Tectono-Magmatic Formation And Gold-polymetallic Mineralization In South Beishan Area, NW China

The Central Asian Orogenic Belt (CAOB), situated between the Siberian, North China, Tarim and East European cratons, represents one of the most important sites of juvenile crustal growth in the Phanerozoic. The interaction between mantle and crust and multiple metallogeny in the processes of the continental growth have attracted many people’s attention.The South Beishan area is located at the south margin of CAOB. It has experienced complicated geological process with multiple accretion and collision events, and hosts a large number of porphyry copper, orogenic gold, magmatic copper-nickel, and epithermal gold deposits.In this paper, the Shijinpo gold deposit, Xiaoxigong gold deposit and Huaniushan gold-polymetallic ore district are detailed studied. On the basis of the accurately age dating, combined with the geology and geochemistry analysis, the genesis of these granitoids and gold deposits, and their temporal and spatial relationships have been evaluated. The main results and conclusions are summarized as following:1. Magmatic formation and geodynamic evolutionLA-ICP-MS U-Pb Zircon dating yields the age of Xiaoxigong K-feldspar granite(841Ma), Huitongshan granite (424.4±2.5Ma), Shijinpo granite (405.5±1.8Ma)、Baishantang rhyolite-porphyry (374.9±2.3Ma), Shibanquan granite (280.5±5.5Ma), Xiaoxigong quartz synenite (246.4±2.5Ma), and Huaniushan granite (225.6±2.2Ma), respectively. Though geochemistryand Sr-Nd-Pb isotope analysis, petrogenesis and tectonic implications of these granitoids have been discussed. Based on the geochronological and geochemical data concerning the other Paleozoic magmatic rocks from the South Beishan area, this paper outlines the evolved geodynamic scenarios inherent in these magmatic sequences. The generation of syn-and post-orogenic granitoids represents horizontal (lateral) and vertical crustal growth. And four magmatic activities stages have been yielded, such as:(1) Neoproterozoic (ca.880Ma to840Ma),(2) Ordovician to Devonian (ca.451Ma to374Ma),(3) Carboniferous to Permian (ca.310Ma to270Ma),(4) Triassic (ca.240Ma to220Ma). The detailed research can be described as follows: ⅰ. Petrology, major and trace elements data all indicate that the Xiaoxigong K-feldspar granite (841Ma) is typical A-type granite which intruded in a post-collisional extensional tectonic setting. It is the oldest A-type granitic pluton in the Beishan area. Combined with the Gubaoquan elogite which has an eclogite-facies metamorphic age of857±71Ma, it indicates an important subduetion-collision and post-collision extension event in the Neoproterozoic.ⅱ. The Paleozoic granitoid intrusions in the Beishan area are mainly I-type granite, with some adakites, Nb-enriched basalts and A-type granite,suggesting subducted oceanic slab-melting was frequent during451Ma to270Ma.iii. Triassic granitic plutons have also been identified, such as the Huaniushan granite(225.6±2.2Ma), Xiaoxigong quartz synenite(246.4±2.5Ma),and Jingchanggou granite. They belong to the high-K calc-alkaline and shoshonitic series and have alkalic and alkali-calcic characteristics, and are weakly peraluminous-metaluminous. These granitoids are mainly highly fractionated I-type granite and A-type granite, suggesting a post-orogenic extensional setting during the Triassic.2. Ore genesis studies of gold-polymetallic depositsThe Shijinpo gold deposit is hosted in the early Devonian granite, and consists primarily of auriferous quartz veins that confined to the EW-trending faults.Laser incremental heating40Ar/39Ar analysis of hydrothermal sericite yields a plateau age of364.6±3Ma (2σ), which suggests that the gold mineralization took place in the late Devonian. A comprehensive fluid inclusion study shows that in the early stage there are three major types of fluid inclusions: liquid-rich, gas-rich aqueous fluid inclusions and CO2-bearing fluid inclusions, with intermediate homogenization temperature (270℃to340℃), and low salinity (4.65%to9.21%NaCl). Fluid boiling are evidenced by the co-existence of three types of fluid inclusions with similar homogenization temperatures. Fluid pressures declined from ca.113MPa to ca.11MPa through the early stage to the late stage, and the ore-forming fluids are assumed to undergo boiling in a transitional setting (from compression to extension). Oxygen and hydrogen isotopes for ore fluids range from1.39‰to3.39‰and-71‰to-99‰, respectively, suggesting that the ore fluids are mainly sourced from metamorphic water, and mixed with meteoric water later. Measured δ34S values for sulfide minerals range from4.44%o to11.33‰. Both Precambrian basement and granite might be the sulfur sources with respect to the sulfur isotopic composition of the ores, Precambrian metamorphic basement and Paleozoic granites. On the basis of our researches and the regional geology, we propose that the Shijinpo gold deposit is an orogenic gold deposit.The Xiaoxigong gold deposit is hosted in Mesoproterozoic