Rare Earths in fluorite deposits of Elika Formation (East of Mazandaran Province)

The Central Alborz in eastern Mazandaran province is host to the most important carbonate-hosted fluorite deposits in Iran, such as Pachi-Miana, Sheshroodbar, Era and Kamarposht. In these deposits, mineralization occurs in the upper parts of the middle Triassic Elika formation (Vahabzadeh et al., 2009 and references therein). These deposits have long been studied, and various models are presented for ore genesis. Nevertheless, ore genesis in these deposits is still unclear. The present study of the geochemistry of the REEs of these deposits is intended to improve genetic models.
Three hundred samples were taken from above mentioned deposits. Samples were categorized into 5 groups: (1) fluorite ore types, (2) ore-stage calcite, (3) carbonate host rocks, (4) basaltic rock around the deposits, and (5) shale of the Shemshak formation. Fourteen pure fluorite samples, 4 samples of pure calcite, 4 samples of carbonate host rock, 1 sample of basalt and 1 sample of shale were analyzed for REEs by ICP-MS at West Lab in Australia.
Analytical data on fluorite from the Elika deposits show very low REE concentrations (0.5-18ppm), in calcite(0.5-3ppm) in carbonate host rocks – limestone (1.8-7ppm), and in dolomitic limestone 6.5ppm, compared with upper Triassic basalt (43ppm) and shale (261ppm). REE in fluorite of these deposits are strongly enriched (10 3 to 10 6 times) relative to normal sea water, ore stage calcite and carbonate host rocks, especially for mid-REEs (Eu, Gd) and heavy REEs (Lu, Yb, La/Yb=~0.05).
Also, LREEs depletion (La/Sm= 2-10) and HREEs (La/Yb=0.01-0.08) relatively enrichment of fluorites compared with limestone (La/Sm=2.5-4, La/Yb=0.1-1.5) and dolomitic limestone (La/Sm=4.28, La/Yb=0.07-0.4) host rocks as well as positive Eu anomaly are the most important REEs signatures in fluorites.
Fluorite elsewhere in the world with low total REE conten thas been interpreted to have a sedimentary origin (Ronchi et al., 1993; Hill et al., 2000; Sasmaz et al., 2005). Strong enrichment of REEs in fluorite and carbonate host rocks worldwide, relative to normal sea water indicates that diagenetic and/or hydrothermal processes have contributed to the process. Depletion of LREEs and moderately strong HREE enrichment in fluorite relative to carbonate host rocks is interpreted to be post-sedimentation (Ronchi et al., 1993;Hill et al., 2000). Thisis supported by the hydrothermal character of the fluorite in the Elika deposits and similarity between REE profiles and those of fluorine-rich MVT deposits with hydrothermal origin (Chesley et al.,1994; Bau et al., 2003). Positive Eu anomalies in fluorite elsewhere suggest deposition reduced conditions and temperatures~250°C (Bau et al., 2003; Sverjensky, 1989). The present study indicates that low total REEcontents in fluorite precipitated from reduced hydrothermal solutions could be caused by (1) increasing pH of the ore-forming solution during interaction with carbonate host-rock, (2) gradually decreasing F concentration in hydrothermal solutions due to different generations of fluorite mineralization, and (3) low REE contents of carbonate hostrocks.