Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 94

4. The process may be repeated several times as new pulses of magma enter the
high-level magma chamber, creating a complex, multiphase system of intrusions
and ore bodies.
Analysis: Porphyry Deposits
Source of metals - mainly the granitic magma
Source of S - mainly magmatic
Source of fluid - magmatic and ground water
Cause of fluid circulation - expulsion of fluid from the magma, convection
of heated groundwater
Precipitation process - cooling, change in fluid composition, mixing with
other fluids
4.3.3 Sedimentary Exhalative (SEDEX) Deposits
The precipitation of sulfides from black smokers is not the only occasion where we
can observe a process that forms an ore body. Almost 100 million tones of sediment
containing 2% Zn, 0.5% Cu and significant amounts of the Au and Ag has
precipitated from hot dense brine that accumulated in the “Atlantis II Deep”, a
10 km diameter depression on the floor of the Red Sea. Were this deposit on land
and in a politically stable part of the world, it would constitute a very attractive ore
body of the type we refer to as a SEDEX or sedimentary exhalative deposit.
Another example is the Salton Sea, a large shallow lake in southern California
that formed in 1905 when a canal transporting water from the Colorado River
breached and flooded a saltpan. The water became brackish as it dissolved the
salt, and large-scale hydrothermal circulation was set up as water in the underlying
sedimentary basin was heated by the high prevailing geothermal gradient and the
conduits of local active volcanoes. At depth the circulating fluid, a hot (up to 350 C)
dense Na-Ca-K-Cl brine, has dissolved Fe, Mn, Pb, Zn, and Cu from the lacustrine
sediments that underlie the lake. When the fluid mixes with cooler, dilute surface
waters about 100 m below the surface, it precipitates these metals in veins of sulfide.
The two processes recorded in Red Sea and Salton Sea examples - precipitation of
sulfide-rich chemical sediment and interaction of sediments with circulating hydro-
thermal fluids - are key elements to the formation of SEDEX deposits.
The definition of these deposits is not straightforward because in many respects
they form a continuum with VMS deposits. They typically occur as tabular bodies
composed predominantly of Zn and Pb sulfides (sphalerite and galena) and they
usually contain economically important amounts of Ag. The Zn and Pb sulfides are
interbedded with iron sulfides (pyrite and pyrrhotite) and with generally fine-
grained detrital or chemical sediments. They are believed to have formed from
hydrothermal fluids that were expelled from mostly reduced sedimentary basins in
continental rifts. Two important subtypes are the “Broken Hill type”, which is
associated with bimodal volcanic rocks and Fe- or Mn-rich chemical sediments, and