts with increase in silver. The extrapolated lattice parameter for the theoretical freibergite (Ag10(Fe,Zn)2Sb4S13) end member is of the same order as tetrahedrite.
Chapter 6 Rift Basins of the Sudan
Abstract The Sudanese rift structures form intracontinental basins, bordered on all sides by anorogenic terrain. These basins are seen as the result of a multistructural system of rifts which appear to have been activated several times since the Palaeozoic. Improved methods of dating and extensive geophysical surveys have led to a better understanding of the mobile belts and the Basement rocks in the general environs of the basins, and of the elevated blocks and highs surrounding the sedimentary basins. Detailed mapping and extensive drilling in the sedimentary basins led to a better understanding of the Phanerozoic. The rapid rate of uplift and subsidence have assisted in the rapid accumulation and filling of the basins with unconsolidated sediments, ranging from a few hundred metres to some thousand metres. The rift structures contain sediments of several age groups, origin and mode of deposition. Some of them are as old as the Palaeozoic passing through to Mesozoic, Tertiary and Quaternary (Bahr El Arab rift and Blue Nile rift). The thickness of the Tertiary sediments exceeds 15 km in Bahr El Arab rift. Palaeozoic sediments were found to occupy NE-SW grabens. Jurassic? and Cretaceous sediments were deposited in NW-SE troughs. Tertiary sediments filled grabens that range in thicknesses up to 15 km. Highly saline ground-water bodies which coincide with hydrogeologically closed basins occupy the flowing end of each of the rift basins. These saline bodies have been interpreted as buried saline lakes, sabkhas or playas. Alternating wet and dry periods eventually filled up the basins with intercalating layers of fresh and saline layers. The filling up of the basins led to the interconnection of the hydrological regime and the development of the integrated River Nile. Rift structures in Sudan form the major ground-water basins, outside these basins ground water is found only in small quantities and is of poor quality. The rift basins also act as reservoirs for hydrocarbon accumulation; all the known oil fields of inland Sudan are located in these basins.