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YORKSHIRE GEOLOGICAL SOCIETY - THE WEARDALE GRANITE

Next Meeting: Saturday 28th January 2012: 2.00pm -5.00pm, Great North Museum:Hancock, Barras Bridge, Newcastle upon Tyne NE2 4PT:

Foundations of the Northern Pennines: Rookhope - 50 years on: The Sir Kingsley Dunham Meeting: Joint meeting of the Natural History Society of Northumbria, the Yorkshire Geological Society and Friends of Killhope

It is now fifty years since the Weardale Granite, the existence of which was first predicted in the 1930s by Kingsley Dunham, then a Durham University post-graduate student, was finally proved by drilling the Rookhope Borehole. The varied lines of reasoning that first suggested that a granite may lie beneath the Northern Pennines, and the detailed results of the drilling, including the unexpected age of the granite, soon become established as one of the classical stories of British geology. The project was a geological adventure that radically changed perceptions, not just of northern England geology, but which led to major advances in the understanding of ore-forming and related processes worldwide.

Fifty years on, the scientific legacies of the borehole are as relevant as ever, underpinning research into new areas of understanding, some of which may have the potential for economic benefits undreamt of when the granite was first predicted. To mark this significant anniversary, the Natural History Society of Northumbria has joined with the Yorkshire Geological Society and the Friends of Killhope to hold a joint meeting at the Great North Museum:Hancock on the afternoon of Saturday 28th January 2012. In addition to reviewing the ongoing significance of the borehole, each of the afternoon’s four talks will focus on an aspect of northern England geology which arises from insights provided by the borehole. Topics will include new interpretations and models for the origins of mineralisation, the possibilities for economically viable geothermal resources and a review of the potential for future mineral exploration and working.

The Yorkshire Geological Society occasionally dedicates one of its scientific meetings as The Sir Kingsley Dunham Meeting in recognition of Sir Kingsley Dunham’s distinguished contributions to geological science, particularly in respect of his seminal work on Pennine ore deposits and his former Presidency of the Society. There can be no more fitting a topic for this dedication than the theme of this meeting.

Speakers and their Abstracts

Martin H P Bott, Department of Earth Sciences, University of Durham: North Pennine Mineralization

A Hercynian or later granite beneath the Alston Block was suggested by Kingsley Dunham 80 years ago in explanation of the zonal pattern of mineralization centred on Upper Weardale and Tynehead. 30 years later, gravity surveys gave convincing evidence for the Weardale granite with cupolas underlying the mineral zones. In 1960, the Rookhope borehole proved an Early Devonian granite, stimulating major re-appraisal of the origin of the mineralization and of the influence of the granite on the stability of the Alston Block and subsidence of nearby deep Northumberland and Stainmore basins. I review the discovery of the Weardale granite, and its thermal history in relation to origin of the associated mineralization.

Heat flow of ~95 mW/m2 was measured in the Rookhope borehole which is about 65-75% above the continental average and the values in adjacent regions. This is explained by the high content of long-lived radioactive isotopes in the granite extending over its 10 km depth. Temperature-depth gradients are 32°C/km in the granite and 44°C/km in the Carboniferous due to differing thermal conductivity. Conductive heat flow appears to dominate. Two sets of observations, however, suggest that heat flow several times higher than the present value occurred locally for a few million years above the Rookhope and Tynehead cupolas, both before and after emplacement of the Great Whin Sill. First, Creany showed that vitrinite petrography of thin Namurian coals indicates temperatures of 187°C or higher above the Rookhope and Tynehead cupolas prior to emplacement of the Great Whin Sill. Second, fluid inclusions in vein minerals such as quartz, fluorite and barite yield temperatures of crystallization ranging up to 210°C during the cooling phase of a thermal high subsequent to emplacement of the sill, with temperature in the barite zone of less than 50°C suggesting normal heat flow beyond the cupolas. The sharpness of these high thermal anomalies recording heat flow several times the present value indicates that they originated mainly from hydrothermal convection in the cupolas below rather than just thermal conduction. They may represent earlier and later stages of a single thermal event, but accurate dating of the mineralization is required to clarify. A modern example of this thermal scenario is provided by drilling for the Hot Dry Rock geothermal plant at Soultz-sous-Forêts in the Rhinegraben, where an excessively high conductive geothermal gradient occurs in the impervious cap above granite; in contrast, an excessively low gradient occurs in the granite itself where hydrothermal convection carries most of the upward flow of heat.

