Ulf Kempe

Tetrad effect in rare earth element distribution patterns: a method of quantification with application to rock and mineral samples from granite-related rare metal deposits

Abstract In some geological environments, the tetrad effect can be observed as a split of rare earth element (REE) patterns into four rounded segments. A new method is proposed to quantify the sizes of the individual segments, and for the first time, the significance of observed tetrad effects is evaluated by taking analytical errors into account. The outlined method was applied to lanthanide patterns of whole-rock and fluorite samples collected from granite-related rare metal deposits. The REE patterns of the granite and greisen samples investigated exhibit significant tetrad effects that may not be accounted for by analytical uncertainties. It is shown that the study of whole-rock samples is insufficient to determine whether this effect is developed during fractional crystallization or is due to other processes such as fluid-rock interaction. A concave tetrad effect mirroring the pattern of the whole-rock samples was not observed in the REE patterns of related vein fluorite samples. Therefore, it is unlikely that the convex tetrad effect in the samples from the magmatic environment can be explained by removal of a respective complementary REE pattern by a coexisting hydrothermal fluid, as previously suggested. It is proposed that the tetrad effect formed within the magma-fluid system before emplacement in the subvolcanic environment where phase separation caused a split of this system into fluid and magma subsystems. Alternatively, the tetrad effect may also be inherited from an external fluid influencing the system during or after the emplacement of the magma. On the basis of the fluorite data, it is shown that the behavior of Eu in the fluids is not related to the tetrad effect. Consequently, different physico-chemical factors control the occurrence of both phenomena. Y was found to be strongly enriched in samples precipitating from hydrothermal fluids that experienced prolonged interaction with the wall-rocks, whereas the tetrad effect in the fluids vanished with time and increasing distance from the ore-bearing granite. Thus, these different geochemical parameters can be used to reconstruct different aspects of the fluid evolution within this type of deposit.

Spectroscopic methods applied to zircon

Natural and synthetic (pure and doped) zircon (ZrSiO4) have been studied with a variety of spectroscopic techniques. These techniques are based on different physical phenomena, for instance transitions between spin states of nuclei and electrons, energetic transitions of valence electrons, intra-molecular vibrations, or vibrations of atoms and molecular units in the lattice. All of the diverse spectroscopic techniques, however, have in common that they probe energy differences between a ground and excited states, mostly upon interaction of the mineral with incident radiation. Such interactions are not only determined by the excited elementary particles or molecules themselves but depend greatly on their local environments (i.e. number, type, valence and geometrical arrangement of neighboring atoms). Spectroscopic techniques are thus sensitive to the local structure and provide information on the short-range order. Most research on zircon crystals using spectroscopic techniques was done to study their “real structures,” that is the characterization of deviations from “perfect” zircon. Such features include the incorporation of non-formula elements, structural defects and the presence of inclusions and other impurities. Correspondingly, most of the spectroscopic investigations can be assigned to two major groups. The first group represents studies done to characterize the structural position and local environment of non-formula elements when incorporated in the zircon lattice, and accompanying effects on physical properties. The second group comprises studies subjected to the real structures of “metamict” zircon samples, i.e., changes of the zircon structure caused by the impact of self-irradiation and upon recovery from radiation damage (Ewing et al., this volume). It is most obvious that a spectroscopic bulk or point analysis will first of all yield a spectrum (i.e. a plot of the intensity of the respective physical parameter versus wavelength, frequency or wavenumber), and this is what is used in most studies. In addition, image generation based on …

Genetic significance of the trace element content in metamorphic and hydrothermal quartz: a reconnaissance study

Abstract A reconnaissance study on trace elements in metamorphic and hydrothermal quartz was carried out using quartz samples from the tin district Erzgebirge, Germany, the gold mineralization at Kasperske Hory, Czech Republic, and the gold-quartz vein deposits Muruntau and Myutenbai, Uzbekistan. A new method of sample preparation has been developed to prepare pure quartz samples by combining conventional hand-picking with microscopic and spectroscopic studies as well as acid wash/etch procedures. Preparation of monomineralic samples was followed by sample dissolution and measurement by ICP-MS. The metamorphic quartz has very low concentrations of Li (≤0.4 ppm), Al (≤30 ppm), K (≤35 ppm), Rb (≤50 ppb), Sr (≤0.3 ppm), and Y (≤15 ppb). Moreover, it is characterized by light rare earth element enriched lanthanide distribution patterns lacking Eu anomalies. The low element concentrations in metamorphic quartz are interpreted to result from recrystallization. Metamorphic quartz from alteration halos enveloping tin and gold deposits has distinctly different trace element signatures. These differences are related to the hydrothermal overprint of the pre-existing metamorphic quartz by the mineralizing fluids. Hydrothermally altered metamorphic quartz from tin deposits has elevated concentrations of Li (≥0.9 ppm), Al (≥50 ppm), K (≥45 ppm), Rb (≥250 ppb), and Y (≥40 ppb) whereas altered metamorphic quartz from gold deposits is characterized by elevated concentrations of Sr (≥0.5 ppm). The rare earth element distribution patterns of altered metamorphic quartz show variable enrichments of the heavy rare earth elements and frequently display positive Eu anomalies. Hydrothermal vein quartz from the gold deposits usually has elevated Al (≥50 ppm) and Sr (≥0.6 ppm) contents. The lanthanide distribution patterns exhibit variable enrichments of the heavy rare earth elements and commonly show positive Eu anomalies. The elevated Sr concentrations in the quartz from gold deposits are best explained by Sr release during feldspar alteration in the wall rocks. This mechanism may also account for the relative enrichment of Eu in the mineralizing fluids although other processes may not be unambiguously ruled out.

