Geochemistry Of Ore-forming Processes In The Yangshan Gold Belt, West Qinling, Central China

The Yangshan gold deposit belt is located in Wenxian arc structural belt, whichbelongs to the Mian-Lue suture of the West Qinling orogeny tectonically. Therefore,this gold deposit belt is believed to be an ideal region for exploring the genesis of golddeposits developed in composite orogenenic belts. The key scientific problemsinclude the source of vast quantity of gold and the mechanism for its accumulation.We conducted detailed field work on this gold ore belt, and collected samplessystematically for multi-isotope determination and further tracing the sources ofore-forming fluid and materials. Below are the main conclusions we have in thisstudy:(1) Based on ore microscopic observations, we divide the evolution of this golddeposit belt into3phases and5stages, including depositional-diagenetic phase, earlystage in ore-forming phase, main stage in ore-forming phase, late stage in ore-forming,and post ore-forming phase.(2) Analysis for C-H-O isotopic data indicates that the ore-forming fluid of thisdeposit belt mainly consists of metamorphic water mixing with minor magmatic andmeteoric water. In the early stage of the ore-forming phase, the fluid is dominated byevolved magmatic water; in the main stage, the ore-forming fluid is evolvedmetamorphic water formed via water-rock reaction between metamorphic water andthe surrounding Devonian strata during fluid migration; the ore-forming fluid for thelate stage is a mixture of metamorphic and magmatic water with some addition ofmeteoric water, while abundant meteoric water was added into the fluid in postore-forming phase, as evidenced by the migration of H-O isotopic composition towardmeteoric water line in the plot.(3) S-Pb isotopic data for pyrites show the ore-forming materials of this golddeposit belt are mainly derived from those Devonian strata and with minor frommedium-acidic dikes, but appear to have no obvious relationship with the Bikougroup. For the depositional-diagenetic phase, those pyrites yield typically sedimentarysulfur with a broad range of values. The sulfur data for the early stage of the ore-forming phase varies at2.84-3.24‰, which is indicative of magmatic source. Thesulfur isotopic values for the main stage are from7.63‰to10.93‰, which isconsistent with those of the Devonian strata (10.1‰and10.9‰). Also, the sulfurisotopic data in the late stage are similar to those of the main stage, varying at5.81~7.91‰. These two together imply those Devonian strata provide the ore-formingmaterials, which is further confirmed by Pb isotopic data from various kinds ofwall-rock and sulfide minerals at different stages of the ore-forming phase.(4) Based on integrated zircon U-Pb dating, monazite U-Th-Pb dating, and K-Ardating, it is revealed that the main metallogenetic timing is close to the maindiagenetic timing, both around185Ma.(5) The Yangshan gold deposit belt iscompared with Carlin-type and orogenicgold deposits in terms of geological conditions and sources of the ore-forming fluidand materials constrained by multiple isotope analysis, and is ascribed to be orogenicgold deposits.