Geochronology and geochemical characteristics of granitoids in the Bastielieke tungsten polymetallic deposit in the southern margin of Altay: Implications for tungsten mineralization
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巴斯铁列克钨多金属矿床位于新疆阿尔泰造山带南缘,是近年来在区内发现的首例中型钨多金属矿床。矿体主要产于二叠纪花岗岩与上志留统-下泥盆统康布铁堡组火山-沉积岩接触带的矽卡岩中。钨矿化与矿区花岗质岩石有明显的空间关系。然而,与钨矿化有关的花岗质岩石成因尚不清楚。本文对矿区出露的与矿化关系密切的黑云母花岗岩、二长花岗岩和二云母花岗岩进行了锆石U-Pb 年代学和岩石地球化学研究。3个样品的LA-ICP-MS锆石U-Pb加权平均年龄分别为282.3±3.2Ma、284.3±2.2Ma和284.8±2.3Ma,属早二叠世岩浆活动的产物,与成矿年龄一致。所有岩石具有高硅(SiO2=73.6%~78.3%)、富碱(K2O+Na2O=5.15%~9.62%)富钾(K2O/Na2O>1.1)、贫钙(CaO=0.19%~0.75%)和钛(TiO2=0.04%~0.24%)、弱过铝-强过铝质(A/CNK=1.01~1.39)特征。这些岩石稀土元素总量(∑REE)变化较大(变化于20.3×10-6~328×10-6),但二云母花岗岩显示轻重稀土元素分异不明显((La/Yb)N=0.96~2.06)、Eu强烈负异常(δEu=0.07~0.41)的深V型稀土元素分布特征,黑云母花岗岩和二长花岗岩显示轻稀土略富集((La/Yb)N分别为2.8~5.5和4.8~7.4)且Eu负异常(δEu=0.33~0.39和0.34~0.63)明显的右倾型稀土元素分布特征。所有样品均显示相对富集Rb、Th、U、Pb元素和相对亏损Nb、Ti、P、Sr、Ba元素,但二云母花岗岩中W含量(4.6×10-6~9.4×10-6)相对低于黑云母花岗岩和二长花岗岩中W含量(分别为15.1×10-6~168×10-6和8.4×10-6~16.0×10-6)。所有样品的锆石具有正的高εHf(t)值(+3.8~+11)和相对年轻的亏损地幔模式年龄(Hf的tDM2为0.60~1.0Ga)。以上特征说明,这些岩石属高钾钙碱性分异I-A过渡型花岗岩。结合区域地质背景,认为这些岩体是二叠纪时期后碰撞伸展环境下两个独立岩浆事件的产物,母岩浆均来源于新生地壳熔体与幔源岩浆,经过高度分异演化后结晶形成矿区岩石。花岗质岩浆活动为巴斯铁列克钨矿床提供了成矿物质,岩浆演化过程(结晶分异与熔体-流体作用)对成矿元素有富集作用。;The medium-sized Bastielieke tungsten polymetallic deposit is a newly discovered deposit in the Kelan Basin at the southern margin of Altay Orogenic Belt. The skarn hosted the orebody which is at the outer contact zone between the granitoids and volcanic-sedimentary rocks of the Upper Silurian to Lower Devonian Kangbutiebao Formation. Two-mica granite, biotite granite and monzogranite outcrop in the Bastielieke ore district showing obvious spacial relation to the hosted ore deposits. The petrogenesis of these rocks associated with the tungsten mineralization is indefinitive. LA-ICP-MS zircon U-Pb geochronology and geochemical analysis of major and trace elements and zircon Hf isotope are performed to discuss their petrogenesis. These granites show consistent weighted mean 206Pb/238U ages of 282.3±3.2Ma, 284.3±2.2Ma and 284.8±2.3Ma, respectively, which indicate that the Permian magmatic events at the southern margin of Altay is consistent to the ages of mineralization. These granitoids exhibit high SiO2 concentration (73.6%~78.3%), high total alkali (Na2O+K2O) (5.15%~9.62%) concentration and low CaO (0.19%~0.75%) and TiO2 (0.04%~0.24%) concentrations. In addition, they are enriched in potassium (K2O/Na2O=1.08~2.06) with high A/CNK values (1.01~1.39). The total REE contents of these granites vary from 20.3×10-6 to 328×10-6 with different chondrite-normalized REE patterns. The two-mica granite is characterized by low (La/Yb)N ration (0.96~2.06) with pronounced negative Eu anomaly from 0.07 to 0.41, and showing a deep V-type pattern. The biotite granite and monzogranite display enrichment of LREE with (La/Yb)N ration ranging from 2.8 to 5.5 and 4.8 to 7.4, and moderate negative Eu anomalies ranging from 0.33 to 0.39 and 0.34 to 0.63, respectively. All samples display noticeable negative anomalies of Nb, Ti, P, Sr, Ba and strong positive anomalies of Rb, Th, U and Pb. The two-mica granite has lower W concentrations varying from 4.6×10-6 to 9.4×10-6 than those in the biotite granite and monzogranite from 15.1×10-6 to 168×10-6 and 8.4×10-6 to 16.0×10-6, respectively. In addition, zircon εHf(t) values range from +3.8 to +11, two stage model ages (tDM2) ranging from 0.6Ga to 1.0Ga. The petrologic and geochemical