Cretaceous crustal thinning in North Africa: Implications for magmatic and thermal events in the Eastern Tunisian margin and the Pelagic Sea

The integrated use of geological, geophysical, and geochemical data from Eastern Tunisia onshore and offshore samples indicate a crustal thinning induced from the Tethyan rifting. This is responsible for the subsequent evolution of the North African passive margin during the Late Cretaceous, and the creation of the fold–thrust belt and associated foreland deformations. This thinned crust was an area of mantle upwelling that favoured the increase of isotherms, the uprise of basalt magma, and the circulation of hydrothermal fluids. The Cretaceous magmatism generated a major hydrothermal event characterised by the circulation of hot fluids along faults and a relatively high heat flow in the basin. Temperature elevation and hydrothermal conditions led to alteration of basalts and generated a new mineral equilibrium around the enclosing sedimentary deposits.     

银额盆地哈日凹陷下白垩统热水沉积岩特征及成因

银额盆地哈日凹陷下白垩统具有独特的沉积构造和矿物组成.通过开展岩石矿物和地球化学研究,论证其热水成因,探讨沉积环境和形成模式.研究区下白垩统发育方沸石、菱铁矿和白云石等矿物,富含Fe、Mn和Ba等元素,具有高∑REE、LREE富集和负Eu异常的稀土元素特征.结合Fe vs.Mn vs.（Cu+Co+Ni）×10和Ni vs.Co vs.Zn三角图解,证实下白垩统普遍含有热液成分.微量元素及碳氧同位素分析指示热水沉积岩形成于缺氧、封闭的咸水湖泊环境;氧同位素计算形成温度为43.94~86.08 ℃.研究表明,丰富的断裂系统为湖水下渗和热液喷流提供通道,湖水与热液的对流活动为湖盆持续输入热量和成矿离子.此类湖相热水沉积持续时间长、影响范围广,主要受物源供给和热液性质控制,代表一类特殊的沉积模式.      

Deep hydrothermal activity in hydrocarbon reservoirs,South Kuwait

Fluid inclusion (FI) data provide insights into the origin of hot fluids in Cretaceous and Jurassic reservoirs in south Kuwait. FI data for Cretaceous reservoirs show anomalously high temperatures exceeding 200 °C compared to samples from the Jurassic. In addition, formation water chemical geothermometry data also show anomalous water-rock equilibration temperatures. The preservation of high temperatures indicates rapid migration from depth through deep-seated fault systems. This implies that very hot fluids probably migrated up-fault from near basement to the Cretaceous section. This approach helps to understand the role of hydrothermal activity associated with basement faults in south Kuwait.     

Hydrothermal dolomitization of the carbonate Jurassic succession in the Provençal and Subbriançonnais Domains (Maritime Alps,North-Western Italy)

The present article illustrates a straightforward case of hydrothermal dolomitization, affecting Jurassic platform limestones of the Provençal and Subbriançonnais Domains (Maritime Alps, North-Western Italy). Dolomitized bodies are randomly distributed within the host limestone, and are commonly associated with dolomite vein networks and tabular bodies of dolomite-cemented breccias discordant with respect to bedding. Main dolomite types are a finely to medium-crystalline replacive dolomite and a coarsely-crystalline saddle dolomite occurring both as replacive and as cement. Stratigraphic constraints indicate that dolomitization occurred during the Cretaceous, in a shallow burial context, and was due to the circulation of hot fluids (temperature about 200 °C, as indicated by fluid inclusion microthermometry) through faults and related fracture networks. Hydrothermal dolomitization therefore indirectly documents a Cretaceous fault activity in the Maritime Alps segment of the European Tethyan passive margin.     

