Ping Shen

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active volcanoes have been studied using geophysical methods. However, the internal structure of ancient volcano complexes remains poorly understood. We investigated ancient volcano complexes by comparing magnetotelluric (MT) observations from Zhibo (ZB) ancient volcano with active mid-oceanic ridge volcanoes from Iceland and intracontinental volcanoes from north China. The MT responses of magma chambers in these active volcanoes showed similar low-resistivity values ranging from several to tens of Ω·m, indicating a comparable resistivity of the active magma. Assuming that the ancient active volcano chambers had a similar resistivity to that of current active volcanoes, we reconstructed the ancient Carboniferous volcano complex in ZB using the ratio of the lower portion of the MT responses from ZB ancient volcanic edifices and active volcanoes. The results implied the existence of fossil magma chambers at a depth of 5 to 7 km marking the site of a former volcanic center. This finding supports the magmatic origin of the ZB volcanic rock-hosted iron deposits.

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active volcanoes have been studied using geophysical methods. However, the internal structure of ancient volcano complexes remains poorly understood. We investigated ancient volcano complexes by comparing magnetotelluric (MT) observations from Zhibo (ZB) ancient volcano with active mid-oceanic ridges volcanoes from Iceland and intracontinental volcanoes from north China. The MT responses of magma chambers in these active volcanoes showed similar low-resistivity values ranging from several to tens of Ω·m, indicating a comparable resistivity of the active magma. Assuming that the ancient active volcano chambers had a similar resistivity to that of current active volcanoes, we reconstructed the ancient Carboniferous volcano complex in ZB using the ratio of the lower portion of the MT responses from ZB ancient volcanic edifices and active volcanoes. The results implied the existence of fossil magma chambers at a depth of 5 km marking the site of a former volcanic center. This finding supports the magmatic origin of the ZB volcanic rock-hosted iron deposits.

The Paleozoic Aktogai Group in Kazakhstan ranks among the 30 largest porphyry Cu deposits globally. The Aktogai deposit is the largest one in the Aktogai Group and is characterized by intensive potassic alteration where the dominant orebody occurred. However, its mineralization processes still need to be clarified. Our investigation focused on the texture, trace elements, fluid inclusions, and in situ oxygen isotopes of the quartz from the ore-related tonalite porphyry and associated potassic alteration at Aktogai to trace the deposit’s mineralization processes. Ti-in-quartz thermobarometry, fluid inclusion microthermometry, and geological characteristics indicate that the ore-related magma at Aktogai originated from a shallow magma chamber at ∼1.9 ± 0.5 kbar (∼7.2 ± 1.9 km) and intruded as the tonalite porphyry stock at ∼1.7–2.4 km. The potassic alteration and associated Cu mineralization comprise five types of veins (A1, A2, B1, B2, and C) and two types of altered rocks (biotite and K-feldspar). Among them, nine types of hydrothermal quartz were identified from early to late: (1) VQA1 in A1 veins and RQbt in biotite-altered rocks; (2) VQA2 in A2 veins and RQkfs in K-feldspar altered rocks; (3) VQB1 in B1 veins and VQB2E in B2 veins; and (4) quartz associated with Cu-Fe sulfides (VQB2L, VQBC, and VQC) in B and C veins. Titanium contents of the quartz decreased, while Al/Ti ratios increased from early to late. Fluid inclusion microthermometry and mineral thermometers reveal that VQA1, RQbt, and hydrothermal biotite formed under high-temperature (∼470–560 °C) and ductile conditions. VQA2, RQkfs, VQB1, and hydrothermal K-feldspar formed during the transition stage from ductile to brittle, with temperatures of ∼350–540 °C. The rapid decrease in pressure from lithostatic to hydrostatic pressure led to fluid boiling and minor involvement of meteoric water (∼11–14%) in the mineralizing fluid. Extensive recrystallization in VQA1 to VQB1 was associated with repeated cleavage and healing of the intrusion. With cooling, K-feldspar decomposition and hydrolysis increased. Fluid cooling and water-rock reactions resulted in the co-precipitation of Cu-Fe sulfides, white mica, chlorite, VQBC, and VQC at temperatures of ∼275–370 °C and brittle conditions. The Paleozoic Aktogai deposit exhibits formation depths and fluid evolution processes similar to Mesozoic and Cenozoic porphyry Cu deposits worldwide. The close association between Cu-Fe sulfides and later quartz formed under intermediate-temperature conditions at Aktogai implies that Cu-Fe sulfides are not precipitated under early high-temperature conditions in porphyry Cu deposits.

