Geological Survey of Denmark and Greenland Bulletin 41, 2018, 43-46 43 During the 2013 field season, siliciclastic and carbonate rocks of the lower Palaeozoic sedimentary succession of the Frank- linian Basin in Amundsen Land, central North Greenland, were collected for whole-rock geochemical analysis. These data are evaluated here in an attempt to identify possible hydrothermal signatures related to sediment-hosted Zn-Pb mineralisation, similar to that found in correlative strata at the large Citronen Fjord deposit located c. 100 km to the east- north-east. In this paper, we use the term Sedex in a broad sense to describe stratiform, sediment-hosted deposits that formed either by syngenetic (exhalative) processes or by sub- sea-f loor replacement coeval with sedimentation (e.g. Emsbo et al. 2016); the term Mississippi Valley-type (MVT) is used for non-stratiform Zn-Pb deposits that formed epigenetically during late diagenesis or tectonism (e.g. Leach et al. 2010). Regional setting The Late Precambrian to Devonian Franklinian Basin extends c. 2000 km from the Canadian Arctic Islands to eastern North Greenland (Higgins et al. 1991). In eastern North Greenland, this basin fill overlies the Proterozoic In- dependence Fjord Group and the Hagen Fjord Group, cor- responding to the passive continental margin of Laurentia. The Franklinian Basin is characterised by a transition from a deep-water trough, with mainly fine-grained siliciclastic strata, separated from shelf carbonates to the south (Fig. 1; Higgins et al. 1991). As summarised in Kolb et al. (2016), Zn-Pb mineralisation in the Franklinian Basin resulted from two different events: early exhalative and/or sub-sea- f loor replacement in deep-water siliciclastic rocks, and late epigenetic MVT mineralisation in shelf carbonates. The present study concerns the potential for Zn-Pb mineralisa- tion in the Lower Ordovician to Lower Silurian Amundsen Land Group, in Amundsen Land. In the study area, the Amundsen Land Group comprises black bedded chert and laminated mudstone, commonly siliceous, with subordinate thin-bedded siliceous turbidites and greenish siltstone; lo- cally, thick redeposited chert and limestone conglomerate interbedded with thick calcareous turbidites are present (Fri- derichsen et al. 1982). The chert contains radiolarians (Hig- gins et al. 1991), implying that biogenic silica is responsible for the quartz-rich nature of these rocks, and the siliceous mudstone. Approximately 100 km east-north-east of the study area, in northern Peary Land, correlative siliciclastic rocks host the large undeveloped, sediment-hosted Citronen Fjord de- posit (Fig. 1; van der Stijl et al. 1998), with reported total resources (measured + indicated + inferred), at a 2.0% Zn cut off, of 132 Mt with 4.0% Zn and 0.4% Pb (Ironbark Zinc Base-metal and REE anomalies in lower Palaeozoic sedimentary rocks of Amundsen Land, central North Greenland: implications for Zn-Pb potential Diogo Rosa, John F. Slack and Hendrik Falck Greenland 20°W 82°N 30°W 100 km Nava rana Fjo rd esca rpme ntNava rana Fjo rd esca rpme nt Peary Land Johannes V. Jensen Land A B Amundsen Land Silurian (sandy turbidite) Cambro-Ordovician (siltstone, mudstone) Early Cambrian (mudstone, sandstone, conglomerate) Basement D ee p- w at er de po sit s Sh el f de po sit s Silurian (carbonates) Cambro-Ordovician (carbonates, minor mudstone) Early Cambrian (sandstone, mudstone) Franklinian Basin Proterozoic (sandstone, carbonates, dolerite, basalt) Quaternary overburden Kap Washington volcanic rocks Cretaceous–Cenozoic (lava, pyroclastic rocks) Wandel Sea Basin Carboniferous–Cenozoic (fluvial/marine sandstone, carbonates, shale) Fig. 1. Geology of central North Greenland, showing locations of sampled section in Amund- sen Land (A) and of Citronen Fjord Zn-Pb depo- sit (B); modified after Escher & Pulvertaft (1995). © 2018 GEUS. Geological Survey of Denmark and Greenland Bulletin 41, 43–46. Open access: www.geus.dk/bulletin http://www.geus.dk/bulletin 4444 2012). In the model of Slack et al. (2015), this deposit formed predominantly by exhalative processes. Younger epigenetic, carbonate-hosted, MVT Zn-Pb oc- currences, found in the carbonate shelf in southern Peary Land, are related to the migration of basinal brines expelled by tectonism and/or hydraulic head caused by Ellesmerian orogenic uplift during the Middle to Late Devonian (Rosa et al. 2016). In Amundsen Land, no carbonate shelf exists, so this mineralisation style is not expected to be present, al- though effects of the Ellesmerian orogeny are well expressed by open to recumbent folds and local thrust faults. Methods All samples were collected along one section across strata of the Amundsen Land Group at WGS84 longitude 35°.3647 W and latitude 82°.9655 E (Fig. 1). Twenty-two samples of silty limestone, dolomitic mudstone and mudstone were analysed using a variety of methods. All data are from