Background picture Granite from St.Stythyans Parish.
News from Other Societies
Other societies with active Cornish members in Cornwall.
The Ussher Society
The Russell Society. www.russellsoc.org
The Carn Brea Mining Society
St.Just Mines Research Group.
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
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
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
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
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.