AUTHIGENIC DEPOSITS
G. M. McMurtry, University of Hawaii at Manoa,
Honolulu, HI, USA
Copyright & 2001 Elsevier Ltd.
Introduction
There are many kinds and forms of authigenic mineral
deposits in the deep-sea. Here we concentrate on the
more abundant and, in some cases, potentially economic types, namely ferromanganese deposits, phosphorites, marine barite, and the authigenic silicate
minerals in deep-sea sediments. Hydrothermal deposits
and a class of sedimentary iron and manganese oxides
associated with seafloor hydrothermal activity – the
metalliferous sediments found as a basal sequence in the
deep-sea sedimentary column and in deep basins adjacent to active spreading centers – are described elsewhere in the encyclopedia. Ferromanganese deposits are
here divided into nodules and crusts on the basis of
morphology and environment of formation. The sources of the major and minor metal enrichments in these
deposits are contrasted between continental weathering
and submarine volcanism, with important enrichment
influences from the marine biosphere. Marine barite has
a similar mixed origin with strong biological influence.
Knowledge of the origins of submarine phosphorites
has been greatly aided by studies of recent seafloor
deposits. There are a variety of authigenic silicate
minerals in deep-sea sediments. The more abundant
zeolites and clays such as smectite form mainly from
diagenetic alteration of metastable volcanic glasses, but
many other reaction paths are known and suggested.
Ferromanganese Deposits
Marine ferromanganese deposits are complex assemblages of authigenic minerals and detrital components. They are broadly characterized by shape and
by environment of formation as deep-sea nodules and
crusts. Ferromanganese nodules are commonly found
on the abyssal seafloor at and near the top of the
underlying pelagic sediment cover. Crusts are usually
found on the steep slopes of islands and seamounts
but are also found with the pelagic sediment on the
deep-sea floor where sedimentation is extremely low.
Occasionally, nodules may grow to coalesce into a
pavement or crust of ferromanganese deposits. Conversely, crusts can grow around small fragments of the
usually lithified sediment or basalt substrate of a
seamount or island slope to create ‘pseudo’ or ‘seamount nodules’ that are otherwise indistinguishable
from crustal pavements.
Ferromanganese Nodules
Deep-sea ferromanganese nodules are classified as three
major types on the basis of their origin: hydrogenetic,
early diagenetic, and mixed hydrogenetic–early diagenetic. In this same scheme, crusts are described as primarily hydrogenetic, with comparatively minor
diagenetic and hydrothermal occurrences (Figure 1).
Hydrogenetic nodules obtain their ferromanganese
oxides as precipitates from sea water. However, they are
rarely if ever pure and also contain a variety of detrital
minerals and biogenic debris from the pelagic sediment
and fallout from the overlying water column and atmosphere. Early diagenetic nodules obtain their ferromanganese oxides from metals dissolved from the
sediment and transported within the pore waters of the
upper, peneliquid sediment layer (Figure 1). Because
manganese and minor metals such as nickel and copper
are more readily mobile in pore waters under suboxic
seafloor conditions, diagenetic nodules are enriched in
these metals relative to concentrations of iron and cobalt. The hybrid or mixed type nodule has both
hydrogenetic and diagenetic components. These nodules are usually found at the sediment–water interface
where their outer layers record their most recent position. The upper, relatively smooth hydrogenetic top is
composed of iron- and cobalt-rich ferromanganese
oxides from the bottom water and the lower, relatively
bumpy or ‘botroyidal’ diagenetic bottom is composed
of manganese-, nickel-, and copper-rich ferromanganese
oxides from the sediment.
Attempts have been made to classify nodules on
the basis of morphology and to relate their occurrence to the composition of the underlying sediment.
Such field classifications were made to predict metal
enrichment and genesis, but reliance upon morphological data for other than abundance information
from either seafloor photography or remote acoustic
imaging has not yet been widely practiced in exploration. Nodule abundance appears to correlate
more strongly with siliceous ooze than with red
clays, and both siliceous ooze and red clays have
higher nodule abundance than does carbonate ooze.
