Post‐emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa

Abstract

The common serpentine-diopside matrix assemblage in volcaniclastic kimberlite (VK) at the Venetia Mine, South Africa is ascribed to a secondary origin, because of post-emplacement serpentinization and associated hydrothermal metamorphism. Volcaniclastic deposits with 20-30% porosity infill kimberlite pipes in the waning stages of kimberlite eruptions. Olivine macrocrysts are typically rimmed by talc and are pseudomorphed by lizardite, with minor magnetite. The fine matrix consists of mixtures of lizardite, chlorite, smectite, brucite, calcite, titanite and andradite, an assemblage which either pseudomorphed microcrysts or in-filled voids. Locally we recognize microcryst pseudomorphs rich in sub-microscopic mixtures of lizardite with smectite, and other microcryst pseudomorphs and void-filling matrix rich in chlorite and lizardite. Interstitial lizardite and associated phyllosilicates (brucite, smectite and chlorite) crystallized progressively from meteoric or hydrothermally derived pore waters, and Si 4+ and Mg 2+ released into the fluid phase during serpentinization of olivine macrocrysts. Radial-fibrous fringes of diopside microlites around crystals display void-filling textures because of unrestricted growth into pore spaces. Secondary diopside is attributed to Si 4+ , Mg 2+ and Ca 2+ cations released into the fluid phase by interaction with olivine, calcite and plagioclase in siliceous xenoliths. The paucity of primary, finegrained groundmass phases resistant to alteration, for example, perovskite and spinel, precludes an origin for the intergrain matrix as altered interstitial ash, glass or a late-stage kimberlite melt. Isovolumetric replacement of olivine results in a volume increase of 60% so that pore spaces in the original deposit can be easily filled up with serpentine. The source of Al 3+ to form chlorite and smectite is attributed to alteration of plagioclase in xenoliths which comprise 20-30 vol.% of the deposit. Titanite, hydro-andradite and second-generation diopside precipitate as hydrothermal minerals from calcium-bearing serpentinizing fluids in replacement reactions and as void-filling minerals. Consideration of mineral equilibria in the CaO-MgO-SiO 2 -H 2 O-CO 2 system constrains the common matrix assemblage of lizardite and diopside in X CO 2 )-T space. At 300 bar, the assemblage is stable only at temperatures below 370°C and X CO 2 < 0.01. This upper limit on temperature is well below the plausible solidus of ultrabasic magmas. Furthermore, the requirement of trace CO 2 in the fluid phase implies a post-emplacement external source rather than ÔautometamorphismÕ from kimberlite-derived fluids, because of high P CO 2 commonly inferred for kimberlite magmas.

Maar-diatreme volcanoes: A review

“…This restriction also rules out, by definition, the existence of any non-kimberlite diatreme, which we view as a major conceptual problem. And in practice, even for kimberlites, such a restriction is deeply flawed because one of these "defining" characteristics, the microlitic diopside, is proposed by Stripp et al (2006) to be the product of secondary aqueous alteration at temperatures below 370ºC.…”

“…We are unconvinced by this model for development of tuffisitic kimberlite and tuffisitic kimberlite breccia, and prefer a more conventional fragmentation style. The origin of these textures needs to be interpreted in light of the clear evidence for strong and pervasive alteration (e.g., Stripp et al, 2006;Cas et al, 2008a;Kurszlaukis and Lorenz, 2008).…”

Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions

“…The inter-grain pore-space is filled with isopachous radiating acicular diopside microlites, massive serpentine and clay minerals ( Fig. 10d; e.g., Stripp et al 2006). Country rock xenocrysts include feldspar, amphibole, quartz and biotite.…”

“…The diopsideserpentine-calcite assemblages in the pore-spaces of the massive volcaniclastic kimberlite and matrix-supported volcaniclastic kimberlite lithofacies are interpreted as posteruption cements precipitated by hydrothermal fluids that circulated through the hot and porous deposit after the eruption. Recent work by Stripp et al (2006) indicated that the diopside-serpentine assemblage forms at temperatures between ∼250-380°C from meteoric-derived fluids. This assemblage provides a minimum estimate on the emplacement temperature of massive volcaniclastic kimberlite.…”

Diamond resorption features as a new method for examining conditions of kimberlite emplacement

“…The ratio of the two main volatiles H 2 O and CO 2 and the timing of their exsolution from the magma could vary in different kimberlites affecting their geology (Skinner and Marsh 2004) and diamond population (Fedortchouk et al 2010). Loss of volatiles during kimberlite eruption and development of secondary minerals prior or post-kimberlite emplacement (Mitchell 2013;Stripp et al 2006) hide the primary composition of volatiles. Fluids play the main role in the process of diamond resorption (Fedortchouk et al 2007;Khokhryakov and Pal'yanov 2010), and the morphology of dissolution features on diamond can help to reveal the primary composition of kimberlitic volatiles (Fedortchouk et al 2010).…”