New insights into the behaviour of Cu and Zn isotopes in ore-forming systems from the Alexandrinka VHMS deposit, S. Urals

Thomas F D Mason, SG Tessalina, Dominik J. Weiss, MSA Horstwood, JJ Wilkinson, RR Parrish

Research output: Contribution to journalMeeting abstract

Abstract

New analytical protocols for high-precision Cu and Zn
isotope measurements of silicate and sulphide samples using
multiple-collector inductively coupled plasma mass
spectrometry (MC-ICP-MS) have been validated using ore
materials from the Alexandrinka Volcanic-Hosted Massive
Sulphide (VHMS) deposit, Southern Urals. The Alexandrinka
ores show significant spatial and mineralogical isotope
heterogeneity, with a spread in Cu and Zn isotopic
compositions of 0.36 and 0.44 ‰ per atomic mass unit
(amu-1), respectively, relative to the long-term reproducibility
of 0.035 ‰ amu-1 (2SD).
For Cu, samples from the stockwork zone and the paleohydrothermal
vent complex show no significant isotopic
variability, suggesting bulk isotopic fractionation of Cu during
the genesis of primary Cu minerals was negligible, or that
such fractionation effects were easily homogenised by
continued fluid flow. By contrast, Zn isotopic compositions in
the stockwork show a dependency on mineralogy, with
sphalerite-bearing samples yielding isotopically heavy
compositions of ca. +0.2 ‰ amu-1 relative to chalcopyrite.
This behaviour is tentatively linked to equilibrium isotopic
partitioning of Zn between sphalerite and chalcopyrite during
primary mineralisation. Zinc isotopes also show a trend
towards heavier compositions from core to periphery across a
preserved hydrothermal chimney wall by up to +0.13‰ amu-1.
The exact cause of this variation remains unclear, although it
may indicate a temperature and/or compositional dependence
on Zn isotopic discrimination effects associated with sulphide
precipitation. Secondary Cu sulphides in clastic ores derived
from the degradation of the hydrothermal vent system exhibit
a range of Cu isotopic compositions that are skewed towards
isotopically light values relative to primary chalcopyrite by up
to –0.29 ‰ amu-1. No similar trend occurs with Zn isotopes.
This behaviour is consistent with Cu isotopic fractionation
associated with the reduction of Cu(II) to Cu(I) during
secondary supergene mineralisation within the clastic sulphide
pile.
Original languageEnglish
Article number3.6.42
Pages (from-to)A297-A297
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume68
Issue number11 supplement
DOIs
Publication statusPublished - Jun 2004
Externally publishedYes
EventGoldschmidt Conference (14th : 2004) - Copenhagen, Denmark
Duration: 5 Jun 200411 Jun 2004

Cite this