Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 59

province. It is not entirely coincidental that one of the largest continental flood basalts
hosts one to the largest ore deposits; but, as we shall see, this is not the whole story.
Figure 3.11b , a geological map of the region, shows the vast extent of the flood
volcanic province - it covers an area similar to that of Western Europe. The
deposits are located in the northern part of the province, at a place where later
deformation has brought to the surface the base of the lava pile and the sedimentary
rocks onto which they erupted. (Without this deformation the deposits would have
remained several kilometres below the surface, hidden from prospectors and prob-
ably unmineable). The sedimentary sequence is invaded by a vast complex series of
sills, as shown in Fig. 3.12 , and these sills host the ore deposits. More detailed
descriptions of the geology of the Norilsk regions and its ore deposits are provided
by Naldrett ( 2004 ) and Czamanske et al. ( 1995 ).
A schematic section through an ore-bearing sill (Fig. 3.13 ) illustrates its complex
geometry and lithology. The ore deposits are found in thicker-than-normal parts of
the sill, and these segments are crudely differentiated, from olivine-enriched
“picritic” lower portions to leucogabbroic upper portions. The ore occurs as
remarkable, metre-thick layers of massive sulfide at the base of the intrusion, as
disseminated sulfide in the interior, and as veins and lenses throughout the intrusion
and penetrating into the wall rocks.
The mineralogy of the ores is similar to that at Kambalda, but they have higher
Cu contents. This is related to the compositions of the magmas from which they
Fig. 3.12 Cross section through volcanic pile and underlying sill complex