The exploration of gold by magnetic method in Hired Area, South Khorasan, a case study



Hired is a large gold prospecting area which is located in South Khorasan province, in the east of Iran. Gold mineralization is found in 4 target areas covering about 24 km2. The host rocks are mainly Tertiary volcanics, some Jurassic and Cretaceous sediments. Sub-volcanic rocks are of magnetite series (gabbros to diorite) and ilmenite series (granite-granodiorite-monzonite) which have intruded the Tertiary and older rocks. Important styles of mineralization are: stockwork, skarn, vein, and replacement. Stockwork gold mineralization has only pyrrhotite and is found within or around the granite-granodiorite-monzonite. Detail core logging was carried out in 4 target areas with respect to gold content, mineral paragenesis, types of veinlets, rock types, and measuring the magnetic susceptibility. Within the stockwork mineralization at east of target 1, there is good correlation between gold grade, amount of pyrrhotite, and magnetic susceptibility. At Hired, sub-volcanic rocks of the ilmenite series (granite-granodiorite-monzonite) have susceptibility less than 40 × 10-5 SI. The magnetite series (gabbros to diorite) have susceptibility of 150 – 900 × 10-5 SI. Stockwork gold mineralization has susceptibility of 200-3500 × 10-5 SI. Therefore the host rocks, granite-granodiorite-monzonite, have low magnetic susceptibility. Based on this sharp magnetic contrast ground magnetic was selected as a suitable geophysical method. The aim was to use the magnetic method for drill target identification of gold ore east of target 1. The total magnetic field intensity (TMI) was measured in 780 points along 25 lines. Measurement spacing was 2 meter. magnetic lines, contour maps, and images revealed large anomalies at east of target 1 representing the magnetic responses of gold ore. The depth of the anomalies is estimated to be shallow as indicated by their expression on the first vertical derivative map. This prediction is consistent with mineralogical observation on the surface and the continued map. The locations of the anomalies on the Reduced To the Pole (RTP) map are proposed as suitable points for drilling target.