The source of the heat may have been high density Whin magma underplating the demonstrably lower density solid Weardale granite, consistent with the absence of Whin dykes above the granite. Phacolithic underplating might also explain the Teesdale dome.

Joe Cann, School of Earth and Environment, University of Leeds: The significance of the Rookhope Borehole and the North Pennine Orefield to models of the origins of lead ores

The origin of limestone-hosted lead-zinc ores found in many places around the world has long been disputed. The dispute is still not settled, but the results of the Rookhope borehole had an important impact on the problem. Hydrothermal ores had traditionally been related to igneous intrusions, and especially to granites. Such a relationship is clear in the Cornish orefield, where zones of mineralisation surround the granite intrusions. For example, tin ores are found close to the granites and lead ores farther away, suggesting an origin from granitic fluids that cool with distance as they percolate through the rocks. In Cornwall this model still holds, and the same model seemed applicable when Sir Kingsley Dunham mapped the mineral zonation of the North Pennine orefield. The ore fluids must have emanated from an underlying granite intrusion.

Further reinforcement came from Martin Bott’s gravity survey, showing a negative anomaly that coincided with the zonation of ores. The Rookhope borehole found the granite at the predicted depth, but at the same time showed that the simple model did not hold, since the granite is overlain unconformably by the Carboniferous sediments that host the ores, and are 50 million years or so younger than the granite. What alternative can there be? Detailed investigation of the ores showed that they formed from fluids that had reached temperatures of as much as 200°C, and that the fluids had been about six times as saline as seawater. Similar salinities are common in other limestone-hosted lead-zinc ore fluids, but the temperatures in the North Pennines are rather higher than is common.

Once freed from the need for fluids from a granite intrusion, alternative models were developed for the origin of limestone-hosted lead-zinc ores. One group of models relate the ores to hot fluids expelled from the deep levels of sedimentary basins. Such ideas arisen in other lead-zinc orefields, at first in outline, and later using computer models of fluid flow. Certainly such a model could be applied to the North Pennine orefield, deriving fluids from the deep Solway Basin to the northwest that contains evaporites at depth. An alternative group of models would source the ore fluids as surface seawater that had penetrated by thermal cracking deep into hot crystalline basement. At those depths the fluids would have been heated and enriched in metals and then have risen to shallow levels again, convecting through the rock. In this case questions arose about whether fluids could penetrate deep enough into the basement to be heated to 150-200°C. The high heat production in the Weardale Granite would certainly reduce the necessary depths. The talk will explore the historical background and will assess the likely origin of the ores by comparing these two groups of models.

Jon Gluyas, Department of Earth Sciences, University of Durham and Paul Younger, School of Civil Engineering and Geosciences, University of Newcastle upon Tyne: Getting into Hot Water: the geothermal potential of NE England

In 1961 the Rookhope Borehole proved the presence of the Weardale Granite, whose presence had first been postulated to account for the origins of Northern Pennine mineralisation, and subsequently to explain the results of detailed gravity and magnetic studies. Surprisingly, however, the top of the granite was found to be overlain unconformably by Lower Carboniferous sediments and, although it had clearly been exposed during Carboniferous times, it was still hot. It was later recognised that this heat was not residual after emplacement but resulted from the radiogenic decay of thorium and other elements.

The fiftieth anniversary of the Rookhope borehole in 2011 was marked by the drilling of the third modern geothermal well on the Weardale system by a Newcastle and Durham universities joint project. The two universities are leading the way in the UK’s quest for geothermal energy – ahead of commercial concerns in Cornwall. Two wells have been drilled at Eastgate. Both penetrated the Weardale Granite. Eastgate 1 (2004) was drilled to 995m, entering the granite at about 275m and the flow rate tested at two intervals. An open fracture encountered at 411m flowed at 37m3/hour (at 46ºC) and the interval below at 22m3/hour. The former value is the highest ever recorded from a granite. The thermal gradient was determined to be 38ºC/km. The nature of the fracture system was uncertain. The well had planned to intersect the Slitt Vein, part of the northern, bounding fault system to the Weardale (Block) Granite. However, it was also considered possible that flow was coming from a weathering induced fracture system near the top of the granite.

