Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 75

Using a variety of geological and geochemical techniques, particularly the analysis
of fluid inclusions in ore and gangue minerals, it has been possible to identify the
following types of fluid:

￿ Magmatic fluids released at various stages during the cooling and crystallization
of granitic magmas
￿ Meteoric (rain) water
￿ Seawater
￿ Connate water, the interstitial water in pore spaces in sedimentary basins
￿ Metamorphic fluids, which are released by dehydration reactions in deeper
crustal sections.

The temperatures of hydrothermal fluids, as determined from studies of fluid
inclusions in ore and gangue minerals, range from more than 600 C in magmatic
fluids to as low as 50-70 C for the fluids that deposit Pb-Zn sulfides or uranium
minerals in sedimentary piles. Some examples are listed in Table 4.1 .
The solubility of metals in pure water is very low, even at moderate to high
temperature, and if the fluid is to be capable of transporting and re-depositing ore
minerals, it must contain salt and other anions in solution. For many years it was
very unclear how elements such as Pb or Au could be transported in hydrothermal
fluids. Experiments carried out in the 1960s to 1980s yielded solubilities that were
well below those required to form even modest-sized ore bodies. For example, the
solubility of Zn in a slightly acid solution at 100 C is about 1

10 5 g L 1 . If such
a fluid were to form an ore deposit, the amount of fluid that must pass through the
deposit is about 100,000 km 3 , a volume greater than that of the Caspian Sea! More
recent work has shown that the solubility of ore metals is greatly increased in saline
solutions by the formation metal complexes. The major natural ligands are hydrox-
ide, Cl, S and in some cases HCO 3 /CO 3 and F. Figure 4.2 shows how the concen-
tration of Zn increases by more than 5 orders of magnitude as the Cl content
increases from the low values present in rain or sea water to the higher values
present in saline fluids. In this case the high solubility results from the formation of
high-order chloride complexes; in other fluids complexes with various S species are
important. The nature of metal complexes depends on the metal-ligand chemical
affinities: so-called “hard metals” like Al, REE, Zr, U, preferentially form
complexes with ligands like OH, F, and CO 3 , whereas “soft metals” like Au, Pt
strongly prefer HS or H 2 S. Most base metals will be complexed largely with Cl.
Discussions of these issues are found in Brimhall and Crerar ( 1987 ) and Chenovoy
and Piboule ( 2007 ).
The compositions of hydrothermal fluids are listed in Table 4.1 . The source of
the Cl and S in these fluids was initially seawater, but to explain the high
concentrations it is thought the Cl and F were introduced into the fluids indirectly
by dissolution of evaporates and the sulfur by interaction with sedimentary sulfate
or with reduced species such as pyrite and other diagenetic sulfides or with
S-bearing organic materials or sour gas.