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Baro-acoustic decrepitation in Magnetite
Comparing and contrasting skarn magnetite and BIF
magnetite
Summary
Numerous magnetite samples from around the globe have been
analysed. Magnetite is often found in skarns,
which result from hot hydrothermal fluids interacting with various
host rocks, and it is expected that fluid inclusions would
commonly occur in such magnetite samples. Skarn deposits
frequently contain other valuable minerals and are exploited for
many ores including gold, copper and phosphate.
And magnetite is also commonly found in banded
iron formation (BIF) deposits. These deposits are exploited
for iron and typically lack other metals. They are formed as
sea-floor deposits of proterozoic age, when the atmosphere lacked
oxygen and iron was more mobile (as Fe++) than in
present day oceans. Subsequent upgrading of these ores by various,
often disputed, mechanisms including metamorphism result in
removal of silica to give economically useful iron deposits of
either haematite of magnetite.
Baro-acoustic decrepitation of magnetite samples shows
interesting differences between these two types of deposits and
helps to understand the genesis and subsequent upgrading processes
involved in formation of these ore systems.
Results are shown here from 7 samples collected from 2 major
european BIF deposits and from Kiruna, Sweden. These deposits are
being mined for iron.
I have included Kiruna, Sweden with the BIF deposits here, although
it is not usually considered to be a BIF and its genesis is
controversial. The ore is massive magnetite with phosphate, unlike
conventional BIF deposits. Many authors consider this to be a
magmatic deposit, either intrusive or extrusive. But it is
concordant with the volcano-sedimentary strata and an alternative
explanation is that it is a type of SEDEX (sedimentary - exhalative)
deposit formed from ferruginous fluids and deposited in a surficial
sedimentary environment. Any of these genetic models for the
formation of the Kiruna ore would result in a lack of high
temperature fluid inclusions so its decrepitation response would be
similar to BIF deposits.
At Kiruna, analyses h2602 and h2603 were of magnetite collected at
the working ore face in the mine in November 2012 at the 1165m
underground level. These 2 samples were some 5 metres apart. They
have only low decrepitation from 550 C to 750 C and both samples
give similar results as expected. Analysis h160 was collected around
1998 from magnetite at the main crusher. This has a similar
temperature range, but much grater decrepitation intensity.
Analysis h2543 from the Frunze underground mine at Krivoy Rog,
Ukraine, has similar decrepitation temperatures to the Kiruna
samples. But the other sample from Krivoy Rog, sample V3, analysis
h2591, shows very little decrepitation which was continuing to
increase even at 800 C. Sample V3 was from the Yugok open pit,
approximately 20 Km SSW of the Frunze mine sample.
The 2 analyses from the Kola peninsula, Russia,
were collected from the Olenegorsk open-pit, but had probably been
transferred there from the nearby Bauman open-pit. These both show
no decrepitation at all. Visually, these samples also had a very
high silica content. Such low decrepitation is typical of unaltered
sedimentary deposits because they lack high temperature fluid
inclusions.
The very variable amounts of decrepitation suggest that some samples
have been upgraded and fluid inclusions formed during metamorphism.
Although the Kola peninsula samples have been metamorphosed (to a
higher grade than the Krivoy Rog samples), these samples had no
decrepitation, perhaps because the inclusions were small or
contained only low density fluids.
Magnetite from BIF iron mines in north America show similar
decrepitation to the above european samples with decrepitation
mostly above 500 C. The decrepitation intensity is low to medium,
except for analysis h47 which has been divided by 2 to fit on this
graph and actually has intense decrepitation of over 4000 counts at
580 C.
Baro-acoustic decrepitation of magnetite from skarn and
carbonatite
The following samples of magnetite are from skarns or
carbonatites which are mined primarily or solely for products
other than iron, which include gold, copper, phosphate and
rare-earths. The deposits at Mountain Pass and Agrium are
carbonatites and the other deposits are ferruginous skarns in all
of which the involvement of hydrothermal fluids is suspected,
though not always obvious.
It is notable that these samples often show lower temperature
decrepitation, even as low as 250 C at the Agrium carbonatite.
They also tend to have more intense decrepitation than the BIF
magnetite samples. Skarns are expected to be deposited from or
strongly affected by hydrothermal fluids and this is evident from
the great diversity and intensity of the decrepitation.
To better show the difference in decrepitation intensity between
skarn and BIF magnetite, the BIF results have been replotted below
at the same y-axis scale as used in the skarn plot. The european
BIF samples show obviously less decrepitation than skarn
magnetites.
The north American BIF deposits also have mostly lower
decrepitation than skarns, with the exception of h47, which is
halved in this plot.
Comparison with surficial laterite magnetite
For comparison with hydrothermal and BIF magnetite, samples of
surficial laterite nodules from Darwin were also analysed. The
nodules were hand selected highly ferruginous, rounded pisolites
about 5-10 mm across.
The crushed samples were separated into a magnetic and non-magnetic
fractions comprised of iron oxides, with rare quartz grains in the
non-magnetic fraction. The magnetic fraction gave no decrepitation
at all and the non-magnetic fraction has very minor decrepitation
caused by traces of quartz and other silicates.
There would be no high temperature fluid inclusions in this
magnetite formed by supergene weathering processes and decrepitation
would not be expected. The observed absence of decrepitation shows
that surficial magnetites and iron oxides do not give a
baro-acoustic decrepitation response, in contrast to hydrothermal
and skarn derived magnetites.
Skarn and carbonatite magnetite samples give intense and often
complex decrepitation, reflecting the complexity and variability of
the hydrothermal fluids from which they formed. In contrast, BIF
magnetites which lack metals other than iron, typically show much
lower decrepitation, and also less complex patterns, suggestive of
regional metamorphic fluid events. While supergene lateritic
magnetite formed from simple fluids at only surface temperatures,
gives no decrepitation at all, confirming that the decrepitation
results are not merely caused by mechanical or crystallographic
effects of magnetite.
The decrepitation of magnetite helps to discriminate between
sedimentary, metamorphic and hydrothermal origins. And within
hydrothermal skarn systems, the detail in the decrepitation data
could assist in targeting economically interesting zones.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
