زمين شناسي كانسنگ و مطالعات ميانباره اي سيال در محدوده طلاي ارغش، جنوب باختر نيشابور، شمال خاور ايران

Ore Geology and Fluid Inclusion Studies of Arghash Gold Prospect, Southwest Neishabour, Northeastern Iran

Arghash gold district is located in the eastern Sabzevar zone. The basement of this zone consists of Precambrian metamorphosed rocks and Paleozoic- Mesozoic epicontinental sediments. The basement is covered by Upper Cretaceous ophiolitic melange and Tertiary magmatic and sedimentary rocks. Arghash district includes five gold-bearing vein systems, Au-I-Au-V, and one Sb vein, occurring in Tertiary intermediate to silicic volcanic rocks, tuffs, granite, and diorite. Intensive alteration, dominated by clay minerals, is confined to 1 to 5 m from the veins. The intensive argillic alteration is bordered by irregular zones of moderate to weak argillic and propylitic alterations. Mineralization is mostly confined to veins. Pyrite is the main sulfide mineral and includes four generations: 1) disseminated euhedral to anhedral, fine- to coarse-grained pyrite (Py-I), locally associated with minor chalcopyrite, marcasite, tetrahedrite-tennantite, and arsenopyrite; native gold grains occur in quartz associated with the pyrite; 2) framboidal pyrite (Py-II) which contains up to 960 ppm Au; 3) arsenian pyrite overgrowths (Py-III) which contain up to 1980 ppm Au; and 4) fracture-filling, anhedral, barren, late stage pyrite (Py-IV). Gold occurs as nanoparticles as well as unstable solid solutions in the framboidal and arsenian pyrite. Homogenization temperature (Th) and salinity were measured on fluid inclusions in several quartz and calcite samples. Th varies from 186° to 357°C, and 169° to 313°C, and salinity from 0.2 to 5.3 and 0.7 to 1.9 wt. percent NaCl equiv. for quartz and calcite, respectively. The low salinity character of the ore forming fluid is consistent with the ore mineralogy and metal contents. The variation in salinity and Th could be explained by a combination of boiling and mixing (dilution) of a hotter and more saline fluid with a cooler and less saline fluid. These processes led to the deposition of gold in the veins. The ore mineralogy, textures, alteration, homogenization temperatures, and salinities, are typical of low-sulfidation epithermal precious metal deposits.