High Heat Production Granites Research Articles

Investigation of radiogenic heat production in granites of the Goiás Tin Province, Central Brazil

This study defines the radiogenic heat production of A-type granites in the Goiás Tin Province (GTP), Central Brazil, using airborne gamma-ray spectrometry. Pedra Branca Massif and Serra Dourada Granite are rich in tin, rare earth elements, and they exhibit anomalous radiogenic heat (5.5–15 µW/m³). They are therefore classified as high heat production granites (HHPG). By integrating radiogenic heat data, RGB imaging, magnetometry, density model and geological information, we associated anomalous radiogenic heat with mineralized regions present in the granites found in the GTP. Our methodology was validated using geological information, density model and other granites worldwide. It proved to be effective for targeting HHP granites.

Record of a pioneering geothermal borehole in London

This paper records the results of a pioneering exploration borehole drilled in 1984 to a depth of 403.84 m in the London Basin and underlying London–Brabant basement high. The key objectives of the project were to measure the equilibrium temperature in the Paleozoic basement, estimate the temperature gradient after climate impact corrections and assess the heat flow in the area. From this, estimates of temperature at depths of 2000 m could be made with a view to evaluating potential geothermal resources. From the downhole measurements, a temperature gradient of 16°C km −1 was estimated with a temperature range between 46°C and 51°C at a depth of 2000 m, considered to be consistent with other UK sites where there is no contribution from high heat production granites. The results of the Thamesmead exploration borehole suggested that the geothermal potential of the Devonian and the Paleozoic basement below London is poor, and no further work was carried out. The work has remained unpublished until now but in view of the current renewed interest in geothermal resources it is appropriate to make the work public. Although the results were not encouraging, they do remain as a contribution to the revitalized assessment of geothermal resources in the London area and the UK in general.

Thermophysical properties and geochemical characteristics of granites in the Tengchong area: indications for resource potential of hot dry rocks

Ascertaining regional radiogeochemical characteristics and lithospheric thermal structure characteristics is paramount for identifying the resource potential of hot dry rocks. This study conducted thermal conductivity testing and whole-rock analyses of outcrop granites of different formation epochs from the Tengchong area. Moreover, combined with the previously published whole-rock data and the data related to radioactive heat production rates, this study conducted geological, geophysical and geothermal analyses. Key findings are: (1) Granites in the Tengchong area exhibit an average radioactive heat generation rate of 5.54 μ W m −3 . Among them, the Late Cretaceous granites have an average radioactive heat generation rate of 8.08 μ W m −3 , suggesting high heat production granites. (2) Granites in the Tengchong area present an average thermal conductivity of 3.135 W.m −1 K −1 , relatively akin to the average values of granitic rocks worldwide. Compared to granites of other epochs, the Late Cretaceous granites manifest significantly higher thermal conductivities, with an average of 3.28 W.m −1 K −1 . (3) There is a negative correlation between the mica content and the thermal conductivity of the granites in the Tengchong area. (4) The Tengchong area holds potential for hot dry rocks exploitation and is inferred to have a lithospheric structure characterized by hot mantle and cold crust.

High heat production of the Late Mesozoic Luanchuan granites from East Qinling Orogen, China: Implications for petrogenesis and mineral exploration

