Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 51

Komatiite
Picrite
Basalt
% partial melting
Fig. 3.7 Controls on the chalcophile element contents of mantle-derived melts
and 20% for basalts, 20-30% for picrites and up to 60% for komatiites, and the
composition of the magma depends strongly on the types of minerals that remain in
the residue. The overall character of the melt is controlled by the silicate phases in
the residue of melting while the contents of ore metals like Ni, Cu and the PGE
depend on the sulfide. In normal mantle peridotite, sulfur is present as sulfide,
which enters to liquid at the start of the melting process and is totally exhausted
when the degree of melting exceeds 20-30%. As shown in Fig. 3.7 , sulfide is
retained in the residue during the formation of low-degree melt like basalt but is
exhausted in high-degree melt like komatiite. Nickel, Cu and the PGE are all
strongly chalcophile, which means that when sulfide is retained in the residue, it
holds back these elements: the resultant magmas contain only low concentrations.
High-degree melts, on the other hand, acquire their full component of these metals,
and partly for this reason are the most prone to form ore deposits.
When the magma enter the crust and starts crystallize, an immiscible sulfide
liquid will separate from the silicate liquid if the concentration of sulfur exceeds the
sulfide solubility. The situation can be compared with crystallization of salt from
brine. Only if concentration of salt is high enough will the brine become saturated
and salt crystallize. But saturation can also be reached if the brine evaporates, which
decreases the amount of water and increases the salt concentration in the remaining
brine; or the solubility of salt can be decreased by changing the temperature or
pressure, or by adding to the solution other components that decrease the solubility.
The same principles apply to the separation of a sulfide liquid from a silicate liquid.
Experimental studies have shown that the solubility of sulfide depends on external
parameters such as temperature and pressure, and on the composition of the melt.
Table 3.1 summarizes these factors.
During fractional crystallization of magma, the temperature drops, the Fe con-
tent usually varies little and the Si content increases. The process will therefore lead
eventually to sulfide saturation and the separation of sulfide liquid. The process can