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt ☆

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.

Relevance of Cathodoluminescence for the Interpretation of U-Pb Zircon Ages, with an Example of an Application to a Study of Zircons from the Saxonian Granulite Complex, Germany

In the last few years, an increasing number of papers dealing with applications of cathodoluminescence (CL) imaging — using both optical microscopes (0M-CL) and systems operating with secondary electron microscopes (SEM) or electron microprobes (SEM-CL) — to investigate the internal structures of zircon have been published. CL has been demonstrated to be a powerful tool for investigating zircons. This method yields high-resolution images of internal structures that often cannot be detected with other techniques (e.g., HF etching, Normarski interference). In most previous publications, CL images were interpreted assuming that CL is exclusively generated within the micro-areas that are irradiated with the electron beam (e.g., Hanchar and Miller 1993; Vavra et al. 1996). We want to demonstrate here that this assumption may in several cases lead to some misinterpretation of the internal structure of zircon.

Magmatic and metasomatic processes during formation of the Nb-Zr-REE deposits Khaldzan Buregte and Tsakhir (Mongolian Altai); indications from a combined CL-SEM study

Abstract Cathodoluminescence (CL) imaging and spectroscopy, as well as backscattered electron imaging, were used to assign the occurrence of several mineral phases and rock structures in altered nordmarkites and calcite-bearing granites from the Nb-Zr-REE deposits from Khaldzan Buregte and Tsakhir (Mongolian Altai) to three events: (1) intrusion of barren nordmarkites; (2) intrusion of small bodies of calcite- bearing granites with metasomatic alteration of the wall-rocks; and (3) alteration by F-rich fluids. Unusual red and yellow CL caused by Fe3+ and Mn2+ emission centres were detected in microcline and albite. Fe3+ centres were also established (along with others) in quartz, zircon, and possibly in fluorite. Magmatic and metasomatic rock structures and internal structures of the minerals coexist in the samples. The primary magmatic features were in part preserved during alteration. In contrast, the internal and the centre structures may be changed during alteration even in non-replaced mineral phases. Euhedral minerals may be formed by secondary processes as shown for lath-shaped albite. The occurrence of pseudomorphs, the inheritance of elements during replacement, and the mechanical effects of secondary minerals on earlier mineral phases during metasomatic growth are proposed as criteria for the reconstruction of the mineral succession in altered rocks. Snowball structures may be formed as a result of metasomatic alteration rather than as a magmatic intergrowth.

Unusual rare earth element fractionation in a tin-bearing magmatic-hydrothermal system

Normalized rare earth element patterns of rock and mineral samples from evolved granitic systems and associated hydrothermal tin deposits frequently show an unusual split into four consecutive curved segments, referred to as tetrads. In the present contribution, the simultaneous occurrence of complementary convex and concave tetrads is described for the first time for vein fluorite, suggesting that this unusual trace-element signature may develop during a single evolutionary stage within the hydrothermal environment. Based on geochemical analysis of vein fluorite, fluid inclusion studies, and thermodynamic modeling, it is shown that fractionation of the rare earth elements can be linked to fluid immiscibility and preferential partitioning of these elements between vapor and coexisting liquid. The findings provide the first direct evidence constraining the geological conditions responsible for the occurrence of the tetrad effect in tin-bearing magmatic-hydrothermal systems and require a reassessment of the current understanding of the origin of tin deposits.

Fluid regime and ore formation in the tungsten(–yttrium) deposits of Kyzyltau (Mongolian Altai): evidence for fluid variability in tungsten–tin ore systems

