Geochemical Characteristics of Granitoids (Ho Gneiss) from the Pan – African Dahomeyide Belt, Southeastern, Ghana: Implications for Petrogenesis and Tectonic Setting

The Pan-African Dahomeyide belt exposed in Southeastern, Ghana, consist of granitoid gneisses locally known as Ho gneiss. These rocks are thought to be part of the West African Craton (WAC) which was reworked during the Pan-African Orogeny, and may be time equivalent with the Kara gneisses. Petrographical and whole rock geochemical analyses have been used to evaluate the characteristics, petrogenesis and mode of emplacement of the granitoids. The new data reasonably suggest that the Ho gneiss consist mainly of biotite augen gneisses of both mafic and felsic rock suites. Geochemically, these rocks show tonalitic to quartz monzonite but mainly granodiorites affinities. They are metaluminous to weakly peraluminous, I-type, magnesian to ferroan and calcic to alkali calcic. With these varying features, the granitoids mimic mantle derived magma source which mixed or mingled with crustally derived melt. The rocks display varying REEs and trace elements patterns but, their LREEs and LILE enrichment with  the noticeable enrichment in Rb, Ba, K and especially Pb and negative Ti, Ta, Nb observed among majority of the samples are typical signature of ‘‘arc rocks’’ or continental crustal materials. Their incompatible trace element ratios, such as Th/U (1.07 – 13.87), K/Rb (272 – 574.47), Th/Yb (0.79 – 15.09), Ta/Yb (0.25 -0.64 ), Ce/Pb (mainly 1.62 – 7.88 ) and high Ba/Nb (19.55 – 314.17, with TA = 1565.00), are similar to those of the continental crust. The rocks are characterised by subduction related Sr/Y content (< 100), except sample MA8 that shows high concentration of Sr/Y (227.71). The magnesian affinity reflects relatively hydrous, oxidizing source which is consistent with origins that are broadly subduction related. Their high – K nature also points to an important petrogenetic role of remelting and differentiation of arc – accretionary complex crust. These geochemical signatures are likely to be related to metasomatism of the sub – continental lithospheric mantle via crustal recycling. In conclusion, the granitoids may have from melting of igneous source in a subduction related environment. Keywords: Pan-African belt, Dahomeyide, Southeastern Ghana, Ho gneisses, Geochemistry, petrogenesi

Stable and radiogenic isotopes as tracers for geochemical processes in mineralogically-complex mine waste environments : Insights from 13C, 2H, 18O, 34S and 87Sr/86Sr.

