702
the hot, Cl-rich, metal-bearing brines to migrate laterally upward. As they pass black shales, the brines may take up additional metals, before they escape along more permeable layers toward the basin margins. Alternatively, the brines may
migrate upward along syngenetic (i.e., post-rift) faults.
Cooling, changing Eh and pH conditions, and mixing
with shallow pore fluids from other sources cause the
precipitation of metals, mostly as low soluble sulfides
(e.g. those of Pb, Zn, and Fe). However, several elements, such as Ba, Mn, and U, behave differently.
Variations in the chemical conditions over time lead to
special successions of ore minerals (ore mineral
paragenesis ), as in the case of the common cement
minerals.
Diagenetically generated ore deposits are found in
siliciclastic and carbonate host rocks. The ores may fill
macropores and voids, as weH as faults and fracture
systems. Carbonates with initially high porosities, such
as oolitic wackestones, packstones, and bioherms, and
rocks with karst cavities and massive dolomites, are
particularly promising host rocks. The ascending
highly saline fluids may react with shallow evaporites
and dolomites and dissolve and brecciate part of these
host rocks.
One can distinguish between several phases and types of ore
genesis, for example between early diagenetic cementation of
pores (disseminated ores), replacement of carbonate,
sulfidization of carbonates, secondary concentration of primary ores (formation of discontinuous flat beds and nests),
and filling of fractures.
The precipitation of metal sulfides in areas outside ofthe
sulfate reduction zone is difficult to understand, possibly because two different types of pore fluids are needed. One pore
fluid has to provide the metals in solution (in part as chloride
complexes at high temperatures or as organic complexes),
while the other (probably from shallow sources) has to deliver sulfur in the form of HS' from the sulfate reduction
zone. However, an alternative model has been proposed (see,
e.g., Roberts and Sheahan 1988), in which sulfur migrates
with the metals to the site of deposition.
Precipitation of metal sulfides may be followed by the
formation of barite and other compounds characteristic of
oxidizing conditions. The deposition of metalores in carbonates and shallow evaporites is frequently associated with
dolomitization, dissolution and/ or brecciation, and occasionally with silicification.
The special conditions of ore precipitation at the contact
with evaporites have been described, for example, from the
GulfCoast basin ofthe United States. Here, the intrusion of
salt into the overlying argillaceous sediments had several
significant etfects (Holmes 1986). As in the Mediterranean,
a 125 m thick Tertiary/Quaternary sediment section on top of
drilled salt diapirs displays upwardly decreasing pore water
salinity. In addition, the increased heat flow through salt dehydrated the clay minerals at the salt-sediment interface at
relatively shallow burial depth. The chemical reactions between the pore water containing sulfate and the sediments
rich in organic matter created a diagenetic sulfide-rich front.
Within this zone, metals such as zinc and lead were concentrated. Anhydrite residue from evaporite dissolution, transformation of anhydrite to gypsum, and newly precipitated
carbonate form the cap rock ofthe salt domes.
Chapter 13 Diagenesis
For further details and examples see, e.g., Amstutz et al.
(1982), Maynard (1982), Guilbert and Park (1986), Friedrich
and Herzig (1988), Muchez et al. (1994), Warren and Kempton (1997).
13.8.3 Pore Water Expulsion and Hydrothermal
Systems at tbe Sea Floor
Most ofthe hydrothermal systems described in the literature, ineluding those at active spreading centers on
mid-oceanic ridges (e.g., Fyfe and Lonsdale 1981), are
fed either by sea water or meteoric water circulating in
fracture zones or highly permeable layers through various rocks. If there is no difference in hydraulic head,
such systems are driven by a magmatic heat source or
by local differences in the geothermal gradients. Hot
water is forced to move upwards to the sea floor or
land surface and discharges through vents or springs,
but this water has to be simultaneously replaced by
downward moving sea water or meteoric water.
However, hydrothermal systems can also originate,
partially or entirely, from the expulsion of pore water
from young, soft sediments. Two types of such systems occur in the present oceans. Their origin is
closelyrelated to the topics ofthis chapter. Whether or
not they significantly contribute to the formation of
mineral deposits is not yet sufficiently known.
Expulsion of pore water from young spreading
trougbs. In young, narrow ocean basins and pull-apart
basins (cf. Chap. 1 and Sects. 12.1 and 12.8), the accretion of new oceanic crust does not take place at
sediment-starved oceanic ridges, but in spreading
troughs collecting large quantities of sediment.
This can be observed, for example, in the central
graben ofthe Red Sea and in special basins in the Gulf
of Califomia (Guaymas basin). Since these spreading
troughs represent the deepest depressions in the total
depositional area and are situated elose to terrigenous
sediment sources, they are commonly characterized by
high sedimentation rates. In such cases, the upwelling
basaltic magma intrudes into soft sediments and tends
to generate horizontal sills as soon as the magma pressure exceeds the lithostatic pressure and tensile
strength ofthe sediments (Fig. 13.31a).
During Leg 64 of the Deep Sea Drilling Project in the Gulf
of California, such basaltic sills were encountered in several
drillholes in the Guaymas basin at burial depths between
approximately 50 and 350 m (Curray, Moore, et al. 1982;
Einseie 1982b and 1986). Two of the shallowly buried sills
were still hot. Downhole measurements showed that the porosities of the sediments were reduced by 20 to 40% within
several tens of meters above and especially below the sillsediment contacts (Fig. 13.3lb). Sediment porosity is reduced by "thermal tamping" (Walker and Francis 1987),
which is caused by heat transfer from the hot magma into the
neighboring sediment, as weil as by the emplacement of
heavy magrnatic loads on top ofhighly porous sediments. In
the hot, Cl-rich, metal-bearing brines to migrate laterally upward. As they pass black shales, the brines may take up additional metals, before they escape along more permeable layers toward the basin margins. Alternatively, the brines may
migrate upward along syngenetic (i.e., post-rift) faults.
