1. Clumped isotope constraints on water:rock ratios and the evolution of fluids
associated with late Variscan hydrothermal veins and mineralization
Paul F Dennisa
, Daniel Myhilla
, Neil Allanacha
, Stuart Vinena
, Alexandra Forman and Alina Marcaa
a
School of Environmental Sciences, University of East Anglia, United Kingdom, p.dennis@uea.ac.uk
We present new clumped isotope (Δ47) data for hydrothermal calcite veins from the Lower
Carboniferous limestone of the Peak District, U.K and the Clare Basin, Ireland. Whilst many of
these veins are barren, others are intimately associated with Mississippi Valley Type (MVT) base
metal mineralization such as at Ecton Hill, Staffordshire and Dirtlow Rake, Derbyshire.
Clumped isotope values for carbonate minerals allows us to determine both the precipitation
temperature of the mineral and the 18
O isotopic composition of the parent fluid. The degree of
ordering of the heavy isotopes 13
C and 18
O in the carbonate lattice is a function of temperature. At
low temperatures the isotopes are ordered, or “clumped”, together. At higher temperatures the
isotopes become increasingly more randomly distributed. Measuring the deviation from a
stochastic distribution allows us to estimate the mineral growth temperature. With the bulk
oxygen isotope composition of the carbonate, δ18
Ocarb, we can back out the δ18
Ofluid value.
Veins in the Clare Basin precipitated at a temperature between 100° and 160°C, and for the Peak
District between 30° and 100°C. A striking feature of the data sets for both the Peak District and
Clare Basin is that veins, including multiple samples from single veins, plot on well-defined two
end-member mixing lines in T-δ18
Ofluid space. The implication is that the veins precipitate from a
mixed fluid comprised of: (i) a hot, isotopically evolved end-member (T>160°C, δ18
Ofluid >
+12‰VSMOW) and; (ii) a cooler, isotopically depleted fluid more characteristic of meteoric
groundwaters (T <40°C, δ18
Ofluid < -5‰VSMOW). It is important to note that the hot end-member has
temperatures significantly greater than those we have recorded for diagenetic components in the
host limestones in either the Clare Basin or the Peak District (max. temperatures = 85°C).
The isotopic composition and temperatures of the fluid mixtures vary depending on the mixing
ratio between the two end-members. The δ18
O value of the precipitated vein carbonates occupy a
restricted range between +20‰ and +24‰VSMOW. We interpret this as indicating fluids evolved
under conditions of low water:rock ratios from an original meteoric fluid. This is consistent with
δ13
C values for the vein and host limestone are indistinguishable (δ13
C = +2 to +3‰VPDB).
We are surprised to find that vein precipitation temperatures are in accord with a simple two end-
member mixing model, with temperature acting as a conservative property. We envisage a simple
physical model in which hydraulic fractures propagate rapidly as a result of sharp increases in
pore fluid pressures associated with seismic pumping or seismic valve type activity, providing
flow paths for fluids sourced from deeper in the basin. Mixing of a hot fluid with local pore water
results in carbonate supersaturation and implies carbonate precipitation must occur over very
short time scales (10’s – 100’s years) as a result of mixing a Ca rich, carbonate poor, low pH
basin derived fluid with a ‘local’ meteoric fluid that has evolved towards carbonate saturation in a
system closed with respect to CO2. Hydraulic fracturing may also result in fluid effervescence and
loss of CO2 to the gas phase. Vein zoning with respect to precipitation temperature and fluid
isotope composition may result from repeated crack-seal type process.
It has not escaped our attention that our observations and interpretations of rapid carbonate
precipitation from fluids in a rock buffered system are at variance with previous suggestions that
both long times (105
- 106
years) and very large water rock ratios (104
- 105
) are required for
precipitation of calcite in veins. This has implications for understanding MVT type base metal
mineralization.
Mineral Deposits Studies Group – 38th
Annual Meeting – 18-19 December 2014 - Southampton