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5mm
Trevalganite – composed of a fine grained matrix (<1 mm), mostly made up of tourmaline with some
quartz. Quartz appears to replace orthoclase phenocrysts up to 60 mm, with tourmaline rims. There are
also smaller quartz phenocrysts up to 7 mm. Small vugs occur which are 2-10 mm in size and are filled
with tourmaline.
Trevalganite & Belowdanite – Hydrothermal replacement of granites in Cornwall?
Results
Trevalganite: groundmass of schorl and quartz, with the feldspar phenocrysts replaced by quartz. Rock has ~61% quartz with
~38% schorl. Accessory minerals (<1%). XRF data normalised (without Boron), showed approx.74% SiO2, 13% Al2O3, 9%
Fe2O3, with minor (<1% >0.1%) Na,Ca,P,W,Mg,S and trace (<0.1%) Ti,K,Mn,Co,Zn,Zr.
Belowdanite: groundmass of quartz and schorl, with the feldspar phenocrysts replaced by schorl. Rock has ~78% quartz with
~20% schorl. Accessory minerals (<2%).
XRF data normalised (without Boron), showed approx.91% SiO
2
, 4% Al
2
O
3
, 2%
Fe
2
O
3
, with minor (<1% >0.1%) W,Na,Ti,Mg,S and trace (<0.1%) Ca,P,Co,Mn,Zr,As,Zn,Sr,Nb.
Discussion
• Trevalganite & Belowdanite represent part of the same spectrum of composition.
• Both rocks display two-stage pneumatolysis with silicification and boron metasomatism. Trevalganite shows early silicification of
the orthoclase feldspar phenocrysts (and potentially orthoclase in the groundmass), followed by replacement of mafic minerals
(micas) and albite feldspars by tourmaline. Belowdanite shows early boron metasomatism replacing orthoclase phenocrysts and
groundmass, followed by pervasive silicification. The different make-up of the two rocks is probably due to the relative timing of
the two metasomatic waves.
• Previous mention of Trevalganite can be found in TRGS 1905, Reid & Flett 1907 and on the BGS 2015 website. These suggest
a range of hydrothermal alteration processes of granite from minor to fully altered, usually near granite contacts.
• Trevalganite shares much of its appearance with primary schorl/quartz magmas (such as at Porth Ledden, Cape Cornwall), but
rare contact data (Reid & Flett, 1907) noting gradational contacts supports a metasomatic origin as opposed to a primary one.
• No field contact found for Trevalgonite or Belowdanite, Trevalgonite was mined for tin in early 1900s but was of little value.
(Reid & Flett 1907, p.87). Field contact data would be the next step.
Rollinson, G.,*
1
LeBoutillier, N.,
2
Simons, B.,
1
Yeomans, C.
1,3
& Hughes, S. P.
1
1
Camborne School of Mines, University of Exeter,
2
Consultant Mining Geologist,
3
British Geological Survey
*Email: G.K.Rollinson@exeter.ac.uk
Belowdanite – composed of a fine grained matrix (<1 mm), predominantly quartz with some tourmaline.
Large tourmaline crystals appear to pseudomorph previous orthoclase phenocrysts, and are up to 60 mm.
There are also glassy quartz phenocrysts up to 5 mm in size.
Acknowledgements: Thanks to Steve Pendray (polished thin sections) & Malcolm Spence (XRF/XRD) at CSM UoE labs.
References
• BGS 2015. http://tinyurl.com/hnqobn9
• Transactions of the Royal Geological Society of Cornwall. 1905. The Ninety-first annual Report Penzance. Vol. XIII. p.630.
• Reid.C. & Flett. J.S. 1907. The Geology of the Lands’ End District. Memoirs of the Geological Survey. p.45,53-57,87,91.
Analysis
Samples were examined in hand specimen. Mineralogical analysis consisted of optical microscopy of polished thin sections,
QEMSCAN®, & bulk powder XRD. Whole rock geochemistry was obtained by bulk powder XRF.
Introduction
Two unusual quartz-tourmaline rock types associated with the granite in Cornwall have been investigated.
These are Trevalganite and Belowdanite, named after their type localities. Each is composed of schorl
tourmaline and quartz; however, the proportions of these minerals vary between rock types and they both
display unique textures. The aims of this investigation are to:
1. Elucidate the processes that led to the formation of these rock types
2. Understand why there is stark contrast with the textures between the two rock types
Samples
Trevalganite is associated with the
Land’s End Granite in West Cornwall,
located on the NE contact with the host
Mylor Slate Formation at Trevalgan Cliff
near St Ives [SW 4891 4083].
Belowdanite is found on Belowda
Beacon [SW 9713 6243]. This is within a
small granite stock to the north of the
main St Austell Granite.
Unfortunately, in both situations the rock
types were not observed in situ so
detailed field observations of the structure
and any contacts were not recorded.
Both of these rock types are associated with porphyritic biotite granite. Any discrete variations of granite
textures in the area
were not observed due to poor exposure.
Information from: Bott et al. (1958); Exley and Stone (1964, 1982); Hawkes and Dangerfield (1978); Dangerfield and Hawkes (1981); Exley et al.
(1983); Stone and Exley (1985); Leveridge et al. (1990); Floyd et al. (1993); Manning et al. (1996); Manning (1998); Mullis et al. (2001); Simons (2015).
50 mm
30
mm
1 m
approx.
From BGS iGeology (2015)
From BGS iGeology (2015)
5mm
5mm
Trevalganite
Belowdanite
QEMSCAN
®
mineral map
(10µm resolution)
Polished thin section
Qz
Sch
QEMSCAN
®
mineral map
(10µm resolution)
Polished thin section
XRF
QEMSCAN®
SiO
2
Trev
Bel
SiO
2
Al
2
O
3
Fe
2
O
3
Qz
Qz
Tour
Tour
Qz
Sch
5mm
05 10 20km
St. Austell
Bodmin
Land’s End
Tregonning-
Godolphin
Carnmenellis
Dartmoor
St. Agnes
St. Michael’s Mount
Belowda Beacon
Castle-an-Dinas
Carn Marth
Carn Brea
Kit Hill Gunnislake
Hingston Down
Hemerdon
Tourmaline Granite
Biotite Granite
Topaz Granite
Coarse-grained, porphyritic
Coarse-grained, poorly porphyritic
Medium- to coarse-grained, poorly porphyritic
Fine-grained, variable phenocrysts
Coarse-grained, small phenocrysts
Fine-grained, equigranular
Globular quartz
Coarse-grained, porphyritic
Fine- to medium-grained, equigranular
Key
Medium-grained, poorly porphyritic
Medium-grained, small phenocrysts
Cligga
N
0 3 km
Isles of Scilly