Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 60

Main Body

Weak sulfide
mineralization

Leucogabbro and upper
taxitic gabbrodolerite

Residual Series
rocks

Peripheral Sills

Upper
Copper Ore

Olivine-bearing
gabbrodolerite

Sparse sulfides
in Olivine
gabbrodolerite

Olivine gabbrodolerite

Contact and Lower
olivine gabbrodolerite

Picritic gabbrodolerite

Disseminated Ore

Stringer Ore in contact
gabbrodolerite

Lower Taxitic gabbrodolerite

Massive Ore

Copper Ore

Fig. 3.13 Cross section through an ore-bearing sill

form; the Ni-rich, Cu-poor ultramafic magmas from Kambalda produced ores with
Ni/Cu ratios of about 10 whereas the basaltic magmas at Norilsk, which have lower
Ni and higher Cu, produced ores with ratios closer to 2.
In a broad sense the origin of the ores is like that at Kambalda. An immiscible
sulfide liquid segregated from the silicate liquid and the dense droplets settled to the
base of the intrusions. But what caused the sulfide to segregate from magma of
basaltic composition? In addition, the Siberian large igneous province is only one of
many such provinces, and despite the best efforts of mineral exploration companies
who have actively explored the others, it is the only one known to contain a large
magmatic ore deposit. Why is this?
Part of the explanation lies in the enormous volume of erupted lava, and the high
magma fluxes involved in their emplacement. Recent age dating has shown that the
vast majority of the lava pile was emplaced in a geologically short time period, most
probably less than one million years, with the implication that an enormous volume
of hot mantle melted rapidly, and that large amounts of hot magma flowed rapidly
through the crust to the surface. Such circumstances favour crustal interaction, and
indeed abundant geochemical data provide convincing evidence that many of the
Siberian flood basalts have assimilated large amounts of continental crust. Yet close
inspection of the data shows that the key to the ore forming process is not the
assimilation of normal granitoid crust but a process that took place at shallower
levels. The critical evidence is shown in Figs. 3.12 and 3.15 .
The first diagram shows the sedimentary sequence that underlies the flood
basalts and is invaded by the ore-bearing intrusions. The uppermost formation
consists of Permian terrigenous sediments, the lower formations of Silurian to
Devonian carbonates, marls, and evaporates (Fig. 3.14 ). It is commonly believed
(e.g. Arndt et al . 2003 ;Lietal . 2009 ; Naldrett 1992 ) that these rocks played a