Eu geochemistry in magmatic rocks of continental marginal volcanic belts

The example of eastern Russia is employed to analyze the influence of diverse factors on the development of negative Eu anomalies in magmatic rocks at continental marginal volcanic belts. A strong correlation is identified between the relative value of Eu minima and the degree of Fe oxidation, Fe/(Fe+Mg) of mafic minerals, and an increase in the Eu minimum from basic to acid rocks. It is demonstrated that the value of the Eu minimum hardly depends on either the crystallization depth of the rocks and/or their alkalinity. Within individual tectono-stratigraphic zones, the value of Eu anomalies is independent of the age of the rocks, their affiliation with certain associations, and the basement structure of the volcanic zone. Rocks with clearly pronounced negative Eu anomalies belong exclusively to the ilmenite series that are formed under reduced conditions. The rocks of the magnetite series, which are formed under oxidized conditions, contain trivalent Eu, which behaves analogously to other REE.     

Petrogenesis of Ordovician magmatic rocks in the southern Chiapas Massif Complex: relations with the early Palaeozoic magmatic belts of northwestern Gondwana

The recent discovery of Early Ordovician S-type granites in the southwest of the Chiapas Massif Complex adds a new perspective to the Palaeozoic history of the Maya block, inasmuch as no rocks of such age had previously been reported in this region. New geologic mapping west of Motozintla, Chiapas, revealed pelitic to psammitic metasedimentary successions (Jocote Unit) intruded by granitoids and metabasites. The Jocote Unit is unconformably underlain by the newly defined Candelaria Unit, which comprises deformed calc-silicate rocks and interlayered folded amphibolites. The Candelaria Unit is the oldest rock succession so far recognized in the southern Maya block. We used laser-ablation multicollector inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating to determine the ages of the rock, yielding Early Ordovician (ca. 470 Ma) and Late Ordovician (ca. 450 Ma) ages.

Major and trace element geochemistry, as well as Nd and Sr isotope data, suggest that folded amphibolites of the Candelaria Unit are mantle-derived and are probably related to rifting. The Early Ordovician bimodal magmatism of the Jocote Unit is more strongly differentiated; it reflects crustal contamination and volcanic-arc chemical signatures. A granitic stock (Motozintla pluton) intruded the area in the Late Ordovician. Its geochemical composition indicates less crustal contamination and a mixed signature between volcanic-arc and within-plate settings. Magmatic rocks analogous in age and chemical character crop out in the Rabinal and the Altos Cuchumatanes areas of Guatemala, suggesting the existence of a semi-continuous Ordovician magmatic belt from Chiapas to central Guatemala. Similar but somewhat younger granites also occur in the Maya Mountains of Belize, suggesting that magmatism migrated in the Silurian from the Chiapas–Guatemala belt towards the Maya Mountains.     

内蒙古温都尔庙群铁矿带构造解析

温都尔庙群铁矿分南带和北带,原始铁矿层为华北板块北缘陆坡带火山喷发-沉积产物,通过构造解析方法,恢复含铁层位及铁矿体构造形态,还原了褶皱控矿过程.研究认为,南带和北带的铁矿带分属于同斜地背斜的南北两翼,为同层褶曲构造现象,经历多期褶皱作用以及后期热液作用改造形成多个矿体并存.     

