The effect of particle size distribution on the geochemistry of stream sediments and heavy minerals in the Kuh-Zar copper-gold mineralization, South of Damghan

The Kuh-Zar copper-gold mineralization is located in 110 km south of Damghan at Torud-Chah Shirin volcanic-plotonic belt. Stream sediments are used as useful technique in the regional geochemical exploration. Mineralogy, geochemistry and particle size of stream sediments reflecting the composition of source rocks, mechanical and chemical weathering, morphological and hydrological features of the basin, sorting, and climate, as well as several other factors. It is important to consider the influence of geochemical and mineralogical controls on particle size distribution of stream sediments. Studies of distribution of trace elements in relation to the size fraction of stream sediments generally show that several elements, including Mo, Cu, Zn, Mn, and Fe are concentrated in the finest fractions of the sediments. Therefore the majority of stream sediment surveys have been based on the collection of < 200 µm materials. The Forum of European Geological Survey standard sieve mesh is < 150 µm. However, in present study geochemical distribution of elements investigate in various size fractions of stream sediments to obtain optimum mesh size.
Material and In order to achieve the scope of this study were collected samples from stream sediments, igneous rocks, and silicic veins. The number of 11 thin, thin-polished, and polished samples was studied by optical microscope. To study the effect of particle size distribution on stream sediment geochemistry, 6 stations was selected on the base of lithological, alteration, mineralization, tectonic, and watershed criteria. Each silt sample in every site consisted of 25 sub-samples that were collected along some 30–50 m from active part of stream channel. Silt samples at the field have been screened by a sieve of 2 mm to remove coarse sand. Each sample has been screened with a series sieve from 0.063 to 2 mm (ASTM codes). The ratio of size fractions was determined by weighing of each fraction. All of size fractions were digested in HNO3+HCl (aqua regia) and then analyzed for multi-elements by Varian 735-ES ICP-OES at Zarazma laboratory in Tehran. For measuring the concentrations of Au, fire assay preparation method was employed and the final aliquote was analyzed by Perkin-Elmer 5300 AAS at Zarazma laboratory. Along with silt geochemical samples, 6 heavy mineral and 3 lithogeochemical samples are also studied. Finally, based on the results interpretations have been made.
Discussion of Results & The Kuh- Zar is one of the most important prospecting areas for copper-gold in the northeast of Iran. Geology of the area consists of intermediate to mafic lava with middle-upper Eocene age. The Oligocene granodiorite and diorite were injected into Eocene volcanic series. Intrusive rocks lead to alteration and mineralization of copper and gold. The study of particle size distribution shows that 2-0.425 mm and 0.180-0.125 mm size fractions are forming the maximum and minimum weight percent of stream sediments, respectively. Geochemical data surveying demonstrate that the Au, Ag, As, Cu, Pb, Sb, Mo, and S are considerably enriched at the 4, 5, and 6 sampling stations. Concentration of these elements in 0.425-0.180 mm, 0.180-0.125 mm, and 0.125-0.063 mm mesh size is more than any other fraction. The < 0.150 mm is not representative size fraction of sediments in this area. Gold concentrate in a distance about 700 meters from mineralized source rocks in the Kuh-Zar stream sediments, whilst optimum distance for concentration of Cu, Pb, Ag, As, Sb, and Mo is about 1200 meters. The heavy mineral technique same geochemical surveys suggest that the 0.425 to 0.063 mm size fractions are useful for prospecting of gold and associated elements in the Kuh-Zar area.