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

Abstract

Abstract. The aim of the present communication is to emphasize that some variations of the measured 613C and 6180 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%0 613C and 4%0 6180. 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 613 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 613C and 6180 values.Changes in the stable isotope composition of host and gangue carbonates of Mississippi Valley-type (MVT)

The Bou Dahar Jurassic carbonate-hosted Pb–Zn–Ba deposits (Oriental High Atlas, Morocco): Fluid-inclusion and C–O–S–Pb isotope studies

“…The δ 18 O values of carbonates display a general downward trend in the following sequence: limestone (22.7 to 25.7‰, average = 24.2‰) → ore-calcite (18.4 to 21.6‰, average = 20‰) → post-ore calcite (15.4‰). This trend resembles similar trends that have been observed at other MVT deposits (e.g., Frank and Lohmann, 1987;Nesbitt and Muehlenbachs, 1994;Spangenberg et al, 1995) in which isotopically lighter O is observed in paragenetically late carbonates. Lighter δ 18 O values of post-ore calcite, relative to the ore-calcite, can be attributed to either an increase in temperature, or the introduction of isotopically lighter fluid.…”

Germanium- and gallium-rich sphalerite in Mississippi Valley–type deposits: the San Vicente district and the Shalipayco deposit, Peru

“…Fontboté 1993;Leach et al 2005;Merino and Canals 2011;Kelka et al 2017), breccia, and vein textures and structures. Zebra, vein, and breccia fabrics occur also abundantly in barren parts of the ore-bearing units with similar textures (Fontboté and Gorzawski 1990;Fontboté et al 1995;Spangenberg 1995;Badoux et al 2001). Volumetrically minor Fe-poor orange and yellow sphalerite is found in steep veins that cross-cut the main mineralization and belongs to a minor second mineralization step in the San Vicente deposit that, as explained below, has a particular geochemical signature.…”

“…Ore formation is interpreted to have resulted mostly from the ingression of metal-bearing basinal brines that encountered H2S formed by thermochemical sulfate reduction (TSR) in hydrocarbon traps located in dolomitized oolitic bars Gorzawski 1990, Fontboté et al 1995;Spangenberg 1995;Spangenberg and Macko 1998;Spangenberg et al 1999).…”

The Genesis of the Mississippi Valley-Type Fluorite Ore at Jebel Stah (Zaghouan District, North-Eastern Tunisia) Constrained by Thermal and Chemical Properties of Fluids and REE and Sr Isotope Geochemistry

“…The banded dolomites are known in the literature as "franciscaine" (Jacquin, 1970;Trona, 1973), "banded spar" (Martini, 1976), "Zebra texture" (Beales and Hardy, 1980), "diagenetic crysztallization rhythmites or DCRS" (Fontboté, 1981;Fontboté and Amstutz, 1983), "Zebra rock" (Spangenberg et al, 1993;Spangenberg et al, 1995). This petrographic type of dolomite is similar to those present within the gangue of several Pb-Zn-Ba-F Mississippi Valley type (MVT) deposits, where they always accompany the ore minerals (Beales and Hardy, 1980;Ohle, 1985;Fontboté and Gorawski, 1990;.…”