The Mineralization And Metallogenic Process Of Orogenic Gold Deposits, Western Qinling Region

As the main part of the Qinling orogenic belt which has been undergone all tectonicmovements after Archeozoic, the Western Qinling region has a complex tectonicevolution history and are abundance of noble metal, non-ferrous metal and ferrousmetal mineral resources.Tens of gold deposits were found in the west Qinling region, classified into allkinds of metallogenic belts, minerlization series and ore deposit types on the basis oftectonic units, mineral assemblages,ore-beating formation, regional characteristics.Their distribution can be grouped under two types, the orogenic type and the Carlintype, according to their characteristics and the geological setting. The orogenic typegold deposits are distributed in the middle of the west Qinling region, while the Carlintype are distributed in the south of the west Qinling region and the northwest ofSongpan-Ganzi orogenic belt.The study area is located in the middle of the west Qinling region, bounded on thesouth by Lixian-Shanyang fault, and on the north by Shang-Dan fault, fromShaliangzi in Zhouzhi county of Shanxi province on the east to Lixian and Minxiancounty of Gansu province in the west. Many gold deposits, such as Maanqiao,Shuangwang, Baguamiao, Pangjiahe, Liba, Luoba, Jinshan, Maquan, Anjiacha,Mingzhushan, Zaorendao and so on, as well as tens of lead-zinc deposits are locatedin this area. These gold deposits are typical orogenic gold deposits, with similargeological and geochemical characteristics that associated with the orogenesisspatial-temporally. Gold metallogenesis in the level of ore belt, ore deposit andorebody are controlled by multi-order tectonic system resulted from orogenesis. Thedistribution of the gold deposits are controlled by the deep-large regional faults, andthe orebodys occur in the derived fault systems. Most deposits are controlled by largeshear zones and the shear folds and shear fractures. The Indonisian and Yanshanianmagmas in the west Qinling region are the major ore-control factors, not only haveclosely relationship to the orogenic gold deposits spatial-temporally, but may havebeen the source of the heat, the fluids, and even the materials of the deposits.The ores are mainly fracture zone altered-rock type and quartz-vein type. Theformer are pyrite-phyllic alteration tectonites, there are no obvious boundary betweenthe ores and the country rocks. The later type deposits consist of gold and silverbearing veins, contacting the country rocks with distinct boundary. All the depositshave the same associations of ore-forming elements, auriferous minerals andalterations. The monocrastal minerals of gold are native gold, calaverite and electrum.Besides the native gold, pyrites and pyrrhotites are main auriferous minerals, insidewhich the aurum are in the form of fracture aurum, intercrastalline aurum and auruminclusions.The abundance of gold in the stratohorizon where the mineraliztion occurs vary from different orefields, and is generally once to twice higher than the gold abundancein the upper crust, which indicates that the ore-forming matters in these deposits havea diversified source.The stable isotopic ratio of sulfur, carbon, hydrogen and oxygen from severaltypical gold deposits in the west Qinling region shows that the sulfur and the carboncame from the lower crustal layer or from the mantle. The ore-forming fluids aremainly magma origin, mingled with meteoric and metamorphic water to some degree.The total values of rare earth elements in the gold deposits vary greatly, rich in lightREE, with Eu depletion. The characteristics of REE in the ores is different from thosein country rocks and magmas, the hydrothermal fluids shows the features of mantleorigin.The contents of trace elements in the deposits are higher than the Clarke values,especially the value of Au, Ag, S, Pb, As, Bi, B, Hg and Se. The assemblage of theseelements is relatively complex, which may be related to long term activity of thehydrothermal fluids. The contents of trace elements in the ores are different fromthese in the country rocks. The country rocks contain more sedimentary components,while the ores contain more eruption components, which imply the metals in the oresare not only come from the old metamorphic basement rocks, but also from themagma. The metal sulfides have both sedimentary and hydrothermal characteristics.This suggests that multi-phase and multiple geologic processes were happened. Someore-forming matters have sedimentary origin, magma origin or metamorphic origin.PGE were found in Baguamiao and Shuangwang deposit, also indicates that themetallogenesis is associated with the deep-origin fluids.The composition of the ore-forming fluids in this area are mainly CO₂-NaCl-H₂O,with a little N₂, CH₄ and CO. The value of w(NaCl)_cq has a peak varying from 8.00%to 10.00%, the fluids is a H₂O-CO₂ system with low salinity. The ore-formingtemperatures of the deposits vary in a wide range, mainly from 260 to 400℃, with apeak from 320 to 340℃. The ore-forming pressures were estimated from 32 to 160MPa, according to the P-V-T relationship of the fluid inclusions. The fluids show amoderate-high temperature, moderate-low salinity, weak acid and reductiveore-forming condition, which is favorable for the transform from SO₄^2- toS^2-. Theore-forming elements are mainly transported in the form of HS complex. The carbonhas adsorption to aurum, and the dissociation of carbon is help to the dissociation andprecipitation of gold.The immiscrible fluid inclusions were found in the orogenic gold deposits in thewest Qinling region. The immiscribility of the gold ore-forming fluids was resultedfrom the depressurization and ebullition when the fluids migrated from ductile toductile-brittle and brittle structures, where the elements transported in the form ofmetal-HS complexes were enriched. The fluids acted as a significant role to theactivity, the migration and the enrichment of the ore-forming matters.The age of the magma and the mineraliztion in the study area is from 214 to 168Ma. The mineraliztion is later than the activity of the magmas. And the deposits areformed in early Yanshan epoch, just after the collision of the continents and theorogenesis. The collision and orogenesis, the intrusion of magmas and the formation of ore deposits were occurred sequentially.The Indonisian movement and Yanshannian movement caused the violent collisionand orogenesis in the west Qinling region, followed by the activity of multiphase deepfaults. Granite magmas remelted and intruded, followed the migration of deep fluids,mixed with the fluid with the different origin, took part in the metallogenesis andresulted to the large gold deposits. Generally, the orogenic gold deposits were formedin the shear zones in the orogenic belts, while the Carlin type gold deposits occurredin the shear faulted systems at the margin of fault basin.