呷村-有热矿区含矿带精细结构对比及其找矿意义

四川省白玉县呷村银多金属矿床是我国著名的VMS矿床之一,具有容矿火山岩系+硫化物矿体+喷气岩的“三位一体”特征,以及下部筒状或漏斗状网脉状矿（西矿带）和上部层状块状矿（东矿带）的“双层结构”样式。其南部有热矿区找矿虽有进展,但突破不大。为确定有热矿区的找矿方向,本文对呷村矿床和有热矿区含矿带进行了精细结构对比,认为有热北部19-31线由块状硅化流纹质火山岩+条纹条带状流纹质凝灰岩+千枚状绢云母凝灰岩组成火山喷发单元,以强烈的硅化为特征,发育大量的脉状-网脉状构造,与呷村西矿带完全可以对比;而有热南部39-55线则由块状“黄矿”矿石与流纹质火山碎屑岩组成喷气一火山单元,但缺乏重晶石、硅质岩等喷气沉积岩;以“黄矿”为主,仅形成铅锌贫矿体,与呷村矿床东矿带富含重晶石、以“黑矿”为主,形成银铜铅锌富矿体的特征有较大区别。同时,有热块状矿体上部还存在厚达70-130m的流纹质火山岩和脉状-网脉状矿体,据此认为有热矿带总体上可与呷村西矿带对比,代表热液喷流成矿系统的热水补给通道相,其中的块状（黄矿）矿体具微“双层”结构特征,有可能为早期热流成矿系统的另一个独立的“微”VMS矿床。由此判断有热矿区东部流纹质火山岩与碳质板岩的转换界面处深部存在类似呷村东矿带的以“黑矿”块状矿体+重晶石组合为代表的隐伏矿体,从而共同组成一个“下黄上黑”共两层块状矿体的“堆垛式”结构型式。寻找以“黑矿”-重晶石组合为代表的隐伏银铜铅锌块状矿体是有热矿区今后取得找矿重大突破的关键所在。     

矿田构造变形岩相带的地球物理资料解译与找矿应用

基于岩(矿)石物性参数和矿床成因类型建立的地球物理勘查模型,在深部找矿预测中出现了多解性的问题,急需找到地球物理方法能够高精度识别的地质体目标.多年的找矿实践表明,矿田构造变形岩相带就是一个重要的选项,业已取得显著的找矿效果.目前,大比例尺的矿区地球物理勘查工作较多,而中比例尺的矿田地球物理研究比较薄弱,且两者均缺乏分...     

理解岩浆系统的物理过程

岩浆系统是一种复杂性动力系统,暗示岩浆系统的演化具有多重分支现象,即其演化路径包含一系列平衡过程与突变事件.因此,岩浆系统的物理过程是理解岩浆形成与演化及其相关地质过程的关键.邓晋福教授及其学术团队长期坚持岩浆系统物理过程的研究,在火成岩成因、火成岩构造组合、岩浆活动与成矿作用的关系和相关的地球深部过程等研究领域都做出了杰出的贡献.本文集刊登了部分相关的研究成果,以强调岩浆系统物理过程的重要性.我们撰写此文,目的是简要介绍本文集的主要内容和强调岩浆系统物理过程的研究意义.     

Chronology of hydrothermal and magmatic activity in the Dukat gold-silver ore field

The previously published and newly obtained geological and geochronological (Rb-Sr and Ar-Ar) data show that the igneous rocks and products of hydrothermal alteration in the Dukat ore field pertain to two ore-forming magmatic-hydrothermal systems (OMHSs). The igneous rocks of the Early Cretaceous rift-related OMHS are represented by potassium rhyolites of the Askol’d Formation with Rb-Sr ages of 124 ± 3 and 119.3 ± 3.4 Ma and intercalating amygdaloidal basalts. The products of the hydrothermal activity of this OMHS are the metasomatic anatase-chlorite assemblage of the root zone, which replaces potassium rhyolites, and shallow-seated quartz-adularia and quartz-carbonate-feldspar veinlets retained in rhyolite fragments in Late Cretaceous conglomerate and breccia. The Late Cretaceous OMHS was related to the origination of the Okhotsk-Chukotka volcanic belt and consists of calc-alkaline basaltic andesites of the Tavvatum Formation and moderately silicic K-Na rhyolites of the Nayakhan Formation with a Rb-Sr age of 84 ± 4 Ma. The Late Cretaceous postmagmatic hydrothermal activity in the Dukat ore field resulted in the formation of preore metasomatic rocks and orebodies of the unique Dukat Au-Ag deposit. The first stage of the Late Cretaceous hydrothermal activity gave birth to preore propylites with a Rb-Sr isochron age of adularia samples estimated at 85 ± 1 Ma and quartz-chlorite-sulfide and Ag-bearing quartz-chlorite-adularia orebodies with Rb-Sr isochron ages of adularia estimated at 84 ± 1 and 86.1 ± 4 Ma. The second stage was marked by the formation of garnet-bearing propylites and quartz-rhodonite orebodies with a Rb-Sr age of 73 ± 3 Ma. Further hydrothermal activity occurred after a break related to structural rearrangement of the ore field and was expressed in the replacement of propylites by products of argillin alteration and Ag-bearing Mn hydroxides. Paleogene basaltic dikes and related subeconomic mineralization concluded magmatic and hydrothermal processes in the Dukat ore field.     