Ferromanganese nodules and crusts are mainly
composed of three types of manganese oxide minerals:
vernadite or d-MnO 2 ; birnessite; and todorokite. These
325
G. M. McMurtry, University of Hawaii at Manoa,
Honolulu, HI, USA
Copyright & 2001 Elsevier Ltd.
Introduction
There are many kinds and forms of authigenic mineral
deposits in the deep-sea. Here we concentrate on the
more abundant and, in some cases, potentially economic types, namely ferromanganese deposits, phosphorites, marine barite, and the authigenic silicate
minerals in deep-sea sediments. Hydrothermal deposits
and a class of sedimentary iron and manganese oxides
associated with seafloor hydrothermal activity – the
metalliferous sediments found as a basal sequence in the
deep-sea sedimentary column and in deep basins adjacent to active spreading centers – are described elsewhere in the encyclopedia. Ferromanganese deposits are
here divided into nodules and crusts on the basis of
morphology and environment of formation. The sources of the major and minor metal enrichments in these
deposits are contrasted between continental weathering
and submarine volcanism, with important enrichment
influences from the marine biosphere. Marine barite has
a similar mixed origin with strong biological influence.
Knowledge of the origins of submarine phosphorites
has been greatly aided by studies of recent seafloor
deposits. There are a variety of authigenic silicate
minerals in deep-sea sediments. The more abundant
zeolites and clays such as smectite form mainly from
diagenetic alteration of metastable volcanic glasses, but
many other reaction paths are known and suggested.
Ferromanganese Deposits
Marine ferromanganese deposits are complex assemblages of authigenic minerals and detrital components. They are broadly characterized by shape and
by environment of formation as deep-sea nodules and
crusts. Ferromanganese nodules are commonly found
on the abyssal seafloor at and near the top of the
underlying pelagic sediment cover. Crusts are usually
found on the steep slopes of islands and seamounts
but are also found with the pelagic sediment on the
deep-sea floor where sedimentation is extremely low.
Occasionally, nodules may grow to coalesce into a
pavement or crust of ferromanganese deposits. Conversely, crusts can grow around small fragments of the
usually lithified sediment or basalt substrate of a
seamount or island slope to create ‘pseudo’ or ‘seamount nodules’ that are otherwise indistinguishable
from crustal pavements.
Ferromanganese Nodules
Deep-sea ferromanganese nodules are classified as three
major types on the basis of their origin: hydrogenetic,
early diagenetic, and mixed hydrogenetic–early diagenetic. In this same scheme, crusts are described as primarily hydrogenetic, with comparatively minor
diagenetic and hydrothermal occurrences (Figure 1).
Hydrogenetic nodules obtain their ferromanganese
oxides as precipitates from sea water. However, they are
rarely if ever pure and also contain a variety of detrital
minerals and biogenic debris from the pelagic sediment
and fallout from the overlying water column and atmosphere. Early diagenetic nodules obtain their ferromanganese oxides from metals dissolved from the
sediment and transported within the pore waters of the
upper, peneliquid sediment layer (Figure 1). Because
manganese and minor metals such as nickel and copper
are more readily mobile in pore waters under suboxic
seafloor conditions, diagenetic nodules are enriched in
these metals relative to concentrations of iron and cobalt. The hybrid or mixed type nodule has both
hydrogenetic and diagenetic components. These nodules are usually found at the sediment–water interface
where their outer layers record their most recent position. The upper, relatively smooth hydrogenetic top is
composed of iron- and cobalt-rich ferromanganese
oxides from the bottom water and the lower, relatively
bumpy or ‘botroyidal’ diagenetic bottom is composed
of manganese-, nickel-, and copper-rich ferromanganese
oxides from the sediment.
Attempts have been made to classify nodules on
the basis of morphology and to relate their occurrence to the composition of the underlying sediment.
Such field classifications were made to predict metal
enrichment and genesis, but reliance upon morphological data for other than abundance information
from either seafloor photography or remote acoustic
imaging has not yet been widely practiced in exploration. Nodule abundance appears to correlate
more strongly with siliceous ooze than with red
clays, and both siliceous ooze and red clays have
higher nodule abundance than does carbonate ooze.
Ferromanganese nodules and crusts are mainly
composed of three types of manganese oxide minerals:
vernadite or d-MnO 2 ; birnessite; and todorokite. These
325