Ebrahim Rastad
Richard J. Goldfarb
Jafar Abdollah Sharif
Farhang Aliyari
2014
<p id="sp0005"><span>The Qolqoleh orogenic gold deposit in the northern part of the Sanandaj–Sirjan metamorphic belt in northwestern Iran is hosted by a steeply dipping sequence of greenschist facies Cretaceous volcano–sedimentary rocks, including mafic to intermediate metavolcanic rocks, sericite and chlorite schist, and marble. Geochemical and petrochemical data including the ∑</span> <span>REE, (La/Yb)</span><sub>N</sub><span> and Eu/Eu* ratios were obtained from country rocks, ore-enveloping alteration zones, and mineralized zones to assess the nature of the trace element and rare earth element (REE) interaction between the wall rock and the mineralizing fluid.</span></p><p id="sp0010">Quartz–sulfide veins at the deposit are characterized by a pyrite–pyrrhotite–chalcopyrite–sphalerite–arsenopyrite–native gold assemblage. Alteration halos border the mineralized zones and broadly comprise: (1) an outer carbonate–chlorite alteration zone in all rock types, particularly in chlorite schist; (2) a middle sericite–carbonate alteration zone in the sericite schist; and (3) an inner quartz–sulfide alteration zone in sericite schist and mafic to intermediate metavolcanic rocks.</p><p id="sp0015">The geochemical data indicate that the concentrations of Al<sub>2</sub>O<sub>3</sub>, P<sub>2</sub>O<sub>5</sub>, TiO<sub>2</sub><span>, Y, and Zr are relatively constant, suggesting that these elements were the least mobile during hydrothermal activity. Using Al</span><sub>2</sub>O<sub>3</sub><span> </span>as the immobile component, there is evidence for mobility of trace elements, particularly light REE, TiO<sub>2</sub>, and Zr in the altered wall rocks. The altered rocks show a relatively light REE depletion ((La/Yb)<sub>N</sub> ≅ <span>9.41), which clearly correlates with the grades of gold mineralization and intensity of the alteration (3</span> ppm Au). The depletion of light REE is best indicated by a decrease in (La/Yb)<sub>N</sub><span> as shown by ratios of 10.5 to 11.8. Wall rock decarbonation reactions during infiltration of the mineralizing fluid resulted in differential mobilization of REE, from a fluid with initially low REE content.</span></p><p id="sp0020"><span>The overall trace element geochemistry of the altered wall rock is controlled by the initial composition of the wall rocks and the ore-fluid composition. Hydrothermal ore-forming fluids are recognized as CO</span><sub>2</sub>-rich near-neutral reduced fluids with high values of H<sub>2</sub><span>S, K, and S content. Observed variability in alteration halos at the Qolqoleh deposit points to major differences in REE and trace element content in original host rocks that have interacted with a relatively similar ore fluid. Therefore, depending on the composition of each host rock lithology, the geochemistry of hydrothermal alteration (e.g., ∑</span> REE content and (La/Yb)<sub>N</sub><span> ratios) and alteration mineralogy including the carbonate–sericite–quartz–sulfide assemblages may be used as a primary tool for lithogeochemical exploration for gold deposits in northwestern Iran.</span></p>
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10.1016/j.gexplo.2014.01.007
en
Elsevier
Geochemistry of hydrothermal alteration at the Qolqoleh gold deposit, northern Sanandaj–Sirjan metamorphic belt, northwestern Iran: Vectors to high-grade ore bodies
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