Morphological, thermoelectrical, geochemical and isotopic anatomy of auriferous pyrite from the Bagrote valley placer deposits, North Pakistan: Implications for ore genesis and gold exploration

The Bagrote valley in North Pakistan, belonging to the Kohistan island arc, is well-known for regional placer gold mining. However, no economically feasible in situ hydrothermal gold deposits have been discovered in this region due to rugged terrain and remote nature of its location in the western Himalaya. The streams draining the Main Karakoram Thrust (MKT)/Shyoke suture zone carry placer gold in sediments as well as old river terraces, although the primary source remains unknown. In this paper, we employ a multiparametric approach including, X-ray Diffraction (XRD) analysis, thermoelectricity, major and trace element geochemistry and isotopic characteristics of pyrite associated with placer gold with a view to identify the nature of the unknown deposits and ore forming fluids on the catchment of the Bagrote valley. Pyrite in the Bagrote valley placers is euhedral to subhedral indicating the proximal gold sources. The high rate of occurrence of N-type thermoelectric coefficients (89%) with low P-type (11%) and crystallization temperature (290 degrees C 380 degrees C) combined with chemical features indicate that the pyrite was derived from porphyry or epithermal type of magmatic hydrothermal gold deposits from the hinterlands of the Bagrote valley. The X-Ray elemental maps show that Fe, As, Mo and Ni are homogenously distributed from core to rim suggesting stable crystallization condition without any alteration by later fluids. The calculated chemical formula of pyrite of our samples is [Au0.0006Fe] S2.0041, plots of Au-As and Au-Fe shows that gold occurs in pyrite as micro to nano inclusion as Au-0. The delta(34) SV-CDT values of pyrite range from - 0.6 parts per thousand to 0.9 parts per thousand with an average of -0.02 parts per thousand, indicating the derivation of sulfur from a homogeneous magmatic source. The Pb isotope data indicates that the Pb was sourced from orogenic-type source, with minor contribution of lower crust. The narrow variations in Pb-206/Pb-204 and Pb-208/Pb-204 values suggest a single lead source. The low, medium and high Mo/Ni ratios reflect a mixed provenance for the auriferous pyrite. The average value of gamma (71.8%), of pyrite computed from thermoelectric parameters (XnP), suggests that the dominant part of the primary source that contributes to the placers might have already been eroded. However, the proximal source and with high content of gold in the pyrite grains (up to 1160 ppm) suggest the possibility of significant economic mineralisation below the present erosion level of the deposits in the hinterlands of Bagrote Valley.