Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore

Journal Article · Tue Nov 01 04:00:00 EST 1988 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(88)90036-1· OSTI ID:7018000

Okita, P M; Maynard, J B ^[1]; Spiker, E C; Force, E R ^[2]

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1. Univ. of Cincinnati, OH (USA)
2. Geological Survey, Reston, VA (USA)

Unlike other marine-sedimentary manganese ore deposits, which are largely composed of manganese oxides, the primary ore at Molango (Hidalgo State, Mexico) is exclusively manganese carbonate (rhodochrosite, Mn-calcite, kutnahorite). Stable isotope studies of the carbonates from Molango provide critical new information relevant to the controversy over syngenetic and diagenetic models of stratiform manganese deposit formation. Negative {delta}{sup 13}C values for carbonates from mineralized zones at Molango are strongly correlated with manganese content both on a whole rock scale and by mineral species. Whole rock {delta}{sup 13}C data fall into three groups: high-grade ore = {minus}16.4 to {minus}11.5{per thousand}; manganese-rich, sub-ore-grade = {minus}5.2 to 0{per thousand}; and unmineralized carbonates = 0 to +2.5 {per thousand} (PDB). {delta}{sup 18}O data show considerable overlap in values among the three groups: +4.8 to {minus}2.8, {minus}5.4 to {minus}0.3{per thousand}, and {minus}7.4 to +6.2 (PDB), respectively. Isotopic data for individual co-existing minerals suggest a similar separation of {delta}{sup 13}C values: {delta}{sup 13}C values from calcite range from {minus}1.1 to +0.7{per thousand} (PDB), whereas values from rhodochrosite are very negative, {minus}12.9 to {minus}5.5{per thousand}, and values from kutnahorite or Mn-calcite are intermediate between calcite and rhodochrosite. {sup 13}C data are interpreted to indicate that calcite formed from a normal marine carbon reservoir. However, {sup 13}C data for the manganese-bearing carbonates suggest a mixed seawater and organic source of carbon. The presence of only trace amounts of pyrite suggests sulfate reduction may have played a minor part in oxidizing organic matter.

• OSTI ID: 7018000
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 52:11; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBON ISOTOPES
CARBONATE MINERALS
CARBONATES
CHALCOGENIDES
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
CHEMISTRY
DEVELOPING COUNTRIES
DIAGENESIS
GEOCHEMISTRY
GEOLOGY
IRON COMPOUNDS
IRON SULFIDES
ISOTOPE RATIO
ISOTOPES
LATIN AMERICA
MANGANESE CARBONATES
MANGANESE COMPOUNDS
MANGANESE ORES
MEXICO
MINERALIZATION
MINERALS
NORTH AMERICA
ORES
ORGANIC MATTER
OXIDATION
OXYGEN COMPOUNDS
PETROGENESIS
PETROLOGY
PYRITE
REDOX REACTIONS
REDUCTION
SULFIDE MINERALS
SULFIDES
SULFUR COMPOUNDS
TRANSITION ELEMENT COMPOUNDS