The relationship of mantle-derived fluids to gold metallogenesis in the Jiaodong Peninsula: Evidence from D–O–C–S isotope systematics

The largest gold district in China is the Jiaodong Peninsula, where three types of gold deposits are recognized: quartz vein, fracture-altered and breccia types. The first two developed along a group of NE-trending faults and are hosted by granitic intrusions, dated at 160 to 150 Ma (biotite granite) and 130 to 126 Ma (granodiorite), and by metamorphic rocks of the Precambrian crystalline basement. The breccia-type gold system is mainly located around the northern margin of the Jiaolai Cretaceous basin, where mineralisation is controlled by both detachment fractures and NE-trending faults. This study is based on stable isotope determinations from ten gold deposits, including Linglong, Jiaojia, Sanshandao, Cangshang, Wang'ershan, Dayigezhuang, Denggezhuang, Pengjiakuang, Fayunkuang and Dazhuangzi, as well as the Linglong Jurassic biotite granite, Guojialing Cretaceous granodiorite and Archean gneiss. The stable isotope systematics reflect the style of the three types of gold deposits, but also show that they belong to the same metallogenic system, in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids. The ore-forming age is later than both the Jurassic biotite granite and Cretaceous granodiotite, but overlaps with the 121 to 114 Ma ages of lamprophyre and diabase dykes. The hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar, and represent a CO₂ and potassium-rich system. This fluid system is interpreted to be the consequence of Cretaceous lithospheric thinning, asthenospheric upwelling and mantle degassing in Eastern China.     

New evidences of hydrothermal fluids circulation at the Devonian Kess Kess mounds,Hamar Laghdad (eastern Anti‐Atlas,Morocco)

New evidences based on a combination of field and laboratory investigations reinforce the hypotheses that the circulation of warm fluids has remarkably contributed to the origin and development of the Devonian Kess Kess mounds of the Hamar Laghdad Ridge (eastern Anti‐Atlas, Morocco). The limestones of the Hamar Laghdad Ridge were deposited above a structural high generated by calc‐alkaline volcanic activity that has probably fuelled the circulation of warm fluids throughout the overlying carbonate units. The geological and palaeontological attributes described throughout the succession of the Hamar Laghdad Ridge (from the Lochkovian to Frasnian intervals) are interpreted as the result of hydrothermal processes related to a volcanic system. In particular, these attributes seem consistent with a chemo‐physical environment fuelled by the circulation of warm and late magmatic fluids. These attributes include a very low oxygen stable isotope signature (δ¹⁸O ~ −10‰) for carbonates. Evidences for a late magmatic fluid circulation consist of volcanic glass and pyroclasts replacement with hydrothermal minerals such as quartz, anatase and clinochlore. Fluids circulating through veins and pores into sediments, and venting to the seafloor, probably induced the formation of cavities where monospecific trilobite communities were detected. The partially silicified trilobite remains are associated with traces of goethite. This iron‐bearing oxide mineral is also present in the upper part of the Hamar Laghdad Ridge. All these attributes are here interpreted as possible evidences for a low‐temperature hydrothermal venting system active during the Lochkovian–Frasnian time span. This study combines an updated revision with new petrographic, geological and geochemical results aimed at providing an overall framework on the origin and early diagenesis of the Devonian succession of Hamar Laghdad. Copyright © 2014 John Wiley & Sons, Ltd.     

Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems

The circulation of hydrothermal fluid through the upper oceanic crustal reservoir has a large impact on the chemistry of seawater, yet the impact on dissolved organic carbon (DOC) in the ocean has received almost no attention. To determine whether hydrothermal circulation is a source or a sink for DOC in the oceans, we measured DOC concentrations in hydrothermal fluids from several environments. Hydrothermal fluids were collected from high-temperature vents and diffuse, low-temperature vents on the basalt-hosted Juan de Fuca Ridge axis and also from low-temperature vents on the sedimented eastern flanks. High-temperature fluids from Main Endeavour Field (MEF) and Axial Volcano (AV) contain very low DOC concentrations (average = 15 and 17 μM, respectively) compared to background seawater (36 μM). At MEF and AV, average DOC concentrations in diffuse fluids (47 and 48 μM, respectively) were elevated over background seawater, and high DOC is correlated with high microbial cell counts in diffuse fluids. Fluids from off-axis hydrothermal systems located on 3.5-Ma-old crust at Baby Bare Seamount and Ocean Drilling Program (ODP) Hole 1026B had average DOC concentrations of 11 and 13 μM, respectively, and lowered DOC was correlated with low cell counts. The relative importance of heterotrophic uptake, abiotic sorption to mineral surfaces, thermal decomposition, and microbial production in fixing the DOC concentration in vent fluids remains uncertain. We calculated the potential effect of hydrothermal circulation on the deep-sea DOC cycle using our concentration data and published water flux estimates. Maximum calculated fluxes of DOC are minor compared to most oceanic DOC source and sink terms.     

