Browsing by Author "Nanyaro, Japhet Tulule"

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    Proterozoic gold-base metal veins in the Mpanda mineral field, Western Tanzania
    (University of Dar es Salaam, 1987) Nanyaro, Japhet Tulule
    Research work was carried out on gold veins and gold veins and gold-bearing base metal veins, collectively referred to as the gold-base metal veins - in the Mpanda Mineral Field (MMF), western Tanzania. The research covered wide-ranging aspects of the gold-base metal mineralization: petrography and geochemistry of the veins and the spatially associated granites; wall rock alterations; petrography and microthermometry of the fluid inclusions from the veins; sulphur and lead isotopes in the veins and granites; and Rb-r isotopic age determination on the granites and associated gneisses. The gold-base metal veins were emplaced in shear zones and related fissures which crass-cut Early Proteroeroic, high grade metamorphic rocks of the Ubendian System and post-Ubendian granites. The sheared rocks show progressive deformation and alteration zones subdivided into cryptic and visible alteration zones. The visible zone comprising sericite and Fe-rich chlorite and quartez} occurs adjacent to most mineralized veins while the cryptic zone (characterized by chlorite, epidote-zoisite, and calcite) occupies a broad area tbeyond the visible zone) which merges with the country rocks. The visible zone resulted from the influx of hydrothermal fluids (CO2-H2O) from depth which caused partial partial destruction of the cryptic alteration zone. During this process, the wall rock gained, among others, K, Rb, Si, CO2, H2O, and Cl, and Iost Fe, Mg, Mn, Ca, Ba, Cu, Pb, Z0, etc. Some of the elements lost from the walI rocks apparently became constituent components of the mimeraIized veins. Based on the mineral paragenesis and chemical data, the MMF veins fall into three subtypes of vein-deposits; gold, gold-base metaI, and base metal. The veins, however, have many common characteristics, e.g. omnipresent quartz plus siderite as the gangue mineraIs; uniform Pb isotopic ratios; and simiIar fluid inclusion properties. The data, therefore, indicate a common origin for the mineralized veins in the MMF. The differences in ore contents may be ettributable to the types of dominant Au/base metaI complexing ligands in the ore fIuid during the mineraIization process. The granites from the MMF are marginally peraluminous (Al2O3/(CaO + K2O + Na2O): 1.06 to 1.24 mol. %; normative corundum: O.94 to 3.34) and moderately potassic (K2O/Na2O:1.35 to 2.O wt. %). The granites show low initial Sr ratio (0.7025), a uniform whoIe rock sulfur isotope composition (mean F34S of +4.8 %.), and upper crust Whole rock lead isotope ratios (206Pb/ 204Pb: 15.B4 to 23.86; 207Pb/204Pb: 15.49 to 16.37; 208Pb/204Pb: 42.96 to B0.81). The granite data are interpreted as indicating an upper mantla/lower crust origin for the granite magma which became contaminated by upper crust material prior to consolidation. A whole rock Rb-Sr isochron gives an emplacement age for the MMF granites of 1846 ± 37 Ma (MSWD: 2.77). Pb-Pb data of galena Pb from the ore veins give a model age of 1567 Ma (STDEV. = 34) , interpreted as the time when the Pb was extracted from the source rocks and precipitated in the veins. The galena Pb isotopic ratios are uniform and have upper crust characteristics (206Pb/204Pb 16.26 to 16-39; 207Pb/204Pb: 15.49 to 15.598 208/204Pb =36.82 to 37.39). The data point to homogeneous crustal sources for the lead or extraction of the Pb from heterogeneous sources and subsequent thorough mixing before precipitation. The ore Pb isotopic characteristics differ sharply from those of the granites, ruling out a direct genetic relationship of the rocks and the vei ns. The sulfide sulfur 2 isotope composition ranges widely among the MMF deposits s34S-values -2 to +8.3 .3, but with a tight cluster for individual deposits. The sulfur data further indicate that S isotopic equilibrium was not attained by the mineralizing fluid. The origin of the vein sulfur is therefore, unclear, but a contribution from an upper mantle source is probable. The upper mantle S was then modified during the transportation deposition process leading to a broad scatter of 3'-values around O %.. in the vein-deposits. On the basis of the current data, the previously suggested derivation of the ore-vein fluids from the granitic magma is unable to account far the peculiarities of the MMF vein deposits. A self-consistent genetic model is proposed for the MMF mineralization in which crustal rocks contributed the bulk of the ore elements (Pb, Fe, Cu, Au, Ag, Si, etc.) and deep-level sources supplied carborn-dioxide and some reduced sulfur. In this modeI, an infIux of carbon-dioxide, from a deep-level melt, dehydrated the overlying crustal rocks, and flushed-out water and the ore elements from the rocks into the shear conduits. Cooling and fluid/wall rock reactions are among the factors which caused the ore-vein deposition as the hydrothermal fIuid flowed to the upper levels of the crust.