83
4 Characteristics and Formation Process
several million to tens of millions of tonnes. The volumes, weights and concentrations of exploitable elements from such deposits are identical to those of many
land-based mines. The surface of the mounds consists of scree of broken chimneys
cemented by subsequent hydrothermal circulation; they are topped with a complex
of active chimneys over an area of tens of square meters. On some sites, deposits are
organised in and around depressions forming the crater of the volcano. Mounds are
formed by the accumulation of broken chimneys at the surface, but also by internal
precipitation of minerals, and by replacing the underlying bedrock. The end result
is an accumulation of massive sulphides.
Chemical Composition
The composition of hydrothermal sulphide deposits can vary considerably according to the geodynamic environment, the nature of basement rocks affected by
hydrothermal circulation, the water depth, the phase separation processes and the
maturity of deposits. The main factor is the nature of the basement rocks and of
metals that are pre-concentrated before their extraction by hydrothermal circulation.
Thus, even although the general principle of hydrothermal circulation applies to all
sites, there are considerable differences in the compositions of deposits. In general,
compared to nodules and manganese crusts, these ores have a high potential since
the total of copper + zinc often exceeds 10 %. It is very high (20 %) in back-arc
basins and around 15 % on slow (Atlantic) and fast (East Pacific) spreading ridges.
Furthermore, most sites are also highly enriched in silver and often gold. Some specific sites in the Atlantic associated with mantle rocks are also enriched in cobalt. It
should be noted however that these estimations are based on two-dimensional sampling carried out from submersibles. A real estimation in the economic sense of the
term would require the vertical variations in composition to be known, for which
drilling must be carried out; we know however that, in fossil deposits, deep areas
are generally the richest in copper. This point has been confirmed on the Middle
Valley site, at the base of which a deep copper zone is found, mainly composed of
copper sulphide.
Seawater/Basalt Interaction (Deposits Rich in Cu, Zn, Ag) This is the most common type. It is found on fast-spreading ridges and on some slow-spreading ridges
and back-arc basins. Hydrothermal reactions occur in basalt. The deposits are
mainly composed of iron, zinc and copper sulphides. Compositions of sulphides
vary little from one site to another. However, depending on the depth and boiling
phenomena, the composition of fluids may vary considerably. Salinities lower than
seawater correspond to the condensation of the vapour phase produced during phase
separation, while fluids more saline than seawater correspond to the salt concentration in residual brines. Metals tend to concentrate in brine phases. The youngest
and most immature deposits at the ridge axis (in particular on fast-spreading ridges)
are enriched in zinc and silver, while older deposits, generally outside of the axis,
are enriched in copper, with zinc being mainly concentrated at the summit and at
4 Characteristics and Formation Process
several million to tens of millions of tonnes. The volumes, weights and concentrations of exploitable elements from such deposits are identical to those of many
land-based mines. The surface of the mounds consists of scree of broken chimneys
cemented by subsequent hydrothermal circulation; they are topped with a complex
of active chimneys over an area of tens of square meters. On some sites, deposits are
organised in and around depressions forming the crater of the volcano. Mounds are
formed by the accumulation of broken chimneys at the surface, but also by internal
precipitation of minerals, and by replacing the underlying bedrock. The end result
is an accumulation of massive sulphides.
Chemical Composition
The composition of hydrothermal sulphide deposits can vary considerably according to the geodynamic environment, the nature of basement rocks affected by
hydrothermal circulation, the water depth, the phase separation processes and the
maturity of deposits. The main factor is the nature of the basement rocks and of
metals that are pre-concentrated before their extraction by hydrothermal circulation.
Thus, even although the general principle of hydrothermal circulation applies to all
sites, there are considerable differences in the compositions of deposits. In general,
compared to nodules and manganese crusts, these ores have a high potential since
the total of copper + zinc often exceeds 10 %. It is very high (20 %) in back-arc
basins and around 15 % on slow (Atlantic) and fast (East Pacific) spreading ridges.
Furthermore, most sites are also highly enriched in silver and often gold. Some specific sites in the Atlantic associated with mantle rocks are also enriched in cobalt. It
should be noted however that these estimations are based on two-dimensional sampling carried out from submersibles. A real estimation in the economic sense of the
term would require the vertical variations in composition to be known, for which
drilling must be carried out; we know however that, in fossil deposits, deep areas
are generally the richest in copper. This point has been confirmed on the Middle
Valley site, at the base of which a deep copper zone is found, mainly composed of
copper sulphide.
Seawater/Basalt Interaction (Deposits Rich in Cu, Zn, Ag) This is the most common type. It is found on fast-spreading ridges and on some slow-spreading ridges
and back-arc basins. Hydrothermal reactions occur in basalt. The deposits are
mainly composed of iron, zinc and copper sulphides. Compositions of sulphides
vary little from one site to another. However, depending on the depth and boiling
phenomena, the composition of fluids may vary considerably. Salinities lower than
seawater correspond to the condensation of the vapour phase produced during phase
separation, while fluids more saline than seawater correspond to the salt concentration in residual brines. Metals tend to concentrate in brine phases. The youngest
and most immature deposits at the ridge axis (in particular on fast-spreading ridges)
are enriched in zinc and silver, while older deposits, generally outside of the axis,
are enriched in copper, with zinc being mainly concentrated at the summit and at