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

Abstract

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 4.3 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 0.1–120ppm, As, 3–1165ppm, Ba 9–2820ppm, Ca 0.07–25%, Cd 0.005–210ppm, Hg 0.9–24ppm, Mg 0.03–11.75%, Pb 15–15,000ppm, Zn 65–15,000ppm and Sb 1–305ppm 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×Cd)/(Pb×Zn), (Ag×As)/(Pb×Zn), (Ca×Mg)/(Pb×Zn), (Sb×As)/(Pb×Zn), (Hg×Sb)/(Pb×Zn), (Sb×Ba)/(Pb×Zn) and (Ba×Ca)/(Pb×Zn), correlate with three exploration zones at the average depth of 10, 200 and 310m above and below the main ore zone. All the composite lithogeochemical halos, in particular (Ag×Cd)/(Pb×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.