Post-obduction fluids in the Lizard Complex, Cornwall; implications for cross-course mineralisation

Adularia vein from the Lizard, Cornwall.
The globular structures are probably
pseudomorphs after analcime.
Bigger view.

Introduction

We aim to test the basin compaction hypothesis by making use of high resolution dating techniques (Ar-Ar on adularia, and potentially U-Th-Pb on calcite) coupled with detailed structural characterisation to compare the timing and duration of vein formation with the known chronology of offshore basin development. Stable isotope and fluid inclusion studies will allow us to compare the composition of fluids in the Lizard veins with those offshore.

The project would suit an enthusiastic student interested in isotope geochemistry, mineralisation and structural geology/tectonics and gives them scope to develop promising aspects that they find interesting, such as numerical modelling. The project will provide training in a wide variety of techniques applicable to fluid-rock interaction and structural based fieldwork. We already have some superb sample material, but the student will have plenty of opportunity for fieldwork in sunny Cornwall. This project will equip the student for a career such as geochemistry, mineralisation or waste disposal.

The project

The Cornubian orefield is one of the most extensively studied mineralized areas in the world, its cumulative metal production being worth at least 23 billion $ at modern prices. However, many aspects of the orefield are still not completely understood. Recent accurate dating work has confirmed earlier indications that the Pb-Zn-dominated "cross-course" veins are Mesozoic in age (236±3 Ma; Scrivener et al., 1994) and therefore considerably younger than the Permian mainstage Sn-bearing mineralisation (Chesley et al., 1993; Chen et al., 1993). On the basis of chemical and isotopic data a basinal brine origin has been suggested for the cross-course fluids (e.g. Wilkinson et al., 1995; Gleeson et al., 2000). The cross-course fluids are suggested to have been expelled from offshore basins into the Variscan basement by late Permian-Triassic ENE-WSW extension (Shail & Alexander, 1997). This may have been part of a much wider rift-related fluid flow and mineralising event that occurred over much of the N. Atlantic region (Halliday & Mitchell, 1984).

However, a number of questions concerning the cross-course mineralisation remain unanswered:

1. What were the sources of the metals and sulphur, were they derived locally, from the basins, or the aquifer rocks?
2. What was the cause of precipitation of ore minerals?
3. What is the explanation of the anomalously low hydrogen isotope ratios (dD) of the cross-course mineralisation, the origin for which remains enigmatic (Wilkinson et al., 1995; Gleeson et al., 1999)?
4. Were the cross-courses formed in a single episode, or over a protracted period of time?
5. What were the geometries and kinematics of fracture formation and reactivation during the mineralisation episodes(s) and how has this influenced fluid transport?

In the Lizard complex, Halliday & Mitchell (1976) have dated adularia in veins at 210-220 Ma; similar to the age suggested for the cross-course veins. The adularia forms part of a post-serpentinisation fracture-hosted mineralisation episode which has structural characteristics similar to the cross-course mineralisation (Power et al., 1997). Indications are therefore that the post-serpentinisation mineralisation in the Lizard Complex is formed from the same fluids that formed the cross-course mineralisation. This mineralisation in the Lizard therefore provides a key link between the offshore basins to the south and the cross-course mineralisation to the north, and will help us to address the questions posed for the cross-course mineralisation.

The project involves characterising the fluids, fracture network and tectonics responsible for the Mesozoic mineralisation in the Lizard by:

1. Fieldwork to collect data on vein geometries, kinematics, cross-cutting relationships and vein samples for further study.
2. Petrographic examination of vein paragenesis, augmented by XRD, CL, SEM and EMP.
3. Fluid inclusion microthermometry to determine compositions, salinities and temperatures.
4. O, H, C and S stable isotope measurements to determine temperatures, the source of these constituents and the degree of fluid-rock interaction.
5. K-Ar, Ar-Ar and Rb-Sr dating techniques to precisely determine their age distribution.

Results from this study will be compared with the cross-course and offshore fluids for which a large database already exists (Gleeson et al., 2000 and references therein). If deemed necessary, the student will have the opportunity to carry out numerical fluid flow modelling to simulate the formation of the cross-course mineralisation.

About the Supervisors:
Gawen Jenkin has been a lecturer in Applied Geology at Leicester since 1997 and specialises in fluid inclusion and isotopic studies of fluid-rock interaction.
Robin Shail has been a lecturer in Structural Geology at CSM since 1990 and has worked extensively on regional tectonics and reactivation of the Variscan basement in Cornwall.

To apply, please send your CV and a letter of application, complete with the names and addresses of 2 academic referees, to Dr G.R.T. Jenkin, Department of Geology, University of Leicester, LEICESTER, LE1 7RH. Email: grtj1@le.ac.uk. Tel: 0116 252 3934

No, not a cave in a cliff, but adularia crystals up to 3 mm long,
lining a vug in breccia from the Lizard, Cornwall.
Bigger view.

References

• Chen Y, Clark AH, Farrar E, Wasteneys HAHP, Hodgson MJ & Bromley AV (1993). Diachronous and independent histories of plutonism and mineralization in the Cornubian Batholith, southwest England. J Geol Soc Lond, 150, 1183-1191.
• Chesley JT, Halliday AN, Snee LW, Mezger K, Shepherd TJ & Scrivener RC (1993). Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization. Geochim. Cosmochim Acta, 57, 1817-1835.
• Gleeson SA, Wilkinson JJ, Boyce AJ, Fallick AE, Stuart FM (1999). On the occurrence and wider implications of anomalously low dD fluids in quartz veins, South Cornwall, England. Chem. Geol. (Isot. Geosci.) 160, 161-173.
• Gleeson SA, Wilkinson JJ, Shaw HF, Herrington RJ (2000). Post-magmatic hydrothermal circulation and the origin of base metal mineralization, Cornwall, UK. J. Geol. Soc. Lond., 157, 589-600.
• Halliday AN & Mitchell JG (1976). Structural, K-Ar and 40Ar-39Ar age studies of adularia K-feldspars from the Lizard Complex, England. Earth Planet. Sci. Lett., 29, 227-237.
• Halliday AN, Mitchell JG (1984) K-Ar ages of clay-size concentrates from the mineralisation of the Pedroches Batholith, Spain, and evidence for Mesozoic hydrothermal activity associated with the break up of Pangaea. Earth Planet Sci Lett 68:229-239
• Power MR, Shail RK, Alexander AC, Scott PW (1997). Alteration and vein mineralisation within the Lizard Complex, South Cornwall: Constraints on the timing of serpentinisation. Proc. Ussher Soc., 9, 188-194.
• Scrivener RC, Darbyshire DPF, Shepherd TJ (1994) Timing and significance of crosscourse mineralization in SW England. J Geol Soc Lond, 151, 587-590.
• Shail RK & Alexander AC (1997). Late Carboniferous to Triassic reactivation of Variscan basement in the western English Channel: evidence from onshore exposures in south Cornwall. J. Geol. Soc. Lond., 154, 163-168.
• Wilkinson JJ, Jenkin GRT, Fallick AE, Foster RP (1995) Oxygen and hydrogen isotopic evolution of Variscan crustal fluids, south Cornwall, U.K. Chem. Geol. (Isot. Geosci.) 123, 239-254.