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Variation of carbon and oxygen isotopes in the alteration halo to the Lady Loretta deposit - implications for exploration and ore genesis

conference contribution
posted on 2023-05-24, 10:32 authored by Peter McGoldrickPeter McGoldrick, Kitto, PA, Ross LargeRoss Large

Carbon and oxygen isotope analyses of carbonates were measured on twenty-nine sediment samples within the geochemical halo of the Lady Loretta stratiform sediment-hosted Zn-Pb-Ag deposit. Previous research indicates that the sediments in the inner halo are siderite-bearing and pass outwards to ankerite-bearing and then dolomite-bearing further from the deposit.

This isotope study shows that the inner zone siderites are distinctly heavier in oxygen isotopes (δ18Osid = 23.2 to 28.3‰) and lighter in carbon (δ13Csid = –1.9 to –3.2‰) than the outer zone dolomites (δ18Odol = 16.5 to 18.2‰ and δ13Cdol = –0.9 to –1.5‰). Equilibrium fluid calculations suggest that the siderite halo formed at temperatures near 100°C from a fluid with δ18O = +6‰ and δ13C = –6‰, having the characteristics of an evolved basinal brine. In contrast, the outer dolomite sediments formed in equilibrium with a lower temperature (~50°C) fluid, with δ18O = –11‰, δ13C = –5‰, suggesting either a meteoric or evaporated seawater origin.

The data also suggest that C/O isotopes on carbonates have the potential to be an exploration indicator of favourable stratigraphy for Zn–Pb–Ag mineralisation. For example, within a given district, carbonate sediments which show the heaviest δ18O values and lightest δ13C values may indicate proximity to mineralisation.


Publication title

Water-rock interaction : proceedings of the 9th International Symposium on Water-Rock Interaction (WRI-9)


Greg B Arehart & John R Hulston






School of Natural Sciences


A.A. Balkema

Place of publication


Event title

9th International Symposium on Water-Rock Interaction (WRI-9)

Event Venue

Taupo, New Zealand

Date of Event (Start Date)


Date of Event (End Date)


Repository Status

  • Restricted

Socio-economic Objectives

Zinc ore exploration

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