This paper addresses the complex hydrothermal evolution of Metasomatic Iron and Alkali-Calcic (MIAC) mineral systems based on a review of the lithogeochemical footprints of IOA and IOCG deposits in the northern Norrbotten province (Sweden) and the Cloncurry district (Australia). The use of Na-Ca-Fe-K-Mg molar barcodes on a lithogeochemical diagram tailored for these mineral systems allows to depict the evolution of MIAC systems along diagnostic metasomatic paths from high (HT) to low temperature (LT) alteration facies as follows: 1) HT or LT Na alteration (300–1000 °C); 2) HT Ca Fe alteration (400–1000 °C); 3) HT K Fe alteration (350–450 °C); 4) HT K and HT K-Ca-Mg alteration; 5) LT K Fe, Na-Ca-Mg-Fe, and/or Na-Ca-Mg alteration (≤ ~350 °C); and 6) epithermal alteration (≤ 150 °C) and later stage hydrothermal veining. A distinct range of whole rock compositions and metal associations characterizes each alteration facies and can be captured by diagnostic molar barcodes and alteration indices. In northern Norrbotten, the IOA deposits are hosted in HT Ca Fe alteration facies but regionally intensely albitized regions are overprinted by K Fe alteration. The IOCG deposits are hosted in MIAC systems with zones of early Na ( Ca) alteration related to the regionally extensive albitite or scapolite alteration (Facies 1) and localized skarns. These are overprinted by HT Ca Fe alteration (Facies 2) and HT to LT K Fe alteration (Facies 3 and 5). The Cu Au mineralization is not systematically associated with the iron oxide-rich breccias and the intense K-feldspar- or sericite-rich K Fe alteration typical of many IOCG deposits worldwide. Instead, the lesser intensity of alteration and the abundance of mafic and ultramafic rocks in the environment lead to pattern enriched in Mg with relic of amphibole-rich alteration remaining in the assemblage as demonstrated for the Nautanen North IOCG deposit (Sweden). Consequently, the geochemical footprints of the Norrbotten Cu Au deposits are distinct from magnetite-group (e.g., Great Bear magmatic zone, Canada) and hematite-group (e.g., Olympic Dam, Australia) IOCG deposits even if they have all the known alteration facies of MIAC systems. Conversely, IOCG deposits in northern Norrbotten show similarities to certain deposits in the Cloncurry district of Australia. In both regions, the IOCG deposits are associated with HT Ca Fe and K Fe alteration facies that commonly overprint early Na and/or Na Ca alteration. In northern Norrbotten, IOA deposits are characterized by early Na alteration evolving towards Na Ca alteration, then Fe-rich Ca Fe alteration. These hydrothermal alteration types are subsequently superimposed by later K Fe alteration. We conclude that the use of Na-Ca-Fe-K-Mg molar barcodes provides new insights to understand the evolution of MIAC systems and is a powerful approach for unraveling superimposed alteration trends, which can serve as an exploration targeting tool from the district- to the deposit-scale in complex metasomatized areas.

This paper presents a comprehensive review of Paleoproterozoic (c. 1.8 Ga) lithium-caesium-tantalum (LCT)-type pegmatite mineralization in central Fennoscandia corresponding to the geologically correlative Bothnian Basin and Ostrobothnia areas of east-central Sweden and west-central Finland, respectively. A summary of the geological, structural and petrological characteristics of 19 key deposits and prospects is given based on previous descriptions and new field observations. These features are then integrated with historical and new lithogeochemistry data to assess deposit commonalities from a mineral systems perspective and discuss pegmatite petrogenesis. Identified mineral system components may aid the selection of relevant regional- to deposit-scale mappable criteria for mineral prospectivity mapping, exploration information systems analysis, or Li pegmatite exploration in central Fennoscandia and comparable regions.

Geologically, central Fennoscandia is dominated by stratigraphically correlative c. 2.0 – 1.9 Ga clastic metasupracrustal rocks that constitute the Bothnian supergroup in Sweden and parts of the Western Finland supersuite in Finland. These rock packages formed when continental-derived sedimentary ± epiclastic material and lesser mafic volcanic rocks were deposited in one or more evolving intra-arc marine basins during early subduction-related accretionary orogenesis. Continued accretion from c. 1.94 – 1.85 Ga resulted in mafic – felsic magmatism, basin inversion, polyphase deformation, and regional metamorphism. A renewed phase of continental arc-type magmatism from c. 1.81 – 1.78 Ga developed a paired I- and S-type granitoid belt, with peraluminous granites and pegmatites preferentially emplaced in the arc hinterland, and produced a relatively short-lived (c. 20 – 30 Myr) Li metallogenic event.

Key features of LCT-type pegmatites include mica schist, metapelite and/or amphibolite host rocks, preferential occurrence within lower-medium amphibolite facies metamorphic domains, emplacement within preexisting folds or deformation zones, mineralized pegmatites forming isolated bodies or clustered groups with subvertical to subhorizontal sheet-like or branching network geometries, heterogeneous (indistinct) regional mineralogical-chemical zonation relative to coeval granites, variably developed pegmatite internal textural-mineralogical zonation, typical ore assemblages of spodumene ± petalite ± lepidolite ± columbite-tantalite ± cassiterite ± pollucite, and narrow tourmaline ± mica ± quartz alteration haloes along pegmatite contacts with associated Li, Rb, Cs, Sn, Ta, Nb and Tl enrichments.