data indicate that these rocks are high-K calc-alkaline and weak peraluminious to strongly peraluminous, which is similar to the geochemical behavior of the fractionated I-A type granitoids. Combined with regional geology, we suggest that they are derived from melting of the juvenile crust and mantle, and followed by subsequent crystallization fractionation in deeper magma chamber in a post-collision environment by two independent magmatic events. These granites provide key ore components for the formation of Bastielieke tungsten polymetallic deposit and preconcentrate tungsten by crystallization fractionation and melts-fluid exsolution.Andradite
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Recent potassium-argon age determinations of 48 and 50 m.y. for biotite in rocks of the Lowland Creek Volcanics in southwestern Montana agree with previous K-Ar age of 49 m.y. for biotite of a dike rock correlated with lavas of the upper part of the volcanics. Together, these determinations establish the age of the volcanics as early Eocene rather than late Oligocene, as they had been tentatively designated.
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Abstract Cu and Fe skarns are the world's most abundant and largest skarn type deposits, especially in China, and Au‐rich skarn deposits have received much attention in the past two decades and yet there are few papers focused on schematic mineral deposit models of Cu–Fe–Au skarn systems. Three types of Au‐rich deposits are recognized in the Edongnan region, Middle–Lower Yangtze River metallogenic belt: ∼140 Ma Cu–Au and Au–Cu skarn deposits and distal Au–Tl deposits. 137–148 Ma Cu–Fe and 130–133 Ma Fe skarn deposits are recognized in the Edongnan region. The Cu–Fe skarn deposits have a greater contribution of mantle components than the Fe skarn deposits, and the hydrothermal fluids responsible for formation of the Fe skarn deposits involved a greater contribution from evaporitic sedimentary rocks compared to Cu–Fe skarn deposits. The carbonate‐hosted Au–Tl deposits in the Edongnan region are interpreted as distal products of Cu–Au skarn mineralization. A new schematic mineral deposit model of the Cu–Fe–Au skarn system is proposed to illustrate the relationship between the Cu–Fe–Au skarn mineralization, the evaporitic sedimentary rocks, and distal Au–Tl deposits. This model has important implications for the exploration for carbonate–hosted Au–Tl deposits in the more distal parts of Cu–Au skarn systems, and Fe skarn deposits with the occurrence of gypsum‐bearing host sedimentary rocks in the MLYRB, and possibly elsewhere.
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Skarn deposits occur throughout the world and have been mined for a variety of elements. This paper describes the basic stages of skarn formation and the main causes of variation from the general evolutionary model. Seven major classes of skarn deposits (Fe, W, Au, Cu, Zn, Mo and Sn) are briefly described, and relevant geological and geochemical features of important examples are summarized in a comprehensive table. The important geochemical and geophysical parameters of skarn deposits are discussed, followed by a summary of important petrologic and tectonic constraints on skarn formation. Finally, exploration models are presented for several major skarn types, with a plea for field mapping as a fundamental basis for future studies.
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