Geological-hydrogeological characteristics and paleohydrogeological reconstructions of the Balei Gold Field,Transbaikalia

The Balei epithermal vein gold-quartz deposit is located in the Lower Cretaceous sedimentary sequence. The detailed geological and hydrogeological study of the deposit allowed us to reconstruct the paleohydrogeological conditions of the ore formation. This was particularly promoted by the preservation of the initial geological and hydrogeological structure—a graben-like depression hosting a subsurface water basin. The ore field was formed due to the Early Cretaceous high-temperature hydrothermal activity. The ore-bearing solutions originated via the interaction of the deep fluids with rocks and local subsurface waters. The solutions boiled at the depth of ∼600 m and gold was deposited from the degassed liquid phase.     

Fluid–rock interactions associated with regional tectonics and basin evolution

An integrated approach consisting of fracture analysis, petrography, carbon, oxygen and strontium‐isotope analyses, as well as fluid‐inclusion micro‐thermometry, led to a better understanding of the evolution of fluid–rock interactions and diagenesis of the Upper Permian to Upper Triassic carbonates of the United Arab Emirates. The deposited carbonates were first marked by extensive early dolomitization. During progressive burial, the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture‐filling dolomite, quartz and calcite cements. After considerable burial during the Middle Cretaceous, sub‐vertical north–south oriented fractures (F1) were cemented by dolomite derived from mesosaline to hypersaline fluids. Upon the Late Cretaceous maximum burial and ophiolite obduction, sub‐vertical east–west fractures (F2) were cemented by dolomite (Dc2) and saddle dolomite (Ds) derived from hot, highly saline fluids. Then, minor quartz cement has precipitated in fractures from hydrothermal brines. Fluid‐inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot (200°C) saline brines (20 to 23% NaCl eq.). Through one‐dimensional burial history numerical modelling, the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200°C. Tectonically‐driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the Late Cretaceous maximum burial and lasted until the Oligocene–Miocene compressional tectonics and related uplift. The circulation of such hydrothermal brines led to partial dissolution of dolomites (Dc2 and Ds) and to precipitation of hydrothermal calcite C1 in new (mainly oriented north–south; F3) and pre‐existing, reactivated fractures. The integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history. Hence, this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes, which may subsequently affect reservoir properties.     

Mass Change Calculations as Applied to the Search for Volcanogenic Massive Sulfide Deposits: An Example from Malusok, Siocon, Zamboanga del Norte, Mindanao, Philippines

Abstract. The Malusok volcanogenic massive sulfide (VMS) deposits comprise two adjacent ore bodies, the Main Malusok and the Malusok Southeast ore bodies, hosted within Cretaceous metamorphic rocks. Owing to the structural and metamor-phic overprinting combined with intense hydrothermal alteration, primary textures of the Malusok volcanic rocks have been obliterated. The stratigraphic correlation of the Main Malusok and the Malusok Southeast ore bodies show that both deposits are essentially confined within a single stratigraphic interval. The lithogeochemical analysis of the Malusok samples shows that constituent lithologies have precursor compositions ranging from sub-alkaline basalts to rhyodacites. Field and mass flux data suggest that the Main Malusok VMS deposits were derived as a consequence of axial hydrothermal activity. The Malusok Southeast ore bodies represent satellite deposits generated by off-axis hydrothermal activities from vents aligned along a NW-SE trend with the Main Malusok zone. This alignment represents an ancient fissure that served as a pathway for the upwelling metalliferous hydrothermal fluids. In searching for lateral extensions of these VMS deposits, this NW-SE alignment should serve as a possible exploration guide.     