The Awulale Iron Metallogenic Belt (AIMB) has developed many medium–large iron deposits, of which the Zhibo iron deposit is selected as the research object in this paper. The Zhibo deposit’s ore primarily consists of magnetite as the main mineral, accompanied by extensive epidotization. The mineral assemblage includes diopside, albite, actinolite, epidote, chlorite, K-feldspar, quartz, calcite, chalcopyrite, and pyrite. Magnetite is classified into two groups based on sulfide content and mineral assemblage (MagI for sulfide-free and MagII for sulfide-rich ores). Two-stage mineralization of magnetite has been identified based on mineral assemblages and paragenesis, including the magmatic stage MagI and hydrothermal stage MagII. Mag I shows inhomogeneous backscattered electron (BSE) textures and consists of BSE-light and -dark domains (Mag I-L and MagI-D). Seven subtypes of magnetite have been recognized in this deposit. MagI-L and MagI-D have formed in the magmatic stage and show BSE images in light and dark colors, respectively. MagI-L is anhedral to subhedral and is inclusion-free. MagI-D has mainly replaced MagI-L along fractures and contains inclusions and pores. MagII has formed in the hydrothermal stage and is characterized by coupled dissolution–reprecipitation (DRP) textures. It can be divided into five sub-generations, that is, MagII-1, MagII-2, MagII-3, MagII-L, and MagII-D. MagII-1, MagII-2, and MagII-3 comprise the core–mantle–rim texture, while MagII-L and MagII-D comprise the core–rim texture. MagII-1 is BSE-light and is enriched with inclusions and pores. MagII-2 has partly replaced MagII-1 and exhibits oscillatory zoning under BSE imaging. It also contains inclusions. BSE-light MagII-3 occurs as overgrowth along MagII-2 margins and is inclusion-free. MagI magnetite is enriched with V, Cr, and Ni, whereas MagII is enriched with W, Ta, Nb, Sr, Sb, Sn, Y, Zr, Mg, Al, and Ti, indicating a decreased temperature of magnetite formation. MagI-L crystallizes from the original magma, while MagI-D is formed from the residual magma enriched with incompatible elements. MagII crystallizes from later multiple hydrothermal activities through the dissolution of early magnetite and the re-precipitation of later magnetite or from MagI-D which has later undergone a hydrothermal overprinting process. According to the texture and chemical composition of magnetite from the Zhibo deposit, we suggest that the Zhibo iron deposit was formed from the initial magmatic origin and then underwent a hydrothermal overprinting process.

