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is less obvious. Evacuation of pore fluids during compaction of sediments no doubt
plays a role, but in many cases this is too slow a process to produce the sudden
fluxes of warm fluids that are implicated in the formation of certain deposits.
Tectonic loading associated with mountain building at the basin margins is
implicated in some examples, but again this process is too slow to have the required
effect. To form the Pb deposits in carbonate sequences more obscure processes such
as the influx of warm seawater onto and thence into the basin has been suggested.
As for the cause of the influx, changes in ocean circulation or melting of continental
glaciers have been proposed.
4.2.4 A Site and a Mechanism of Precipitation
The crux of the ore-forming process is the precipitation of the ore metals or
minerals. What is required is a mechanism that causes the metals to come efficiently
out of solution and concentrate in a restricted volume of rock. The most common
cause of precipitation is cooling of the solution, which decreases the solubility of
the metals. Cooling takes place when hot magmatic fluids enter cool wall rocks,
when fluids emerging from a seafloor spring mix with cold seawater or when warm
basin fluids mix with cooler near-surface waters. Associated with many of these
cooling events is dilution of the hydrothermal brines and since this decreases the
concentration of the complex-forming anions, this also decreases the solubility of
the metals, leading to their precipitation. Another process is reaction with wall
rocks, which changes the fluid composition. Particularly important are redox
reactions, which happen when oxidized basin waters come into contact with
reduced materials such as hydrocarbons or organic-rich shales. This type of inter-
action is crucial in the formation of most uranium deposits and many of the base-
metal deposits in sedimentary basins.
There are two main types of deposition site; open fractures and zones of
replacement. Many hydrothermal deposits form at shallow levels in the crust
where fractures remain open and in such cases much of the mineralization consists
of ore minerals that precipitated in such fractures. Ore bodies formed this way
consist of a multitude of veins and patches of ore minerals dispersed through the
host rock. Cavities and caves in limestone reefs and kast facies, and interstitial
space in breccias, are important sites of deposition of Pb sulfides. And finally the
ocean water in which the sulfides of VMS deposits accumulate could be considered
a special case of open cavity precipitation.
Hot hydrothermal fluids are chemically aggressive and capable of reacting with a
wide range of rock types. Alteration zones surround most hydrothermal systems
and ore minerals occur in many of these zones. In some cases the minerals are
disseminated or restricted to veins; in other cases wholesale replacement of the
original rock is evident.
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