Geochemical Features and Tectonic Settings of Upper Ordovician Marine Volcanic Rocks on North Margin of Qaidam

The geochemical and petrological features show that the Upper Ordovician marine volcanic rocks on north margin of Qaidam can be classified into three main kinds of petrotectonic assemblages. The three assemblages exhibit features of tectonic melange. And they indicate that there exists an ancient ocean basin on north margin of Qaidam during Late Ordovician period. At the last stage of Ordovician period, the ocean basin on north margin of Qaidam closed gradually, then Qaidam continental block subducted continually and Qilian continental block thrusted onto Qaidam continental block in the direction from north to south, formed the orogenic belt of continent to continent collision type. As a result, a three unit tectonic model is proposed. The three units are: the overlying Qilian crustal wedge, the collision tectonic melange belt on the north margin of Qaidam and the Qaidam underthrusted wedge.

Large and superlarge silver deposits associated with volcanic rocks in China

This paper deals with the geology and geochemistry of the Gacun and Laochang large-sized marine volcanic rock-type Ag deposits in the Sanjiang(Tri-River) area of southwestern China and of the continental volcanic-subvolcanic rock-type Ag deposits in the Tianshan area of Xinjiang, and in the East area,China. It is considered that the marine volcanic rock-type Ag deposits occur mainly in the second-ordered volcano-sedimentary basins developed in island-arc and rift tectonic environments.The Ag deposits show an obvious zonation,with vein-network mineralization in the lower parts and hot water sedimentary rock-hosted stratified mineralization in the upper parts. From the Earth抯 surface downwards the ore-forming elements follows the order of As(Au)→Ag,Pb,Zn→Cu. The whole rock Rb-Sr isotopic isochron age of layered orebodies in the Gacun deposit is 204±14 Ma, indicating that the main stage of mineralization is Late Triassic in age. The continental volcanic-subvolcanic (porphyry) rock-type Ag deposits were formed later than the country rocks. The ores exhibit disseminated, veinlet disseminated, network and lumped structures. In addition, this study also deals with the geochemical characteristics of the continental volcanic-subvolcanic rock-type Ag deposits and the relations between Ag deposits or silver itself and fluorite, halogen-family elements and manganese.