Their geologist said several times that the idea that mineralisation might be stronger at depth in the n. Pennines was not new and that Dunham had suggested it.
It surprised me at the time as I had never picked that up although sphalerite values increasing with depth is something that appears in Dunham regards the Rampgill Mine and also on mine reports for St Peters in East Allendale.
Sorry for the late reply. Essential gardening had to be done this week before the rain turns my garden on the London Clay into something like the Western Front.
Dunham reviewed the prospects at depth for all three Pennine orefields (Alston Block, Askrigg Block and Derbyshire) in a YGS paper in 1988. His overall conclusion was that in all three areas the mineralisation was of considerable lateral extent but only limited vertical extent and that in the case of the Askrigg Block erosion had reached below the base of the mineralised zone. He discussed the Alston Block in more detail in his 1990 BGS memoir (Chapter 15) and drew attention to the possibilities of mineralization in the Whin Sill, the beds around the horizon of the Robinson Limestone (because of thick sandstone units within the cyclothems) and in the Melmerby Scar Limestone. However, it is clear that he was thinking in terms of vein oreshoots (for example comparing prospects in the Whin with Settlingstones) and not in terms of the type of massive replacement orebodies envisaged by MINCO.
Dunham always stressed that the orezones in the NPO were generally of limited height and that the flow of mineralising fluids had been mainly in a lateral sense with a particular concentration at about the horizon of the Great Limestone. He believed that this was due to a combination of the thickness of that unit, its susceptibility to metasomatic replacement (flats) and the fact that it occupied a key structural position when the veins were formed during the uplift of the Teesdale Dome with vein fractures narrowing downwards and passing into the bedding upwards, which is why the veins do not outcrop on the high fell tops. Because of this effect, he believed there was no reason to expect veins to be mineralised at deeper levels except in the vicinity of the major emanative centres, which he lists as Tynehead, Groverake, Stotfield Burn, Cambokeels and Burtree Pasture on the basis of the mineralogical evidence with fluorite giving way to quartz and galena giving way to pyrite, pyrrhotite and chalcopyrite at depth.
There do seem to be occasional reversals of the zoning pattern with galena passing down into sphalerite, which in theory should not happen, but it may just reflect local conditions. Given the complex plumbing system in the ribbon oreshoots it is always possible that in some cases the mineralising fluids actually moved downwards or alternatively it may be just an effect produced at the extreme edge of an oreshoot.
Dave Greenwood