Geogases in the Kaapvaal Craton – Origin and modes of transport

Introduction The deep gold mines of the Witwatersrand Basin, South Africa, have gained recent attention because of reports of deep microbial communities persisting to depths of >3 kilometers - an exotic outpost of the Earth's deep biosphere [1,2]. Prerequisites of such investigations are studies on the age of the deep fracture water [3] and the origin of associated abiogenic CH4- [4] and H2-rich gases [5]. Here we report about insights obtained from two new gas data sets (I, II) with respect to the dynamic of the ultra-deep fluid system [6,7]. Methods (I) The isotopic composition of neon dissolved in deep fracture water and in fluid inclusions of quartz host rock was analyzed, and the results correlated with the isotopic signatures of the biogenic and abiogenic H2- and CH4-rich sample aliquots. (II) In the other project the geogas composition of the free gas was monitored by means of a mass spectrometer and gas sensitive sensors installed just inside the Pretorius Fault Zone at 3.45 km depth in the TauTona Mine. Data from a 4-year observation period are cross-correlated on the minute-by-minute basis with seismic data obtained from a 3-dimensional array of highly sensitive seismometers. Results (I) Highest ever observed 21Ne/22Ne ratios combined with an anomalous 21Ne/22Ne production ratio imply that the dissolved neon is a 2 Ga metamorphic signal stored in fluid inclusions and accumulating in fracture water since geological times as a result of water rock reaction processes. (II) Geogas concentration variations (4He, H2, CH4, CO2) strongly correlate with seismic signals resulting from mining related blasts and induced earthquakes in the TauTona Mine. The fracture gas permeability of ~ 5*10e-10 m² is estimated on the basis of observed gas flux rates. Conclusion (I) 21Ne/22Ne ratios may serve as search proxy for regions of the Archean Earth's crust where investigations of the deep biosphere might be focused. (II) Methodological improvements enabled the simultaneous ga