Regional variations in fluid formation and metal sources in MVT mineralization in the Pennine Orefield, UK: Implications from rare earth element and yttrium distribution, Sr-Nd isotopes and fluid inclusion compositions of hydrothermal vein fluorites

Abstract

The Pennine Orefield is one of the most important ore fields for Pb-Zn-Ba-F mineralization in Great Britain. It is subdivided into the Northern Pennine Orefield (NPO), consisting of the Alston and Askrigg Blocks, and the Southern Pennine Orefield (SPO). The Alston Block is underlain by the early Devonian Weardale Granite and the Askrigg Block by the coeval Wensleydale Granite. The potential relationship between the batholiths and the mineralization is a matter of debate. We here studied the rare earth elements and Y (REY) geochemistry, Sr-Nd isotopes and fluid inclusion (FI) compositions of fluorites from the two structural blocks in the NPO and found that the fluorite mineralization in these blocks differ substantially. The REY in Askrigg fluorites show features that are characteristic for leaching of adjacent Lower Carboniferous limestones. In contrast, Alston fluorites have significantly higher REY concentrations, lack REYSN limestone signatures and show a decoupling of redox-sensitive Eu from its trivalent REY ‘neighbours’. Neodymium isotopes indicate a similar crustal source of REY in both blocks, but higher REY concentrations and lower Y/Ho ratios suggest Lower Carboniferous shales as potential REY source in the Alston Block. The fluids that precipitated the Alston fluorites experienced temperatures > 250 °C prior to mineral formation, as evidenced by Eu geothermometry. Fluorite formation, however, occurred at much lower temperatures, as suggested by homogenization temperatures in FI, that fall within ranges of 105–159 °C in Alston and 99–160 °C in Askrigg fluorites. Mineralization of the Mississippi-Valley Type usually lack association with igneous activity. We show that some of the fluids responsible for the NPO mineralization were influenced by magmatic sources. The REY systematics in Alston fluorites may be linked to an interaction of the Permian-age Whin Sill dolerite with the basement granite, which heated fluids and focussed fluid flow into the overlying sedimentary rocks. In the Askrigg Block, where such a dolerite intrusion was not described, fluorites lack any positive EuSN anomalies, indicating that these fluids had never been subjected to temperatures exceeding 200–250 °C.