Vertical lithogeochemical halos and zoning vectors at Goushfil Zn-Pb deposit, Irankuh district, southwestern Isfahan, Iran: Implications for concealed ore exploration and genetic models

摘要

The vertical lithogeochemical halos of indicator elements in host rocks over a blind Zn-Pb deposit are developed by upward migration of geochemical halos. This may prove to be a useful exploration guide for targeting undiscovered Zn-Pb mineral systems. The main objective of this investigation is to assess the vertical distribution patterns of the composite lithogeochemical halos for detecting possible exploration guides as well as a possible genetic model of Zn-Pb sulfide mineralization. The Goushfil Zn-Pb deposit is one of the most important Lower Cretaceous carbonate and shale-hosted deposit in the Irankuh district, west-central Iran. The deposit contains about 43 million tonnes of sphalerite-galena-barite ore with an average grade of 8% Zn and 2.4% Pb. In order to examine the exploration significance of the vertical lithogeochemical halos, about 274 rock samples from 26 boreholes were sampled and analyzed by ICP-MS for a multi-element package. The results demonstrate that the highly anomalous values for Ag 0.1-120 ppm, As, 3-1165 ppm, Ba 9-2820 ppm, Ca 0.07-25%, Cd 0.005-210 ppm, Hg 0.9-24 ppm, Mg 0.03-11.75%, Pb 15-15,000 ppm, Zn 65-15,000 ppm and Sb 1-305 ppm are developed in faulted-or fracture-controlled hydrothermally dolomitized and jasperoid carbonates and shales. The newly proposed multiphase composite geochemical halos defined by ratios of (Ag x Cd)/(Pb x Zn), (Ag x As)/(Pb x Zn), (Ca x Mg)/(Pb x Zn), (Sb x As)/(Pb x Zn), (Hg x Sb)/(Pb x Zn), (Sb x Ba)/(Pb x Zn) and (Ba x Ca)/(Pb x Zn), correlate with three exploration zones at the average depth of 10,200 and 310 m above and below the main ore zone. All the composite lithogeochemical halos, in particular (Ag x Cd)/(Pb x Zn) in both rock and soil samples tend to increase toward the shallow depths, but decrease as they approach the main Zn-Pb ore toward the deeper parts of the ore zone. This geochemical response of the composite halos could be used for targeting undiscovered Zn-Pb sulfide mineralization. The low contents of Cu and Fe in the mineralized rocks indicate a low modal content of chalcopyrite and pyrite in the precursor sulfide ore, which correlate with those of the Mississippi Valley Type (MVT) Zn-Pb deposits. This is also highlighted on the log (Pb + Zn)-Pb/Pb + Zn, Cu + Pb + Zn-Cu and homogenization temperature-salinity variodiagrams, on which the Goushfil data plot close to the MVT-type Zn-Pb mineralization. (C) 2015 Elsevier B.V. All rights reserved.