Quantifying the release of base metals into hydrothermal ore forming systems: A SIMS investigation
of potential source rocks for ophiolite-hosted Volcanogenic Massive Sulphide systems
Simon M. Jowitt1*, Gawen R. T. Jenkin1, Laurence A. Coogan2, Jon Naden3
1
Department of Geology, University of Leicester, University Road, Leicester, UK, LE1 7RH
School of Earth and Ocean Sciences, University of Victoria, British Columbia, Canada, V8W 3V6
3
British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, UK, NG12 5GG
simon.jowitt@sci.monash.edu.au
2
Understanding source-deposit relationships in Volcanogenic Massive Sulphide (VMS) systems is
important for mineral exploration, elucidating seafloor hydrothermal processes, and determining
ocean–crust fluxes. Although it is clear that hydrothermal fluids strip metals from oceanic crust and
redeposit them in orebodies, the mineralogical controls on metal release are poorly understood. Basemetal depleted epidosite zones, found at the bottom of the Sheeted Dyke Complex of ophiolites, may
represent source rocks for ophiolite-hosted VMS deposits. We have investigated the release of base
metals during epidosite alteration in the Spilia-Kannavia epidosite zone (SKEZ) of the Troodos
ophiolite, Cyprus. Here, increasing alteration is characterised by a progression from an actinoliteplagioclase-rich diabase to rocks consisting entirely of epidote and quartz (end-member epidosite
facies) via a transitional facies where chlorite peaks in abundance.
Compared to fresh basalt protolith compositions, whole rock Zn and Co concentrations
decrease systematically (from ~35 to 0 ppm Zn and ~66 to 20 ppm Co) with increasing epidositisation
whilst Mn and Ni decrease to close to their lowest levels (from ~0.17 to 0.08 wt% MnO and ~20 to 2
ppm for Ni) during diabase and transitional diabase-epidosite alteration. Cu is uniformly depleted (<4
ppm) across all facies. What is not clear from the whole rock data is whether 1) the base metals are
dominantly held in certain minerals, such as chlorite, and released as these minerals dissolve, or 2) the
base metals are held in a numerous minerals and released by progressive leaching that is coincident
with mineralogical changes. To test these models, silicate minerals from alteration facies from fresh
basalt (protolith) through to most altered were analysed by Secondary Ion Mass Spectroscopy (SIMS;
Co, Ni, Cu, Zn) and Electron Probe Micro Analysis (EPMA; Mn). SIMS was undertaken using a
Cameca-1270 Ion Microprobe at the University of Edinburgh. Calibration was performed using
Corning glass standards corrected with ion yields appropriate for olivine, clinopyroxene, plagioclase,
chlorite, amphibole and epidote. EPMA was undertaken at the University of Leicester using a Jeol
JXA-8600S Superprobe.
Within fresh basalts, the most important silicate hosts of base metals are olivine and pyroxene;
however, these minerals do not host enough Cu and Zn to mass balance whole-rock concentrations
suggesting that these metals are also hosted by sulphides (Cu) or Fe-oxides (Zn). Within the SKEZ,
chlorite and amphibole are the only silicates that retain significant amounts of Co (≤241 and ≤101 ppm
respectively), Ni (≤165 and ≤97 ppm) and Zn (≤157 and ≤95 ppm), although epidote hosts some Mn
(≤0.58 wt% Mn2O3). The proportion of whole rock Ni in chlorite remains stable (77-97%) across all
epidosite facies despite a reduction in the amount of chlorite (from a modal proportion of 35% to
<5%). The proportion of whole-rock Co in chlorite is similarly stable although as the amount of
chlorite becomes very small, Co starts to be released. These changes result in increasing protolithnormalised Co (from 4.4 to 7.5) and Ni (3.6 to 5.4) ratios in chlorite, whilst Zn decreases (1.8-0.5),
indicating preferential sequestration of Ni and Co but release of Zn. The amount of Cu hosted by any
silicate mineral is low (generally ≤12 ppm) and Cu loss across all facies is due to early alteration of
magmatic sulphides.
Four base metal releasing processes occurred during formation of the SKEZ:
A)
Breakdown of clinopyroxene, olivine, sulphide and Fe-oxides to release most of the Cu and
some Mn, Ni, Co and Zn.
B)
The dissolution of amphibole, plagioclase and chlorite during epidotisation, releasing Co, Ni
and Zn.
C)
Sequestration of Ni and Co and release of Zn by chlorite.
D)
The release of very minor amounts of all base metals bar Mn by epidote.