Modelling the structural controls of primary kaolinite formation

Abstract

An abundance of kaolinite was formed within the St Austell granite pluton of Cornwall, southwest England, by the hydrous dissolution of feldspar crystals. The permeability of Cornish granites is low and alteration acts pervasively from discontinuity features, with montmorillonite recognised as an intermediate assemblage in partially kaolinised material. Structural features allowed fluids to channel through the impermeable granite and pervade deep into the rock. Areas of high structural control are hypothesised to link well with areas of advanced alteration. As kaolinisation results in a loss of competence, we present a method of utilising discontinuity orientations from nearby unaltered granites alongside the local tectonic history to calculate strain rates and delineate a discrete fracture network. Simulation of the discrete fracture network is demonstrated through a case study at Higher Moor, where kaolinite is actively extracted from a pit. Reconciliation of fracture connectivity and permeability against measured subsurface data show that higher values of modelled properties match with advanced kaolinisation observed in the field. This suggests that the technique may be applicable across various industries and disciplines.Keywords: Kaolinite, Discrete Fracture Networks, Structural Influence, Kaolinisation, 3D Modelling 1-IntroductionKaolinite is found extensively across the St Austell granite pluton of southwest England, formed from the primary, in-situ hydrous alteration of Na-feldspar and K-feldspar (Brown, 1953;Fuge and Power, A C C E P T E D M A N U S C R I P T Both equations result in the partial leaching of a silica by-product (Charoy, 1981) and an increase in hydrogen ion activity (Exley, 1976). As hydrogen ions dissolve within the circulating fluids, they corrode feldspar cleavage planes and allow ions to redistribute into silicate layers (Guilbert and Sloane, 1968). However, the change from feldspar to kaolinite is not as straightforward as the equations imply. Intermediate assemblages of alternative clay types, such as illite and smectite, are commonly found within partially kaolinised granites (Psyrillos et al., 1998(Psyrillos et al., , 2003Bristow et al., 2000; Papoulis et al., 2004;Suringar, 2004). Papoulis et al. (2004) suggest that exposure length, permeability of the deposit, and the space available for the reactions are all factors that will influence the formation of clay. Partially kaolinised samples from St Austell are found to contain Na-montmorillonite, a smectite clay (Scott et al., 1996; Psyrillos et al., 1998;Ellis and Scott, 2004), due to preferential dissolution of plagioclase end-member albite (Exley, 1976; Psyrillos et al., 1998;Bristow et al., 2000).Granite is relatively impermeable due to its interlocking crystalline texture. In order to create the wide expanses of kaolinite found around St Austell, alteration fluids must have utilised major discontinuity networks within the rock mass (e.g. joints, fractures, faults) to circulate. This is evidenced through the fact t...

Landscape evolution of the granitic Criffel–Dalbeattie hills, south-west Scotland

“…Hall and Gillespie, 2017). These zones indicated as faults (Figure 3a) or structural trends (Figure 8) likely provided loci for preferential transformation into deeply weathered saprolite similar to the weathered beds currently found exposed in granitic terrain in SW England (Tierney and Glass 2016). However while fracture induced weathering appears plausible as an explanation for the incised valleys and basins on the CDGP, deeply weathered profiles were not observed during this work.…”

Lab-Scale Permeability Enhancement by Chemical Treatment in Fractured Granite (Cornubian Batholith) for the United Downs Deep Geothermal Power Project, Cornwall (UK)

“…Products of hydrothermal alteration are present, such as Kaolinite (likely Dickite at reservoir depth [15][16][17][18], greisenisation (paired with tourmalinisation and cassiterite veins [19], Tourmaline veins (related to the circulation of boron-enriched fluids in hydrothermal convection cells [20], Chlorite [20], hydrothermal W/Sn/Cu/As/Zn ore minerals (mainly in the metasediments [4,13] and Quartz, Fluorite or Barite veins [15,21]. Cornwall is intersected by several fault systems, such as the Porthtowan Fault Zone (PTF), which are mainly strike slip faults, roughly trending NW-SE (NNW-SSE) and locally referred to as 'cross courses' [9], connecting ENE-WSW-striking extensional faults [13] or dykes, called 'Elvans'.…”

“…Products of hydrothermal alteration are present, such as Kaolinite (likely Dickite at reservoir depth [15][16][17][18], greisenisation (paired with tourmalinisation and cassiterite veins [19], Tourmaline veins (related to the circulation of boron-enriched fluids in hydrothermal convection cells [20], Chlorite [20], hydrothermal W/Sn/Cu/As/Zn ore minerals (mainly in the metasediments [4,13] and Quartz, Fluorite or Barite veins [15,21]. [5], sampling locations and relevant project locations; PTF simplified after Reinecker et al [22]; meta sediments, dykes and non-magmatic geological units in grey.…”