Komatiites
Fonte:
CSIRO
Komatiites
are a remarkable class of ultramafic (very magnesium-rich) lavas which are, with
very few exceptions, restricted to the first half of the earth's history. A
remarkable global outpouring of komatiites occurred around 2700 million years
ago, and komatiites of this age host a significant proportion of the world's
sulfide nickel resources.
Komatiites were exeptionally hot. The most extreme examples
probably erupted at temperatures in excess of 1600 degrees C. At this
temperature, the lavas would have been extremely fluid, with viscosities
approaching that of water. However, our research leads us to believe that they
were erupted by much the same mechanisms that govern modern basalt lava flows.
Nickel sulfide deposits in komatiites occur largely within
linear, olivine-choked lava pathways which may originally have formed as lava
tubes, within regionally extensive flow fields (see diagram below). The origin
of these deposits remains controversial, but several lines of evidence strongly
favour a hypothesis referred to variously as "ground melting", "thermal erosion"
or "substrate erosion". According to this hypothesis, komatiite lavas melted and
eroded the ground they flowed over, causing the lavas to become contaminated by
this molten substrate. Where the substrate contained high proportions of sulfur,
this caused an immiscible suflide melt to form, in the same way a molten sulfide
matte forms in a blast furnace, with the komatiite lava being analogous to the
slag. The immiscible sulfide melt scavenged Ni, Cu and platinum group metals
from the silicate melt, forming an "ore magma". Orebodies formed where this ore
magma pooled and froze at the floor of the flowing lava. The erosion process,
and the accumulation of sulfide ores, are restricted to the major flow pathways
within the lava flow lobe, as illustrated
Figure 1. Schematic diagram illustrating the genesis of
sulphide ores in komatiite lava-flows.
The Ni-Cu-PGE Group (formerly known as the Magmatic Ore
Deposits Group) has carried out an extended program of research on the
characteristics and origin of these deposits, which include some of the world's
most important Ni resources (see below). Our main lines of enquiry have been:
The volcanology of komatiites - how were they erupted, and
under what conditions could they erode their substrates? Can this knowledge be
used to guide exploration in metamorphosed and deformed terrains?
Lithogeochemical indicators - can chemical indicators of
mineralising processes be detected in komatiite suites, and if so, can they be
used to prioritise exploration targets?
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