Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 52

Table 3.1 Controls on the solubility of sulfide in silicate melts
Factors that increase sulfide solubility
- Increasing temperature
- Increasing Fe content
Factors that decrease sulfide solubility
- Increasing pressure
- Increasing Si content
With increasing oxygen fugacity, the speciation of sulfur changes; at low fO 2 , it dissolves as
sulfide; at higher fO 2 it is present as sulfate and the solubility is far higher
be accelerated if the magma is contaminated with granitoid country rock, which
increases Si and decreases Fe; or it can be caused by the assimilation of sulfide- or
sulphate-bearing sediments, which directly increases the S content of the melt.
Mavrogenes and O'Neill ( 1999 ) showed that sulfide solubility varies inversely
with pressure. This means that magma that formed at high pressure deep in the
mantle is capable of dissolving less sulfide than the same magma at lower pressure
in the crust. The consequence is that most magmas from deep in the mantle are
moderately or highly undersaturated in sulfide when they intrude into the crust. For
such a magma to segregate sulfide liquid, either it must crystallize almost
completely (in which case the sulfide will be trapped between abundant crystal
and cannot accumulate to form an ore deposit) or the system must be perturbed so
that the sulfide segregates sooner.
3.4.2 Controls on the Segregation and the Tenor of Magmatic
Sulfide Liquid
The value of a sulfide ore deposit varies widely, depending on the concentrations of
ore metals in the sulfide phase. Some large accumulations of sulfide contain very low
concentrations of Ni, Cu and PGE (they consist essentially of pyrrhotite, FeS (1 x)
and pyrite, FeS 2 ) and they do not constitute an ore deposit. Other deposits contain a
high proportion of Ni-rich sulfides like pentlandite (Ni, Fe) 9 S 8 or better still millerite
or nicolite, and they form an ore body even if the amount of sulfide is low.
The ore metals Ni, Cu and the PGE are all chalcophile and have a tendency to
partition more or less strongly into the sulfide. Nickel is lithophile as well as
chalcophile and in normal ultramafic rocks it is distributed between olivine and
sulfide. Copper is moderately chalcophile (the partition coefficient KD sulf-silicate
liquid is about 100), but the PGE are enormously chalcophile, having a KD of 10 4 5 .
This means that any droplet of sulfide will extract most of the Cu and Ni, and
effectively all of the PGE, from the surrounding silicate liquid. If the sulfide
droplets can then be concentrated efficiently, for example by gravitative settling,
then an ore deposit forms.
In practice, other processes intervene. The metal contents of the magma clearly
influence composition of the ore: it is evident that magma containing little to no Ni
cannot produce a nickel deposit. Ultramafic magma has high Ni but low Cu contents