Content uploaded by Beth Simons
Author content
All content in this area was uploaded by Beth Simons on Jan 10, 2017
Content may be subject to copyright.
Recent advances in W-Sn and Rare Metal deposit metallogenesis.
Poster abstract
Sn-W and rare metals (In, Nb, Ta) in the Variscan lamprophyres of SW England:
inheritance from a Gondwanan continental crust and mixing with felsic melts.
B. Simons1*, R.K. Shail1 and J.C.Ø. Andersen1
1Camborne School of Mines, University of Exeter, Penryn Campus, Treliver Road, Penryn, Cornwall, TR10 9FE.
*corresponding author: B.Simons@exeter.ac.uk
High-potassium to ultra potassic lamprophyres and basalts are contemporaneous (293.6-285.4 Ma)
with the early Permian Cornubian Batholith in SW England [1]. Moderately to steeply dipping minette
lamprophyre dykes (LD) outcrop across SW England, with voluminous expressions of lamprophyre
lavas (LL) and basalts within the Crediton Graben, east of the Cornubian Batholith. Lamprophyres are
minettes, with phlogopite mica phenocrysts, rare clinopyroxene (diopside) and alkali feldspar >
plagioclase [e.g. 2,3]. The LL and basalts contain iddingsite pseudomorphs after olivine. There are
quartz xenocrysts in all rock types and enclaves of felsic material, particularly in the LD. All rock types
are strongly enriched in LILE with depletions in Ti, Nb and Sr. Lamprophyres are strongly LREE
enriched (La/LuN 47-172 for lamprophyre dykes, 18-57 for lamprophyre lavas) [3].
The LD and LL are enriched in Li, Be, Sb, Ta, W and Bi relative to primitive mantle abundances and
slightly enriched in Ga, In and Sn. Likewise, the basalts are slightly enriched in Li, Be, Sb, Ta and W
relative to the primitive mantle. Trace element modelling indicates that fractional crystallisation and
assimilation-fractional crystallisation are not dominant processes in controlling variation of
lamprophyre and basalt compositions, and are not sufficient to attain the trace element abundances
observed. Low degrees of partial melting of a phlogopite-garnet mantle (a metasomatised veined
mantle) and mixing with contemporaneous felsic melts are sufficient to obtain the trace element
abundances displayed in the LL and LD. Trace element patterns are consistent with calc-alkaline
lamprophyres across the Variscan, which show depletion in Nb and P, but enrichment in LILE, LREE
and Pb.
The distinctive U-shaped primitive mantle normalised pattern displayed by Li, Be, Ga, In, Sn, Nb, Ta,
W and Bi is also present in the Devonian metasedimentary greywackes of the Gramscatho Formation
and the Cornubian Batholith indicating a prolonged signature prevalent in SW England. Reverse
modelling of Carboniferous basalts indicates that the mantle beneath SW England was already
enriched in Li, Sn, Ta, LREE, U, K and Pb, implying metasomatism of the sub-continental lithospheric
mantle (SCLM) prior to the Variscan Orogeny [3]. This metasomatism likely occurred during Silurian-
Devonian subduction of a marginal basin to the Rheic Ocean beneath SW England. Sediments
subducted during basin closure would have a peri-Gondwanan source, in common with the wider
Variscan, translating into a heterogeneous, but metasomatised, enriched SCLM across Europe. The
same peri-Gondwanan sedimentary rocks are a source for the Cornubian Batholith; there is no
suggestion that the lamprophyres are a dominant source for W-Sn and other rare metals [4].
[1] Dupuis et al., 2015, Journal of the Geological Society
[2] Fortey, 1991, British Geological Survey Report
[3] Simons et al., in prep
[4] Simons et al., in prep