Geochemistry and mineral phases of REE in Jammi carbonatite veins and fenites, southern end of the Sokli complex, NE Finland

Abstract

The Jammi area consists of Archaean mafic volcanic rock and tonalitic gneiss, which were intruded by the Devonian Sokli carbonatite, causing large-scale and intense fenitization of the country rocks. The rocks are albite-fenites, crosscutting late carbonatite veins, with the mineral assemblages of calcite-dolomite-aegirine-albite-apatite-phlogopite. Geochemical analyses indicate that the total content of rare earth elements (REE) in the carbonatite veins is 0.11–1.83%, including 0.11–1.81% LREE and 0.002–0.041% HREE. Seven rare earth minerals have been analysed in detail from the area, including Sr-apatite, monazite, ancylite (Ce), bastnäsite (Ce), strontianite, baryte and brabanite. The Jammi carbonatite and fenite samples display similar REE patterns, with highest enrichment of LREE, and steep slopes towards the HREEs. The REE distribution in the whole-rock data is controlled by carbonates and apatite. REE-rich accessory phases such as monazite and ancylite (Ce) influence the REE pattern only when present in large amounts. Mineralogical and chemical evidence demonstrates that hydrothermal processes were responsible for the REE mineralization. During late-stage processes, apatite and carbonate minerals were replaced by various assemblages of REE–Sr–Ba minerals. Apatite could be a potential source for REE as a by-product of phosphate production in the Sokli area.

Geochemistry of Monazite within Carbonatite Related REE Deposits

“…Without including the exceptions found in Lesnaya Varaka [29], the total REE2O3 contents within monazite range from 49.6 (a) apatite grain completely replaced by monazite in carbonatite from the Sokli complex, Finland [15]; (b) monazite on the edge or included within the heterogeneous apatite in the apatite-dolomite ore from the Seligdar deposit, Russia [11]; (c) monazite inclusions within apatite in association with bastnaesite inclusions from the Naantali carbonatite, Finland [16]; (d) apatite crystal with a LREE-rich rim partly replaced and overgrown by monazite associated with LREE-poor carbonate-apatite in the Matongo carbonatite, Burundi [22]; (e) monazite grains girdling around coarse apatite in the Purulia phoscorite, India [17]; and (f) monazite overgrowth on apatite in the Abyan carbonatite, Yemen Republic [18]. Ap = apatite; Bas = bastnasite; and Mnz = monazite.…”

“…Monazite partially or completely replaces apatite, and the replaced monazite displays a cracked nature [15] (Figure 3a; e.g., Sokli monazite). Secondary monazite usually occurs as small crystals (up to 30 µm in size) on the edge, replacing apatite or is included within apatite crystals (Figure 3b).…”

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

“…The total REE content of samples are from 0.11 to 1.83 wt %, with dominance of LREE (0.11-1.81 wt % LREE and 0.002-0.041 wt % HREE contents [25]. Similar dikes also occur within the carbonatite intrusion in the Kaulus area ( Figure 2).…”

“…The country rocks consist of ultramafic and mafic volcanic units and tonalitic gneiss ( Figure 2). There is a strong connection between the complexity of fenite textures and associated mineralization due to the enrichment of REE in the intermediate and late stage carbonatite magma generations which produced multiple pulses of fenitizing fluids [21,25]. The presence of brecciated zones within the carbonatite and alkaline complex is the result of an explosive release of fluids and volatiles, thus brecciation indicates development of more evolved magma generations and increased potential of Nb and REE enrichments in the source intrusion [25,26].…”

“…Monazite also often occurs in a round, sunflower-like form. In the centre of latter precipitations, strontium-apatite is often present (Figure 3b,c) Bastnäsite-(Ce) [(Ce,La)(CO 3 )F] has been reported as a relatively common product of alteration of allanite and Sr-or F-apatite [25] in the Sokli complex. Individual crystals of bastnäsite and allanite in the carbonatite dikes appear to be acicular or needle-like and they form either radial accumulations or intricate cross-cutting grids within a variety of minerals such as albite and dolomite (Figure 3d).…”