Hydrothermal Deposits Sedex

2. What are SedEx deposits?
• SedEx, or sedimentary exhalative deposits, are ore deposits formed when
hydrothermal fluids enter a water reservoir, such as an ocean, and
precipitate minerals. SedEx deposits are a major source of minerals
including copper, silver, gold and tungsten– and the single most important
source of lead and zinc. SedEx deposits are high grade, with an average
size of approximately 70 Mt, and can host about 12 percent lead and zinc.
Depending on the deposit sub-type they also host variable amounts of
valuable by-products including copper, gold and silver.
• SedEx deposits are easily distinguished from many other deposit types by
the fact that their formation is the result of minerals being deposited
through the discharge of metal-bearing fluids into seawater.

3. Occurrence in world

4. Mineralization types
• Classically, SedEx deposits are considered lead-zinc deposits, due to
the fact that lead and zinc are generally the most prevalent mineral.
In addition, there are a variety of other valuable SedEx deposits. For
example, most of the world’s barite deposits are considered to be
SedEx deposits. The supergiant deposits of the Zambian Copper belt
are considered to be SedEx-style copper deposits. Some geologists
consider the gold deposits of Nevada to be formed by SedEx
processes (this concept is controversial because most gold is clearly of
later epigenetic origin.)

5. Formation and processes
• There are variances in the manner of how SedEx deposits are formed;
however, the general process is the same. They are formed when ore
bearing fluids discharge onto a seafloor and mix with seawater. When
the two fluids mix, a variety of chemical processes take place that
result in the precipitation of minerals on the seafloor. These deposits
are lain down congruent with the stratigraphy of the seafloor, and are
fine grained and finely laminated- characteristics of “sedimentary
deposits.” Concentrated amounts of minerals can be found in “trap
sites,” which are depressed areas of the ocean floor where the
minerals may settle. Occasionally, mineralization develops in the
faults and feeder conduits that fed the mineralizing system.

6. Towards a SEDEX model

7. Formation continues
• There are a few different mechanisms that may create the mineralizing
fluids that form SedEx deposits. They may be from magmatic fluids from
sub seafloor magma chambers and hydrothermal fluids generated by the
heat of a magma chamber intruding into saturated sediments. Some
geologists theorize that seawater convection cells play a big part in SedEx
fluid source. These sea-water convection cells contain dissolved metals
from the sediments they passed through before being heated and forced to
the surface. In the case of these sea-water convention, the mineral types
found in the SedEx deposits are linked to the depth at which the
mineralizing fluids originated. Fluids that come from a shallow depth are
rich in iron and manganese, fluids that penetrate deeper pick up lead and
zinc. Copper is picked up by fluids that reach an even greater depth.

8. Conditions pre Requisite to form
• Firstly, SEDEX deposits are generally formed in fault-bounded sedimentary
basins on continental crust rather than in volcanic piles on oceanic crust.
• For SEDEX deposits, the basin needs to accumulate several kilometres or
tens of kilometres of oxygen lacking sediment, usually shells.
• Secondly, the heat that drives the hydrothermal system is dominantly from
depth and burial rather than a felsic intrusion although there may be a
deep mafic intrusion. For the same reason the metals are not derived from
felsic magmas, so copper is largely missing, instead the metals are purely
derived from leaching of the sediments themselves and lead, zinc and
silver dominate instead.
• To form SEDEX deposits, you require the deep sedimentary basins so
distribution is more limited than that of VMS deposits.

9. Comparison of VMS and SedEx
• Both types of deposits are products of submarine hot-springs
• VMS deposits are volcanic-hosted, dominantly copper and zinc.
• SEDEX deposits are dominantly sediment-hosted, they are dominantly zinc and lead they are
bigger but generally less common than the VMS deposits
• In VMS deposits the metals are usually spatially zoned, with copper near the vent and then zinc
and lead more distant to that These deposit types are major global sources of base metals,
particularly zinc
• VMS deposits are some of the last deposits to from in terms of the upward migration of metals
and mineralization drops out of the fluid either right at the surface of the seafloor or in a few
tens of meters from that. So, they form at even shallower depths than shallow epi thermals.
SEDEX deposits are not directly related to volcanic activity and so don’t fit in directly to this
framework. However, their mode of formation is just similar.
• The main difference between the VMS and SEDEX deposits, it’s that VMS deposits are
dominantly copper and zinc rich and associated with volcanic activity. Whereas SEDEX deposits
are dominantly lead and zinc rich and rely mainly on the heat caused by the depth and burial in
deep sedimentary basins to drive the hydrothermal system.

10. Characteristics and features of SedEx Deposits
• Sedex are dominated by Zn-Pb (with lesser Cu, But commonly Ba and Ag)
Metal association and are also related to hydrothermal fluids venting onto
the sea floor, but without an obvious or direct link to volcanism. Many of
the large SEDEx deposits of the world are Proterozoic in age, although
several examples, such as Red Dog also formed in Phanerozoic times.
Although there is Generally no Spatial or temporal link between Sedex and
VMS Deposits, it is widely held that they represent a continuum and are
conceptually linked by the fact that they formed by the same basic
processes. These processes are active and can be Studied in modern day
environments as discussed below.
• The notion of a continuum between VMS abd SEDEX deposit types is
However, contentious and readers should be Wary of over interpreting the
generic link between the two deposit types.