Genesis and tectonothermal evolution of late Proterozoic stratiform mangenese deposits at Otjosondu, Damara Belt, east central Namibia

Abstract

Abstract provided by author:

Despite the high-grade metamorphic overprint of lithologies in the study area, a sedimentological investigation and interpretation of the lithotypes has turned out to be possible. Based on detailed lithological sections through the ore horizons and their country rocks, nine major lithotypes have been determined. The lithotypes of the manganese ore horizons display Mn/Fe ratios ranging from 0. 01 to 14. 5, and some pure manganese ores bear Ba up to 7. 9 percent. With the aid of their mineralogy and their bulk rock geochemistry, these lithotypes are interpreted as facies types and, taking into account their contact relationships, may serve to reconstruct the depositional setting in which they were deposited and to reveal sedimentary cycles. Taking the meta-quartz-arenites, interpreted as shallow-marine high-energy level quartz sands, as a near-shore reference line, the relative depositional distribution of facies types on a stable continental shelf can be reconstructed. The resulting zonation displays an environment depositing impure sandstone seaward from the quartzarenites, followed by impure manganese ores, pure manganese ores (manganostones) and lastly by iron formation lithologies. This distribution reflects (i) a chemical zonation perpendicular to the continental shelf, namely Fe and chert deposition in the outer and precipitation of manganese formations in the middle shelf area and (ii) a decrease in clastic influx from the continent towards a pelagic middle to outer shelf environmental regime

The ore-forming solution are interpreted to derive from hydrothermal vent systems related to the alteration of oceanic crust. Separation and relative enrichment of Fe and Mn near to the vents, and their differential precipitation on a continental shelf are explained by a model which uses changing Eh conditions at a constant pH level. The basal contact of the ore-bearing horizon at Otjosondu is interpreted to represent the breakup unconformity due to the separation of the Congo and the Kalahari cratons, going along with the initial production of oceanic crust in the Khomas Sea to the south of the Congo craton, where oceanic crustal remnants and associated proximal base metal deposits are still preserved. The associated large-scale transgression in response to continental breakup (i) provided the clastic-starved shelf environment at Otjosondu, (ii) caused the redox interface to be raised onto the shelf and (iii) allowed continuing mixing of bottom-with oxidizing surface-waters to precipitate Fe and Mn differentially in a pelagic regime. Small-scale sedimentary cycles (up to a number of six in one ore horizon) were identified by the amount of terrigenous components in the chemical precipitates and are interpreted to relate to glacial/inter-glacial regressions and transgressions

The tectonothermal evolution in the study area displays a multi-stage ductile deformation and one pulse of high-grade metamorphism. The first deformation event D1 produced a polyharmonic fold generation characterized by the presence of overturned to recumbent folds verging towards the north-east as well as of upright/close and upright/ open folds. An s1 foliation and an I1 lineation are developed, and a high component of simple shear during this event has been deduced. D1 folds were superimposed by an upright, tight D2 fold generation. The polyharmonic style of D1, folds and the small angle of 40-60° between the D1 and D2 axes of maximum shortening are responsible for different mechanisms of superposed folding between these two generations. Interference of upright D2 with recumbent D1 folds resulted in dome-crescent mushroom outcrop patterns. On the other hand, upright/open D1 folds experienced reactivation and rotation according to the D2 maximum shortening direction, imitating primary D2 folds. The transgressive nature of an s2 fabric, however, can reveal the early arrangement of a rotated D1 structure. Any degree of reorientation by the mechanism of fold hinge migration may occur, so that all hinge orientations between the D2 and the D1 trend can be observed in the study area. Upright, close D1 folds did not allow rotation during D2 strain and thus have preserved their original D1 hinge trend. Anticlock-wise rotating axes of maximum shortening from D1 towards D2 are envisaged for the study area. D3 folds are of large scale and high amplitudes, and lack any associated schistosity. Interference of this event with previous fold generations produced large-scale distorted dome-and-basin patterns. Brittle deformation in the study area is manifested by horizontal stress-release fractures and by a set of vertical, left- and right-lateral shear fracture zones, of which the former is dominant

The chronology and patterns of ductile deformation in the study area conform with those described from the western parts of the southern Central Zone. The mechanism of superposed folding described from the study area readily explains why D1 folds in the southern Central Zone of the Damara inland branch are hardly visible, and, if they are, were described to be recumbent in attitude

Peak metamorphic conditions reached 680 ± 20 °C at 3. 6-4. 4 kbar. These data are based on garnet-biotite thermometry (660-700 °C), hornblende-plagioclase thermometry (about 670 °C), the d18O distribution in quartz-haematite pairs (mean = 677 °C) and discontinuous mineral reactions. The ore mineral paragenesis braunite + jacobsite + rhodonite + haematite observed at Otjosondu yield a temperature of formation at or above 660 °C at 5 kbar applying the T-fO2 grid of Dasgupta et al. (1989). In addition, the paragenesis K-feldspar + corundum and the occurrence of two sillimanite generations besides perthite and, on the other hand, the stability of the assemblage muscovite + quartz in feldspar-free lithologies of Otjosondu, allow to bracket the metamorphic conditions in a P-T net of discontinuous reactions. The ore mineral parageneses at given temperatures and the absence of the assemblages braunite + quartz and hausmannite + rhodonite restrict the associated oxygen fugacities during metamorphism in the ore horizon at approximately logfO2 = -5 to -10, which is above the Cu/Cu2O buffer

Using the determination of peak metamorphic conditions in assemblage with field observations, geobarometry, and the analysis of microtextures and fluid inclusion generations, a P-T-D evolution of the Otjosondu area can be designed. Geobarometry on phengitic white micas can be used to roughly delineate the prograde P-T loop. Peak pressures around 5 kbar were attained before peak metamorphism. The high-pressure stage was accompanied by the main deformation event D2, and subsequently, by the mobilization of partial melts from the ore horizon which are manifested by numerous Ba-rich pegmatite veins in the manganese ores. A H2O-rich fluid inclusion generation is interpreted as a remnant of dehydration reactions in the ore horizons, which caused hydraulic facturing processes in the ore horizons and triggered the subsequent production of partial melts intruding the fracture planes. With the production of partial melts, the pressure conditions decreased at rising temperatures. Peak metamorphism occurred under strain-free conditions which allowed equilibrium conditions of all minerals to develop and prominent recrystallization. Equilibrium conditions are furthermore reflected by the d18O- distribution between paragenetic braunites, garnets, aegirine-augites and Ba-rich ternary feldspars. The uplift path of the rocks can be constrained using decrepitation patterns of H2O- and CO2-rich fluid inclusion generations, pointing to high geothermal gradients of 70 °/km and more. The strain-free period is terminated by weak D3 strain at still high temperatures, which triggered secondary recrystallization and recovery textures in quartzites. The ductile deformation ends with brittle deloading joints in quartzites at shallow crustal levels

The P-T-D evolution of the study area conforms with the tectonothermal connotation of the southern Central Zone as a high-temperature/low-pressure metamorphic belt. This P-T-D pathway is the consequence of collisional tectonics in the Damara inland branch between the Kalahari and the Congo cratons and crustal thickening on the latter, to which the D1 and D2 events at Otjosondu are related. Subsequent continental collision in the coastal branch of Damara Orogen is manifested by the related D3 event which decreases in intensity towards the east