The role of bitumen in strata-bound copper deposit formation in the Copiapo area, Northern Chile

In northern Chile, between 27 and 33°S, there are numerous deposits where residual petroleum is associated with Cu-(Ag) mineralisation (the most famous being El Soldado). All of these deposits are hosted by Lower Cretaceous volcanic or volcanoclastic facies along the axis of a former backarc basin. This close relationship suggests that the generation, migration and emplacement of hydrocarbons in the Cretaceous volcanic units is a regional process, associated with the evolution of the Cretaceous backarc basin and points to the importance of pyrobitumen as an exploration tool for similar Cu–(Ag) deposits. The present work analyses four small strata-bound copper deposits located along a north–south belt approximately 10 km east of Copiapó in northern Chile. These deposits are typically hosted by pyrobitumen-rich andesitic volcanic to volcanoclastic rocks intercalated with the marine carbonate Pabellón Formation, the youngest formation within the Chañarcillo Group. The strong genetic and spatial relationships between the pyrobitumen-rich lavas and the mineral deposits allow us to define this volcanic belt as the Ocoita-Pabellón Metallotect. Two hydrothermal events can be distinguished based on the mineralogical, textural, fluid inclusion and isotope data of ore and gangue and on the optical properties of residual petroleum. During the early event, petroleum was mobilised from the source rocks into the primary and secondary porosity of the lavas by Fe-rich hydrothermal fluids, which precipitated pyrite as an early sulphide phase. The second event is characterised by Cu-rich hydrothermal fluids, which induced three successive sub-stages of Cu-sulphide precipitation. The hydrothermal fluids chemically and thermally altered the first-stage bitumen, transforming it into pyrobitumen. The present work documents similarities between the Ocoita-Pabellón Metallotect and the El Soldado ore deposit and emphasises important differences. In the El Soldado host rocks, a petroleum reservoir existed prior to the arrival of the mineralising hydrothermal fluids, the framboidal pyrite was formed by assistance of bacteria, the S of the Cu sulphides was inherited from the pyrite, and the fluid source was basin connate-metamorphic brine. In the Ocoita-Pabellón Metallotect, the hydrocarbons were mobilised into the host rocks by hydrothermal fluids; the pyrite is epigenetic, the δ³⁴S values of pyrite and copper sulphides are very different, with distinctive light δ³⁴S signature of Cu sulphides (δ³⁴S between −44.7 and −17.9‰), and the calculated δ¹⁸O of hydrothermal fluids indicates the participation of meteoric water in the late phases of the hydrothermal system.

Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits, southwestern Japan

The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The ⁸⁷Sr/⁸⁶Sr ratios and δ¹³C–δ¹⁸O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.     

Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits,southwestern Japan

The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The ⁸⁷Sr/⁸⁶Sr ratios and δ¹³C–δ¹⁸O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

尼日尔阿泽里克砂岩型铀矿控矿因素初探

尼日尔（Niger）阿泽里克（Azelik）铀矿位于阿尔利特（Arlit）砂岩铀矿省西南部。在结合前人研究的基础上,通过对阿泽里克铀矿进行野外地质勘查,对各种野外现象进行分析总结,初步探讨了阿泽里克砂岩型铀矿控矿因素。阿泽里克铀矿受构造控制明显,矿化均位于NE与NW向断裂交叉处。矿化同时受沉积控制,矿化层为下白垩统阿萨乌阿（Assaousas,Kla）组砂岩。矿化砂岩以河流相中粗粒砂岩为主,孔隙度、渗透率良好,利于流体在砂岩中流通。矿床热液蚀变现象明显,存在碳酸盐化、褐铁矿化、铜矿化、还原性流体还原作用。     

Fluid Inclusions,C–H–O–S Isotope and Geochronology of the Bujinhei Pb–Zn Deposit in the Southern Great Xing'an Range of Northeast China: Implication for Ore Genesis

