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Magnetite & Haematite
Magnetite and haematite both commonly give good decrepigrams and
decrepitation is particularly useful to investigate these opaque
Magnetite and haematite are often considered to be
homogenous materials, but the decrepitation method reveals them to
be quite variable. These variations can be used as an exploration
tool and have also provided insight into the genesis of these
Tennant Creek Au & Cu mines, NT, Australia
At the Tennant Creek mines, Au and Cu occur within massive
magnetite/haematite/chlorite hosts, there being little or no quartz
present. Intense decrepigrams are produced by both magnetite and
haematite samples shown in the following 2 figures. For comparison,
magnetite from the same BIF formation at Arltunga, some 500km away
was analysed. This sample gave no decrepitation and it is inferred
from this that the Tennant Creek deposits are hydrothermal rather
than of sedimentary origin, counter to other widely adopted genetic
models. In addition, the decrepitation of haematite suggests that
this is a primary phase and not simply a weathering product derived
from the magnetite as that would have destroyed any fluid
Similar mineralization to that at Tennant Creek occurs at the
Upper Beaver mine, Ontario with Au in massive magnetite. Magnetite
samples from 2 close mine shafts show strong decrepitation with
substantial temperature variations. Samples from a bulldozer cut
some 300m away gave no decrepitation and the results can be used to
discriminate between potentially mineralized and barren magnetite.
Lyon magnetite skarn, Nevada, USA.
The Lyon deposit in Nevada is a magnetite skarn with low grade Cu
and Au mineralization. Samples of magnetite from one of the
exploration drillholes show widely varying decrepitation. This
complexity within the magnetite is not apparent either from the core
logging or chemical analyses.
Other magnetite skarns
Other magnetite skarns at Leadville, Colorado and Afton, British
Columbia similarly show strong decrepitation in magnetite samples.
Comparison of various magnetite samples
Magnetite samples show a great range of
variability, which can be used to discriminate between different
types of magnetite and also to discern zonation within
individual magnetite bodies. Note that the carbonatite samples
show unusually low decrepitation temperatures, and the
auriferous magnetites sampled to date frequently have higher
decrepitation temperatures, with decrepitation peaks being
near 650o C.
Concentrates used in the steelmaking industry have also been
analysed and decrepitation is being evaluated as a means of quality
control of ore feed to concentrating plants.
Fresh and re-analysed magnetite
To confirm that the decrepitation sounds measured are not merely
mechanical noises, this magnetite sample was analysed twice in
succession. The second analysis (green) gave only background
counts with a few counts from inclusions which just survived the
first analysis at the 800 C cutoff temperature. This confirms that
the counts being measured are the result of an irreversible
phenomenae, such as inclusion decrepitation, rather than thermal
expansion noises - which would be present in both analyses.