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 fades Cretaceous volcano-sedimentary rocks, including mafic to intermediate metavolcanic rocks, sericite and chlorite schist, and marble. Geochemical and petrochemical data including the Sigma REE, (La/Yb)(N) 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. Quartz-sulfide veins at the deposit are characterized by a pyrite-pyrrhotite-chalcopyrite-sphalerite-arsenopyritenative 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. The geochemical data indicate that the concentrations of Al2O3, P2O5, TiO2, Y, and Zr are relatively constant, suggesting that these elements were the least mobile during hydrothermal activity. Using Al2O3 as the immobile component, there is evidence for mobility of trace elements, particularly light REE, TiO2, and Zr in the altered wall rocks. The altered rocks show a relatively light REE depletion ((La/Yb)(N) congruent to 9.41), which clearly correlates with the grades of gold mineralization and intensity of the alteration (3 ppm Au). The depletion of light REE is best indicated by a decrease in (La/Yb)(N) 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. 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 CO2-rich near-neutral reduced fluids with high values of H2S, 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., Sigma REE content and (La/Yb)(N) 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. (C) 2014 Elsevier B.V. All rights reserved.
