Transcript Controls on the geothermal potential of the buried Kentstown and

Controls on the geothermal potential
of the buried Kentstown and Glenamaddy plutons,
Ireland – implications from hydrothermal alteration
Tobias Fritschle1*, J. Stephen Daly1, Martin J. Whitehouse2,
Stephan Buhre3, Brian McConnell4 and the IRETHERM Team
1 UCD
School of Earth Sciences, University College Dublin, Dublin 4, Ireland *([email protected])
Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Sweden
3 Institut für Geowissenschaften, Johannes Gutenberg-Universität, Mainz, Germany
4 Geological Survey of Ireland, Beggars Bush, Dublin 4, Ireland
2
??
Aims of the study / Outline of the talk
Geochemical evaluation of the available specimens in
terms of their heat production (uranium, thorium,
potassium and density)
HPR [µW/m³]= 0.1326*ρ[g/cm³]*(0.718*U[ppm]+0.193*Th[ppm]+0.262*K[wt%])
(Webb in Manning et al. 2007)
Comparison of the buried granites with exposed analogues
(contemporaneous granites in the Iapetus Suture Zone)
Implications for the geothermal exploitation potential of the
buried Kentstown and Glenamaddy plutons
Granite as a possible target for geothermal exploitation
 Enhanced geothermal systems (EGS) /
Hot dry rock (HDR):
High heat producing rocks are ‘stimulated’ at depth,
using borehole hydro-fracturing techniques, to create
a permeable reservoir in which water can circulate.
Cool water is injected down the injection well, and hot
water, having heated up during passage through the
artificial reservoir, is pumped back up at the
production well.
 Currently five EGS power plants are fully operational.
 About a dozen EGS projects are under construction
(e.g. Australia, England, Germany, Netherlands, USA,…)
(Ove Arup and Partners Ltd. 2011)
 Soultz-sous-Forêts in France was the first to be
commissioned in July 2011, reaching a depth of
5,000m, and yielding a water temperature of 200 °C
and power of 1.5 MWe
Buried
production
(?) granites
Highhigh
heatheat
production
(?) granites
Geological map
(Map courtesy of Geological Survey of Ireland
and Geological Survey of Northern Ireland)
Gravity anomaly map
(Map courtesy of Dublin Institute for Advanced Studies)
Timing of Irish & IoM Late Cal. granites
Modified after Brown (2008) and Holdsworth (2009)
 Very
Late Caledonian
granites
likelytherefore
intruded in
few drill cores
accessible
exposed
multiple
phases
between
– 405 Ma
analogues
rock are
used as425
proxies
 No
Majority
of the Late
Caledonian
graniteswith
have
correlations
of the
heat production
age,
their
latest intrusive
phaseoraround
Ma
geographical
distribution
isotopic410
signature?
Heat production rates
(Recalculated after: 1Genter et al. 1997, Alexandrov et al. 2001, Stussi et al. 2002 and Greksch et al. 2003 ; 2Manning et al. 2007)
Geochemistry reflecting the geothermal potential
 Rb and Nb correlate with the heat production rate
 Both Rb and Nb are enriched in the upper continental
crust. Naturally, contribution of such material may
enrich the abundance of heat producing elements
 I-type and S-types exhibit distinct
slopes in Rb v HPR for Rb >150ppm
 Glenamaddy Rhyolite enriched in
Nb compared to granites
 Unsurprisingly, Rb and Nb generally correlate with
each of the heat producing elements
 Drogheda and Soultz
granites exhibit thorium
enrichment
 Foxdale Granite exhibits
uranium enrichment
Geochemistry reflecting the geothermal potential
 Th/U ratio suggests different underlying causes
for the elevated heat production rates in the
Drogheda and Foxdale granites (red and blue stars)
 I-type granites are enriched in thorium;
S-type granites are enriched in uranium
 S-type granites have a lower Th/U than the crustal
average (Th/U = 3.8) – except for very altered
samples
 The most altered samples correspond with the
highest Th/U ratios
 Sub-parallel trends of decreasing HPR correlate
with increasing Th/U
We suggest this trend is produced by
hydrothermal redistribution of uranium and
that the latter may be a major mechanism
controlling the heat production in a granite.
Hydrothermal alteration in Glenamaddy Granite
Calcite and quartz veinlets in the Glenamaddy Granite showing U-enrichment in a
calcite vein, and uranium precipitation around pyrite interpreted as the result of a
redox reaction of the sulphide with a U-bearing fluid
Glenamaddy pluton
 An
Thermal
conductivity
valuesbetween
for the the
unconformable
contact
sandstone
ranges between
1.7 – 3.1 and
W/mK,
overlying Carboniferous
sandstones
whereas
those
the graniteatand
the rhyolite
wasofintersected
154rhyolite
m
are around 2.5 W/mK
 The overlying sandstone comprises up to
90% angular quartz grains and subordinate
microcline feldspar (both up to 100 µm)
Glenamaddy pluton
 A single drill-core comprises 160 m of
alternating rhyolitic and granitic rock
 Four sections of each granitic and
rhyolitic rock were drilled
 The contact between the granite and
the rhyolite is intrusive
 Large parts of the pluton are strongly
hydrothermally altered and mineralized
Kentstown Granite
 Two boreholes intersected the
Gravity anomaly map
unconformable contacts of the overlying
Carboniferous limestone with the granite at
492 m and 662 m, respectively
 Each of the cores only comprises 15 m of granite
 Granite is strongly hydrothermally altered,
depleted in U, presumably linked to the
Carboniferous-hosted Zn-Pb orefield
 Stratigraphic differences between the west
and east parts of the pluton
(Map courtesy of Dublin Institute for Advanced Studies)
Drilling of Kentstown granite
 GSI drilled Kentstown Granite based on
Tom Farrell’s preliminary MT modelling,
that predicted granite at 370 ± 30 m
 Drilling had to be abandoned in soft
Namurian shales at a depth of c. 300m
(much thicker than expected)
Cover rocks on top of the Kentstown Granite
 Unfortunately, Irish Carboniferous rocks are generally poor
thermal insulators due to diagenesis (occlusion of pore-space)
1000μm
200m
 The Kentstown granite is overlain by Visean limestones in the
west, and by thick Namurian shales (presumably underlain by
limestone) in the east
500μm
 Values for the thermal conductivity of both granite samples and cover
rocks range between 2.3 – 2.7
 The thermal conductivity for the Visean limestone cover appears to
have been increased due to diagenetic compaction and cementation of
the pore spaces.
(after Pickard et al. 1992)
Drilling of Kentstown granite
 GSI drilled Kentstown Granite based on
Tom Farrell’s preliminary MT modelling,
that predicted granite at 370 ± 30 m
 Drilling had to be abandoned in soft
Namurian shales at a depth of c. 300m
(much thicker than expected)
Conclusions
 Trends of increasing HPR with decreasing Th/U are suggested
to relate to the hydrothermal redistribution of uranium.
 Geothermal potential of the buried Kentstown and
Glenamaddy granites requires further investigation
 HPR for Glenamaddy is relatively high (3.3 µW/m³) and
Kentstown (2.3 µW/m³) only moderate, but only 15 m of
altered granite drilled at Kentstown
 Possibility for unaltered granite in the eastern part of the
Kentstown pluton due to the N-S trending fault system
 Deep drilling and further petrophysical research is required for
assessing the dimensions of the plutons and for characterising
the associated fault systems
 Apart from that, the Midlands and Killarney gravity lows
which are likely related to subsurface granites invite for
geothermal research
Gravity
anomaly map
(Map courtesy of Dublin Institute for Advanced Studies)