The aim of the present communication is to emphasize that some
variations of the measured delta(13)C and delta(18)O values are
apparent, and due to analytical interferences caused by the presence of
sulfur and organosulfur compounds in the analyzed carbonates. This is
particularly relevant for isotopic studies on carbonate-hosted mineral
deposits, where the nearly ubiquitous association of the host carbonates
with organic matter and sulfides can certainly affect the metallogenetic
interpretations. In this work two methods were used to overcome the
disturbing effects of sulfides and organic matter: (1) sample
pretreatment following the method proposed by Charef and Sheppard
(1984), combining the oxidation of organic matter with sodium
hypochlorite and trapping of the sulfur species with silver phosphate;
and (2) laser-based microprobe extraction. Apparent isotopic variations
in sparry dolomite from a single hand sample of zebra ore from the MVT
Zn-Pb deposit, San Vicente, central Peru, are as large as 6 parts per
thousand delta(13)C and 4 parts per thousand delta(18)O. These
variations are reduced to several tenths of a per mil when the samples
are pretreated. A careful examination of the effects of treatment with
NaOCl and/or Ag3PO4 in relation to the concentration of sulfide
inclusions indicates that the main disturbing effects for delta(13)C
values are the presence of sulfur species and organic matter, whereas
the delta(18)O values are mainly affected by the presence of sulfides.
Fine- and medium-grained replacement carbonates from MVT and other
sediment-hosted base metal deposits are potentially the most affected
during isotope analysis, due to the common presence of organic matter
and sulfides. Using in situ laser microprobe techniques, it is possible
to determine isotopic variations at a sub-millimeter scale. Our results
show that laser extraction analysis allows a more precise sampling of
the carbonate minerals, and minimizes contamination of the sample with
sulfides and to some extent with intergrown organic matter. However,
there is an isotopic shift associated with the laser extraction
technique, of the order of 0.5-1 parts per thousand for delta(13)C and
delta(18)O values.