Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru

Abstract

The aim of the present communication is to emphasize that some variations of the measured ∂13C and ∂180 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 per mil ∂13C and 4per mil ∂180. These variations are reduced to several tenths of a per rail when the samples are pretreated. A careful examination of the effects of treatment with NaOC1 and/or AgaPO4 in relation to the concentration of sulfide inclusions indicates that the main disturbing effects for ∂13 C values are the presence of sulfur species and organic matter, whereas the 6180 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%o for ∂13C and ∂180 values.