Funds from DECC in 2010 allowed drilling of Eastgate 2. A location was chosen 700m from Eastgate 1 specifically to test the two possible hypotheses regarding the fracture system. The well terminated at 420m and although the temperature was as expected the well failed to flow; clear proof that Eastgate 1 had indeed penetrated a little mineralised section of the Slitt Vein. Further funding from DECC, Newcastle Science City and the British Geological Survey in 2011 resulted in a third well. The well was spudded in central Newcastle on the old Scottish & Newcastle brewery site. Like the Eastgate 1 well, Science Central 1 was planned to intersect one of the major bounding faults to the Weardale Block, this time the 90 Fathom Fault. The well drilled to about 1850m having penetrated almost the whole of the Carboniferous section. Evaluation is currently underway to determine both the thermal gradient and potential flow characteristic. Evaluation of all three wells continues with the intention of determining whether the temperature and flow rates will support district heating schemes. Such schemes could deliver low enthalpy heat with a near zero carbon footprint. Success in Eastgate and Science Central could pave the way for a geothermal revolution in the UK.

Brian Young, Department of Earth Sciences, University of Durham and F.W.Smith, FWS Consultants Ltd, Spennymoor, Co.Durham: The Northern Pennine Orefield - Opportunities and Possibilities

Mineralisation in the Northern Pennine Orefield occurs in numerous veins and related replacement deposits of Mississippi Valley type, hosted in Carboniferous sediments and the Permo-Carboniferous Whin Sill. Over centuries of mining the orefield has yielded around 4 million tonnes of lead, over 2 million tonnes of fluorspar, 1.5 million tonnes of barytes, 1 million tonnes of witherite (almost the entire world total), 0.3 million tonnes of zinc, a large, though unrecorded, tonnage of iron ores, together with very modest amounts of copper ores and small but significant tonnages of silver, recovered as a by-product of lead smelting. The 18th and 19th centuries saw the heyday of mining when, through ambitious programmes of exploration, innovative approaches to mining, mineral processing and smelting, the area achieved a prominent place at the forefront of the world’s lead industry. However, the collapse of world lead prices towards the end of the 19th century forced the closure of all but a handful of mines. An almost contemporary rising demand for spar minerals, particularly fluorspar, and to some extent zinc ores, offered a lifeline for a few mines with reserves of these minerals. Whilst the earliest decades of the 20th century witnessed the final demise of lead mining, by the second half of the century the area had become a major source of fluorspar, an industry that was to outlive the extraction of zinc, baryte and witherite. But, by the 1990s, a fall in the world price of fluorspar was to prove the death knell of the few surviving Northern Pennine mines and, since the closure of Groverake Mine in 1999, save for the extraction of cabinet specimens of fluorite, there has been no commercial mining in the orefield.

Although mining here may have been dormant for over a decade, thoughts of the industry’s resurrection have never been far away. Whereas changing economic, environmental and political climates will inevitably constrain any revival of mining, or even exploration, of fundamental importance is an understanding of whether further workable deposits might exist and whether there may be realistic prospects of locating and exploiting them. Previous attempts to foresee a future for Northern Pennine mining have concentrated on the possibility of locating new deposits of the sorts long known here, or extensions of them. In the light of a modern understanding of the area’s geology and drawing also upon the legacy of observations of generations of Northern Pennine miners and mine agents, all brought into focus through the findings of the Rookhope Borehole, this review will explore the proposition that the Northern Pennine Orefield may be far from exhausted and will examine the prospects for new orebodies, including the possibilities of significant deposits in situations as yet unknown within the orefield. Fifty years on, this important borehole remains a vital milestone in understanding ore forming processes worldwide and may well offer important clues to where we might look afresh in the Northern Pennines for the means of reviving the very industry that helped inspire its drilling.