Microanalytical techniques developed within the past decade (ion
microprobes, laser microprobes) are allowing not only elemental analysis
on a fine scale, as noted above, but also stable isotopic analysis on a fine
(micron) scale. In particular, microbeam studies of the zonation of S
isotopes within and among individual crystals in sulfide ores are
providing important clues to deposit origins. Using secondary ion
microprobe mass spectrometry, McKibben and Eldridge [1990]
found that hydrothermally altered rhyolites within the Valles
[4]
Caldera
contained strongly 
S/
S-zoned authigenic pyrite crystals
(enriched cores, depleted rims) at depths coinciding with elevated Au
contents. They concluded that boiling and oxidative H
S destruction
had caused Au deposition coincident with progressive Rayleigh S isotopic
depletion in the growing crystals. The micron-scale isotopic zoning may
have recorded a large-scale geologic event, breaching of the caldera wall
and draining of the former caldera lake, which triggered the boiling and
Au deposition. Arehart et al. [1993b] also found large variations
and late-stage depletions in 
S/
S values for arsenian pyrite in
fine-grained ores from the Post/Betze sediment-hosted disseminated gold
deposit in Nevada.
Several important advances were made based on conventional (bulk sample) analyses of stable isotopes in ore deposits. An elegant paper by Rye et al. [1992] worked out the stable isotopic systematics of acid sulfate alteration. The characteristic mineral alunite contains four stable isotope sites in its crystal structure, making it a very useful mineral for reconstructing mineralizing conditions and processes. A companion paper by Stoffregen et al. [1994] reported experiments that determined O and H fractionation factors between alunite and water. Ohmoto et al. [1990] reviewed the sulfur isotopic systematics of modern marine sediments and sediment-hosted base metal deposits.