Die Entwicklung der Fluid-Systeme während der oberprotorozoischen [sic! oberproterozoischen] Damara-Orogenese am Südrand des Damara-Orogens, Namibia

Abstract

English summary by author:

A predominant feature are zoned hydrothermal quartz-dolomite bodies of up to 80m height that mineralised from an early, highly saline fluid phase. Single or twinned quartz crystals of the outer zone can reach up to 15m along the c-axis. Towards the centre the quartz is tightly intergrown with large dolomite crystals, reaching up to lm in size. The central zone of the quartz-dolomite bodies consists of a dolomitic breccia. Fluid inclusions of the earliest, highly saline fluid phase show distinct hydrothermal patterns. Microthermometric measurements gave a minimum temperature of formation of 220-280°C for the large quartz-dolomite bodies. Total salinities of the primary fluid inclusions reached up to 65 wt percent NaCl equiv., with very low eutectic temperatures indicating the presence of Ca and Mg2+

The subsequently predominating metamorphic and tectonic fluids formed, depending on the position of the sample, primary or secondary fluid inclusions. The related fluid system was active during the entire compressive phase of the tectogenesis. Microthermometric measurements showed distinctly lower salinities than for the first phase of fluid activity, especially with respect to Ca2+ and Mg2+. Total salinities averaged at about 35wt percent NaCl equiv. Homogenisation temperatures of 220-330°C were distinctly higher than for the first phase of fluid activity

This second fluid phase also resulted in numerous tourmaline mineralisations. Stratiform turmalinites and crosscutting quartz-turmaline veins formed through the mobilisation of boron from evaporitic or volcanic sequences on the continental margins (Kamtsas and Duruchaus formations). Electron microprobe analyses of tourmalines from various localities showed the complex composition of the mineralising fluids. Regional variations of the composition as well as variations within single, zoned tourmaline crystals allow a correlation of the formation of the tourmalines with the metamorphic grade

P-T-x estimations from fluid incluison [sic] studies as well as mineral parageneses of the quartz-dolomite bodies allow the reconstruction of the physico-chemical conditions during the evolution of the fluid systems and a correlation with the course of tectogenesis and metamorphosis at the southern margin of the Damara Orogen:

(a) The compressive phase of the orogeny resulted in extenisve isoclinal, in parts alpinotype deformation in the Damara belt. Thick nappe units were thrust from the Khomas Trough onto the southern continental margin (Kalahari craton). The well developed thrust zones can be traced for more then 400km along the southern margin. The extensive overthrust of nappes mobilised large quantities of fluids from the partly evaporitic sedimetary sequence in the basins along the continental margin. The fluids migrated into the thrust zones as preferred pathways and thus enhanced the nappe movement by increasing the hydrostatic pressure. The resulting relative decrease of compressive stress lead to the opening of fractures, faults and fissures. The highly saline fluids migrated into these tectonically controlled low pressure zones, where the large mineralisations formed due to the changing P-T-x conditions. Together with the fluids a dolomitic breccia mush was also mobilised and migrated into the thrust zones and low pressure regimes

(b) The progressing Orogenic front drove ahead large quantities of tectonometamorphic fluids, which reacted with the rather stationary highly saline fluid system at their interface. The differing composition of the fluids resulted in the seperation of a CO2 rich phase. Increasing pressure and temperature lead to the developement of a typical greenschist to lower epidot-amphibolite facies along the southern margin. Locally even a high pressure facies with kaynite growth has developed. Large portions of the dolomite in the quarz-dolomite bodies were transformed to talc and minor tremolite under these conditions

(c) Most of the primary, high saline fluid inclusions decrepitated partly or completely due to the increasing tectonic or thermal stress. Numerous very small gas or H2O containing fluid inclusions formed in the course of this event of natural decrepitation. A minimum pressure of formation of 5 to 7 kbar can be estimated by intersection of isochores determined from thermometric measurements on these inclusions

(d) The tourmalinites and quartz-tourmaline mineralisations formed in suitable positions from residual fluids