『Abstract
Carbonate and sulfide minerals from the Molango, Mexico, and TaoJiang,
China, Mn deposits disply similar and distinctive δ34S
and δ13C patterns in intervals of manganese carbonate
mineralization. δ13C-values for Mn-bearing carbonate
range from -17.8 to +0.5‰ (PDB), with the most negative values
occurring in high-grade ore zones that composed predominantly
of rhodochrosite. In contrast, calcite from below, within and
above Mn-carbonate zones at Molango has δ13C≒0‰ (PDB).
Markedly negative δ13C data indicate that a large proportion
of the carbon in Mn-carbonates was derived from organic matter
oxidation. Diagenetic reactions using MnO2
and SO42- to oxideze sedimentary
organic matter were the principle causes of such 12C
enrichment.
Pyrite content and sulfide δ34S-values also show distinctive
variations. In unmineralized rocks, very negative δ34S-values
(avg. <-21‰ CDT) and abundant pyrite content suggest that pyrite
formed from diagenetic, bacteriogenic sulfate reduction. In contrast,
Mn-bearing horizons typically contain only trace amounts of pyrite
(e.g., <0.5 wt.% S) with δ34S-values 34S-enriched,
in some cases to nearly the value for contemporaneous seawater.
34S-enriched pyrite from the Mn-carbonate intervals
indicates sulfide precipitation in an environment that underwent
extensive SO42- reduction, and
was largely a closed system with regard to exchange of sulfate
and dissolved sulfide with normal seawater. The occasional occurrence
of 34S-depleted pyrite within Mn-carbonate zones dominated
by 34S-enriched pyrite is evidence that closed-system
conditions were intermittent and limited to local pore waters
and did not involve entire sedimentary basins. Mn-carbonate precipitation
may have occluded porosity in the surficial sediments, thus establishing
an effective barrier to SO42-
exchange with overlying seawater.
Similar isotopic and mineralogic characteristics from both the
Molango and TaoJiang deposits, widely separated in geologic time
and space, suggest they were formed similarly by MnO2
precipitation at the margins of dysaerobic to anoxic marine basins.
Mn-carbonate formed predominantly by early-diagenetic reduction
of Mn-oxides via oxidation of organic matter in near-surface sediments.
In addition to MnCO3 precipitation, organic
matter oxidation reactions resulted in oxidation of FeS to Fe-oxides
such as magnetite, maghemite and hematite. The latter process
explains anomalously low pyrite content and abundant Fe-oxide
minerals in ore zones dominated by rhodochrosite.』
1. Introduction
2. Previous work
3. Geologic setting
3.1. Molango deposit
3.2. TaoJiang deposit
3.3. Comparisons
4. Sampling and analytical methods
4.1. Molango deposit
4.2. TaoJiang deposit
4.3. Geochemistry
4.4. Stable isotopes
5. Results
5.1. Molango, Mexico
5.1.1. Carbon
5.1.2. Oxygen
5.1.3. Sulfur
5.2. TaoJiang, China
5.2.1. Carbon
5.2.2. Oxygen
5.2.3. Sulfur
6. Discussion
6.1. Origin of carbonates
6.1.1 Molango deposit
6.1.2. TaoJiang deposit
6.2. Sulfate reduction and sulfide oxidation
6.3. Geochemical model
7. Conclusions
Acknowledgments
References