Some of the high heat production granites in the continental crust have been suggested to be linked with I‐, A‐, and partly S‐type granitoids which aided in the formation of associated polymetallic deposits. Here, we investigate the Shibaogou and Laojunshan plutons in the Luanchuan region, East Qinling Orogen, China, and integrate these with published geochemical information to compute the rate of heat production per unit rock mass (RHP value, unit μW/m3). In addition, we also present RHP contour maps of all the Late Mesozoic granitic plutons in the Luanchuan region, with a view to evaluate the link among the high heat production granites, their petrogenesis and mineralization. The RHP values of granitoids in the Luanchuan region vary from 1.7 to 14.44, with the Nannihu pluton showing the highest RHP peak value of 6.04, whereas the Huoshenmiao pluton has the lowest RHP peak value of 2.59. The RHP values have a positive correlation with the degree of magma differentiation, as suggested that the Nannihu pluton is a highly fractionated I‐type granite and the Huoshenmiao pluton is a quartz diorite. This is also consistent with U, Th and K which are incompatible elements that tend to enter the melt during magma evolution. Furthermore, the higher RHP value intrusions (e.g., Nannihu, Daping) show close genetic links with high economic deposits, whilst the plutons with lower RHP values are generally related to small ore deposits. We envisage that the high heat production granites underwent protracted cooling process, which promote the convection and circulation of hydrothermal fluids, so that ore‐forming materials (e.g., Mo, W) can sufficiently accumulate and precipitate as economic mineralization. This also indicates that these granitoids with relatively higher RHP values have a greater metallogenic potential. The RHP contour mapping results show high RHP value domains ranging from 2.68 to 4.82 in the southeast and northeast segments of the Shibaogou pluton, whereas the RHP values of other regions in this pluton vary from 2.14 to 2.68. In contrast, the high RHP value regions mostly occur within the western and eastern parts of the Laojunshan pluton, and range from 4.91 to 6.21. Other regions in the Laojunshan pluton have low RHP values in the range of 2.69–4.91. A few Mo‐W deposits are mostly hosted in the southeastern part of the Shibaogou pluton, implying that these regions with high RHP values might have higher metallogenic potential. The contour mapping of RHP maximum, minimum, average and peak values of all the Mesozoic granitic plutons in the Luanchuan region also show high value zones within and/or at peripheral regions of the Nannihu ore fields and these segments between the Nannihu and Yuku ore fields, which suggest that these zones likely have high potential for Mo‐W exploration.

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new UPb zircon age from Pirrit Hills of 178.0±3.5Ma confirms earlier RbSr and UPb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and ~5m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6ppm respectively. Measured heat production of the granite samples is 2.96–9.06μW/m3 (mean 5.35W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70–95mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200–299Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

In situ gamma radiation measurements in the Neoproterozoic rocks of Sirohi region, NW India

Natural gamma ray measurements using a portable device were performed at 157 sites in the area around Sirohi town and Sindreth village in Rajasthan (NW India). This region comprises sedimentary rocks, metasediments, granites and gneisses that bear characteristic GR dose values and U/Th ratios corresponding with their specific geological history. A-type Malani granites and rhyolitic derivates, also referred as high heat production granites, show distinct differences as compared to the S-type Erinpura and Balda granites, most prominent in a high Th content of the former (up to 90 ppm). Sedimentary rocks in the Sirohi and Sindreth area are variable in their signatures reflecting their variable source rocks. In the area between the Balda and Paladi villages, northeast of Sirohi, measurements in vicinity of a N–S running shear zone, have shown U enrichment up to 8 ppm. This shear zone has been synkinematically mineralized with quartz and shows evidence of fluid infiltration into the host rocks in the vicinity of the shear zone. Erinpura granites have been altered due to fluid activity and show a light depletion of K (3.96%) and Th (20.11 ppm) as compared to the unaltered rocks (K, 4.06; Th 24.46 ppm). Enrichment of U (with a mean value of 13 ppm) has also been recorded in the lower clastic unit of the Sindreth Basin, especially within gritty conglomerates wherein migration and precipitation along fault planes is proposed.

The Paleo‐Proterozoic High Heat Production Richardson Granite, Great Bear Magmatic Zone, Northwest Territories, Canada: Source of U for Port Radium?

AbstractAirborne radiometric survey and field studies outlined a large, elongate, high‐level plutonic suite within the Richardson pluton south of the Contact Lake Belt in the Great Bear Magmatic Zone, Northwest Territories, Canada. In terms of content of radioactive elements, the Richardson pluton is composed of two distinct granite types, low heat production (LHP) and high heat production (HHP). Uranium content in the LHP and HHP granites ranges from 3.0 to 4.9 ppm and 6.5 to 24.6 ppm, respectively, showing similarity of the LHP granite to average granites. Geochemical studies indicate that there is a genetic relationship between these two types of granite; the LHP granite was the early product of magma crystallization, whereas the HHP granite is the result of extensive crystal fractionation of biotite, plagioclase and apatite. The presence of magmatic fluorite in granite suggests that high fluorine content lowered the liquidus temperature of magma causing lower temperature fractionation during ascent to high crustal levels, which increased U and Th concentrations in the resultant HHP granite. Weak U mineralization occurs locally as discontinuous quartz ± hematite ± pitchblende veins and veinlets within the HHP granite. Stronger U mineralization (U ± Ag ± Ni ± Co ± Cu) occurred in the past‐producing Contact Lake and Port Radium deposits. It appears that such mineralization may have had a spatial and temporal genetic‐paragenetic relationship with the HHP granite.

