The characteristics of regional-scale fluids, fluid flow and hydrothermal mineralisation in metasedimentary terranes

Abstract

Abstract by author:

This study presents more than 400 O, H, and C isotope data with the principal aim of determining the nature and characteristics of fluid flow during the deformation, metamorphism and plutonism in the Brandberg West area. The siliciclastic turbidites show a general trend of increasing dI8O values across the Zerrissene basin from approximately 10. 5 0/00 in the andalusite zone to as much as 19 0/00 in the chlorite zone. This trend cannot be attributed to systematic variations in rock type or stratigraphy across the basin, and the variations are too large to be accounted for by the effects of metamorphic devolitisation reactions. Instead, the regional d18O pattern is considered to be due to the effects of near-equilibrium polythermal fluid-rock interaction during regional-scale fluid flow through the basin; combined with minor infiltration of residual magmatic fluids out of the marginal Damaran granites and into the contact aureoles

The migrating regional fluid phase was dominantly aqueous, with relatively homogenous chemical (NaCl-H2O±KCl±CaCl2; ~2-7wt percent NaCl equivalent) and oxygen isotope compositions. The stable isotope data suggest that this fluid may have been derived in situ during dewatering and dehydration of the rocks of the Zerrissene basin; reaching a homogenous steady-state chemical and isotopic composition on a regional scale (30 km) through convective circulation in response to a regional thermal gradient. Alternatively, it is proposed that the isotopically homogenous rock-buffered fluid was derived from the higher crustal levels of the basin and flowed up-temperature from the lower grade parts of the basin towards the granite contacts. Theoretical modelling shows that equilibrium fluid-rock interaction during regional up-temperature fluid flow across the Zerrissene basin during peak metamorphism can only account for about 4 0/00 of the variation in the siliciclastic rocks d18O values. Whereas the siliciclastic rocks with the dI8O values greater than about 13o/oo can be explained by the regional-scale up-temperature fluid flow; the rocks with d18O values of less than 13 0/00, which are generally restricted to the contact aureoles surrounding the Damaran granites, are considered to have been isotopically reset by down-temperature infiltration of l8O-depleted magmatic fluids

In general, the carbonate rocks of the Zerrissene Group have preserved their primary isotopic compositions, and show little evidence of post-depositional modification. The contacts between the marbles and the siliciclastic rocks are associated with abrupt changes in host rock and vein d18 O values, indicating a lack of fluid interaction between the siliciclastic and carbonate rocks. Similarly, sharp intraformational d18 O fronts which exist between the various members of the two main carbonate formations, indicate that pervasive fluid flow across the strike of the members was limited, and that the across-bedding flow was confined to the network of discrete fluid channels (vein-filled fractures) which cross-cut the impermeable marbles. The regional-scale flow models proposed in this study are based on lateral fluid flow along the strike of the folded Zerrissene Group rocks. It is envisaged that the impermeable marble horizons acted as aquitards restricting the buoyancy-driven vertical escape of the turbidite-hosted fluids, and that the siliciclastic units were therefore largely disconnected from one another and that they represented isolated sub-horizontal 'compartments' along which the 'trapped' fluids would have had no option but to flow along strike

The best estimate of the timing of regional-scale fluid flow appears to be just after deformation and peak metamorphism occurred; at a time when most of the larger granites were intruding, sometime between about 590 and 550Ma. The isotope studies suggest that the formation of the quartz and calcite veins, and by inference the fluid flow, occurred over an extended period as the terrane cooled from peak metamorphic temperatures, to about 50°C below peak temperatures. Theoretical modelling suggests the time-integrated fluid fluxes during the regional-scale flow were mostly in the order of 0. 5 x l05 to 5 x l05 moles/cm2 and the actual volumetric fluxes were in the order of ~5 x l0 -10 to 5x l0- 11m3/m2/s. These values, and the estimated intrinsic permeabilities in the range of ~l0-18 to 10-20 m2, are fairly typical conditions for regional-scale fluid flow through low-grade metamorphic terranes

