6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
277
Acounts of the diverse pelagic mineral deposits have been given by Cronan (1980,
1986). Of the nonmetallic minerals, barytes (BaSO4) is one of the most common associated with pelagic deposits, though it occurs in several geological settings, both igneous
and sediment related. Bedded barite deposits are found associated with pelagic shales,
limestones, and cherts in Ireland, Canada, the USA, Australia, Thailand, and India (Orris, 1986). It has been argued that the barium is derived from the solution of biogenic detritus (Scholle et aL, 1983b), but some bedded barite deposits are associated with Pb-Zn
exhalative centers.
Studies of modern mid-oceanic ridges have led to the discovery of brine springs,
whose waters are enriched in many metallic ions, notably Cu, Mn, Fe, and Pb. The
emergent fluids are reducing, acid (pH about 4), and have temperatures of up to 350~
These brine springs were first encountered in the Red Sea (Degens and Ross, 1969), but
have subsequently been found on many other mid-ocean ridges. Reactions between the
brines and seawater cause mineral precipitation. Amorphous mineral particles impart
a cloudy appearance to the emergent spring waters, and are deposited as a chimney,
sometimes several meters high, around the spring vent. These are termed smokers.
White smokers and black smokers have been distinguished. Black smokers emit black
clouds of finely disseminated pyrrhotite. They begin with the precipitation of an anhydrite chimney. This is followed by a phase of replacement of the original anhydrite and
direct sulfide precipitation. Bornite, chalcopyrite, and chalcocite are the principal minerals that form (Haymon, 1983). White smokers are rather cooler, with temperatures of
less than 300~ They emit white clouds of amorphous silica, barytes, and pyrite. The resultant chimneys are encrusted with polychete worms and have a peripheral ecosystem
adapted to high temperatures and salinities. Mn-oxyhydroxite crusts develop on the
sea bed within a few meters of the vents (Haymon and Kastner, 1981).
These modern smokers are of great interest because they are analogs of an important group of mineral deposits, variously termed "volcanogenic," "volcano exhalative,"
"sedimentary-exhalative," or simply sedex deposits (Briskey, 1986; Guilbert and Park,
1986, pp. 572-603). These are massive sulfide ore bodies, principally of lead, zinc, copper, and iron, sometimes associated with bedded baryte deposits. They commonly occur
interbedded with volcanic and volcaniclastic rocks, together with pelagic shales, limestones, and cherts. Noted examples include Rammelsberg (Germany), Mount Isa (Australia), Broken Hill (Australia and South Africa), and Silver mines (Ireland). Andrews
et al. (1986) give a fascinating account of the analogy of modern "smokers" with the massive sulfide ores of Ireland. The similarities even include the discovery of paleosmokers
complete with an encrusted fossilized worm tube (Fig. 6.77).
Moving away from the axis of mid-ocean ridges, vast areas of their flanks and of major ocean basins have been found covered by manganese nodules (Fig. 6.78). There is a
gradual change in the ferromagnesian ratio of these nodules away from the ridge axis
(Figure 6.79). Manganese nodules form very slowly from oxygenated seawater of normal salinity (Cronan, 1980). The foregoing brief review shows that pelagic deposits are
of great commercial interest.
6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
The preceding pages have described the sedimentary models that are recognized as major facies generators. It is now appropriate to review the principles behind this approach
277
Acounts of the diverse pelagic mineral deposits have been given by Cronan (1980,
1986). Of the nonmetallic minerals, barytes (BaSO4) is one of the most common associated with pelagic deposits, though it occurs in several geological settings, both igneous
and sediment related. Bedded barite deposits are found associated with pelagic shales,
limestones, and cherts in Ireland, Canada, the USA, Australia, Thailand, and India (Orris, 1986). It has been argued that the barium is derived from the solution of biogenic detritus (Scholle et aL, 1983b), but some bedded barite deposits are associated with Pb-Zn
exhalative centers.
Studies of modern mid-oceanic ridges have led to the discovery of brine springs,
whose waters are enriched in many metallic ions, notably Cu, Mn, Fe, and Pb. The
emergent fluids are reducing, acid (pH about 4), and have temperatures of up to 350~
These brine springs were first encountered in the Red Sea (Degens and Ross, 1969), but
have subsequently been found on many other mid-ocean ridges. Reactions between the
brines and seawater cause mineral precipitation. Amorphous mineral particles impart
a cloudy appearance to the emergent spring waters, and are deposited as a chimney,
sometimes several meters high, around the spring vent. These are termed smokers.
White smokers and black smokers have been distinguished. Black smokers emit black
clouds of finely disseminated pyrrhotite. They begin with the precipitation of an anhydrite chimney. This is followed by a phase of replacement of the original anhydrite and
direct sulfide precipitation. Bornite, chalcopyrite, and chalcocite are the principal minerals that form (Haymon, 1983). White smokers are rather cooler, with temperatures of
less than 300~ They emit white clouds of amorphous silica, barytes, and pyrite. The resultant chimneys are encrusted with polychete worms and have a peripheral ecosystem
adapted to high temperatures and salinities. Mn-oxyhydroxite crusts develop on the
sea bed within a few meters of the vents (Haymon and Kastner, 1981).
These modern smokers are of great interest because they are analogs of an important group of mineral deposits, variously termed "volcanogenic," "volcano exhalative,"
"sedimentary-exhalative," or simply sedex deposits (Briskey, 1986; Guilbert and Park,
1986, pp. 572-603). These are massive sulfide ore bodies, principally of lead, zinc, copper, and iron, sometimes associated with bedded baryte deposits. They commonly occur
interbedded with volcanic and volcaniclastic rocks, together with pelagic shales, limestones, and cherts. Noted examples include Rammelsberg (Germany), Mount Isa (Australia), Broken Hill (Australia and South Africa), and Silver mines (Ireland). Andrews
et al. (1986) give a fascinating account of the analogy of modern "smokers" with the massive sulfide ores of Ireland. The similarities even include the discovery of paleosmokers
complete with an encrusted fossilized worm tube (Fig. 6.77).
Moving away from the axis of mid-ocean ridges, vast areas of their flanks and of major ocean basins have been found covered by manganese nodules (Fig. 6.78). There is a
gradual change in the ferromagnesian ratio of these nodules away from the ridge axis
(Figure 6.79). Manganese nodules form very slowly from oxygenated seawater of normal salinity (Cronan, 1980). The foregoing brief review shows that pelagic deposits are
of great commercial interest.
6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
The preceding pages have described the sedimentary models that are recognized as major facies generators. It is now appropriate to review the principles behind this approach