Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 63

Fig. 3.16 The ore-forming process at Norilsk-Talnakh
associated with a meteorite impact. Most Sudbury ores occur as massive layers and
pods in depressions in the lower contact of the Sudbury Irruptive Complex, a
differentiated intrusion interpreted as the sheet of molten crustal rock that formed
by total melting of the rocks at the site of impact (Fig. 3.17 ). Other deposits occur in
“offshoots” the name given to vein-like intrusions extending outwards from the
margin of the irruptive.
The impact that generated the Sudbury irruptive and its ore deposits was located
at the contact between two crustal provinces, one an Archean granite-greenstone
terrain, the other consisting mainly of sedimentary rocks. The melt sheet
incorporated material from both, including a small fraction of sulfide from mafic
intrusions that are inferred to have been present at the site of impact. The molten
mafic material, being dense, accumulated in the lower part of the melt sheet,
beneath an upper layer of molten felsic rock. The entire melt sheet was extremely
hot (it was probably several hundred degrees above its liquidus) and had particu-
larly low viscosity. Sulfide droplets that segregated from the molten rock were
therefore able to settle efficiently to the base of the intrusion or were injected along
fractures into the enclosing rocks. A more complete description of the Sudbury
deposits is found in Naldrett ( 2004 , Chap. 8).
Two other Ni-Cu sulfide deposits deserve mention, Voisey's Bay in Newfound-
land and Jinchuan in China. The Voisey's Bay deposit is unusual in that it is hosted
in mafic intrusions that form part of an anorogenic suite that includes troctolites and
anorthosites (Li et al . 2000 ), but the ore formed in a manner broadly similar to that a
Norilsk-Talnakh. As magma flowed up through a complex series of intrusions, it