Geochemical (elemental and isotopic) constraints on the genesis of the Mississippi Valley-type zinc-lead deposits of San Vicente, central Peru


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Geochemical (elemental and isotopic) constraints on the genesis of the Mississippi Valley-type zinc-lead deposits of San Vicente, central Peru
Spangenberg J. E.
Fontboté L.
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Department of Earth Sciences, University of Geneva, Switzerland
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Terre & Environnement, vol. 1, 123 pp.
ISBN 2-940153-00-0
Mississippi Valley-type (MVT) zinc-lead deposits and ore occurrences of the San Vicente belt are hosted in dolostones of the eastern Upper Triassic to Lower Jurassic Pucara basin, central Peru. The elemental (Ca, Mg, Fe, Mn, Sr, Na, Ba, Zn, Corg. S and rare earth elements) and isotopic (C and O) compositions of the host and gangue carbonates from 20 localities, including the San Vicente main deposit, minor ore occurrences, and barren localities are used to constrain fluid path-ways, ore precipitation mechanisms, and changes in Eh-pH conditions during mineralization.
The altered replacement dolomite (I) and the white sparry dolomite (II) are both depleted in Fe and REE, and enriched in Mn compared to the host dolomite. They display, at least in the samples of the main deposit, negative Ce and Eu anomalies. These results suggest that the incoming ore fluid was slightly oxidizing, acidic, and poor in REE and Fe and therefore had had limited previous interaction with the carbonate host rocks. This is consistent with its migration along an oxidizing main aquifer, such as the Red Sandstones and other clastic units at the base of the basin.
Mixing between an incoming hot, saline, slightly acidic and oxidizing radiogenic (Pb, Sr) fluid and the native intra-formational alkaline reducing waters explains the overall isotopic variation (-11.5 to 2.5‰ δ13C, -12.5 to -6.4‰ PDB δ18O) and the distribution of REE and other trace elements in the different generations of hydrothermal carbonates (1, II, and III). The mixing of these two different fluids determines the evolving composition of the mineralizing fluid(s).
The ore-stage altered host dolomite and hydrothermal carbonates show narrower isotopic ranges (0.2 to 1.7 ‰ δ13C, -11.4 to -7.3 ‰ PDB δIBO). These patterns are similar throughout the studied area. The measured isotopic covariations of the hydrothermal carbonates, which are consistent with the REE data, indicate that two mechanisms were responsible for precipitation of the ore-stage carbonates and associated sphalerite:
a) Temperature-dependent interaction of the mineralizing fluids and the carbonate host rocks explain the alteration and replacement of the dark dolomite (I) and precipitation of early sphalerite (I) and (II). Subsequently, a second generation of hydrothermal white sparry dolomite (II) and sphalerite (II) formed overgrowths on the altered dolomite (I) by filling secondary porosity under fluid-buffered conditions. This main mineralizing stage was the result of prolonged fluid-rock interaction coupled with minor CO2 degassing.
b) Pressure fall leading to outgassing of CO2 and consequent increase in the pH of the ore fluid caused precipitation of late sparry carbonates (dolomite and calcite Ill) and, likely, sphalerite (III).
The regionally uniform isotopic and elemental patterns, coupled with the mineralogical and petrographic similarities among the different MVT occurrences, reflect that similar mineralizing processes occurs in the entire San Vicente belt. This implies the existence of an interconnected mineralizing hydrothermal system. In addition, the limited isotopic and elemental equilibration between the incoming fluid and the Pucara rocks indicates that access of the corrosive ore fluid to the mineralized zones was likely by permeable channel-ways (faults, basement highs).
The results of the Rock-Eval pyrolisis indicate the occurrence of two types of organic matter in the gangue carbonates of San Vicente: a hyper-mature kerogen (Tmax about 515°C), which likely is thermally altered native organic matter, and an allochtonous thermally labile soluble bitumen (Tmax about 230°C). The presence of native sulfur associated with extremely carbon-light calcites replacing evaporitic sulfates (up to -11.5‰ δ13C), altered native organic matter, and heavier hydrothermal bitumen (from -27.5 to -23.3‰ δ13C) point to thermochemical reduction of sulfate and I or thiosulfate. The Rock- Eval results combined with carbon isotope data suggest that the native organic matter was the main source of reductants in the mineralizing fluid. Alteration of the organic matter disseminated in the host dolomite by the incoming ore fluid liberated hydrocarbons which locally produced reducing conditions that led to sulfate reduction (and reduction of Eu+3 to Eu+2). After further thermal cracking, water-washing, and polymerization of the released hydrocarbons, solid aggregates of hydrothermal bitumen precipitated. Thus, the main flow type of the fluid during ore precipitation was pervasive and, consequently, the ore distribution controlled by porosity (primary or secondary).
The Fe-Mn covariations combined with the Eu anomalies of the hydrothermal carbonates are consistent with a change from a reducing ore-forming stage to oxidizing conditions following ore deposition. The REE enrichment, the Mn depletion, and the positive Eu anomalies of the late-stage vug-filling carbonates indicate that the post-ore "residual" acidic fluids were again oxidizing due to continuous influx of fresh basinal waters. Late REE-rich dolomite III (or calcite) and associated sphalerite III formed from the slightly acidic ore fluid during CO2 degassing, caused in tum by an enhanced hydrothermal porosity. The widespread hydraulic brecciation, upward "escape" veins, and other structures indicate that the fluid was overpressured. An abrupt pressure drop could favor CO2 degassing and therefore may play a major role in the ore precipitation during the late hydrothermal events in San Vicente.
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