Composite soil-geochemical halos delineating carbonate-hosted zinc–lead–barium mineralization in the Irankuh district, Isfahan, west-central Iran

• The response to blind ores is detected by B horizon sampling
• Multi-element anomalies display a NW–SE trend fault zones.
• Ore halos are explored by using Ag*Cd/Pb*Zn and Ba*Ca/Pb*Zn indexes.
• Residual soil anomalies detect concealed fault-controlled Zn–Pb ores.

The Irankuh district in west-central Iran hosts several largely unexposed Lower Cretaceous carbonate-hosted Zn–Pb deposits whose host rocks are overlain by soil cover. This paper documents a detailed soil-geochemical exploration program, comprising 804 residual soil samples that were treated with a microwave acid digestion and analyzed by ICP-MS for a multi-element package. The soils overlying the Zn–Pb–Ba veins and mineralized zones have geochemical contrast and element enrichments that are more than two to twenty-one times greater than the non-mineralized background soil values. The most diagnostic elements are Zn, Pb, Ag, Ba, Hg and Sb which show multi-point anomalies across the entire mineralized zone with concentration ranges of 282–10,000 ppm Zn, 73–10,000 ppm Pb, 2.2–71.9 ppm Ag, 1500–3400 ppm Ba, 0.89–36 ppm Hg and 11–164 ppm Sb, respectively. The newly proposed composite geochemical halos defined by Ag*Cd/Pb*Zn, Ag*As/Pb*Zn, Ca*Mg/Pb*Zn, Sb*As/Pb*Zn, Hg*Sb/Pb*Zn, Sb*Ba/Pb*Zn, Ba*Ca/Pb*Zn and Ag*As* Sb/Pb*Zn*Bi correlate well with the anomalous patterns of the individual elements. The anomalies are coincident with the main faults and fractures, in particular Gushfil and Rumarmar faults of the area. On the basis of the low contents of Cu and Fe and the variodiagram of log (Pb + Zn) versus Pb/Pb + Zn, the composition of the samples suggests epigenetic stratabound Zn–Pb–Ba mineralization of the Mississippi-Valley-Type (MVT). The mode of occurrences of the anomalies is likely controlled by structural replacement in sphalerite, galena, carbonates and barite or adsorption by iron–manganese oxides and clay minerals. Follow-up sampling of gossans in highly faulted-fractured zones may indicate more concealed Zn–Pb mineralization.