Acme Analytical Laboratories Ltd. in Vancouver, British Colum- bia (Canada), except Y and rare-earth elements (REE) that were determined at Activation Laboratories Ltd. in Ancaster, Ontario (Canada). Detailed information on methods, stand- ards, and uncertainties are given on the respective web sites (www.acmelab.com; www.actlabs.com). Complete analyses of all 22 samples are available in Appendix A (online Excel file). Results Several samples have distinctive bulk compositions. For major-element oxides, one of three grey mudstones contains slightly high Fe 2 O 3 T (7.83 wt%) relative to average shale (6.75 wt%; Appendix IV in Krauskopf & Bird 1995); this sample also has elevated MnO (0.14 wt%) in contrast to the other samples that contain <0.05 wt% MnO. The three mudstones have uniformly low total S and organic C (<0.8 wt% and <0.7 wt%, respectively). For metals of economic and explora- tion interest, one mudstone sample is noteworthy for having slightly anomalous Zn (174 ppm), Pb (29.6 ppm), Ni (75.0 ppm) and As (24.7 ppm) relative to average concentrations in shale (Zn = 95 ppm; Pb = 20 ppm; Ni = 68 ppm; As = 13 ppm; Krauskopf & Bird 1995, Appendix IV). One sample of silty limestone has the highest total S (1.27 wt%) and Pb (63.0 ppm) among all 22 analysed samples, the latter con- centration being highly anomalous relative to the average of 3.1 ppm Pb for unaltered limestone (Hartree & Veizer 1982). Abundances of REE vary greatly from 0.6–2.0 × aver- age Post-Archaean Australian Shale (PAAS; Fig. 2). Most of the mudstone and all of the carbonate-rich samples (silty limestone, dolomitic limestone, calcareous shale) display relatively f lat PAAS-normalised patterns, which are typical of sedimentary rocks from throughout the geological record (e.g. McLennan 1989). However, one mudstone and both si- liceous mudstone samples show slight depletion of light rare- earth elements (LREE). Most of the silty limestone samples display slight enrichment of LREE. Calculated Eu anomalies (Eu/Eu*), relative to PAAS, range from 0.90 to 1.51; 20 of 22 samples have positive anomalies, the three highest values (1.41–1.51) occurring in silty limestone. These Eu anomalies are not an analytical artifact of Ba interference on Eu (e.g. Slack et al. 2004), because no correlation exists between Eu/ Eu* and Ba. Also important is the fact that all samples dis- play small negative Ce anomalies (Ce/Ce*), which relative to PAAS vary from 0.81 to 0.95; most are true anomalies (i.e., unrelated to anomalous La enrichment), based on a discrimi- nant plot of Pr/Pr* vs Ce/Ce* (Fig. 3). Field of shaley carbonates (n = 5) in hangingwall of Citronen Fjord deposit Footwall of Citronen Fjord deposit 0.1 1 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sa m pl e/ PA A S Sa m pl e/ PA A S A 0.1 1 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu B Mudstone Siliceous mudstone Silty limestone Calcareous mudstone Dolomitic mudstone Fig. 2. Plots of rare-earth element concentrations of representative sam- ples of early Palaeozoic sedimentary rocks from Amundsen Land Group in Amundsen Land. A: mudstone and siliceous mudstone. B: Calcareous mudstone, dolomitic mudstone, and silty limestone. Field of samples host- ing the Citronen Fjord deposit are included (Slack et al. 2015), for compari- son; note that small positive Eu anomalies for these samples (1.16-1.29) are not evident due to overlapping patterns. Normalisations are to average Post- Archean Australian Shale (PAAS); data from Taylor & McLennan (1985). http://www.acmelab.com http://www.actlabs.com http://www.geus.dk/media/19170/nr41_p43-46-appendix-a-whole-rock-analyses.ods 45 Discussion The presence in one mudstone sample of slightly high Fe 2 O 3 T, Zn, Pb, Ni and As is permissive evidence of a hydro- thermal component being present in the basin. The small LREE depletion in this sample and in the two siliceous mudstone samples (Fig. 2A), likely ref lects the dissolution of detrital apatite, which in low-temperature sedimentary en- vironments occurs by interaction with acidic f luids and not typical seawater-derived pore f luids (see Slack et al. 2017). The geochemical data for this mudstone sample, namely elevated MnO together with very low Mo, record sedimenta- tion and early diagenesis in oxic bottom waters (e.g. Slack et al. 2017). Oxic bottom waters are consistent with the presence of small negative Ce anomalies in this sample, in both siliceous mudstone samples, and in most of the carbonate-rich rocks. These conditions, as well as the apparently low availability of H 2 S in pore f luids beneath the palaeo-sea f loor (total S <0.8 wt%), were also proposed by Slack et al. (2015) for the host sedimentary rocks during initial formation of the Citronen Fjord deposit. However, according to their model for that de- posit, only after emplacement of debris f lows that physically restricted the local basin and sealed off communication