Fractionation of Rare Earth Elements in Greisen and Hydrothermal Veins Related to A-Type Magmatism

Abstract

This study focuses on concentrations and fractionation of rare earth elements (REE) in a variety of minerals and bulk materials of hydrothermal greisen and vein mineralization in Paleoproterozoic monzodiorite to granodiorite related to the intrusion of Mesoproterozoic alkali- and fluorine-rich granite. The greisen consists of coarse-grained quartz, muscovite, and fluorite, whereas the veins mainly contain quartz, calcite, epidote, chlorite, and fluorite in order of abundance. A temporal and thus genetic link between the granite and the greisen/veins is established via high spatial resolution in situ Rb-Sr dating, supported by several other isotopic signatures (δ34S, 87Sr/86Sr, δ18O, and δ13C). Fluid-inclusion microthermometry reveals that multiple pulses of moderately to highly saline aqueous to carbonic solutions caused greisenization and vein formation at temperatures above 200–250°C and up to 430°C at the early hydrothermal stage in the veins. Low calculated ∑REE concentration for bulk vein (15 ppm) compared to greisen (75 ppm), country rocks (173–224 ppm), and the intruding granite (320 ppm) points to overall low REE levels in the hydrothermal fluids emanating from the granite. This is explained by efficient REE retention in the granite via incorporation in accessory phosphates, zircon, and fluorite and unfavorable conditions for REE partitioning in fluids at the magmatic and early hydrothermal stages. A noteworthy feature is substantial heavy REE (HREE) enrichment of calcite in the vein system, in contrast to the relatively flat patterns of greisen calcite. The REE fractionation of the vein calcite is explained mainly by fractional crystallization, where the initially precipitated epidote in the veins preferentially incorporates most of the light REE (LREE) pool, leaving a residual fluid enriched in the HREE from which calcite precipitated. Fluorite occurs throughout the system and displays decreasing REE concentrations from granite towards greisen and veins and different fractionation patterns among all these three materials. Taken together, these features confirm efficient REE retention in the early stages of the system and minor control of the REE uptake by mineral-specific partitioning. REE-fractionation patterns and fluid-inclusion data suggest that chloride complexation dominated REE transport during greisenization, whereas carbonate complexation contributed to the HREE enrichment in vein calcite.

In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults

“…Evaluation of method feasibility. The high level of detail on both spatial and depth scales by the new in situ fault dating technique emphasizes a close assessment of the significance of and relation between individual textural, chemical and geochronological features, following the procedure that in our recent study proved decisive for successful linkage of hydrothermal veins to granitic magmatism 56 . In that study, in situ Rb-Sr dating of muscovite/biotite ± K-feldspar ± calcite ± fluorite ± epidote in greisen and veins emanating from a Mesoproterozoic granite yielded ages (1432 ± 8 Ma) in accordance with crystallization age of the granite (1433 ± 10 Ma), thus confirming the temporal connection between granite, greisen and veins and a correspondence of the U-Pb (zircon) and the in situ Rb-Sr geochronological approaches.…”

Using 87Sr/86Sr LA-MC-ICP-MS Transects within Modern and Ancient Calcite Crystals to Determine Fluid Flow Events in Deep Granite Fractures

“…In brief, according to previous studies, the evolution of the O isotope composition reflects precipitation from hydrothermal fluids in the Paleo-Mesoproterozoic with depleted δ 18 O values (~−27 to −18% PDB) and 87 Sr/ 86 Sr values (0.705-0.710), trending to successively higher values at later events [23,32]. In the Laxemar-Äspö area, in situ Rb-Sr geochronology of vein assemblages of calcite with 87 Sr/ 86 Sr values of 0.707-0.708 reveals formation ages of 1431.7 ± 3.9 Ma and 1432.3 ± 7.5 Ma [38]. These values overlap with or are slightly more evolved than the initial 87 Sr/ 86 Sr (0.7040-0.7081) of coeval granitoids in southern Sweden [85], but not as evolved as the present-day 87 Sr/ 86 Sr of the wall rock in the Laxemar-Äspö area that ranges from 0.713540 in monzodioritic parts, to 0.716009 in granodiorite [32].…”

“…For Laxemar-Äspö, the large span in 87 Sr/ 86 Sr values of the calcite crystals reflects fluid flow and precipitation over much longer time spans. Particularly, in sample KLX09:740 m, the crystal cores have values (0.705-0.707, Figures 6b and 7b) that point to the onset of calcite formation in this fracture already in the Mesoproterozoic (~1431.7 ± 3.9 Ma and 1432.3 ± 7.5 Ma [38]), whereas the younger overgrowths represent a precipitation event with Paleozoic signatures and outermost overgrowths that have overlapping values with the present-day groundwaters. Intermittent crystal growth may thus have occurred for almost 1.5 billion years in this partly open fracture.…”

“…We connect the calcite Sr isotope compositions with those of modern groundwater to determine whether the youngest of these calcite growth zones could be precipitated from the present-day waters. We compare and link the distinguished generations with published geochronological constraints [34,[36][37][38][39] of the same fracture assemblage to assess the overall temporal span of the precipitation history in presently water-conducting fractures. We also investigate the 87 Sr/ 86 Sr variability of modern calcite precipitated from fracture water in a borehole at 415 m depth over the course of 17 years, and compare these 87 Sr/ 86 Sr values with the modern source water.…”

Reconstructing craton‐scale tectonic events via in situ Rb‐Sr geochronology of poly‐phased vein mineralization

“…extension related to Rapakivi granite magmatism in eastern parts of the craton (Söderlund et al, 2008). At Laxemar, there are also veins directly related to hydrothermal fluid circulation and greisenization introduced by local granite intrusions of Danopolonian-Hallandian association (Tillberg et al, 2019). The 1,299 ± 36 Ma K-feldsparfluorite mineralization at Siljan temporally overlaps, and may thus be related to 1.27-1.26 Ga mafic magmatism and related crustal extension near Siljan (Söderlund et al, 2006).…”