The Bujinhei Pb–Zn deposit is located in the southern Great Xing'an Range metallogenic belt. It is a representative medium‐ to high‐temperature hydrothermal vein type deposit controlled by fractures, and orebodies hosted in the Permian Shoushangou Formation. The hydrothermal mineralization is classified into three stages: pyrite ± arsenopyrite–quartz (Stage 1), polymetallic sulfide–quartz (Stage 2), and polymetallic sulfide–calcite (Stage 3). Fluid inclusion petrography, laser Raman analyses and microthermometry indicate that the liquid‐rich aqueous inclusions (L) and vapor‐rich CO₂ ± CH₄–H₂O inclusions (C) occur in the Stage 1 and as medium‐ to high‐ temperature and low‐ to medium‐salinity NaCl–H₂O–CO₂–CH₄ hydrothermal fluids. The liquid‐rich (L) and rare vapor‐rich CO₂ ± CH₄–H₂O inclusions (C) occur in the Stage 2 with medium‐temperature and low‐salinity NaCl–H₂O ± CO₂ ± CH₄ hydrothermal fluids. The exclusively liquid‐rich (L) fluid inclusions are observed in the Stage 3, and the hydrothermal fluid belongs to medium‐temperature and low‐salinity NaCl–H₂O hydrothermal fluids. The results of hydrogen and oxygen isotope analyses indicate that ore‐forming fluids were initially derived from the magmatic water and mixed with local meteoric water in the late stage (δ¹⁸O_H2O‐SMOW = 6.0 to 2.2‰, δD_SMOW = ?103 to ?134‰). The carbon isotope compositions (?18.4‰ to ?26.5‰) indicate that the carbon in the fluid was derived from the surrounding strata. The sulfur isotope compositions (5.7 to 15.2‰) indicate that the ore sulfur was also primarily derived from the strata. The ore vein No. 1 occurs in fractures and approximately parallel to the rhyolite porphyry; orebodies have a close spatial and temporal relationship with the rhyolite porphyry. The rhyolite porphyry yielded a crystallization age of 122.9  ± 2.4 Ma, indicating that the Bujinhei deposit may be related to the Early Cretaceous magmatic event. Geochemical analyses reveal that the Bujinhei rhyolite porphyry is high in K₂O and peraluminous, and derived from an acidic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; it is similar to A2‐type granites, and formed in a backarc extensional environment. These results indicate that the Bujinhei Pb–Zn deposit was a vein type system that formed in Early Cretaceous and influenced by the Paleo‐Pacific tectonic system. Bujinhei deposit is a representative hydrothermal vein type deposit on the genetic types, and occurs on the western slope of the southern Great Xing'an Range. The ore‐forming fluids were medium‐ to high‐temperature and low‐to medium‐salinity NaCl–H₂O–CO₂–CH₄ hydrothermal fluids, which became medium‐temperature and low‐salinity NaCl–H₂O hydrothermal fluids in later stages, and came from magmatic water and mixed with meteoric water, whereas the ore‐forming materials were mainly derived from the surrounding strata. The LA–ICP–MS zircon U–Pb dating indicates that the Bujinhei deposit formed at the period of late Early Cretaceous, potentially in a backarc extensional environment influenced by the Paleo‐Pacific tectonic system.     

洋中脊热液系统中的锇及其同位素

洋中脊热液系统是将相对富集在深部的Os运移到海底表面的重要媒介,同时该过程也是全球Os循环的重要组成部分.在归纳总结洋中脊热液系统各物源组分和产物中Os的化学形态、含量及其同位素组成特征的基础上,探讨了Os在洋中脊热液活动各阶段中的分布演化规律及物源贡献特征.在缺乏沉积物覆盖的洋中脊区域,热液系统中的Os及其同位素组成特征主要受控于海水和不同构造环境下洋壳组分特征的差异以及这两种物源组分混合比例的不同.经历了海底之下的水岩反应后,围岩会将下渗海水中的部分放射性成因Os固定,而将自身富集的非放射性成因Os释放进入热液流体中.堆积在海底之上的各种热液产物中的Os大多来自海水,而海底之下的热液产物则因为海水下渗深度以及海水与热液流体混合程度的差异而体现出宽泛的Os含量和187Os/188Os比值变化范围.      

河北东坪碲金矿床辉钼矿Re-Os及锆石U-Pb年龄研究

东坪碲金矿床是与碱性侵入岩有关的国内规模最大的碲金矿床。虽然前人已对该矿床做过大量研究,但其成矿年龄仍存在较大争议。文章通过对矿床中辉钼矿Re-Os和锆石U-Pb年龄进行研究,对成矿年龄和矿床成因进行限定。矿床中的辉钼矿存在浸染状和细脉状两种产状。浸染状辉钼矿通常产于正长岩,与硫化物共生,Re-Os模式年龄为(401.1±2.1)和(402.8±2.1) Ma。细脉状辉钼矿主要产在石英-硫化物脉中,其内部及裂隙中产出大量自然金,Re-Os模式年龄为(380.3±2.0)和(376.9±2.9) Ma。含金石英脉中的岩浆锆石U-Pb年龄为(394±1.7) Ma,热液锆石U-Pb年龄为(382±4.7) Ma。辉钼矿与锆石年龄一致,且与水泉沟碱性杂岩体侵位年龄相吻合,因此认为东坪碲金矿床存在泥盆纪成矿,可能与水泉沟岩体岩浆活动有关。矿床经历了晚侏罗世—早白垩世岩浆流体的交代、活化、富集过程,最终形成大型碲金矿床。矿床中的碲主要来自上地幔或洋壳物质的部分熔融,水泉沟碱性岩浆出溶的高氧逸度、中偏碱性的流体和脱气作用为碲的迁移和富集提供了重要条件。     