Assessment of the Re decay constant by cross calibration of Re-Os molybdenite and U-Pb zircon chronometers in magmatic ore systems

The past decade has seen renewed interest in ¹⁸⁷Re-¹⁸⁷Os geochronology using a variety of matrices including sulfide minerals, shales and meteorites. The most widely used value of the ¹⁸⁷Re decay constant (λ¹⁸⁷Re) is 1.666 ± 0.005 × 10⁻¹¹ a⁻¹ (±0.31%), which is based on cross calibration of Re-Os and Pb-Pb chronometers for certain meteorites [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re-Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science271, 1099-1102]. However, other recent studies have yielded alternate values of λ¹⁸⁷Re, based upon either direct counting experiments or analysis of meteorites. Here, we provide an independent assessment of λ¹⁸⁷Re, using methodology, sample materials, and preparation of Os standard solutions different from those of Smoliar et al. (1996). Combining Re-Os age data for molybdenite formed in magmatic ore deposits, with the U-Pb zircon age of the magmatic rocks, a refined λ¹⁸⁷Re value is determined by averaging 11 individual cross-calibration experiments spanning ca. 2700 Ma of Earth history. Using the U decay constants of Jaffey [Jaffey A. H., Flynn K. F., Glendenin L. E., Bentley W. C., and Essling A. M. (1971) Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev.4, 1889-1906], a value for λ¹⁸⁷Re of 1.6668 ± 0.0034 × 10⁻¹¹ a⁻¹ is determined. Using the λ²³⁸U value of Jaffey et al. (1971) and λ²³⁵U value of Schoene [Schoene B., Crowley J. L., Condon D. J., Schmitz M. D., and Bowring S. A. (2006) Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Geochim. Cosmochim. Acta70, 426-445], a value for λ¹⁸⁷Re of 1.6689 ± 0.0031 × 10⁻¹¹ a⁻¹ is determined. These values are nominally higher (ca. 0.1 and ca. 0.2%) than the value determined by Smoliar et al. [Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re-Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science271, 1099-1102], but within calculated uncertainty. Further refinement of λ¹⁸⁷Re by cross calibrating the molybdenite and U-Pb zircon chronometers should be possible by utilizing high precision, single-grain, chemical abrasion zircon U-Pb analyses.     

Redox potential of mantle magmatic systems

A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi₂O₆(Cpx) + 2Fe₃O₄(Spl) = 3CaMgSiO₄(Ol) + 3Fe₂SiO₄(Ol) + O₂. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere from ascending mantle plumes.     