Multiscale mineral system components for LCT-type pegmatite mineralization are: (1) originally clay-rich metasedimentary source rocks derived from a mainly felsic volcanic terrane, (2) development and persistence of metamorphic biotite, staurolite, muscovite and cordierite in source rocks as likely carriers of Li, (3) generation of felsic magmas within the thicker, hot, hinterland part of a Cordilleran-type arc system, with anatexis and subduction-related mafic magmatism being key energy drivers, (4) reactivation of earlier formed crustal-scale deformation zones providing pathways for melt migration, (5) reactivation and/or development of second- to third-order deformation zones overprinting earlier folds, resulting in superimposed, multigenerational structural traps, (6) juxtaposition and intersection of structural traps with domains of relatively cool, low-medium amphibolite facies metasupracrustal rocks promoting down-gradient melt emplacement and cooling, and rapid pegmatite crystallisation, (7) late primary to secondary internal muscovite-sericite alteration, spodumene-quartz reprecipitation and Li, Rb, Ta, Nb, Sn remobilization, and (8) down-ice, laterally dispersed pegmatite boulder fans, indicator minerals and geochemical pathfinder haloes associated with Pleistocene glacial overburden.

This study investigates lithium-caesium-tantalum (LCT) pegmatites and associated pegmatites and granites in the Varuträsk area, northern Sweden, using a mineral systems approach (i.e. source-transport-trap). The research incorporates geological mapping both regionally and underground, analysis of geological structures and host rock competency, 3D modelling, lithogeochemistry, and zircon-monazite U–Pb SIMS geochronology to assess the genesis and controls on the Varuträsk LCT pegmatite system. The findings reveal four phases of intrusive magmatism within a supracrustal package predominantly composed of metagreywacke, metabasalt (amphibolite), and black shale, subjected to two fabric-forming deformation phases and at least two folding phases, pre- and syn-plutonism in association with the Svecokarelian orogeny. Pegmatites are controlled by brittle to brittle-plastic structures that intersect the tectonic fabric of the host rock, with host rock competency, characterized by uniaxial compressive strength, being a key factor determining the intrusion angle of the pegmatite. The earliest magmatic phase in the area is represented by a granodiorite pluton with a zircon 207Pb/206Pb age of 1885 ± 3.2 Ma, linked to the main Svecokarelian orogenic cycle. A granodiorite-tonalite intrusion related to the Transscandinavian Igneous Belt has a zircon 207Pb/206Pb age of 1801 ± 1.7 Ma. Three peraluminous S-type granites (plutonic Skellefte suite) yielded zircon 207Pb/206Pb ages of 1795 ± 1.7 Ma and 1792 ± 1.6 Ma, and a monazite 207Pb/206Pb age of 1798 ± 4.6 Ma. Small and irregular to elongated bodies of hypabyssal pegmatitic leucogranite with peraluminous characteristics (hypabyssal Skellefte suite), although not dated in this study, form a geochemical continuum with the Varuträsk LCT pegmatite and less evolved regional pegmatites, and are chemically distinct from the larger S-type plutons. A ‘simple’ muscovite pegmatite dyke, representative of the less evolved regional pegmatites in the area, produced a monazite U–Pb crystallisation age of 1780 ± 6.9 Ma. These results suggest that the hypabyssal pegmatitic leucogranites, regional less evolved pegmatites, and the Varuträsk LCT pegmatite represent the final stage (c. 1.78 Ga) of crustal maturation in the orogenic cycle, marginally postdating the regional migmatization and S-type granite plutonism event. However, further research is encouraged in this study to validate the time gap between plutonic and pegmatitic granites in the Varuträsk area. We propose a mineral system model encompassing a c. 1.80 – 1.78 Ga timeframe that involved initial anatexis of metasedimentary rocks to form S-type granite plutons during the late Svecokarelian orogeny. Subsequently, a resolvable younger magmatic event generated granitic pegmatite melts that utilized pre-existing structures as transport pathways. These melts were trapped in structurally favourable, brittle to brittle-plastic sites and the host rock competency influenced the emplacement angle of the pegmatites.

To achieve the demand for elements used for the green transition energy, such as lithium, it is necessary to recognize the spatial distribution of the concentrations of these elements in different earth materials such as bedrock and soil and to identify areas with anomalous concentrations of such elements (i.e., mineralization) for further exploration and hopefully exploitation. This study carried out multivariate statistical analyses on compositional (i.e., element concentration) data from till samples to recognize areas that likely contain lithium pegmatite mineralization in the Västernorrland region, central Sweden. We applied principal components analysis (PCA) and K-means clustering techniques to reveal regional-scale patterns in the till geochemical data. We demonstrate that these two methods have potential for recognition of geochemical anomalies related to the underlying bedrock geology as well as to mineralization. The results of PCA- and K-means clustering were validated using known occurrences of lithium mineralization. Two different datasets were compared; one containing all available geochemical data and the second containing only available trace elements in the dataset and it was found that anomalous clusters of samples defined by K-means clustering have anomalous multi-element signatures defined by robust PCA. This demonstrated that principal components are the continuous solutions to the discrete cluster members for the K-means clustering. The results show that both PCA and K-means clustering of till geochemical datasets at the early stages of exploration and target generation may reveal useful information that can be used to identify potential areas for more detailed mapping or exploration activities.