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U-Pb geochronology of tin deposits associated with the Cornubian Batholith of southwest England: Direct dating of cassiterite by in situ LA-ICPMS

April 22, 2019

The Cornwall and Devon vein- and greisen-type copper and tin deposits of southwest England are spatially and genetically related to shallow-seated granitic intrusions. These late Variscan intrusions, collectively known as the Cornubian Batholith, extend over 200 km and form a continuous granitic spine from the Isles of Scilly Granite in the west to the Dartmoor Granite in the east. The granitic plutons of the Cornubian Batholith were intruded from ~ 295 to 270 Ma without a major hiatus. Twelve samples of cassiterite (SnO2) were obtained from tin deposits associated with seven different plutons within the Cornubian Batholith for in situ LA-ICPMS U–Pb dating. This study of cassiterite was undertaken to obtain the first results of direct dating of ore mineral to refine the geochronology of tin mineralization in this region. Of the cassiterite samples analyzed, the oldest ages were determined within the Kit Hill and Hingston–Gunnislake Granites in the central part of the Cornubian Batholith. The Hingston–Gunnislake cassiterite, from Drakewalls Mine, was the oldest sample dated at 291.8 ± 3.4 Ma. The next oldest dates, 290.5 ± 2.8 and 288.5 ± 2.9 Ma, were from two cassiterite samples extracted from the adjacent Kit Hill Consolidated Mines within the Kit Hill Granite. At the eastern end of the study area, two cassiterite samples within the Dartmoor Granite produced ages of 286.0 ± 1.8 and 284.1 ± 1.3 Ma. The youngest sample from this study, 275.4 ± 1.6 Ma, is from the Balleswidden Mine within the westernmost Land’s End Granite. The cassiterite dates do not reveal any readily observable relationship between ore ages and geographic relationship from west to east throughout the Cornubian Batholith. Incorporating the associated errors, the geochronology does indicate continuous mineralization within the granites for ~ 21 million years, from ca. 295 to 274 Ma. This span falls within the established period of granitic magmatism of ca. 295 to 270 Ma for the Cornubian Batholith and further confirms the reliability of in situ LA-ICPMS U–Pb dating of cassiterite.