metamorphic rock and Neoproterozoic granite, related to shear zone. Fluid inclusions in quartz of the major metallogenic stage are mainly two-phase vapor-liquid inclusions with small amount of CO2phase. The ore-forming fluids are CO2-H2O-CH4-NaCl system with intermediate homogenization temperature (281℃to325℃), and low salinity (3.2%to9.2%NaCl). Oxygen and hydrogen isotopes for ore fluids range from-2.95%o to13.79%o and-72.5%o to-107‰, respectively, suggesting that the ore fluids are mainly sourced from metamorphic water, and mixed with meteoric water later. Measured δ34S values for sulfide minerals range from1.2%o to12.7‰, suggesting that both Precambrian basement and granite might be the sulfur sources. We further propose that the Xiaoxigong gold deposit was formed in the late-collisional setting during the middle Permian.The Huaniushan gold-polymetallic ore district consists mainly of magmatic-hydrothemal and skarn Au, Mo, Pb-Zn ores, occur in the contact zone of the Huaniushan granite, respectively. LA-ICP-MS zircon U-Pb dating constrains the crystallization of the granite at225.6±2.2Ma (1δ). Five molybdenite samples from the magmatic-hydrothemal molybdenum ores yield Re-Os isochron age of225.2±2.4Ma (2δ), while a molybdenite sample from the skarn gold ores yields a Re-Os model age of143.0±0.3Ma and an isochron age of221.0±3.4Ma, indicating the gold-polymetallic mineralization occuring in a post-orogenic setting during the late Triassic.1)The Huaniushan skarn gold deposit controlled by EW trending faults. The auriferous quartz veins were deposited at intermediate-high temperature conditions (280℃to425℃) from aqueous or aqueous-carbonic fluids with moderate salinity (6.59%to14.04%NaCl equiv). Oxygen and hydrogen isotopes for ore fluids range from-0.01‰to+0.29‰and-109‰to-116‰, respectively, compatible with mixing of magmatic and meteoric components. Measured δ34S values for sulfide minerals are predominantly+4.2l‰to+6.53%o, indicating a deep-seated sulfur source of magmatic origin2) The Huaxishan magmatic-hydrothemal gold deposit was occurring in the Huaniushan granite and along the contact with the Mesoproterozoic clastic rock. The homogenization temperatures and salinities of inclusions in major stage vary from246℃to334℃,16.24%to9.86%NaCl equivalent, and in late auriferous mineralization vary from154℃to214℃,2.41%-6.88%NaCl equivalent. Measuredδ34S values for sulfide minerals are predominantly+2.6%o to+4.25%o, indicating a deep-seated sulfur source of magmatic origin.3) The Huaheitan magmatic-hydrothemal molybdenum deposit was hosted in the granite and along the contact with the Mesoproterozoic hornfels. Fluid inclusion studies suggest that the molybdenum-quartz veins were deposited at high temperature conditions (342℃to583℃) from aqueous or aqueous-carbonic fluids with moderate salinity (15.67%to16.53%NaCl equiv). Measured δ34S values for sulfide minerals are predominantly+3.3‰to+3.9‰, also indicating a deep-seated sulfur source of magmatic origin.4) The Huaniushan Pb-Zn ore bodies occur in the contact zone between the granite and the marine carbonates. The ore bodies occurred mainly in terms of massive ores in big veins and lenticular ore bodies. Fluid inclusion studies suggest that the veins were deposited at intermediate-high temperature conditions (276℃to362℃) from aqueous or aqueous-carbonic fluids with moderate salinity (10.23%to21.11%NaCl equiv). Measuredδ34S values for sulfide minerals from NO.2mining area are predominantly-10.1%o to-3.68%o, which is quite different from the other Au, Mo deposits related to the granite. And δ34S values for sulfide minerals from NO.3mining area vary from0.71‰to4.82‰. According to the deposit geology characteristics, we propose that the Huaniushan Pb-Zn deposit was formed by multiple stage:a) the "ore source-bed" was formed by submarine exhalation-sedimentation in Late Mesoproterozoic, b) the magmatic-hydrothemal processes determine the Pb-Zn enrichment and mineralization in Late Triassic.3. Metallogenic series and metallogenic regularityIn South Beishan area, there are mainly four gold-polymetallic metallogenic series. They are the Cu-Au series formed in the island arc setting during the Silurian to Devonian, the volcano-subvolcano Cu-Au-Fe series formed in continental marginal volcanic arc setting during the Carboniferous to early Permian, the shear zone related Au(Cu) series formed in late-collisional setting during the middle-late Permian, and the potassium granite related Au-Mo-Pb-Zn-(W) series occured in the post-orogenic extensional setting during the late Triassic.By summarizing the regional metallogenic conditions and the temporal and spatial distribution regularities, the regional gold-polymetallic metallogenic model have been proposed. The geochronology study indicates that the Devonian, Permian and Triassic are the main gold mineralization epochs in this area. Five gold-polymetallic ore prospecting areas have been divided, and recommendations on the deployment of prospective areas have been put forward.