Ore belts of the Greater Altai and their ore resource potential

The results of comprehensive geological and metallogenic studies of the Greater Altai are presented. This project has been carried out since 1997 under the guidance of Academician G.N. Shcherba. The importance of these investigations is determined by the need to enhance and further develop mineral resources of nonferrous, noble, rare, and other metals for operating mining and metallurgical enterprises of Kazakhstan. The great body of information on the geology, geophysics, and metallogeny of the region obtained over many years has been integrated on the basis of new global tectonics. The Greater Altai embraces the Hercynides of the Rudny Altai, Qalba-Narym, West Qalba, Zharma-Saur, and the adjacent territories of Russia and China. The present-day tectonic units are considered to be detached blocks of ancient continental massifs that drifted in the Paleoasian ocean and then amalgamated into the structure of the Greater Altai during the Hercynian collision. The tectonic and metallogenic demarcation of the studied territory made possible the recognition of the Rudny Altai, Qalba-Narym, West Qalba, and Zharma-Saur ore belts, different in geology, geodynamic evolution, and metallogeny. The formation conditions and localization of volcanic-hosted massive sulfide, gold, and rare-metal deposits pertaining to certain ore-bearing geochronological levels were specified, and the potential of the region for various mineral resources was estimated.     

晋南平顺地区两岩浆岩带内在联系及找矿意义

平顺铁矿为山西省南部典型的矽卡岩型铁矿床之一,矿区内发育有东西两岩浆岩带,它们的化学性质相似,具有低硅(绝大部分小于56%)、高镁(Mg#=51.47～81.66,平均值为61.42);富集大离子亲石元素(如Sr、Ba、K)和亏损高场强元素(如Nb、Ta、Ti);LREE富集,HREE亏损,无或只有微弱的正铕异常,富集放射性成因Pb,以及第一岩浆岩带橄榄辉长岩含有大量橄榄石、角闪石和辉石的捕掳晶等特点,显示两岩浆岩带具有起源相同、快速侵位的特点。平顺地区6个典型钻孔岩性的化学分析结果说明岩浆后期热液具有从东往西运移的趋势。结合两岩浆岩带侵位特点,提出第二岩浆岩带是由第一岩浆岩带晚期岩浆侧向侵位形成,二者之间在某一范围内深部相连,并可能成矿的观点。这为本区将来深部找矿提供理论依据。     

造山带岩浆铜镍硫化物矿床的混染模式 ——以天山-北山二叠纪铜镍矿为例

造山带铜镍矿床的地幔源区均经历过不同程度的俯冲交代作用和复杂的源区混染历史,造山带铜镍矿带内大量个体差异的矿床为源区和壳内混染的多样性提供了研究实例.中亚造山带天山-北山地区是中国众多造山带内铜镍矿数量最多、分布最广、总体储量最大的地域,这些矿化岩体普遍体现Nb-Ta亏损、高18O和锆石O-Hf同位素离散的特征,该宏大...     

河南舞阳地区赵案庄铁矿床成矿时代及地质意义——中国最古老的岩浆型铁矿床

河南舞阳地区赵案庄铁矿床是产于超基性岩中的隐伏矿床。超基性岩侵位于新太古界太华群赵案庄组透辉斜长角闪片麻岩中。矿体呈似层状,具有多层。矿石成分较复杂,主要矿物为磁铁矿、蛇纹石、氟磷灰石等。矿床规模为中等,其成因是与超基性岩有关的岩浆晚期磷灰石-磁铁矿矿床。在磷灰蛇纹磁铁矿石中选出的锆石样品为变质锆石,利用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICPMS)进行微区原位U_Pb同位素测年,获得不一致线的上交点年龄为(1943±5)Ma(MSWD=2.3)。同时对锆石阴极发光(CL)图像研究,表明该年龄代表锆石遭受强烈变质作用的年龄,可以限定矿体形成时代的上限,即矿体形成时代不晚于1943 Ma。超基性岩浆演化晚期,岩浆充填构造裂隙呈透辉石岩脉产出。透辉石脉与矿体同期并且穿切铁山庙组BIF型铁山铁矿。铁山庙组形成时代在2300~2500 Ma之间。透辉石岩脉形成时代不早于该组年龄,可作为赵案庄铁矿床形成时代的下限。赵案庄铁矿床的形成时代为1943~2300 Ma,为古元古代,这是中国目前发现的最古老的岩浆型铁矿床。该成矿地质时代反映了在华北陆块东南缘曾发生过一次地壳扩张和裂陷活动及华北克拉通裂解事件。     

Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.