Tracing element sources of hydrothermal mineral deposits: REE and Y distribution and Sr-Nd-Pb isotopes in fluorite from MVT deposits in the Pennine Orefield, England

摘要

Fluorite from Mississippi Valley Type (MVT) deposits in the South Pennine Orefield, England, displays significantly different distributions of rare earths and yttrium (REY) compared to fluorite from similar MVT deposits in the North Pennine Orefield. Samples from the South Pennine Orefield display negative Ce and positive Gd and Y anomalies but lack any Eu anomaly, indicating that the REY were mobilized from relatively pure marine sedimentary carbonates. In marked contrast, fluorite from the North Pennine Orefield lacks any Ce and Gd anomalies but shows a pronounced positive Eu anomaly, suggesting that the REY were provided by different source rock(s), that the mineralizing hydrothermal fluid had experienced higher temperatures prior to fluorite precipitation, and that it was derived from deeper crustal levels in the north compared to the south. The isotopic composition of Sr in Blue John fluorite from the South Pennine Orefield suggests that Sr was mobilized from Lower Carboniferous (Tournaisian) limestones, whereas Pb isotopes suggest that in contrast to REY and Sr, Pb was derived from aluminosilicate rocks. Neither Nd nor Sr or Pb isotopes can be used to radiometrically date the formation of Blue John fluorite. All isotope systems studied indicate that the limestone host rock of this fluorite mineralization did not contribute to the trace element budget of the hydrothermal fluid. Our results show that different solutes in a natural water (hydrothermal fluid, groundwater, etc.) may be derived from different sources, and that the study of a small set of elements or isotope ratios may not provide full insight into the genesis or history of a mineralization or a hydrothermal fluid. Our data provide evidence for the uncoupling of Sr, Nd and Pb during fluid-rock interaction and fluid migration, and show that the use of plots such as ~(87)Sr/ ~(86)Sr vs. implied by _(Nd). to learn about mixing relationships (as is commonly done in igneous geochemistry) is unreliable when applied to natural waters and their precipitates.