Trace element geochemistry of Wallrock alteration in the Pennine Orefield's and Cumberland iron field

Abstract

The variations in the trace elements Rb, F, Pb, Sr, Mn, Zr, Zn, Cu, Ni and Fe have been studied in limestone wallrocks next to lead - zinc - fluorite - barite mineralization in the Northern Pennine Orefield and Derbyshire Orefield; and next to haematite orebodies in the Cumberland Ironfieldo. Trace element aureoles have also been investigated in sandstones and shales. An analytical technique for the determination of fluorine in limestones was developed using the optical emission spectrograph, and found to give satisfactory results. Standard X-ray fluorescence techniques were employed for the determination of the other trace elements. The petrology, mineralogy and chemistry of certain aureoles have been examined in detail. The heavy metal content of the limestone wallrocks in the majority of cases, illustrates widespread and irregular dispersion patterns. It is suggested that migration of the metals has taken place through fractures developed prior to, or contemporaneously with, the mineralization. Diffusion of the heavy metals has occurred into massive country rocks at certain localities and results in a logarithmic decrease in concentration away from the ore and more restricted aureoles. The width of the dispersion patterns varies next to the veins, but in general a wider aureole is developed in Derbyshire than in the Alston Block. It is concluded that, at the time of emplacement, the Derbyshire veins were at a higher pressure than the veins in the Alston Block, but that variation in the temperature of the ore solutions was not an important parameter controlling the migration of the heavy metals. The alteration of the limestone wallrocks is discussed and suggestions are put forward to explain the possible origin of some of the replacement minerals. Adjacent to the veins, a depletion in strontium is recorded. This is related to the recrystallisation of the calcite which has liberated strontium from the crystal lattice. The strontium content of the Great Limestone and the D(_2) Limestone, the respective sampling horizons in the Alston Block and the Derbyshire area, are considerably different and it is concluded that this reflects either the original aragonite content of the limestones, or differing conditions of deposition in the two areas. Migration by diffusion into massive country rocks has occurred in the Cumberland Ironfield to far greater distances than in the Pennine Orefields; the depletion of strontium is also more extensive. It is suggested that the ore solutions were emplaced at higher temperatures than in the Pennine Orefields. The trace element aureoles in sandstone and shale wallrocks indicate that the heavy metals have been migrated through fractures, as irregular and widespread patterns are observed. A comparative study was conducted on the trace element variations which occur next to the veins which intersect the Whin Sill (quartz- dolerite) in the Alston Block. Contrary to previously reported widespread dispersion aureoles in igneous rocks, a narrow aureole is formed where diffusion has taken place into the wallrocks. A profound chemical change between the unaltered and altered quartz- dolerite has occurred where CO(_2), K(_2)O and H(_2)O are enriched, while Na(_2)O, MgO, total iron and CaO are depleted. The quartz-dolerite has been converted into a carbonate and clay-rich rock called the White Whin. The chemical and mineralogical variations are related to the variations in the trace elements where Rb, Ba, Pb and Zn are concentrated while Cu, Ni, Zr, Sr, V and Cr are depleted next to the veins. A geological and geophysical survey over altered and mineralised quartz-dolerite dykes at Closehouse mine, Lunedale near Middleton-in- Teesdale has produced results, which, it is hoped, will elucidate the complexities of the area.