Geochemical and stable isotope patterns in altered volcaniclastic and intrusive rocks of the Kruidfontein carbonatite complex, South Africa

Abstract

The inner zone of the Kruidfontein Complex consists largely of carbonatitic volcaniclastic breccia with interbedded, massive to well-bedded, tuffaceous sedimentary units dipping concentrically at > 50° around the margin, the dip becoming shallower towards the centre. A geochemical study has revealed the formation of siderite, ankerite, fluorite, K-feldspar, hematite, quartz, apatite, calcite and chlorite as alteration minerals. Different generations of fluorite have been distinguished by variation in Rare Earth Element (REE) contents and Light REE (LREE) to Heavy REE (HREE) ratios. One or more of these phases correlates positively with the rare-earth minerals, whilst others show a negative correlation. The C and O isotope patterns point to three major stages of development for the inner zone of the complex. Stage one involves carbonatite magma which originated from fractionation with a H 2OCO 2 vapour phase, at 700°C in a closed system with a H 2O/CO 2 molar ratio of 0.5. The second stage of the development is recorded in the intrusive carbonatitic dykes and in at least part of the carbonatitic ash flow tuffs, by a shift to higher δ 18O values without major changes in the δ 13C values. This is a consequence of a secondary alteration for which a temperature of approximately 150°C has been estimated. The isotopic characteristic of the alteration fluid could be determined as δ 18O H2O = 1.2% and δ 13C CO2 = −3.8%; a possible source being a hydrothermal fluid which introduced isotopically light C from the deep subvolcanic area and mixed with high 18O ground water that had equilibrated at about 150°C. The third stage is characterised by siderite mineralisation, with a concomitant precipitation of fluorite. Siderite appears to be secondary to the other carbonates and can thus be assumed to have formed at temperatures at or below 150°C. The secondary nature of the siderite mineralisation is re-inforced by a negative correlation between the whole rock carbonate δ 18O values and the modal siderite content. By application of siderite/water and siderite/CO 2 fractionation factors from Carothers et al. (1988), the C- and O-isotope composition of the siderite precipitating fluid, was approximated as δ 13C CO2 = −8.0% and δ 18O H2O = −1.4%. From these findings it is concluded that siderite found in the Kruidfontein Complex is likely to have been precipitated by a late-stage Fe-bearing hydrothermal fluid, depleted in 18O and 13C. Carbon was provided by a deep-seated source within the subvolcanic region and Fe was leached from rocks that the hydrothermal fluid passed through. The siderite mineralisation and concomitant fluorite formation most probably took place during progressive cooling of the Kruidfontein Complex, as can be deduced from the positive correlation between the δ 18O and δ 13C values of siderite.