| Yangshan gold deposit in Wenxian County, Gansu, is located in Chuan-Shan-GanGolden Triangle region, around the subzone of the West Qinling orogen, which wasenclosed tectonically between the North China block in the north, Songpan-Ganzi foldsystem in the west, and the Yangtze platform in the south. The gold deposit wasinitially discovered in1997by the12thBranch of Armed Gold Police. By the end of2008,96ore veins has been identified, over308tons of metal reserves werecontrolled, and the prospective scale of gold may be beyond more than500tons.Gold ore-body is mainly controlled by secondary thrust faults such asAnchanghe-Guanyiba fault locally. Magmatic dikes from tens centimeters to severalmeters occur commonly in ore district, which implies the genesis of gold deposit isclosely associated with magmatic activities. The host rocks are mainly the MiddleDevonian Sanhekou Formation, which is composed of carbonaceous phyllite, siltysandstone and carbonates. And invisible gold was hosted in disseminated fine-grainedarsenian pyrite and arsenopyrite with minor native gold in quartz veins. In spite ofowning many similar characteristics with the typical Carlin-type gold deposit inNevada, US, Yangshan gold deposit displays the features between Carlin-type andorganic gold deposit. Therefore, some scholars regarded it as a unique style ofYangshan-type.Based on field geological survey and sampling this paper analyzed traceelements of bulk ore rocks by inductively coupled plasma mass （ICP-MS）, thecomponents and contents of organic matters in ore rocks by near-infraredspectroscopy and chromatography. Carbon isotope of carbonate in ore rocks and traceelement geochemistry were also deternined. Additionally, the relationship betweenorganic matter contents and gold degree was surveyed. The results showed that thegold contents in unoxidized carbonanceou phyllite were very low, from3.4to45ng/g. In contrast, the higher degree occured in these highly oxidized ore samples. The totalorganic carbon （TOC） content of bulk sample is not positively related to the golddegree, indicates that the organic matters are not gold carrier. The contents ofdissolved organic carbon （chloroform bitumen A） in ore rock extracted directly bychloroform range from35.65×10-6to79.85×10-6with the average value of54.48×10-6. The dissolved organic carbon content is very low, which indicates that insolubleorganic carbon such as kerogen, also known as dead carbon is dominant The Carbonisotope composition of carbonate （4-5‰v-PDB） likely imply a magmatic origin ofore fliud. Alternative interpretation is the host rock may have been subjected tointensitive thermal metamorphism. The high correlation （r=0.910） between arsenicand gold, shows Arsenic-bearing minerals （such as arsenopyrite, and arsenian pyrite）is the main Au-carrying minerals of Yangshan gold deposit. The S/C ratio and rareearth elements geochemical characteristics of bulk-rock indicate that the ore-formingmaterials of Yangshan gold deposit are significantly derive from sedimentary rockssequence of Sanhekou Formation.Gold adsorption experiment showed that carbonaceous matters extracted fromore rocks still have strong adsorption ability for gold. From which we can deducedthat, during early stage of the sedimentary formation, carbonaceous materials canpre-concentrate gold from sea water to form gold-enriched carbonaceous host rock.Gold desorption behavor from the carbonaceous surface confirmed that S2-has thepowerful capacity to capture gold from the carbonaceous matters surface. The resultsof this work and previous studies indicated that the ore fluids were mainly themagmatic origin.Such summary remarks can be drawed as following: firstly, carbonaceousmaterials may preconcentrate gold from sedimentary rocks during early stage ofdiagenesis, and formed gold-riched source rocks （or host rock）. Secondly, gold wasextracted from carbon-riched sedimentary rocks during late diagenesis and subsequentmetamorphism, and ultimately achieve a necessary part of the further enrichment ofgold mineralization.As a result, we proposed a ore formation model on Yangshan gold deposit.Initially, carbon-riched muds and silty sands preconcentrated gold from euxinicmarine water. Carbonaceous matters enriched gold by complexation, adsorption andother physical and chemical processes. And then, during the early diagenesis, the ofcarbon-riched gold-bearing source rocks formed. Finally, gold was extrated andliberated from the carbon-bearing host by hydrothermal fluids triggered by tectonic movement and magmatic activity. Ore-fluid ascending along the fault systemdeposited gold in the open sections, for example, the axis of anticline, due to thephysical and chemical conditions （such as temperature, pressure, pH, fugacity, etc.）changed. |
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