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.
Descriptions of the Sangan samples are here.
Separated minerals from the Sangan area were also compared with other mineral deposits.
The comparison deposit of Mengku
is a stratabound iron deposit in the Altay area of China,
discussed here, and the Gejui deposit
is a very large tin skarn / greisen in the Yunnan province,
South China, discussed here.
The Mountain Pass deposit in California, USA is a carbonatite and has been a major rare-earth element producer. The Samples from Nevada are from various small iron deposits which have had very limited production.
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.
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 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.
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:
A comparison of many worldwide magnetite samples from BIF and skarn is here
Additional discussion of decrepitation in FeOxides at Mengku, China is here
A discussion of decrepitation in haematite and magnetite is here
Magnetite from the Bergslagen region, southern Sweden