Cooling, changing Eh and pH conditions, and mixing
with shallow pore fluids from other sources cause the
precipitation of metals, mostly as low soluble sulfides
(e.g. those of Pb, Zn, and Fe). However, several elements, such as Ba, Mn, and U, behave differently.
Variations in the chemical conditions over time lead to
special successions of ore minerals (ore mineral
paragenesis ), as in the case of the common cement
minerals.
Diagenetically generated ore deposits are found in
siliciclastic and carbonate host rocks. The ores may fill
macropores and voids, as weH as faults and fracture
systems. Carbonates with initially high porosities, such
as oolitic wackestones, packstones, and bioherms, and
rocks with karst cavities and massive dolomites, are
particularly promising host rocks. The ascending
highly saline fluids may react with shallow evaporites
and dolomites and dissolve and brecciate part of these
host rocks.
One can distinguish between several phases and types of ore
genesis, for example between early diagenetic cementation of
pores (disseminated ores), replacement of carbonate,
sulfidization of carbonates, secondary concentration of primary ores (formation of discontinuous flat beds and nests),
and filling of fractures.
The precipitation of metal sulfides in areas outside ofthe
sulfate reduction zone is difficult to understand, possibly because two different types of pore fluids are needed. One pore
fluid has to provide the metals in solution (in part as chloride
complexes at high temperatures or as organic complexes),
while the other (probably from shallow sources) has to deliver sulfur in the form of HS' from the sulfate reduction
zone. However, an alternative model has been proposed (see,
e.g., Roberts and Sheahan 1988), in which sulfur migrates
with the metals to the site of deposition.
Precipitation of metal sulfides may be followed by the
formation of barite and other compounds characteristic of
oxidizing conditions. The deposition of metalores in carbonates and shallow evaporites is frequently associated with
dolomitization, dissolution and/ or brecciation, and occasionally with silicification.
The special conditions of ore precipitation at the contact
with evaporites have been described, for example, from the
GulfCoast basin ofthe United States. Here, the intrusion of
salt into the overlying argillaceous sediments had several
significant etfects (Holmes 1986). As in the Mediterranean,
a 125 m thick Tertiary/Quaternary sediment section on top of
drilled salt diapirs displays upwardly decreasing pore water
salinity. In addition, the increased heat flow through salt dehydrated the clay minerals at the salt-sediment interface at
relatively shallow burial depth. The chemical reactions between the pore water containing sulfate and the sediments
rich in organic matter created a diagenetic sulfide-rich front.
Within this zone, metals such as zinc and lead were concentrated. Anhydrite residue from evaporite dissolution, transformation of anhydrite to gypsum, and newly precipitated
carbonate form the cap rock ofthe salt domes.
Chapter 13 Diagenesis
For further details and examples see, e.g., Amstutz et al.
(1982), Maynard (1982), Guilbert and Park (1986), Friedrich
and Herzig (1988), Muchez et al. (1994), Warren and Kempton (1997).
13.8.3 Pore Water Expulsion and Hydrothermal
Systems at tbe Sea Floor
Most ofthe hydrothermal systems described in the literature, ineluding those at active spreading centers on
mid-oceanic ridges (e.g., Fyfe and Lonsdale 1981), are
fed either by sea water or meteoric water circulating in
fracture zones or highly permeable layers through various rocks. If there is no difference in hydraulic head,
such systems are driven by a magmatic heat source or
by local differences in the geothermal gradients. Hot
water is forced to move upwards to the sea floor or
land surface and discharges through vents or springs,
but this water has to be simultaneously replaced by
downward moving sea water or meteoric water.
However, hydrothermal systems can also originate,
partially or entirely, from the expulsion of pore water
from young, soft sediments. Two types of such systems occur in the present oceans. Their origin is
closelyrelated to the topics ofthis chapter. Whether or
not they significantly contribute to the formation of
mineral deposits is not yet sufficiently known.
Expulsion of pore water from young spreading
trougbs. In young, narrow ocean basins and pull-apart
basins (cf. Chap. 1 and Sects. 12.1 and 12.8), the accretion of new oceanic crust does not take place at
sediment-starved oceanic ridges, but in spreading
troughs collecting large quantities of sediment.
This can be observed, for example, in the central
graben ofthe Red Sea and in special basins in the Gulf
of Califomia (Guaymas basin). Since these spreading
troughs represent the deepest depressions in the total
depositional area and are situated elose to terrigenous
sediment sources, they are commonly characterized by
high sedimentation rates. In such cases, the upwelling
basaltic magma intrudes into soft sediments and tends
to generate horizontal sills as soon as the magma pressure exceeds the lithostatic pressure and tensile
strength ofthe sediments (Fig. 13.31a).
During Leg 64 of the Deep Sea Drilling Project in the Gulf
of California, such basaltic sills were encountered in several
drillholes in the Guaymas basin at burial depths between
approximately 50 and 350 m (Curray, Moore, et al. 1982;
Einseie 1982b and 1986). Two of the shallowly buried sills
were still hot. Downhole measurements showed that the porosities of the sediments were reduced by 20 to 40% within
several tens of meters above and especially below the sillsediment contacts (Fig. 13.3lb). Sediment porosity is reduced by "thermal tamping" (Walker and Francis 1987),
which is caused by heat transfer from the hot magma into the
neighboring sediment, as weil as by the emplacement of
heavy magrnatic loads on top ofhighly porous sediments. In