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Sangan Iron skarn, Iran
Fatemeh Sepidbar & Kingsley
Burlinson
Fatemeh Sepidbar collected samples from and studied the Sangan
Skarn deposits in Iran. They are mined for Iron with ore reserves
of >1000 Mt at 53% Fe. Decrepitation analyses were carried out
on a few samples to assist with a genetic understanding of these
deposits. Because the deposits lack transparent minerals, the
samples used were of opaque minerals including magnetite,
haematite and garnet upon which conventional fluid inclusion
microscopy methods could not be used.
A full paper describing this study (Sepidbar, F., et al., Mineral
geochemistry of the Sangan skarn deposit, NE Iran: Implication for
the evolution of hydrothermal fluid.) was published in Chemie
der Erde (2017) and is also present here as a pdf (6 Mbyte).
Samples were collected from the A, B and C orebodies as well as
some unspecified locations within the associated volcanic strata.
Map of Iran and geological map of the Sangan region including
location of the Sangan deposit and the Eocene plutonic and volcanic
rocks based on 1:250,000 geological maps of Taybad (Alavi
Naini, 1982 ). Locations of samples from the A, B, and C orebodies
used in this decrepitation study are shown. SMC = Sangan magmatic
complex, SDMB = Sabzevar-Dorouneh magmatic belt.
The intention was to decrepitate magnetite samples to give an
indication of the formation temperature of the magnetite and skarn.
However it was difficult to obtain clean mono-mineralic samples of
magnetite, despite mineral separation operations. Baryte and garnet
were commonly present and occasionally dominant. At orebody A it was
necessary to rely on the decrepitation of garnet as there was
insufficient magnetite in the sample provided.
Estimates of the formation temperatures of 350 C to 450 C were based
on the onset temperatures of the A, B and C deposit samples in the
following diagram.
Ore zone Magnetite samples
Clean magnetite samples were only available from 2 of the ore
bodies, B and C. These two samples are very different. Sample B4
shows only very weak decrepitation which is unlike most skarn
deposits which frequently show intense decrepitation of magnetite.
Sample C4 shows intense decrepitation, but only above 700 C, which
is also unusual for most magnetite samples from other deposits. Note
that the decrepitation counts for sample C4 has been divided by 20
for comparison on this graph and its maximum intensity is 3500
counts at 790 C.
Comparison of magnetite and haematite samples
The only haematite samples available were collected for unspecified
locations within the nearby volcanic and pyroclastic rocks. Both
samples begin to decrepitate at 400 C and have the same formation
temperature however their genetic relationship is unknown. The
decrepitation in haematite is quite weak with a maximum of less than
40 counts.
Haematite samples from the volcanics / pyroclastics
Both of the available haematite samples show a similar decrepitation
response, with only low intensity decrepitation and a possible
formation temperature of about 400 C.
Many skarn magnetite deposits show intense magnetite decrepitation,
seen here in sample QX06 (blue curve) which is from the Qiaoxiahala
Au-Cu skarn deposit in the altay region of China. (Detailed discussion
here) Both of the Sangan magnetite samples (red curve)
lack intense decrepitation between 500 and 700 C seen on many but
not all other skarn magnetite samples. This may be due to formation
from relatively vapour-rich fluid system at a high crustal level.
Baryte (Barite)
The baryte samples from Sangan (red curve) are similar to baryte
from other deposits with intense decrepitation from 300 to 500
C. Carbonate samples also show similar intense decrepitation, which
may be related to fluids trapped on the well developed cleavage
planes in these minerals and not actually indicative of the mineral
formation temperature.
Garnet
Garnet samples show a wide range of decrepitation responses. The
Sangan sample A1 (red curve) is similar to sample MK43 (orange
curve) from the
Mengku Fe deposit in China which is a similar
stratabound but probably hydrothermally upgraded Fe system.
Summary
Despite the less than ideal samples studied at Sangan, the
decrepitation data indicates that the deposits probably formed from
a vapour-rich fluid system at 350 to 400 C. The deposits at Sangan
show similarities with the Mengku region
deposits in the Altay region of China where stratabound
deposits are also garnet bearing and seem to have been influenced by
hydrothermal fluids during or post formation.
Other discussions of magnetite bearing systems are here:
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
