also associated with the silica rich deposits. Chemically, the Fe-nontronite clay is
also silica-rich since it essentially consists of SiO 2 (49–54 %) and Fe 2 O 3
(28–31 %) and it is deprived of Al 2 O 3 (\1 %). This clay is also very low in
transition metal content (\200 ppm) so metals such as Zn, Co, and Ca are sparse.
The formation of the Fe-nontronite has been attributed to two processes: (1)
direct low temperature discharge from hydrothermal fluids circulating within the
oceanic crust and (2) from the interaction with siliceous and carbonaceous ooze
acting as a permeable material through which low-temperature fluids (\60 °C) are
circulating. Temperature determination from oxygen isotopic studies indicates that
the formation of the Fe-nontronite occurred at 25–47 °C (McMurty et al. 1983).
The most plausible hypothesis for the creation of nontronite is related to a
hydrothermal precipitation of low-temperature circulating fluids. Nontronite precipitates directly on the seafloor; however, in the case of the Galapagos Mounds
area, the nontronite is probably precipitated within the sedimentary layer. The
precipitation is the result of the oxidation gradient enhanced by the upward
migration of oxygen-poor hydrothermal fluids and the downward diffusion of
oxygen from seawater (Dymond et al. 1980). This model of direct precipitation for
nontronite is in accordance with the presence of similar products associated with
the sulfide chimneys near 13°N and 21°N on the EPR and on intraplate volcanoes.
Stockwork Mineralization
The pattern of fluid circulation in the lithosphere is usually determined though a
study of low temperature (\400 °C) fluids and the subsequent hydrated mineral
assemblages that have precipitated and/or replaced the more stable original
magmatic minerals. These transformations are marked by changes in mineral
composition and the precipitation of new minerals.
Hydrothermal fluids charged with metallic particles could also precipitate metalbearing compounds along their pathway to the surface. These types of precipitates
are called ‘‘stockwork’’ mineralization (Fig. 6.4b, c). Stockwork is a term used by
mining companies and it usually defines a mineral deposit that has an economic
value formed by a closely spaced network of veins and veinlets cutting through the
surrounding country rock. Often, these mineralized veins crossing various types of
flows (dykes and lava flows) are exposed by faults and fissures on spreading ridge
walls and in transform faults walls as well as in hotspot volcanoes. The mineral
association found in the stockwork will depend on the existing physical and
chemical conditions of the environment where the veins were formed. A low
temperature (\150 °C) and high pH ([6) fluid is likely to precipitate oxides and
hydroxide phases. More acidic (pH \ 5) and higher temperature conditions
([250 °C) are favorable for the precipitation of sulfides. The subsurface of the
ocean floor at the sites of discharge for hydrothermal fluid is marked by an
extensive alteration of the surrounding rock formations (basalt, dolerite, gabbro and
158
6 Hydrothermal Activity and Metalliferous Deposits
also silica-rich since it essentially consists of SiO 2 (49–54 %) and Fe 2 O 3
(28–31 %) and it is deprived of Al 2 O 3 (\1 %). This clay is also very low in
transition metal content (\200 ppm) so metals such as Zn, Co, and Ca are sparse.
The formation of the Fe-nontronite has been attributed to two processes: (1)
direct low temperature discharge from hydrothermal fluids circulating within the
oceanic crust and (2) from the interaction with siliceous and carbonaceous ooze
acting as a permeable material through which low-temperature fluids (\60 °C) are
circulating. Temperature determination from oxygen isotopic studies indicates that
the formation of the Fe-nontronite occurred at 25–47 °C (McMurty et al. 1983).
The most plausible hypothesis for the creation of nontronite is related to a
hydrothermal precipitation of low-temperature circulating fluids. Nontronite precipitates directly on the seafloor; however, in the case of the Galapagos Mounds
area, the nontronite is probably precipitated within the sedimentary layer. The
precipitation is the result of the oxidation gradient enhanced by the upward
migration of oxygen-poor hydrothermal fluids and the downward diffusion of
oxygen from seawater (Dymond et al. 1980). This model of direct precipitation for
nontronite is in accordance with the presence of similar products associated with
the sulfide chimneys near 13°N and 21°N on the EPR and on intraplate volcanoes.
Stockwork Mineralization
The pattern of fluid circulation in the lithosphere is usually determined though a
study of low temperature (\400 °C) fluids and the subsequent hydrated mineral
assemblages that have precipitated and/or replaced the more stable original
magmatic minerals. These transformations are marked by changes in mineral
composition and the precipitation of new minerals.
Hydrothermal fluids charged with metallic particles could also precipitate metalbearing compounds along their pathway to the surface. These types of precipitates
are called ‘‘stockwork’’ mineralization (Fig. 6.4b, c). Stockwork is a term used by
mining companies and it usually defines a mineral deposit that has an economic
value formed by a closely spaced network of veins and veinlets cutting through the
surrounding country rock. Often, these mineralized veins crossing various types of
flows (dykes and lava flows) are exposed by faults and fissures on spreading ridge
walls and in transform faults walls as well as in hotspot volcanoes. The mineral
association found in the stockwork will depend on the existing physical and
chemical conditions of the environment where the veins were formed. A low
temperature (\150 °C) and high pH ([6) fluid is likely to precipitate oxides and
hydroxide phases. More acidic (pH \ 5) and higher temperature conditions
([250 °C) are favorable for the precipitation of sulfides. The subsurface of the
ocean floor at the sites of discharge for hydrothermal fluid is marked by an
extensive alteration of the surrounding rock formations (basalt, dolerite, gabbro and
158
6 Hydrothermal Activity and Metalliferous Deposits