Comparisons with other low-grade metamorphic belts suggest the main conditions favouring the regional-scale fluid flow in the Brandberg West area were the presence of a well developed and steep temperature gradient, the high crustal level of the terrane, enhanced rock mass permeability due to coeval metamorphism and regional deformation, the presence of marble aquitards and a lack of previous episodes of metamorphic dehydration. Although these factors are specific to the Zerrissene basin, several of them probably represent general conditions necessary for pervasive regional-scale fluid flow through meta-sedimentary terranes

Additional objectives of this project have included suggesting potential origins for the Damaran granites, characterising the nature and origins of the Sn-W deposits, assessing the potential for turbidite-hosted gold mineralisation in the Brandberg West area, and possible stratigraphic correlations of the Zerrissene Group with other Pan-African sequences in the Damara Province

Most of the Pan-African granites of the Brandberg West area have calculated magma dI8O values of between 9 and 1l°/oo, straddling the I-type-S-type boundary. The Doros monzonite has more I-type d18O values at its centre than its rim, suggesting some degree (calculated to be ~42 percent) of country rock assimilation along the edges of the pluton. The O and Sr isotope data suggest the Doros syenite (and possibly also the Durissa Bay diorite and Voetspoor syenite) could have been derived as an admixture of relatively unaltered mantle melt and the Zerrissene Group siliciclastic rocks

The Brandberg West area is host to several hydrothermal Sn-W deposits. The results of the isotope study, the nature of the mineralisation, and the associated thermal metamorphism and alteration suggest that the deposits were sourced from Pan-African granite magmas, and that the Brandberg West, Frans, Goantagab and Gamigab deposits were emplaced at progressively higher levels above the hidden granite cupolas

The cassiterite and wolframite-bearing greisen-type veins at the Brandberg West mine appear to have developed during several phases of mineralisation and wall rock alteration. The early phases were associated with the high temperature (~500°C) quartz-muscovite greisen formation and the thermal metamorphism of the country rocks. The subsequent phases of vein formation were associated with a progressive decrease in temperature and the cassiterite and wolframite mineralisation appears to have occurred at temperatures of approximately 300-350°C. The siliciclastic host rocks within the Brandberg West mine have O isotope compositions which are significantly lower (3-3. 5 0/00) than their unmineralised counterparts from the surrounding region indicating significant fluid-rock interaction during mineralisation. Despite the vein-controlled nature of the mineralisation, the modification in the isotopic compositions of the siliciclastic wall rocks suggests the flow of the mineralising fluids was pervasive and was probably concomitant with the wall rock alteration. Calculations indicate that the host rocks experienced a high open-system fluid to rock ratio of ~1. 4 and a minimum total time-integrated fluid flux of approximately 190kg H2O/cm2

The Brandberg West deposit is situated in the apex of an F1 antiform, at the interface of siliciclastic and carbonate units. The impermeable carbonate rocks appear to have acted as a physical barrier to the rising hydrothermal mineralising solution and thereby effectively trapped the mineralisation in the apex of the F1 antiform. The general paucity of quartz veins cross-cutting the carbonate rocks strengthens the argument that they acted as barriers to fluid migration during the formation of the Sn-W deposits. Carbon isotope data indicate that the CO2 in the mineralising fluid was derived from the host carbonate rocks, indicating an interaction between the marbles and the mineralising fluids. This suggests that the carbonate rocks may not only have provided a physical barrier, but may have caused ore deposition through chemical reactions

The magnitude and nature of the variations in the composite d13C profile through the Zerrissene Group suggests that the middle siliciclastic unit and the overlying carbonate unit may be tentatively correlated with the glaciogenic Chuos Formation diamictite and the post-Sturtian (~750-700Ma) cap carbonates, respectively

The generation and large-scale movement of predominantly aqueous (+CO2) hydrothermal fluids, the lithological, structural and metamorphic setting of the Zerrisene Group turbidites, and the chemistries of the Orogenic fluids indicate that there exists at least the potential for gold mineralisation in the Zerrissene basin