Hydrothermal alteration of kimberlite by convective flows of external water

Abstract

Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130–400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water–rock ratios (estimated at <0.2). Such low water–rock ratios result in only small changes in stable isotope compositions; for example, δO18 is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.

A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

“…Its origin in kimberlites, however, is still contentious. Two dominant models assume serpentine as being either late-stage magmatic mineral crystallizing from deuteric fluids (e.g., [33,101,113]) or hydrothermal mineral crystallizing from impregnating low-T fluids of external (non-magmatic) source, i.e., groundwaters (e.g., [114][115][116]). Both models accept that serpentine forms at least partly as a replacement product of olivine and other Mg-silicates.…”

Fenites associated with carbonatite complexes: A review

“…This process is analogous to the kimberlite alteration model proposed by Afanasyev et al (2014), in which fluids flowing through the pipe are proposed to reduce breccia porosity and permeability by alteration to fine hydrous minerals. This process suppresses fluid flow and effectively seals off unaltered sections, very applicable to fenites which display highly heterogenous intensities of metasomatism.…”

“…These breccias are often interpreted as hydrothermal products resulting from the escape of volatiles from magma during boiling stages, leading to intense hydraulic fracturing (Burnham, 1985;Pirajno, 2015). Diatremes are often filled with highly permeable and porous vent breccias and tuffs (Afanasyev et al, 2014;Elliott et al, 2015) that provide efficient intrusive pathways and hosts for later carbonatite magmas. This process can form REE-enriched (up to ∼ 3 wt% TREO) carbonatite stockworks such as the Bull Hill phonolitic diatreme at Bear Lodge, USA (Moore et al, 2015) and diatreme breccias at Kaiserstuhl, Germany (Hornig-Kjarsgaard, 1998).…”

Diatremes Act as Fluid Conduits for Zn-Pb Mineralization in the SW Irish Ore Field

“…It is well established that diatreme-filling deposits have high initial porosity and permeability (Sparks et al, 2006;Stripp et al, 2006), allowing enhanced hydrothermal fluid flow (Afanasyev et al, 2014). Further, diatreme-forming eruptions produced radial fracture networks (cf.…”

“…Our data indicate that diatreme emplacement strongly influenced mineral deposit formation at Stonepark. Diatremes, such as those preserved at Stonepark (Elliott et al, 2015), are typically infilled with porous and permeable material (e.g., White and Ross, 2011;Afanasyev et al, 2014) and are associated with fracture networks and breccia bodies related to their explosive emplacement (Sparks et al, 2006). We propose that the relatively permeable diatremes served as conduits for mineralizing fluids, forming more efficient fluid pathways from the basement than the extensional fault systems widely observed within the Irish ore field (e.g., Silvermines, Navan, Lisheen).…”