Geochemical and geochronological analysis of post-impact hydrothermal alteration in the Morokweng impact structure, South Africa

Abstract

This study investigates evidence of post-impact hydrothermal activity in the 369 m long M4 drillcore, located 18 km NNW of the centre of the 80 km-wide, 146 Ma, Morokweng Impact Structure (MIS), South Africa. The core intersects fractured, faulted, brecciated and cataclased Archaean granitoid gneisses and minor dolerites that are intruded by mm- to m-wide pseudotachylite and suevite dykes, for which a peak ring setting has been proposed. Petrographic evidence indicates that impact-related shock features and breccias are overprinted by a dominantly zeolite-smectite paragenesis that contains some evidence of high-temperature (>350 °C) minerals such as actinolite, clinopyroxene, andradite and titanite, as well as minor amounts of calcite and anhydrite. Biotite shows varying degrees of alteration to chlorite. Pyrite occurs as part of the post impact assemblage locally, but the dominant Fe-bearing phases are magnetite, haematite and iron hydroxides such as goethite. The impact-related structures and the post-impact hydrothermal veins are cut by younger, mostly subhorizontal, mm- to cm-wide, calcite veins in the upper parts of the core that contain minor amounts of zeolite. The calcite vein textures vary from blocky coarse grained to fibrous/stretched to elongate-blocky and blocky/fine-grained and cross-cutting relationships indicate multiple veining events. Smectite-illite K-Ar ages range from 105.80 ± 4.43 Ma (214.40 m) to 68.88 ± 48.87 Ma (121.8 m), 59.45 ± 7.74 Ma (216 m) and 57.41 ± 7.14 (119 m). The correlation of age with grain size suggests that these ages might reflect resetting of the original impact hydrothermal assemblages formed at 146 Ma, although new mineral growth during discrete post-impact hydrothermal events cannot be completely ruled out. Four calcite veins analyzed via LA-ICP-MS yielded U-Pb ages of 66.46 ± 0.74, 16 ± 47, 28.1 ± 1.1, and 13.95 ± 4.32 Ma. The broad agreement between the oldest U-Pb calcite age and three of the smectite-illite ages suggests a significant hydrothermal event close to the Cretaceous-Palaeogene boundary. Stable isotope analysis indicates that the older calcite veins record heavier ẟ18O and varying ẟ13C values ranging from 21.45 to 27.39 ‰ and -21.80 to -9.24 ‰, respectively, whereas the younger veins record lighter ẟ18O and varying ẟ13C values of 19.00 to 22.04 ‰ and -19.79 to -3.99 ‰, respectively. Negative ẟ 13CPDB values are consistent with a meteoric groundwater influence enriched in dissolved organic carbon; however, the combined ẟ 13CPDB–ẟ 18OSMOW values suggest connate waters that interacted with a marine carbonate source. The ẟ 13CPDB–ẟ 18OSMOW values and age values vary between samples. The temperature of crystallization of the vein carbonate is estimated between 8.5 to 140.7 and > 200℃, using the stable isotope thermometer. Rare earth element (REE) patterns in the calcite veins are also sample-specific and show little to no correlation with the REE patterns of the host rocks, suggesting derivation from an external source and little to no interaction between the fluids and the wallrocks. One sample exhibits a pattern enriched in light rare earth elements relative to middle and heavy rare earth elements, with a negative Ce, slightly positive Gd, and positive Y anomalies, suggestive of precipitation under oxidizing conditions. However, two samples show flat REE patterns with strong positive Eu and Y anomalies, indicative of oxygen-poor hydrothermal fluids, while the fourth sample shows a flat REE pattern with a positive Y anomaly. These patterns indicate different fluid sources and may imply temporal variation in fluid composition related to different veining events. In conclusion, the calcite veins in the upper parts of the M4 drillcore appear to represent polyphase fluid infiltration events unrelated to the Morokweng impact event, and most likely are associated with discrete events relating to the poorly understood Late Cretaceous to Cenozoic tectonic and exhumation/burial history of the region.