Mining and its related activities generate large volumes of mine wastes such as tailings that can have negative environmental implications. One of such mine wastes of potential environmental concern is the historical Yxsjöberg Cu-W-F skarn tailings in Sweden, which encompasses a complex mineralogy including sulphides, carbonates, silicates, oxides / wolframite and halides. Some of these minerals contain high contents of potentiallytoxic elements such as Be, Bi, Cu, F, Zn and W; hence posing a significant threat to surrounding soils, aquatic ecosystems and drinking water quality due to their weathering. Potential remediation strategies for this site require a detailed understanding of the mobilization and transportation of contaminants in and from the tailings to the surrounding environments. Therefore in this present work, chemical and isotopic (18O, 13C, 34S, 87Sr/86Sr) composition of minerals, tailings and water-soluble (WS) fractions of the tailings were used to gain comprehensive insights into geochemical processes including mineral weathering and precipitation, trace elemental sources and sulphide oxidation reaction pathways within the tailings. Furthermore, chemical composition and water mixing analyses, aimed at quantifying the elemental contributions of the tailings and tailings groundwater to the downstream surface waters were carried out using 18O, 2 H and 87Sr/86Sr isotope data of ground and surface waters collected during 6 different sampling campaigns from May to October 2018. Subsequently, the consistency of the mixing analyses was evaluated via a simple model. Biogeochemical processes regulating the stable carbon isotope signatures of dissolved inorganic carbon; DIC (δ13CDIC) of the ground and surface waters were also investigated. Three distinct geochemical zonation namely; oxidised (OZ), transition (TZ) and unoxidised zones (UZ) based on pH, elemental contents and colour, were observed in the tailings as a result of their long term storage and exposure to oxidising conditions. The OZ was characterised by a low pH (3.6 - 4.5), depletion of S as well as Be, Co, Cu and Zn in both the bulk tailings and WS fractions, particularly in the upper OZ (UOZ). On the other hand, the WS fractions of the lower OZ (LOZ), which included the oxidation front, recorded elevated concentrations of these trace metals and SO4 2- .Mineralogical observations and elemental contents of the tailings as well as the 34S and 18O signatures of SO4 2- ( 18OSO4 and 34SSO4) of the WS fractions in the OZ, TZ and upper UZ (UUZ) pointed to the historical and extensive weathering / oxidation of danalite and pyrrhotite by Fe3+ (i.e. at pH ≤ 3) in the UOZ, particular during their early stages after deposition when the sulphide surfaces were fresh. Very radiogenic 87Sr/86Sr ratios coupled with elevated concentrations of silicate-associated elements such as Al, Fe, K and Mg in the WS fractions of the OZ indicated the weathering of biotite, K-feldspar and muscovite. Weathering of Ca-bearing minerals as well as the dissolution of secondary minerals (e.g. gypsum) in the LOZ resulted in high Ca/K ratios. In the TZ and UUZ, the WS fraction 87Sr/86Sr and 34SSO4 values as well as consistent peaks of Ca, Be, Mn, SO4 2- and Zn suggested the dissolution of gypsum with a similar isotopic composition as danalite. Danalite was weathered in the OZ and hence Be was assumed to have been mobilised from this zone and trapped secondarily in gypsum in the UUZ. The 18OSO4 and 34SSO4 signatures of the WS fractions in the middle UZ suggested their mobilisation from the current oxidation front and represented mixed signals from the incomplete oxidation of pyrite, pyrrhotite and chalcopyrite via atmospheric oxygen (O2), resulting in the potential formation of intermediate sulphur species such as elemental S. Negative 18OSO4 and 34SSO4 signatures of the lower UZ were attributed to their probable release from processes such as carbon-bonded sulphur mineralisation and hydrogen sulphide (H2S) oxidation in the forests surrounding the mine site. Recorded negative δ13C signatures of some carbonates (average = -2.7 ‰) in the tailings compared to that of primary calcite (δ13C= +0.1 ‰)signaled the precipitation of secondary carbonates. The δ13C values of these secondary carbonates were ascribed to a mixture of C sources from atmospheric CO2, degraded organic matter and primary calcite dissolution in the tailings. The water mixing analyses indicated that the elemental contributions of the tailings groundwater to the downstream surface waters were small (1 -17 %), resulting in low dissolved concentrations of various elements in the latter relative to that of the former and thus a low negative environmental impact. The results of the mixing analyses and the model suggested that the low elemental concentrations of the surface waters were due to various potential retention mechanisms such as precipitation, sorption and reductive processes within the tailings and at the outlet of the tailings. The δ13CDIC values of the groundwater samples were attributed to mixed C signals from the primary calcite and potential secondary carbonate dissolution in the tailings as well as degradation of the vegetation and sewage sludge on the tailings, the peat underneath the tailings and the surrounding forests. The δ13CDIC signatures of the downstream surface waters seemed to be dependent on the climatic seasons and groundwater contributions. However, limitations with respect to unavailable data on DIC concentrations as well as a myriad of potential biogeochemical processes that could influence the DIC pool and δ13CDIC values of the surface waters made it difficult to pinpoint the major regulating process (es) of the δ13CDIC signatures. Nonetheless, the results of this study shows that the use of the 18O, 34S, 87Sr/86Sr and to a lesser extent 13C isotopes, coupled with the chemical and mineralogical data offer better insights into discriminating between various elemental sources and related geochemical processes, especially in mineralogical-complex setting such as skarn tailings. Furthermore, the results of the mixing analysis and the model data emphasises the importance of a thorough understanding of the hydrogeochemical processes along groundwater flow paths, as these processes can modulate the amount of dissolved elements reaching the surface waters

Biogeochemical Controls on 13CDIC Signatures from Circum-Neutral pH Groundwater in Cu–W–F Skarn Tailings to Acidic Downstream Surface Waters

Regular features of ground and surface waters affected by drainage from mine waste include their acidity and elevated concentrations of dissolved metals, with their attendant negative effects on drinking water quality and aquatic life. One parameter that aids in buffering these waters against acidity and sustains aquatic life is dissolved inorganic carbon (DIC). In this study, the chemical and isotopic (δ13C) composition of primary calcite and DIC (δ13CDIC) in groundwater and surface waters within and downstream, respectively, of abandoned Cu–W–F skarn tailings at Yxsjöberg, Sweden, were used to trace the biogeochemical processes controlling their respective δ13CDIC signatures. In addition, the δ13C signatures of the inorganic (carbonate) fractions of the tailings were used to verify the formation of secondary carbonates within the tailings. Lower average δ13C values of the carbonate fractions (δ13Ccarb = −2.7‰) relative to those of the primary calcite (δ13C = +0.1‰) from the orebodies from which the tailings originated pointed to the precipitation of secondary carbonates. These lower δ13Ccarb signatures were assumed to represent mixed-source C signals involving isotopically light CO2 from the atmosphere, the degradation of organic matter in the upper part of the tailings and HCO3− from calcite dissolution. The groundwater δ13CDIC values (−12.6‰ to −4.4‰) were far lower than the hypothetical range of values (−4.6‰ to +0.7‰) for primary calcite and secondary carbonate dissolution. These signatures were attributed to carbonate (calcite and secondary carbonate) dissolution and the degradation of dissolved organic carbon (DOC) from various organic sources such as peat underneath the tailings and the surrounding forests. Downstream surface water samples collected in May had low δ13CDIC values (−16‰) and high DOC (14 mg C/L) compared to the groundwater samples. These signatures represented the oxidation of the DOC from the wash out of the mires and forests during the snowmelt and spring flood. The DOC and δ13CDIC values of the surface waters from June to September ranged from 6–15 mg·C/L and −25‰ to −8.6‰, respectively. These signatures were interpreted to reflect mixed C sources, including carbonate dehydration by acidity from Fe3+ hydrolysis due to the mixing of groundwater with surface waters and the subsequent diffusive loss of CO2 (g), aquatic photosynthesis, photooxidation, DOC degradation, as well as microbial respiration. Although the 13CDIC signatures of the downstream surface waters seemed to be seasonally controlled and influenced by variable groundwater contributions, the lack of data with respect to DIC concentrations, coupled with multiple potential biogeochemical processes that could influence the DIC pool and 13CDIC values, made it difficult to identify the major regulating process of the 13CDIC signatures. Therefore, other complimentary isotopes and elemental concentrations are recommended in order to decipher the dominant biogeochemical process

