Abstract

The western part of the Mid-Proterozoic Namaqua mobile belt consists of a series of tectonostratigraphic terranes, including the Aggeneys terrane. In the centre of the Aggeneys terrane occurs a duplex of five fold nappes, represented by the Aggeneys Hills. One of these, the Quarry fold nappe, contains sections through a distinctive volcano-sedimentary sequence correlated with the Koeris Formation that elsewhere overlies the Hotson Formation with ore-bearing assemblages. A description of the stratigraphic and petrological characteristics of the volcano-sedimentary sequence can supply guides to underlying Broken Hill type deposits in other parts of the Aggeneys terrane.

The exposed portion of the Koeris Formation that was mapped in detail in the Quarry fold nappe, is 220m thick and is interpreted to be stratigraphically inverted because of large-scale thrusting and folding. The sequence starts with Mg-rich mafic lavas (now amphibolite with hornblende and cummingtonite), followed by a sedimentary sequence (now quartz-muscovite schist). The succeeding metarhyolite and felsic breccia are overlain by mafic metalava and breccia with intercalated clastic products (now quartz-muscovite schist with mafic lenses). A subsequent major clastic incursion represented by conglomerate (called the main conglomerate), is terminated by massive amygdaloidal lava flows (termed the main amphibolite). The main amphibolite is followed by sedimentary rocks (now schist and quartzite) that are in turn overlain by lava (now homogeneous amphibolite). The sequence ends with volcanoclastic conglomerate/agglomerate succeeded by fine-grained clastic deposits (now quartz-mica schist).

Regional metamorphism at amphibolite grade yielded peak temperatures in the range of 400 to 700°C at ≥4 kbar. However epidote-rich parageneses (such as epidote-garnet-quartz) that locally transect the regional fabric, indicates the introduction of Ca, Si and Fe subsequent to the metamorphic peak. Vestiges of earlier alteration are represented by crystallographically zoned cordierite and anthophyllite in rocks of appropriate composition. In most of the exposed sequence, primary features such as vesicles and pillow structures have been retained in spite of the metamorphism, except in zones of high strain. The same can be said of the chemical composition of samples representing typical amphibolite and metarhyolite, matching average tholeiite and rhyolite/rhyodacite.

The lavas were generated from an upper mantle source near to 1 600 Ma ago, during extensional crustal conditions subsequent to the infilling of cratonic sedimentary basins in an alluvial-fan/fluvial environment. The volcanism was followed by the Namaqua Orogeny, a long-lived compressional period that included terrane amalgamation and felsic magmatism at ~1 200 Ma ago. The orogeny was terminated during renewed extensional conditions, when easterly orientated shear zones of regional extent were formed near to 1 100 Ma ago.

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