Lyon Nevada, Cu, Au, Fe prospect is near Yerrington (?). and is a skarn
type deposit. Samples were collected from a drillhole CL301 at depths
from
359 to 409 feet. Note the degree of variation over this short
distance.
Zonation of magnetite is also common at Tennant Creek, NT.
Kingston Summit, Nv (on Ca border?) is a defunct Fe(?) mine in a skarn
magnetite. Samples from this mine show even more intense decrepitation
than the Lyon samples.
These samples collected from the roadside show intense decrepitation
in magnetite, with very variable patterns. Samples 1340 and 1345 are
the
same sample, with 1345 being washed in acid to remove potentially
interfering
carbonates. In this case the 2 results are the same and so carbonate
contamination
has not been a problem.
Iron ore samples - Norway, Sweeden, Quebec and
Minnesota
Assorted iron ore samples from Sweeden, Quebec and Minnesota often show
intense decrepitation. In this plot, sample 137 is from Rana Gruber,
Norway;
147 is from Mt Wright mine, Quebec; 160 is from Kiruna, Sweeden ; 47 is
the old ore from Minnesota and 41 is minnesota taconite (current ore).
(No geological sample descriptions available - these are samples of
mill feed)
Humboldt area, Nv - Buena Vista pits
This was mined for iron ore, but is now defunct. The ore is dominantly
magnetite, with scapolite common. It has been suggested to be of
magmatic
origin but may be a replacement deposit. The temperature of such
replacement
is disputed and there is no syngenetic quartz from which to derive
fluid
inclusion temperatures. The decrepitation response in magnetite
samples
varies widely but does suggest the involvement of high temperature
fluids.
The following plots have been divided up with high intensity and low
intensity
samples in seperate plots for scaling convenience.
Note the lower temperature of the gypsum samples compared to magnetite.
American Ore pit, near Buena Vista pits at Humboldt
Good samples were hard to find here, but these 2 samples have much
lower
decrepitation and perhaps higher temperature than the nearby Buena
Vista
samples.
Non-magnetic samples from this mine show very weak decrepitation only
and
magnetic samples have no decrepitation recognizable above
background.
All samples were collected from the (extensive) old mine dumps as no
pit
access was possible.
This old iron mine is now defunct and is hosted in rhyodacite. Most
samples
from the mine itself had very low decrepitation. Samples were also
collected
from loose float material on the road access near the mine. These may
be
of alluvial origin, but these showed the best decrepitation responses
in
the area. Note that 1324 (non-magnetic) and 1325 (magnetic) are splits
from the same hand specimen and show essentially identical
decrepitation
response despite the change from magnetite to haematite.
Samples from the mine benches show low intensity decrepitation at
almost
background levels with barely recognizable peaks near 650-700 C.
It was difficult to find magnetic samples in hand specimen at this
mine. The magnetic samples collected show no significant difference in
decrepitation from the non-magnetic samples, with the exception of
sample
1336. However, even this sample was of only low decrepitation activity
(<300 counts max.) compared to the magnetite float sample from the
access
road float sample from the access road
(1325, above) or from other deposits.
On Bench 4 of the mine, 2 non-magnetic samples were collected at the
same sample site and these show marked difference from no response to
quite
intense despite their identical location.
Mountain Pass, Ca. feldspar-magnetite
from
costean
Two feldspar magnetite samples were each separated into magnetic
(magnetite)
and non-magnetic (feldspar) fractions for analysis. The magnetite gave
very intense decrepitation, with an unuaually narrow temperature range
of decrepitation. Note that a magnetite sample from a carbonatite pipe
near Kapuskasing Ont. also gave a narrow decrepitation temperature
range.)
The co-existing feldspar however, gave almost no decrepitation, and
that
decrep[itation was at much higher temperature than the magnetite. One
sample
of baryte was also analysed and it gave intense decrepitation at a
markedly
lower temperature. The genetic relation between the baryte and
feldspar-magnetite
is unclear.
The old workings occur on a single ridge, and all the FeOx samples here
are non-magnetic. Only one sample (1356) was weakly magnetic. These
samples
show lvery weak decrepitation only.
To the north, the next hill shows abundant brecciation
textures
and although there are no old workings, the samples give reasonably
intense
decrepitation over a wide temperature range. Samples 1357 and 1358 are
from the ridge to the north which shows abundant brecciation and
samples
1359 and 1360 are from the ridge to the east, where brecciation was not
observed.
One sample from this deposit was weakly magnetic and shows oly low
decrepitation
intensity
Upper Beaver, Ontario. Au in magnetite
host
These old workings are in massive magnetite which has intense
decrepitation.
Nearby barren, magnetite bearing host rocks usually have negligible
decrepitation.
Tennant Creek, NT, Australia. Au, Cu in
magnetite-haematite-chlorite
host rocks.
The Tennant creek goldfield has been proposed to be of sedimentary
origin,
with haematite secondary after primary magnetite. However, the rich ore
bearing magnetites have intense decrepitation at high temperatures and
many haematite samples also decrepitate. Consequently it is suggested
that
these deposits have a hydrothermal origin.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
