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Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia
Corresponding author: e-mail,
Ross.Large{at}utas.edu.au
A study of carbon and oxygen isotope ratios in
fine-grained sedimentary dolomite in the Barney Creek Formation of the
McArthur basin demonstrates the presence of an extensive isotope halo
surrounding the giant stratiform HYC Zn-Pb-Ag deposit. Dolomite within the
halo shows an 18O-enriched and 13C-depleted isotope
signature (
18O
= 23 to 26
SMOW,
13C
= –2 to –3.5
PDB), relative to normal Proterozoic sedimentary dolomite beyond the halo (18O
= 20–23
and 13C
= 0 to –2).
The C-O isotope halo within the dolomitic siltstones extends at least 15 km
southwest of the HYC deposit and approximately coincides with a previously
defined lithogeochemical halo of elevated Fe, Mn, Zn, Pb, and Tl. Dolomitic
siltstone lamellae within the stratiform Zn-Pb-Ag ores at HYC exhibit an
isotopic range similar to that of the halo dolomites, suggesting that the
ore and halo equilibrated with the same hydrothermal fluid.
Modeling of isotopic exchange accompanying fluid-rock
interaction suggests that the halo dolomites equilibrated with
low-temperature fluids (50°–120°C),
which were enriched in 18O (18O
= 5 ± 5)
but with an average crustal carbon isotope signature (13C
= –6 ±
1). Our
preferred interpretation is that the oxygen and carbon isotope halo at HYC
is related to the development of an extensive brine pool. This pool was
deepest in the vicinity of the HYC deposit adjacent to the Emu fault, and it
became shallower to the southwest away from the fault. Using the carbon
isotope fractionation equation between dolomite and HC
, it is possible to estimate the temperature variation at the base of the brine pool during the accumulation of the Barney Creek Formation. Brine pool
temperatures were highest in and adjacent to the HYC deposit (40°–70°C)
and decreased to values of 17°
to 30°C remote
from the deposit. These temperatures are similar to those recorded in the
Red Sea brine pool associated with the Atlantis II metalliferous sediment
deposit.
Based on our work at HYC and Lady Loretta, strata-bound 18O-enriched carbonate lithogeochemical halos may be a characteristic of the Proterozoic stratiform Zn-Pb-Ag deposits of northern Australia. These halos are more extensive than the narrow 18O-depletion halos recorded in dolomites surrounding skarn, Mississippi Valley-type and Irish-style Zn-Pb deposits. This fundamental difference in the isotopic halo characteristics of these groups of zinc deposits is probably related to the low-temperature, synsedimentary brine pool origin of the North Australian SEDEX deposits in contrast to the various replacement and open space fill origins of skarn, Mississippi Valley-type, and Irish-style deposits.
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