Richard Scrivener

Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. (1) exoskarns 2. (2) high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias 3. (3) polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization 4. (4) late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40Ar39Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40Ar39Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210°C myr−1 to ~60°C myr−1 with a mean of 100°C myr−1. These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The mineralization history is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorite from stage 3 polymetallic mineralization give ages of 259 ± 7, 266 ± 3 and 267 ± 12 Ma, postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and cooling of the host granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

Samarium-Neodymium Direct Dating of Fluorite Mineralization

The direct dating of many styles of hydrothermal mineralization has proved difficult, limiting understanding of the geological processes that lead to crustal fluid flow and the formation of major ore deposits. The hydrothermal mineral fluorite (CaF2) displays large variations in rare earth element (REE) abundance and samarium/neodymium ratios within a single vein. Samarium-neodymium dating of fluorite from the classic granite-hosted tin deposits of southwest England demonstrates its use as a precise chronometer of mineralization. The concentrations of light rare earth elements (LREEs) in the fluorites are highly variable and suggest the coeval precipitation of an LREE-rich phase as the most likely cause of the extreme variation in samarium/neodymium ratios.

Timing and significance of crosscourse mineralization in SW England

Rb-Sr isotope analyses of inclusion fluids from quartz have demonstrated a Triassic age (236 ± 3 Ma) for N–S-trendmg Pb–Zn–F vein mineralization in the Tamar Valley district of the Cornubian orefield. Consideration of the stratigaphy and structure of the Permo-Triassic basins both onshore and in the English Channel suggest that this mineralization results from the formation of fractures during regional extension-driven subsidence, and the subsequent ingress of basinal brines to the Variscan basement.

The tourmaline-bearing granite pluton of Bodmin (Cornwall, UK): magnetic fabric study and regional inference

The magnetic fabrics and microstructures of the Variscan granite pluton of Bodmin (Cornwall) have been studied. Its low susceptibility magnitude, consistent with its lack of magnetite, comes from biotite and tourmaline. As the magneto-crystalline behaviour of tourmaline is ‘inverse’ compared with that of the phyllosilicates, the magnetic fabric was remeasured after heating the specimens to enhance the biotite magnetic signal by the growth of mimetic magnetite. The roughly similar magnetic fabrics before and after heating demonstrate that the effect of tourmaline is not important as long as the tourmaline content does not exceed that of the phyllosilicates. Regionally, this study reveals a well-defined NNW trend of the magnetic lineations with opposite plunges on each side of a separation line crossing the pluton centre. This pattern, very similar to that documented in the nearby Carnmenellis pluton, is ascribed to unroofing during emplacement. It agrees with the late to post-magmatic NW stretch that characterizes the Devonian to Carboniferous formations located to the north of this region, the thrust-and-fold structure of which is attributed to southward compression. We conclude that syntectonic emplacement may have characterized the whole Cornubian batholith at the very end of the Variscan orogeny in Cornwall.

U-Pb and Rb-Sr isotopic systematics of fluids associated with mineralization of the Dartmoor granite, southwest England

Abstract Quartz veins from early hydrothermal mineralization within the 280 Ma Dartmoor granite contain abundant fluid inclusions, which were sampled using crush-leach techniques and analyzed for Rb, Sr, U, and Pb content and isotopic ratios. The present-day lead isotopic signature of some of the crushleach fluids is similar to that of granitic alkali feldspars from Dartmoor and other localities in southwest England, although some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field upon correction of the present-day fluid Pb ratios for in situ U decay over ca. 280 Ma, but the lead isotope ratios of the less-radiogenic fluids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid mixing due to leaching of secondary fluid inclusions, differential leaching of U and Pb, the presence of an undetermined U-rich included phase (or fracture-filling) within the quartz, or a combination of the above. The variation of fluid U/Pb ratios also suggests some complications regarding either the selective retention of U and Pb on quartz surfaces, or the presence of submicroscopic U-rich inclusions. Therefore, we suggest that in situ decay corrections to lead isotopic data obtained using crush-leach techniques be applied with great caution. The fluid 87 Rb/ 86 Sr values vary little (1.40–1.74), as do their present-day strontium isotope signatures ( 87 Rb/ 86 Sr = 0.71814–0.71968). When corrected for in situ 87 Rb decay over 280 Ma, the fluids contain significant excess radiogenic Sr (Sr i = 0.7118–0.7141), relative to the Dartmoor granite S i (=0.7101). If the Sr in the fluids is derived exclusively from the granite, or some granite-related source, and closedsystem evolution of the Rb-Sr isotopic system is assumed, the amount of excess 87 Sr is a function of the time difference between Sr closure in the granite and Sr closure in the vein. Age estimates from the intersection of granite and fluid inclusion Sr evolution curves are in agreement with published 40 Ar- 39 Ar ages of late dikes and sericitic alteration at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Dartmoor granite are themselves related to granite-derived, late-stage aplitic melts.