POST PALEO-ISLAND ARC EPOCH TENSION ZONES AND MARINE VOLCANIC ROCK GOLD DEPOSITS OF BEISHAN AREA, GANSU PROVINCE

1. Post paleo-island arc epoch tension zones
The studied area was situated at the intersection of Tarim and Kazakhstan plates during early Paleozoic period. In late paleozoic (D₂-D₃), the south Mongolian ocean crust under-thrust form north to south, which resulted in the collision of the two plates with Siberian plate. The late Paleozoic area zone and back-arc-basins were formed, in the area, followed by a period of mid-plate tectonic activity. There were three extension-closure structure movements (O-S, C, and P), of which the third had the largest scale and strength, formed a large scale volcanic eruption and magmatic intrusion, and provided a favourite condition for gold mineralization. Four tension zones are recognized by study. They are; (a) Ganquan-Daqishan tension-depression trough; (b)Jinwozi tension-depresion zone; (c)Mazhuangshan-Nanjinshan tension-depression basin; (d) Baishan-langwashan rift zone.
2. Characteristics of marine volcanic rock gold deposits
The gold deposits of this area belong to marine volcanic rock type. The mineralization system is submarine volcanic eruption hydrothermal circulation system. For a nother type of gold deposit related to granitoid,its mineralization system is postmagmatic hydrothermal sys tem. The acid-intermediate to acid volcanic rocks and related intrusive rocks were formed in about the same time and associated spacially. Their REE composition, REE pattern and microelement characteristics are similar. Therefore, it is believed that the acid to acid-intermediate volcanic rcks and acid-intermediate granitoid in this area share the same material source and mineralization geology environment.
The marine volcanic rock gold deposits of this area occur in volcanic dome and volcanic sedimentray basin within the post island arc epoch tension zones. The ore-bearing stratum is lower Carboniferous Baishan formation. Hydrothermal alteration are mainly secondary quartzification sericitize and pyritization.