喇嘛苏铜锌矿位于中亚成矿域西段, 为新疆西天山地区相对较早发现的斑岩-矽卡岩铜锌矿床。本文对喇嘛苏铜锌矿矽卡岩和矽卡岩化大理岩中的矽卡岩矿物进行了岩相学及主微量成分分析, 首次发现了喇嘛苏铜锌矿床发育的黑榴石和富钛钙铝榴石。对矽卡岩中发育震荡环带的钙铁榴石进行主量元素和电子探针mapping分析, 发现钙铁榴石发育不均一的核部, 环带部分表现为Al-Fe元素此消彼长的耦合震荡关系; 对钙铁榴石从核部中央向边缘环带进行定向原位微量元素LA-ICPMS分析, 发现∑REE表现为连续略有降低的趋势、δEu表现出震荡变化。黑榴石和富钛钙铝榴石表现为核幔结构, 显示流体氧逸度的降低, 流体从超常富集Ti转变为富集Ti。对富钛钙铝榴石进行原位微量元素LA-ICPMS测试, 发现富钛钙铝榴石∑REE高于钙铁榴石(钙铝榴石: 58.4×10^-6~408.4×10^-6; 钙铁榴石28.2×10^-6~108.7×10^-6), 富钛钙铝榴石具有强烈正Eu异常(δEu=11.3~15.0), 钙铁榴石和富钛钙铝榴石均表现为富集轻稀土而亏损重稀土。喇嘛苏铜锌矿中的原生辉石为透辉石, 后经退蚀变流体交代发生Mg元素丢失和Fe元素富集, 转变为次透辉石-铁次透辉石, 随交代程度加深, 进一步转变为铁角闪石, 最后在退蚀变流体中直接沉淀出铁镁钙闪石。在背散射下, 原生透辉石、次透辉石-铁次透辉石、铁角闪石、铁镁钙闪石表现为亮度不同的明暗结构。矽卡岩矿物的显微结构和主微量成分表明, 蚀变流体为酸性和富Cl、REE、Ti的流体, 蚀变阶段早期为氧化性流体, 晚期为还原性流体。喇嘛苏铜锌矿流体中CH₄、C₂H₆等还原性物质、共生的磁黄铁矿-磁铁矿低氧逸度组合及还原状态矽卡岩矿物组合表明喇嘛苏铜锌矿可能为一还原性斑岩-矽卡岩铜锌矿。

新疆阿尔泰可可托海3号脉矿床是世界瞩目的伟晶岩型稀有金属Li-Be-Nb-Ta-Rb-Cs-Hf矿床。尽管前人对3号脉伟晶岩矿床已经进行了大量的研究工作,然而在伟晶岩稀有金属成矿潜力以及伟晶岩成因问题上仍存在一些争议。本文以发育在可可托海矿区不同规模的伟晶岩脉以及淡色花岗岩为研究对象,利用石英SEM-CL和原位微量元素技术手段,查明各伟晶岩脉和花岗岩的稀有金属禀赋差异及成因联系。研究结果表明,石英的Li和Al含量可以用来指示伟晶岩的矿化潜力。与1号和2号伟晶岩脉相比,3号伟晶岩脉具有更宽的分异范围,以石英中Ge/Ti变化于1.83×10^-6~159×10^-6范围为特征,更高的Li含量,其中外侧带平均Li含量为39×10^-6,因而成矿潜力最大。此外,证实了白云母钠长花岗岩为矿化花岗岩,其微量元素组成和演化程度与3号脉伟晶岩外侧带相当,而白云母碱长花岗岩为贫矿花岗岩。更重要的是,本文利用各伟晶岩结构分带内的石英中Ge/Ti-Ge以及Ge/Ti-Al/Ti图解,确定了各伟晶岩脉具有相似的演化趋势,指示它们可能源于相同的花岗岩母岩。与世界上典型的RMG(花岗岩浆演化后残余熔浆结晶)和DPA(直接深部地壳熔融)成因的伟晶岩相比,可可托海矿区含矿伟晶岩和花岗岩中石英微量元素与RMG成因伟晶岩存在地球化学亲和性,这指示它们来源于花岗质岩浆的残留熔浆。