A simple model for evaluating isotopic (18O, 2H and 87Sr/86Sr) mixing calculations of mine : Impacted surface waters

This study was aimed at identifying and quantifying mixing proportions in surface waters downstream of historical Cu-W-F skarn mine tailings at Yxsjöberg, Sweden, using 18O, 2H, and 87Sr/86Sr isotopes. In addition, a simple mathematical model was developed to evaluate the consistency of the mixing calculations. Hydrochemical and isotopic data from 2 groundwater wells, 6 surface water and 2 rainwater sampling sites, spanning 6 sampling campaigns between May and October were used. Three mixed surface waters downstream of the tailings were identified, namely: C7, C11 and C14. C7 was directly influenced by groundwater from the tailings whereas C11 was also subsequently influenced by C7. C14 on the other hand, had contributions from C11. Sequential mixing calculations indicated that the contribution of the groundwater to C7 ranges from 1 to 17%. The subsequent contribution of C7 to C11 varied from 49 to 91% whereas C14 had contributions of C11 ranging between 16 and 56%. A strong agreement between the model data (MD) and measured raw data (RD) for C11 and C14 indicated the accuracy of the mixing calculations. Variations between the MD and RD at C7, however, was mainly due to sorption and reductive processes underneath the tailings, which tend to attenuate the amount of dissolved ions reaching the surface waters, resulting in a low ionic contribution of the tailings groundwater to the surface water. The low ionic contribution of the groundwater to C7 suggested that although the tailings impoundment is of environmental concern, its impact on the downstream surface waters is small. The results of this study suggest that mixing calculations in surface waters involving a closed system such as groundwater (as an end-member) must be treated with caution. It is recommended that the interpretation of such mixing results must be coupled with detailed knowledge of the potential hydrogeochemical processes along its flow paths.Validerad;2020;Nivå 2;2020-04-29 (alebob)</p

Sr/Ca and 87Sr/86Sr : A tracer for geochemical processes in mine wastes

Understanding geochemical processes in mining environments are essential to waste management decisions including remediation. In an attempt to understand geochemical processes, chemical data have mostly been used but these have oft en led to inaccurate conclusions. Th erefore, in this work 87Sr/86Sr, Sr/Ca and other elemental ratios (Ca/K and Rb/Sr) in leachates were employed to constrain the geochemical processes in an abandoned tungsten (W) tailings in Yxsjöberg, South-Central Sweden. Th e results of this study indicate that coupling chemical ratios with 87Sr/86Sr ratios off er better insights in discriminating between diff erent geochemical processes in mine wastesISBN för värdpublikation: 978-0-620-80650-3Min-Nort

Sr/Ca and 87Sr/86Sr : A tracer for geochemical processes in mine wastes

Understanding geochemical processes in mining environments are essential to waste management decisions including remediation. In an attempt to understand geochemical processes, chemical data have mostly been used but these have oft en led to inaccurate conclusions. Th erefore, in this work 87Sr/86Sr, Sr/Ca and other elemental ratios (Ca/K and Rb/Sr) in leachates were employed to constrain the geochemical processes in an abandoned tungsten (W) tailings in Yxsjöberg, South-Central Sweden. Th e results of this study indicate that coupling chemical ratios with 87Sr/86Sr ratios off er better insights in discriminating between diff erent geochemical processes in mine wastesISBN för värdpublikation: 978-0-620-80650-3Min-Nort

Min-North : Development, Evaluation and Optimization of Measures to Reduce the Environmental Impact of Mining Activities in Northern Regions

The Min-North (Development, Evaluation and Optimization of Measures to Reduce the Environment Impact of Mining Activities in Northern Regions) project was a trans-national cooperative project, with participants from the Geological Survey of Finland (GTK), University of Oulu (UO), UiT The Arctic University of Norway (UiT), Luleå University of Technology (LTU) and SMEs from Sweden, Finland and Norway. The project was funded by Interreg Nord and Norrbottens länsstyrelse. The participants have expertise in mine waste management, mine water treatment and geophysics. The overall aim of the project was to enhance the development of environmental protection technologies. An associated goal was to deepen cross-border cooperation by creating a larger critical mass of researchers in mine waste management and local SMEs in the Northern regions with greater capacities to disseminate and implement new methods, products and services. The project ran for 36 months from the 1st of January 2016 to the end of December 2018.  Min-Nort