Primary geochemical haloes and alteration zoning applied to gold exploration in the Zarshuran Carlin-type deposit, northwestern Iran

Abstract

The Zarshuran gold deposit as sediment-hosted Carlin-type has a total reserve of 120 million metric tonnes and an average grade of 3.5 g/t Au. The dominant hydrothermal alterations associated with gold mineralization are argillic and silicic as well as decalcification of the carbonate host rocks. The exploration geochemistry was applied to characterize the mineralization in the subsurface and peripheries of the open-pit mine at Zarshuran in order to enhance the proved reserves of the gold ores. A sampling network of a 10 × 10 m grid (n = 1720) was applied to almost unweathered surface outcrops to determine the primary geochemical haloes in the study area. Initially, the cut-off grades utilized in the exploratory data analysis were applied to identify the threshold values of the trace elements with lognormal distribution. Then, the central log ratio transformation was used to determine the robustness against outliers to characterize the closure problems with compositional data during Principle Component Analysis (PCA). The supra-ore and near-ore geochemical haloes are recognized by the indicator elements like As-Sb-Hg-Te-Au-Ag-Fe-Mn-Ba-F which are associated with mineral assemblages such as arsenical pyrite, orpiment, realgar, cinnabar, barite, and fluorite. The sub-ore geochemical haloes are characterized by index element association like Cu-Mo-Bi-W-Sn-Co-Ni-V-Cr. The ores at Zarshuran are enriched in Au, As, Sb, Ag, Te, Tl, Zn, Pb, and F but depleted in Cu, W, and Sn, as were observed in many other Carlin-type gold deposits. The enrichment factor of indicator elements (except S, Ag, and Sb) gradually decreases in value from the highly altered and mineralized zones (in proximity of the mine) toward the zones with least alteration and mineralization (in the distal parts of the mine). According to PCA analysis, two main types of ore mineralization occurred at Zarshuran, (1) Au mineralization associated with As, Sb, and Hg, and (2) Cu-Zn-Pb base-metal mineralization related to Ag. The ore-forming index was used to determine the primary axial haloes in both shallow (<−80 m) and deep (>−120 m) levels. The results of this study show that the main ore body may extend eastwardly and downwardly to greater depths, as was verified by the data obtained from diamond drill cores in the eastern part of the mine.