Modelling the thermal evolution of a collisional Precambrian orogen: High heat production migmatitic granites of southern Finland

We present results of thermal and geochemical modelling of Paleoproterozoic southern Finland, which is characterized by formation of high heat production migmatitic granites, post-orogenic granitoids, and high temperature–low pressure metamorphism at 1.85–1.78 Ga during late Svecofennian time. Metamorphic grade is mostly amphibolite facies (4–5 kbar, 650–700 °C), but also granulite facies (6 kbar, up to 800 °C) occurs. Our results suggest that the thermal evolution of the area can be modelled as a process of rapid crustal thickening (up to 70 km) in a plate collision at ca. 1870–1860 Ma ago, followed by conductive heating of the crust, simultaneous exhumation and partial melting of middle crust, and emplacement of the melts in the upper crust as late orogenic high heat production granites at about 1825 Ma ago. The upper crust collapsed gravitationally at about 1820 Ma but the crust was still thick (57 km) when the main orogenic era terminated about 1800 Ma ago. In modelling the thermal evolution we applied crustal heat production values constrained by geochemical data, and a conservative estimate, i.e., relatively low value for mantle heat flow density. The modeled temperatures at 30–50 km depths in the thickened crustal stack exceed the dehydration melting temperatures of micas (at 40 km 750 °C for muscovite and 800 °C for biotite, respectively) 25–35 Ma after the collision and thickening of crust. The results explain the high temperature–low pressure metamorphism in southern Finland as an inevitable consequence of crustal thicknening leading to an increase of the total heat production of the crust. Crustal thicknening is the main heat source for melting and metamorphism of the orogeny. Advective and latent heat brought by the melts further increased the temperatures at the emplacement depth and contributed to the granulite facies metamorphism that is spatially related to the migmatitic granites. Geochemical models suggest that the high concentrations of U and Th in the migmatitic granites can be attributed to about 20 vol% partial melting of deeply buried sediments and volcanic rocks. About 36–71% of the original U, 18–54% of Th and 34–46% of K, respectively, in the source rock types entered the melt phase. No anomalous concentrations of these elements in the source rock are required. In addition to explaining the migmatitic late orogenic granite and post-orogenic granitoid genesis, the collision model implies that the middle-lower crust and the upper mantle continued warming until the Mesoproterozoic times, which suggests a causal link between the Svecofennian thermal evolution and the extensive bimodal rapakivi granite and anorthosite magmatism in the Fennoscandian Shield about 200–250 Ma later.

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

The Moorby Microgranite: a deformed high level intrusion of Ordovician age in the concealed Caledonian basement of Lincolnshire

SUMMARY A granophyric microgranite has been proved in the sub-Carboniferous basement at Moorby, near Horncastle in south Lincolnshire. The microgranite is interpreted as a high level intrusion which has suffered strong alteration and deformation subsequent to emplacement, including the development of a spaced pressure-solution foliation and brecciation. The microgranite has yielded a U-Pb zircon age of 457 ± 20 Ma (late Ordovician) interpreted as the age of emplacement. A Rb-Sr whole-rock isochron age of 400 ± 9 Ma (early Devonian) is strongly discordant with the U-Pb age, and is interpreted to reflect thorough post-emplacement hydrothermal disturbance of the Sr isotope system, either associated with the development of the tectonic foliation during the Acadian phase of the Caledonian Orogeny or subsequent uplift. Geochemical data suggest that the affinities of the Moorby Microgranite lie with the arc-related Ordovician intrusions of the Lake District (e.g. the Ennerdale and Eskdale plutons) rather than with the early Devonian high heat production (HHP) granites such as Shap, Skiddaw and Weardale. While the physical properties of the Moorby Microgranite are comparable to those of the HHP granites, gravity modelling suggests that the microgranite is unlikely to be a major component of the inferred Caledonide Wash - north Norfolk Batholith.