Hydrothermal fracture mineralization in the stable cratonic northern part of the Baltic Paleobasin: sphalerite fluid inclusion evidence

Long‐distance fluid migration defines the diagenetic history of unique Ediacaran sediments in the East European Craton

“…Also, in central Estonia, ca. 250 km SW from the St. Petersburg region, Eensaar et al., (2016) documented 64°C–200°C temperatures from fluid inclusions in sphalerite, crystallized at the same time (early Middle Devonian) as the illitization of smectite in the Ordovician bentonites of the Baltic Basin (Środoń, Clauer, et al, 2009). Along with the occurrence of authigenic K‐feldspar, the data presented above support the interpretation that was developed for the lower Palaeozoic bentonites in the Baltic Basin (Chaudhuri, Środoń, & Clauer, 1999; Somelar, Kirsimäe, Hints, & Kirs, 2010; Somelar et al., 2009; Środoń, Clauer, et al, 2009); hydrothermal K‐rich fluids, generated at the orogenic belts NW of the craton, migrated across the EEC and produced illitization.…”

Heterogeneous REE + Y distribution in Early Paleozoic shelly phosphorites: Implications for enrichment mechanisms

“…Redistribution of REE+Y during late-stage diagenesis and metamorphism is largely controlled by the stability of the host-phases but large-scale redistribution of bulk rock REE+Y is rare (Chakrabarti et al, 2007;Ohr et al, 1994), unless influenced by hydrothermal alteration (e.g., Chetty and Gutzmer, 2012). Resetting of the REE+Y signatures due to hydrothermal alteration in Estonian shelly phosphorite seems unlikely given the overall tectonic stability and shallow burial of the sedimentary sequences in the northern Baltica Paleobasin with only limited evidence for hydrothermal activity (Eensaar et al, 2017;Gaškov et al, 2017;Somelar et al, 2010), excellent preservation of conodonts with CAI values <1.5 (Heinsalu et al, 2003), and the absence of Eu-anomalies, that would be indicative of Eu 2+ /REE 3+ decoupling in high temperature (>200 °C) processes (Bau et al, 2010). However, even in sediments not influenced by hydrothermal activity, protracted uptake of REE+Y by biogenic apatite can still occur during late-stage diagenesis (Herwartz et al, 2011;Kowal-Linka et al, 2014) and can, in some cases, account for >50% of the REE found in apatite (Kocsis et al, 2010).…”

Illitization of the lower Cambrian (Terreneuvian) Blue Clay in the northern Baltic Palaeobasin

“…Using Ar diffusion modelling, they suggested that the BC in the Lontova Formation was heated to a temperature of 130-140 °C for a duration of 2-5 Myr or the temperature may have been as high as 170 °C during a shorter period of time (< 1 Myr), in relation to largescale fluid migration and dolomitization in the Silurian-early Devonian. Indeed, recent studies (Eensaar et al 2017a(Eensaar et al , 2017bGaškov et al 2017) have shown evidence of hydrothermal activity in the northern BP and have delineated a Zn-Pb mineralization event driven by the intrusion of highly saline low to moderate temperature (60-200 °C) fluids, the maximum age of which can be constrained by crosscutting relationships to the early Middle Devonian (Eifelian). However, this event is tied to thin crosscutting sphalerite-(galena)-calcite veins and scattered fracture zones with no evidence of significant wallrock alteration (Eensaar et al 2017a).…”

“…Indeed, recent studies (Eensaar et al 2017a(Eensaar et al , 2017bGaškov et al 2017) have shown evidence of hydrothermal activity in the northern BP and have delineated a Zn-Pb mineralization event driven by the intrusion of highly saline low to moderate temperature (60-200 °C) fluids, the maximum age of which can be constrained by crosscutting relationships to the early Middle Devonian (Eifelian). However, this event is tied to thin crosscutting sphalerite-(galena)-calcite veins and scattered fracture zones with no evidence of significant wallrock alteration (Eensaar et al 2017a). Moreover, the large scatter of Rb-Sr and K-Ar ages of the BC finest fractions excludes the possibility of short (<5 Myr) abnormal heat and/or hydrothermal pulses responsible for the BC illitization that would have not been registered in organic material maturation data but have been re sponsible for illitization, which in this case should be characterized by a welldefined isotope signal.…”