Characteristics of the Ahmadabad hematite/barite deposit, Iran – studies of mineralogy, geochemistry and fluid inclusions
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Date
2018-04
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Instytut Geologii UAM
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Abstract
The Ahmadabad hematite/barite deposit is located to the northeast of the city of Semnan, Iran. Geostructurally, this
deposit lies between the Alborz and the Central Iran zones in the Semnan Subzone. Hematite-barite mineralisation
occurs in the form of a vein along a local fault within Eocene volcanic host rocks. The Ahmadabad deposit has a simple
mineralogy, of which hematite and barite are the main constituents, followed by pyrite and Fe-oxyhydroxides such
as limonite and goethite. Based on textural relationships between the above-mentioned principal minerals, it could be
deduced that there are three hydrothermal mineralisation stages in which pyrite, hematite and barite with primary
open space filling textures formed under different hydrothermal conditions. Subsequently, in the supergene stage,
goethite and limonite minerals with secondary replacement textures formed under oxidation surficial conditions. Mi-
crothermometric studies on barite samples show that homogenisation temperatures (T H ) for primary fluid inclusions
range from 142 to 256°C with a temperature peak between 200 and 220°C. Salinities vary from 3.62 to 16.70 NaCl wt%
with two different peaks, including one of 6 to 8 NaCl wt% and another of 12 to 14 NaCl wt%. This indicates that two
different hydrothermal waters, including basinal and sea waters, could have been involved in barite mineralisation. The
geochemistry of the major and trace elements in the samples studied indicate a hydrothermal origin for hematite and
barite mineralisation. Moreover, the Fe/Mn ratio (>10) and plots of hematite samples of Ahmadabad ores on Al-Fe-Mn,
Fe-Mn-(Ni+Co+ Cu)×10, Fe-Mn-SiX 2 and MnO/TiO 2 – Fe 2 O 3 /TiO 2 diagrams indicate that hematite mineralisation in the
Ahmadabad deposit occurred under hydrothermal conditions. Furthermore, Ba and Sr enrichment, along with Pb, Zn,
Hg, Cu and Sb depletion, in the barite samples of Ahmadabad ores are indicative of a low temperature hydrothermal
origin for the deposit. A comparison of the ratios of La N /Yb N , Ce N /Yb N , Tb N /La N , Sm N /Nd N and parameters of Ce/Ce*
and La/La* anomalies of the hematite, barite, host volcanic rocks and quartz latite samples to each other elucidate two
important points: 1) the barite could have originated from volcanic host rocks, 2) the hematite could have originated
from a quartz latite lithological unit. The chondrite normalised REE patterns of samples of hematite barite, volcanic host
rocks and quartz latite imply that two different hydrothermal fluids could be proposed for hematite and barite miner-
alisation. The comparison between chondrite normalised REE patterns of Ahmadabad barite with oceanic origin barite
and low temperature hydrothermal barite shows close similarities to the low temperature hydrothermal barite deposits.
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Keywords
geochemistry, trace and rare earth elements, Semnan
Citation
Geologos, 2018, 24, 1, s.55-68
Seria
ISBN
978-83-232187-4-6
ISSN
1426-8981