『Abstract
Laminated to massive rhodochrosite, hausmannite, and Mn-silicates
from the Smith prospect and Manga-Chrome mine, Sierra Nevada,
California were deposited as ocean floor sediments associated
with chert and shale. The principal lithologies at Smith are chert,
argillite, rhodochrosite-, hausmannite- and chlorite-rich layers,
and relatively uncommon layers of jacobsite. The Manga-Chrome
mine also contains layers rich in manganoan calcite and caryopilite.
Tephroite, rhodonite, spessartine, and accessory aleghanyite ando
sonolite formed during metamorphism. Volcaniclastic components
are present at Manga-Chrome as metavolcanic clasts and as Mn-poor,
red, garnet- and hematite-rich layers. There is no evidence, such
as relict lithologies, that Mn was introduced into Mn-poor lithologies
such as chert, limestone or mudstone. Replacement of Mn-poor phases
by Mn-rich phases is observed only in the groundmass of volcanic
clasts that appear to have fallen into soft Mn-rich mud.
Manganiferous samples from the Smith prospect and Manga-Chrome
mine have high Mn/Fe and low concentrations of Ni, Cu, Zn, Co,
U, Th and the rare-earth elements that are similar to concentrations
reported from other ancient Mn deposits found in chert-greenstone
complexes and from manganiferous sediments and crusts that are
forming near modern sea floor vents. The Sierra Nevada deposits
formed as precipitates of Mn-rich sediments on the sea floor,
probably from mixtures of circulating hydrothermal fluids and
seawater. The composition of a metabasalt from the Smith prospect
is consistent with those of island-arc tholeiites. Metavolcanic
clasts from the Manga-Chrome mine are compositionally distinct
from the Smith metabasalt and have alkaline to calc-alkaline affinities.
A back-arc basin is considered to be the most likely paleoenvironment
for the formation of the Mn-rich lenses at the Manga-Chrome mine
and, by association, the Smith prospect.
Layers of rhodochrosite, hausmannite and chert preserve the composition
and some textures of the sedimentary protoliths at both Sierra
Nevada deposits. Jacobsite-rich layers probanly represent a Fe-rich
protolith. Caryopilite and manganoan calcite represent additional
protoliths at the Manga-Chrome mine.
The metamorphic assemblage prehnite-chlorite-epidote-calcite in
a metabasalt from the Smith prospect constrains regional metamorphic
conditions to a maximum temperature of 325゜C and a pressure of
2 kbar. Slightly higher temperatures are indicated by the presence
of actinolite in another metabasalt. Compositions of Mn-rich minerals
in Smith samples are consistent with these metamorphic conditions.』
Introduction
Previous work
Purpose of present study
Geologic setting
Analytical methods
Petrology
Chert and phyllite
Meta-igneous rocks
Mineralogy and textual characteristics
Mn-rich lithologies
Clasts and hematite-rich layers
Major and trace element chemistry
Country rocks
Mn-rich lithologies
Discussion
Origin of layering
Protoliths
Components
Depositional environment
Comparison with the Buckeye mine
Acknowledgments
References