Petrological, geochemical and geochronological characterisation of heat-producing granites

Abstract

Granitic magma generation has the ability to strongly differentiate the continental crust and concentrate elements such as the heat-producing elements (HPE) uranium (U), thorium (Th) and potassium (K). These elements can be enriched above average upper continental crustal values, and these granites are referred to as moderately (4-8 µW/m 3 ) or high (>8 µW/m 3 ) heat-producing granites (HHPGs); when overlain by insulating sedimentary cover these may be considered a target for enhanced geothermal systems (EGS).The composition of the source, and differentiation during magma ascent and emplacement, can influence the magmatic elemental budget, as can the accessory mineral assemblage, tectonic setting and history of regional magmatism. It is of importance to investigate whether there is a combination of these factors that promotes HPE enrichment and, if so, can these factors be used to provide a criteria for HHPGs prospective for future EGS development?To address the relative influence of factors on the ability to affect elemental enrichments three HHPG examples with an established history in EGS development studies were selected: the Big Lake Suite (BLS), South Australia; Cornwall, UK; and Soultz-sous-Forêts (Soultz), France. These sites represent the three most common granite types: A-, S-and I-type, respectively. Comparing their mineralogy, major and trace element abundances, and zircon geochemistry and associated U/Pb ages will inform the timing and controls on enrichment at each location. iii an enriched source. The emplacement ages for BLS samples range from 325 ± 2 Ma to 312 ± 2 Ma; this spread in ages implies continual granite emplacement over ~13 Myr. Some zircons dating to the time of emplacement record extreme U (>5000 ppm) and Th (4000 ppm) enrichments.Inherited zircon populations were also dated in European analogues: 363 ± 9 Ma, 408 ± 9 Ma and 420 ± 6 Ma (Cornwall) and 426 ± 14 Ma ages (Soultz) are associated with granite emplacement in a syn-collisional environment during closure of the Rheic Ocean. Although some inherited grains inCornwall record high Th contents (~800 ppm), extreme enrichments (>6000 ppm U) are analysed in some emplacement-aged grains. Th enrichment (~1600 ppm) is observed in emplacement-aged zircons in Soultz.Zircon locations in thin section were studied to address where these extreme enrichments could have developed. Zircons bordering mineral boundaries or adjacent to phenocrysts likely crystallised in differentiated chemical boundary layers, and elemental abundances in these regions would have been different to those of the pluton as a whole. Localised enrichment could explain the extreme enrichments present in some emplacement-aged zircons; the extreme levels of differentiation required to attain these enrichments -up to 99% -are not plausible on a pluton scale. Zircons armoured within earlier crystallising mafic silicates (e.g., biotite) would not have been exposed to these extreme enrichments, and this could account for the range of elemental abundances recorded in empl...

Chemical and mineralogical characterisation of illite–smectite: Implications for episodic tectonism and associated fluid flow, central Australia

“…Previous work by Deighton and Hill (1998) constrained maturation and expulsion of such reserves to the "mid Cretaceous"; however, the thermal regime responsible for such expulsion is not well constrained. These basins also host one of the most prospective hot dry-rock geothermal resources in world, which may stem from the presence of the highly radiogenic Big Lake Suite A-type granite (up to 144 ppm Th and 30 ppm U; Marshall, 2014). Early workers believed zircons to be the primary sink for heat-producing elements (HPE; K, Th and U) in the Big Lake Suite.…”

Petrogenesis of high heat producing granites and their contribution to geothermal resource in the Huangshadong geothermal field, South China

“…The granitic upper crust has depths of mostly 10-20 km, and its radioactive heat production contributes surface heat flow of 10-20 μW/m 2 , which accounts for 50% of the surface heat in the stable continental crust (Artemieva and Thybo, 2013). High-heat-producing granites, with radioactive heat production rates >5 μW/m 3 , play a critical role in the formation of geothermal resources (Marshall, 2014;Artemieva et al, 2017). For instance, the Cooper Basin, which is subjected to the most recent intrusion of Carboniferous-Permian granodiorites rather than present-day magmatic events (Big Lake Suite, 310-327 Ma; (Marshall, 2014), records the terrestrial heat flow values measuring >100 mW/m 2 .…”

“…High-heat-producing granites, with radioactive heat production rates >5 μW/m 3 , play a critical role in the formation of geothermal resources (Marshall, 2014;Artemieva et al, 2017). For instance, the Cooper Basin, which is subjected to the most recent intrusion of Carboniferous-Permian granodiorites rather than present-day magmatic events (Big Lake Suite, 310-327 Ma; (Marshall, 2014), records the terrestrial heat flow values measuring >100 mW/m 2 . However, the minimum heat flow value is only 33 mW/m 2 , which occurs at the edge of the rock mass in a borehole.…”