Manganese deposits in the Franciscan Complex of the Diablo Range in the central Clifornia Coast Ranges occur as comformable lenses within bedded radiolarian chert-argillite sequences that are, in turn, intercalated within thicker sections of sandstone and shale. The field relationships, composition, and petrographic and isotopic characteristics of the Ladd and Buckeye deposits indicate that the manganese was concentrated by diagenetic reconstitution of siliceous and hemipelagic sediment during burial.
The ore lenses are Mn-rich and Fe-poor assemblages that consist largely of rhodochrosite, manganese silicates, opal-CT, and quartz. Although multiple sets of quartz-rich veins crosscut the red and green thin-bedded chert that hosts the manganese mineralization, no Fe- or Mn-rich feeder system is evident. Highly negative δ-13 carbon values of rhodochrosite samples indicate that CO2 originated from oxidation of methane; less negative values result from mixing of methanogenic carbon and CO2 derived from bacterial degradation of organic matter. δ-18 oxygen values for rhodochrosite samples indicate temperatures of formation in the range 20゜ to 100゜C.
Massive chert beds adjacent to the manganese lenses formed from silica released when siliceous host brocks were partly replaced by carbonate. The oxidation of methane prior to carbonate precipitation may have been accomplished by Mn and Fe oxhydroxides and oxides deposited with the sediment. The mobilization of manganese from biogenic and terrigenous sources in the sediment column into discrete horizons and the fractionation of manganese from iron reflect the presence of oxidation reduction boundaries and gradients in the sediment column. Fluids derived from silica dehydration reactions in the transformation of opal-A to quartz were involved in the transportation of principal components. Their sedimentary and geochemical attributes suggest that the deposits formed in a deep-water environment in a zone of oceanic upwelling at or near a continental margin.』
Review: Evolution of ideas
Regional framework and lithologic associations
Petrography and mineralogy, Ladd-Buckeye district
Isotopic composition of rhodochrosite
Massive chert beds
Timing and depth of rhodochrosite formation
Source of the manganese and manganese-iron fractionation
Tectonic and sedimentary environment of deposition