Thomas Monecke

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.

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.

Shallow Drilling of Seafloor Hydrothermal Systems Using the BGS Rockdrill: Conical Seamount (New Ireland Fore-Arc) and PACMANUS (Eastern Manus Basin), Papua New Guinea

ABSTRACT From September to October 2002, shallow drilling, using the submersible (5 m) Rockdrill of the British Geological Survey and the German R/V Sonne revealed critical information on the subsurface nature of two distinct hydrothermal systems in the New Ireland fore-arc and the Manus Basin of Papua New Guinea. Drilling at Conical Seamount significantly extends the known surface extent of the previously discovered vein-style gold mineralization (up to 230 g/t Au) at this site. Drilling the conventional PACMANUS volcanic-hosted massive sulfide deposit recovered complexly textured massive sulfide with spectacular concentrations of gold in several core sections including 0.5 m @ 28 g/t Au, 0.35 m @ 30 g/t Au, and 0.20 m @ 57 g/t Au. Shallow drilling is a fast and cost efficient method that bridges the gap between surface sampling and deep (ODP) drilling and will become a standard practice in the future study of seafloor hydrothermal systems and massive sulfide deposits.

Primary clinopyroxene spherulites in basaltic lavas from the Pacific-Antarctic Ridge

Fresh glassy basaltic andesite samples recovered from the northern part of the Pacific–Antarctic Ridge contain abundant spherulites consisting of arrays of closely packed clinopyroxene fibers. The spherulites frequently enclose elongated vesicles that are tear drop-shaped or tailed. The long axes of the elongated vesicles were found to be always parallel to the orientation of the surrounding crystal fibers. In several cases, elongated vesicles having different orientations are hosted by a single spherulite. The existence of elongated vesicles provides unequivocal evidence that a significant proportion of the clinopyroxene fibers must have crystallized directly from a supercooled liquid at temperatures above the glass transition. Moreover, the absence of fractures within the spherulites and the surrounding glass as well as the observed nucleation of clinopyroxene fibers at vesicle walls are interpreted to be consistent with a primary origin of the clinopyroxene spherulites. Based on these textural observations it has to be concluded that spherulites do not represent a diagnostic texture for the devitrification of volcanic glass that occurs below the glass transition temperature.

Trace element content of quartz from the Ehrenfriedersdorf Sn-W deposit, Germany: Results of an acid-wash procedure

The trace element content of quartz from the Ehrenfriedersdorf Sn-W deposit has been determined by ICP-MS. It has been tested whether acid-washing of the finely crushed samples modifies the composition of the bulk quartz samples. In the case of the pure quartz analyzed, untreated and treated sample splits show very similar trace element contents suggesting that this method of sample preparation can be used to remove traces of associated minerals such as calcite without influencing the contents of elements required for genetic interpretation. The quartz samples from the Ehrenfriedersdorf deposit show high concentrations of Li, Rb, and K suggesting that these elements were enriched in the ore-forming hydrothermal-magmatic system. High Al concentrations in quartz are interpreted to result from the elevated temperatures of quartz formation. The REE patterns of the quartz samples are similar to those previously observed in fluorite.