Another type of breccia consists of the debris of stockwork mineralization
resulting from the alteration of hydrothermal outcrops during tectonic activity.
Talus piles and landslides were created along spreading-ridge graben walls and
fissures on the ridge axes. This type of breccia is made of veined-rock fragments
where remnants of angular chilled margins and mineral grains are intermixed with
Fe-sulfides. These specific types of breccia are discussed in this chapter’s section
dealing with stockwork.
Ochreous Deposits or Gossan
Variegated purple-red and reddish-yellow Fe-oxyhydroxide-rich material is found
associated with hydrothermal chimneys, as a powdery sediment fill, and is also
observed forming mounds.
It is often difficult to tell the difference between gossan deposits, resulting from
the oxidation of pre-existing sulfide edifices, and ochreous deposits whose origin is
due to slow venting (exhalative discharge) giving rise to primary Fe-oxyhydroxide
deposits. This is mainly true when dead chimneys are composed entirely of Feoxyhydroxides and when no trace of sulfides is detected. In the case when this type
of edifice covers an extended field ([200 m in diameter), such as are encountered
on off-axis seamounts and on intraplate edifices, the ochreous deposits are probably due to a primary exhalative (slow venting) origin due to the circulation of
Fe-rich fluids. In contrast, many of the ridge axis gossan deposits were probably
once ancient sulfide edifices formed from sulfide-rich fluid emanations, which
have become extensively altered, but will still contain trace amounts of sulfides.
The ochreous and gossan deposits often alternate with other more quickly
extruded hydrothermal deposits and form small fields (\20 m in diameter) of
powdery mounds which are located near other metalliferous sulfide edifices.
Gossan and ochreous deposits are found as frequently as sulfide chimneys but they
differ from the sulfides because of their friable nature and due to their intimate
association with Fe–Mn oxides. In fact, Fe–Mn oxides are usually present with this
type of deposit as a coating on top of red and/or yellow colored oxyhydroxide
compounds.
Although both gossan and ochreous deposits contain variable amounts of
amorphous Fe-oxyhydroxide powdery material, they also consist of nontronite
(smectite/clay) and Fe–Mn crusts such as todorokite and birnessite. Hydromica,
illite, and kaolinite also occur as minor constituents of these deposits (Hoffert et al.
1978). Ochreous and gossan deposits contain goethite spherules and amorphous
Fe-oxides, but they differ from other Fe-oxyhydroxide material because they
contain atacamite (due to the oxidation of a Cu-rich deposit), traces of pyrite and
other sulfides (such as covellite and chalcopyrite).
Types of Hydrothermal Deposits
155
resulting from the alteration of hydrothermal outcrops during tectonic activity.
Talus piles and landslides were created along spreading-ridge graben walls and
fissures on the ridge axes. This type of breccia is made of veined-rock fragments
where remnants of angular chilled margins and mineral grains are intermixed with
Fe-sulfides. These specific types of breccia are discussed in this chapter’s section
dealing with stockwork.
Ochreous Deposits or Gossan
Variegated purple-red and reddish-yellow Fe-oxyhydroxide-rich material is found
associated with hydrothermal chimneys, as a powdery sediment fill, and is also
observed forming mounds.
It is often difficult to tell the difference between gossan deposits, resulting from
the oxidation of pre-existing sulfide edifices, and ochreous deposits whose origin is
due to slow venting (exhalative discharge) giving rise to primary Fe-oxyhydroxide
deposits. This is mainly true when dead chimneys are composed entirely of Feoxyhydroxides and when no trace of sulfides is detected. In the case when this type
of edifice covers an extended field ([200 m in diameter), such as are encountered
on off-axis seamounts and on intraplate edifices, the ochreous deposits are probably due to a primary exhalative (slow venting) origin due to the circulation of
Fe-rich fluids. In contrast, many of the ridge axis gossan deposits were probably
once ancient sulfide edifices formed from sulfide-rich fluid emanations, which
have become extensively altered, but will still contain trace amounts of sulfides.
The ochreous and gossan deposits often alternate with other more quickly
extruded hydrothermal deposits and form small fields (\20 m in diameter) of
powdery mounds which are located near other metalliferous sulfide edifices.
Gossan and ochreous deposits are found as frequently as sulfide chimneys but they
differ from the sulfides because of their friable nature and due to their intimate
association with Fe–Mn oxides. In fact, Fe–Mn oxides are usually present with this
type of deposit as a coating on top of red and/or yellow colored oxyhydroxide
compounds.
Although both gossan and ochreous deposits contain variable amounts of
amorphous Fe-oxyhydroxide powdery material, they also consist of nontronite
(smectite/clay) and Fe–Mn crusts such as todorokite and birnessite. Hydromica,
illite, and kaolinite also occur as minor constituents of these deposits (Hoffert et al.
1978). Ochreous and gossan deposits contain goethite spherules and amorphous
Fe-oxides, but they differ from other Fe-oxyhydroxide material because they
contain atacamite (due to the oxidation of a Cu-rich deposit), traces of pyrite and
other sulfides (such as covellite and chalcopyrite).
Types of Hydrothermal Deposits
155