Abstract The Kyzyltau ore field is located in the northern part of the Mongolian Altai Mountains. W(–Y–Be–Mo) mineralization occurs in veins and stockworks in granites, felsic volcanics, basaltic flows, and conglomerates. The main ore minerals in the veins are wolframite, fluorite, beryl and minor molybdenite. Inclusions in hydrothermal quartz, fluorite and beryl from veins, as well as in magmatic quartz in vein-hosting granites of the Ulaan uul, Buraat uul and Tsunkheg deposits were investigated using microthermometry and laser Raman spectroscopy. Pseudosecondary/primary inclusions in the Kyzyltau ore veins are predominantly liquid-rich aqueous inclusions, containing variable amounts of CO 2 as the main gas component, and showing N 2 /CH 4 ratios >1. Secondary inclusions contain no or only trace concentrations of gases, as indicated by clathrate melting. The T h values of pseudosecondary/primary inclusions in veins are between 180 and 433°C. Fluid inclusion data indicate that phase separation processes leading to fluid immiscibility occurred in vuggy quartz II and green fluorite II in the Ulaan uul and Buraat uul ore veins. Phase separation pressures of 100–350 bars were estimated. Fluorite I from the Kyzyltau ore field shows a strong enrichment of HREE and a strong negative Eu anomaly common for rare metal-bearing ore systems. Fluorite II from Buraat uul and Tsunkheg is characterized by a change in the incorporation of rare earth elements (REE) with decreasing HREE contents and a decreasing Eu anomaly. The REE distribution patterns in fluorite II from Ulaan uul remained unchanged compared with fluorite I despite a strong increase in the total REE content. A review of the literature shows that from high-temperature, Fe-rich, K-dominated brines cassiterite ores can precipitate in quartz veins together with Fe–chlorite and Fe–tourmaline (Bolivian type). If phase separation of a gas-rich, low-salinity and Fe-rich fluid occurs, cassiterite–wolframite ores may be deposited (Cornwall/Devon type). The deposits of Kyzyltau are characterized by low-Fe alteration assemblages, a predominance of sodium over potassium, high contents of REE and Y, and a lack of extended tin mineralization despite the tin potential of the ore systems. Fluid inclusion data as well as geochemical and geological indications suggest formation of the tungsten deposits near the tops of Li–F-rich sub-volcanic intrusions. We interpret the pH of the mineralizing fluid to be the main factor controlling wolframite precipitation in the Kyzyltau ore field. Fluid–wall rock interactions, a lowering of the temperature and unmixing processes in the ore fluid generated contributions to neutralization and buffering of the acid CO 2 -bearing fluid into a pH range where tungstates were precipitated. It can be inferred that formation of tungsten ores without precipitation of extended tin mineralization is possible in deposits characterized by high potential tin and tungsten. According to this inference, tin mineralization may be found in a more favourable setting in the vicinity of the Kyzyltau deposits.

High resolution cathodoluminescence combined with SHRIMP ion probe measurements of detrital zircons

Abstract Cathodoluminescence (CL) microscopy and spectroscopy combined with SHRIMP ion probe measurements were carried out on detrital zircons from the Cretaceous Weferlingen quartz sand (Germany) to distinguish and characterize different zircon populations. Investigations by CL microscopy, SEM-CL and BSE imaging show that there are three main types of zircons (general grain sizes of 100-200 μm): (1) apparently weakly zoned, rounded grains with relict cores, (2) well rounded fragments of optically more or less homogeneous zircon grains showing CL zoning predominantly parallel to the z-axis, and (3) idiomorphic to slightly rounded zircon grains typically showing oscillatory euhedral CL zoning. A fourth type of low abundance is characterized by well-rounded grain fragments with an irregular internal structure showing bright yellow CL. High-resolution CL spectroscopic analyses reveal that blue CL is mainly caused by an intrinsic emission band centered near 430 nm. Dy3+ is the dominant activator element in all zircons, whereas Sm3+, Tb3+, Nd3+ have minor importance. Yellow CL (emission band between 500 and 700 nm) is probably caused by electron defects localized on the [SiO4] groups (e.g. related to oxygen vacancies) or activation by Yb2+ generated by radiation. Variations of the integral SEM-CL intensity are mainly controlled by the intensity of the broad bands and the Dy3+ peaks. SHRIMP analysis provides in situ high-resolution U-Pb dating of single zircon grains and confirms different ages for the evaluated different zircon types. The measurements show that the U-Pb ages of the zircons from Weferlingen scatter over a wide range (340 to 1750 Ma), backing up earlier conclusions that the quartz sand from Weferlingen is quite heterogeneous in terms of provenance.

Cathodoluminescence microscopy and spectroscopy of plagioclases from lunar soil

Abstract Cathodoluminescence (CL) microscopy and spectroscopy of single plagioclase grains from lunar soil show that plagioclases from Luna 20 (highland) have more or less homogeneous CL with both blue or green colors, whereas plagioclase grains sampled by Luna 24 (mare) luminesce dominantly green with partially distinct oscillatory zoning. The three main emission bands in the blue (~450 nm), green (~560 nm), and red-IR (~690 nm), mimic the most common emission bands in terrestrial feldspars. Mn2+ is the most important activator element in lunar plagioclases. Variations in the amount of structurally incorporated Mn2+ cause variations in the intensity of the green emission band at 560 nm, in some cases resulting in zoning of the CL intensity within single crystals. Calculations by a combination of quantitative spectral analysis of CL emission and PIXE measurements yield Mn concentrations of 7-47 ppm. The intense intrinsic emission band at 450 nm (probably an Al-O--Al center), which was especially prominent in Luna 20 plagioclases, causes their blue CL color. The occurrence of a CL emission band at ~690 nm in plagioclases from Luna 24 samples confirms that Fe3+-activated CL is common in these grains. The results indicate that at least some of the Fe in Luna 24 plagioclases is Fe3+, whereas all Luna 20 plagioclases have Fe3+-near the CL detection limit of about 0.1 ppm.