Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran |
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| Authors: | H H Asadi J H L Voncken R A Kühnel M Hale |
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| Institution: | (1) Ministry of Culture and Higher Education, Dr. Beheshti Avenue, Shahid Sabounchi Cross, Tehran, Iran, IR;(2) International Institute for Aerospace Survey and Earth Sciences, Kanaalweg 3, 2628 EB Delft, Netherlands, NL;(3) Faculty of Civil Engineering and Geosciences, Department of Applied Earth Sciences, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, The Netherlands e-mail: j.h.l.voncken@ta.tudelft.nl, NL |
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| Abstract: | Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which
have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving
conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with
decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage
of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical
pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite,
and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite
and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe
and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized
native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich
shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many
ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively
high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic
contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.
Received: 13 May 1999 / Accepted: 2 February 2000 |
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