Geology,U-Pb geochronology,and stable isotope geochemistry of the Tunca semi-massive sulfide mineralization,Black Sea region,NE Turkey: Implications for ore genesis

Upper Cretaceous volcano-sedimentary sequences of the Eastern Pontide orogenic belt, NE Turkey, are host to significant VMS mineralization, including near Tunca. The initial stages of felsic volcanism within the mineralized area are marked by the eruption of dacitic lavas and breccias of the Kızılkaya Formation. This was accompanied by the emplacement of domelike hematitic dacites. Autobrecciated and volcaniclastic rocks, both in situ and resedimented, were likely generated from extrusive portions of these dacite bodies. Basaltic volcanism is marked by the eruption of the lava flows and pillow lavas of the Çağlayan Formation. Hiatuses in basaltic activity are marked by thin horizons of volcaniclastics and mudstones. The uppermost felsic volcanic units were accompanied by resedimentation of autoclastic facies from previous volcanism and represent the latest phase of Upper Cretaceous volcanism in the area. The semi-massive sulfide mineralization is associated with a late stage of the initial felsic volcanism. U-Pb LA-ICP-MS zircon dating of a dacitic tuff breccia yielded an age of 88.1 ± 1.2 Ma (Coniacian-Upper Cretaceous), which is interpreted to be the age of the sulfide occurrences.A concentric zoned alteration pattern is observed in the footwall rocks. The alteration pattern is considered to have formed by lateral migration of hydrothermal fluids which had ascended along the discharge conduit. Fluid inclusion data indicate precipitation or mobilization processes within a relatively narrow temperature range of 152–255 °C (avg. 200 °C). The low-salinity fluids in the fluid inclusions, less than 5.9 wt% NaCl equivalent, are consistent with typical modified seawater-dominant hydrothermal vent fluids. Sulfur isotope analysis of the Tunca sulfides yields a narrow range of 1.5–4.1 per mil. These δ³⁴S values are also typical of many VMS deposits. Most of the recorded δ¹⁸O values (+7.1 to +14.0 per mil) are greater than 9 per mil. The most intensely hydrothermally altered rocks tend to have lower δ¹⁸O values relative to the less altered rocks. Collectively, the geologic relationships, mineralization style, and the lack of seafloor ore facies suggest that mineralization is principally of sub-seafloor origin. The most geologically reasonable interpretation of the genesis of the Tunca mineralization is the continuous interaction between the host rocks and seawater-derived fluids, without significant involvement of a magmatic fluid.     

Origin and age of rift related fluorite and manganese deposits from the San Rafael Massif,Argentina

The San Rafael Massif is characterized by widespread fluorite and manganese epithermal ore deposits whose origin has been under debate to the present. Isotopic (Sm/Nd and K/Ar) and geochemical (trace elements and REE) data of fluorite and manganese ore allowed to establish the age and genesis of the deposits and to propose a regional genetic model. The fluorite deposits were formed during the Upper Triassic–Lower Jurassic as a result of the Triassic rifting that launched a hydrothermal activity at regional scale. The hydrothermal fluids had low T and high fO₂ with fluorine probably derived from a mantle source and REE scavenged from the volcanics of the Gondwanan Choiyoi Magmatic Cycle upper section. The manganese deposits were formed by oxidizing hydrothermal fluids that collected Mn from deep sources and also leached REE from the upper section of the Choiyoi Magmatic Cycle during two mineralization episodes. One episode was linked to the rift tectonic setting that remained active up to the Upper Cretaceous and the other was related to an Early Miocene back-arc extensional geodynamic setting. Both manganese and fluorite deposits were formed in extensional tectonic settings within an epithermal environment near the surface, and can be ascribed to the general model of detachment-related deposits.