斑岩型矿床易受后期岩浆、构造、变质等地质作用影响,使得矿床自身发生改造变形。后期的热事件不仅可以将原有矿体再活化,还能够带来新的成矿物质,进而发生新一期成矿作用。不同地质时期的成矿作用在同一空间互相叠加,会形成与斑岩矿床矿化特征截然不同的矿床,称为叠加改造型斑岩矿床。多宝山铜(钼)矿床位于中亚造山带东段,兴蒙造山带内。该矿床在成矿构造地质、矿石组构和成矿期次等方面表现出多期改造与叠加成矿的特征。本次根据野外观察和成矿年代学研究,推测多宝山铜矿至少存在两期成矿作用。对采自矿坑340平台10件叠加矿化类型的辉钼矿样品进行Re-Os同位素分析,获得辉钼矿的模式年龄介于435.6±10.5Ma~446.1±7.1Ma之间,等时线年龄为440.1±4.5Ma (MSWD=0.56)。结合前人获得的年龄资料,确认多宝山叠加成矿与晚奥陶世玄武安山岩应形成于统一的成岩成矿地质事件。不同矿化期次的黄铜矿和黄铁矿原位硫同位素结果显示:典型斑岩型矿脉中黄铜矿δ³⁴S平均值为-2.12‰;叠加矿化黄铜矿δ³⁴S平均值为-1.78‰,黄铁矿δ³⁴S平均值为-1.03‰,均落入幔源硫范围,表明硫主要来自岩浆。而前人在叠加期形成的黄铜矿中,测试到较低的δ³⁴S值(-12.9‰~-5.6‰),这是后期流体易携带轻硫³²S发生迁移并在应力较低的区域内重新沉淀的结果。同位素组成指示叠加成矿期的矿化元素即来源于新的岩浆活动,又继承了先前存在的岩(矿)体。

伟晶岩岩浆-热液演化中流体出溶时间和铌钽矿化过程还存在争议。针对上述科学问题,本文选择了中国新疆阿尔泰造山带柯鲁木特112号钠长石-锂辉石型伟晶岩脉作为研究对象,在野外观察和室内岩相学研究的基础上,以云母的结构和主量成分为切入点,为该伟晶岩脉的结晶演化和铌钽矿化过程提供了一些新约束。112号脉主要发育以下5个结构单元:钠化微斜长石-石英带(I)、白云母-石英-钠长石集合体(II)、钠化块体微斜长石带(III)、石英-钠长石-锂辉石带(IV)和糖晶状钠长石集合体(V)。它具有弱的分带:I和II多出现在脉体边缘或外侧,是贫矿的伟晶岩的"壳";III是贫矿的富巨晶微斜长石的中间上部带;IV是富巨晶锂辉石的中间下部带;V是富铌铁族矿物(CGM)的晚期岩浆残余相。在不同的分带/集合体中共识别出3类白云母:Type A,原生成分均一白云母;Type B,原生被改造的成分变化白云母(BSE下具有亮暗分带);Type C,次生成分均一白云母。研究结果表明,112号脉历经了3个演化阶段:(1)早期流体出溶和流体-围岩(二云母花岗岩)相互作用;围岩发生云英岩化,围岩中黑云母分解产生的部分Fe进入伟晶岩岩浆(围岩混染)。(2)熔体内部分离结晶与钠长石化;外侧I带中部分Type A白云母和所有Type B白云母的亮部具有全脉最高的FeO^T含量(5.20%~5.73%),反映了一阶段的云英岩化过程中围岩Fe的贡献;而从I带至V集合体,Type A白云母和Type B白云母的亮部的FeO^T和MgO含量整体递减,指示分异演化程度增强。(3)二次流体出溶;晶体-流体相互作用导致Type B白云母中的Fe、Mg和F释放进入流体,形成了亮暗成分分带;同时,致使FeO^T、MgO和F含量较低的Type C白云母以交代的形式生长在原生矿物内部。最后,本文认为:一阶段中围岩混染Fe的加入为CGM的沉淀提供了Fe源;二阶段中磷灰石的分离结晶对熔体中F的消耗促进了CGM的沉淀;三阶段中晶体-流体反应会引起已结晶矿物释放Fe或Mn,含Fe或Mn流体与富Nb或Ta熔体结合促使CGM晶出。