with the larger oxic ocean, did the venting of hydrothermal f luids turn the bottom waters anoxic and possibly locally very re- ducing (euxinic) and allow for sulphide preservation. If this model for the redox evolution of the Citronen Fjord deposit is correct, an analogous scenario for Amundsen Land (this study) hinges on verifying the local presence of anoxic to eux- inic bottom waters, a requirement as yet unachieved, without supporting evidence from additional sampling and analyses. The presence of small positive Eu anomalies in most samples is consistent with a hydrothermal component (e.g. Lottermoser 1992). However, other non-hydrothermal pro- cesses can also create small positive Eu anomalies in sedi- mentary rocks, both siliciclastic and carbonate. For example, in organic-rich black shales, small positive Eu anomalies may form diagenetically in euxinic pore f luids (Slack et al. 2017, and references therein), but no evidence of such f lu- ids exists in the geochemically anomalous mudstone, based on its elevated MnO (0.14 wt%) coupled with low organic C (0.37 wt%) and very low Mo (1.75 ppm) contents, which together indicate oxic (not anoxic or euxinic) bottom waters and pore f luids (see Slack et al. 2017). Furthermore, TOC values lack any correlation with metal concentrations. The relatively high Fe 2 O 3 T content of this mudstone sample could be a hydrothermal signature, but might also ref lect a detrital component derived from a Fe-rich source area. Regarding the positive Eu anomalies present in all of the carbonate samples, a possible non-hydrothermal origin for this anomaly may be related to a large clay component (Tostevin et al. 2016), but this explanation is ruled out by the fact that the samples with the highest Eu/Eu* values (1.41–1.51) have uniformly low Al 2 O 3 (0.49–0.62 wt%). Given these observations, we conclude that the small positive Eu anomalies ref lect a hy- drothermal signature, involving the passage of reduced f lu- ids that preferentially carried Eu2+ (Bau 1991). Importantly, a hydrothermal origin has also been proposed by several work- ers for positive Eu anomalies in the carbonate gangue and carbonate-rich wall rocks and country rocks of several strati- form Sedex deposits (e.g. Slack et al. 2004; Frimmel 2009). The inferred hydrothermal component in the early Pal- aeozoic siliciclastic and carbonate rocks of the studied section can be ascribed to either a distal or a proximal source, or both. In the case of a distal source, the likely prolonged (c. 105–106 y) venting of hydrothermal f luids into seawater to form the Citronen Fjord deposit could account for the Eu incorporat- ed into the distal mudstones and carbonates of Amundsen Land during sedimentation, by mixing of hydrothermally derived Eu with seawater. In the latter case, involving a prox- imal source, the observed base-metal and REE anomalies – both Eu and LREE – in the samples analysed here could record a hydrothermal signature from a local system of either syngenetic or epigenetic origin. Given the apparent lack of organic-rich black shales in the study area with anoxic or eu- xinic redox signatures, a syngenetic origin for this postulated Zn-Pb mineralisation is considered unlikely, either by purely exhalative or downward-penetrating brine processes (Emsbo et al. 2016; Sangster 2018). The occurrence of undiscovered MVT Zn-Pb deposits is also possible (Rosa et al. 2016), but this type of mineralisation is characterised by negative, not 0.90 0.95 1.00 1.05 1.10 1.15 1.20 0.6 0.7 0.8 0.9 1.0 1.1 1.2 C e/ C e* PA A S Pr/Pr* PAAS True negative Ce anomaly True positive Ce anomaly Fig. 3. Plot of Ce anomaly (Ce/Ce*) vs Pr anomaly (Pr/Pr*) for analysed samples of early Palaeozoic sedimentary rocks from the Amundsen Land Group in Amundsen Land. Data are normalised to PAAS. Fields after Bau & Dulski (1996). Symbols as in Fig. 2. 4646 positive, Eu anomalies in carbonate host rocks and gangue minerals (e.g. Graf 1984; Souissi et al. 2013). In summary, considering all available field and geochemi- cal data, including the lack of evidence for anoxic or euxinic bottom waters during sedimentation, we suggest that the base-metal and REE anomalies highlighted in this study from the Amundsen Land Group, in Amundsen Land, favour a potential for local Sedex Zn-Pb mineralisation that formed mainly by the sub-sea-f loor replacement of carbonate-rich sediments. Additional sampling and geochemical analysis are recommended for the study area, in order to better evalu- ate this mineral potential. Acknowledgements This work was financially supported by GEUS and the Ministry of Min- eral Resources of Greenland, through the Nordzinc project. Additional support was provided by the Northwest Territories Geological Survey, Canada. Per Kalvig and Erik Vest Sørensen of GEUS are acknowledged for collaboration during field work. 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