Variation of mineralizing fluids and fractionation of REE during the emplacement of the vein-type fluorite deposit at Bozijan, Markazi Province, Iran

Abstract

Fluorite vein mineralization occurs mainly in slates and phyllites of Lower Jurassic Shemshak Formation and scarcely in Triassic limestones in Bozijan, which is situated at the west of Mahallat city in the Markazi Province of Iran. The ore consists mainly of fluorite, quartz, calcite, and iron oxides. The subordinate components are galena, pyrite, manganese oxides, and malachite. Local wall-rock alterations include argilization and silicification. The nature of the mineralization and ore–host rock relationships indicate an epigenetic mode of formation for fluorite mineralization. Fluid inclusions in early fluorites and quartz consist of aqueous and aqueous-carbonic inclusions, thereby showing that during deposition of bulk fluorites, two immiscible fluids are involved: aqueous and carbonic. The aqueous fluid is a mixture of two low- and high-salinity (< 15 and > 26.24 wt.% NaCl equivalent) H 2O–NaCl–(CaCl 2–KCl–MgCl 2) brines, as is evidenced by T h -salinity plots. The aqueous inclusions are homogenized at temperatures between 152.5 and 312 °C. The aqueous-carbonic inclusions exhibit salinity and homogenization temperature in ranges between 22.04 and 24.26 wt.% NaCl equivalent and between 253 and 390.5 °C, respectively. Fluid inclusions in late fluorites show the fluorites were precipitated from a colder (102.4–175 °C) and less saline (15.96–24.45 wt.% NaCl equivalent) fluid than the early fluorites. Rare earth element (REE) analysis in fluorites revealed extremely low values in ranges between 2.87 and 34.39 ppm for early fluorites and between 1.91 and 6.4 ppm for late fluorites, thus indicating that fluorites had been derived from a sedimentary environment. However, Tb/Ca and Y/Ho ratios invariably suggest a hydrothermal origin for Bozijan fluorites. The Ce/Yb ratios and chondrite-normalized patterns revealed that the fluorites (early as well as late ones) are enriched in LREE (light rare earth element) relative to HREE (heavy rare earth element). This indicates that REE leaching from source rocks and fluid migration occurred under high-temperature and low-pH conditions. The enigmatic LREE-enriched late fluorites suggest that deposition of fluorites in Bozijan did not occur during a long-lived episode of mineralization. Europium represents positive as well as negative anomalies that have been probably caused by fluorite precipitation from mixing two fluids possessing opposite Eu anomalies. The Ce/Ce* ratios portray persistent negative Ce anomalies, thus indicating reducing conditions in the hydrothermal fluids. In the (La/Yb)n–Eu/Eu* diagram, data points do not overlap with the fields represented by fluorite-bearing Au–Ag deposits from elsewhere, thus indicating a very low potential for precious-metal mineralization in the studied area. This interpretation is well in line with the low contents of precious metals in the Bozijan deposit, thus suggesting that REE geochemistry is a reliable tool for exploring precious metals in fluorite deposits. The Bozijan deposit is classified here as an “unconformity-related fluorite deposit.” According to a conceptual model, mineralization occurred when the ascending hypersaline brines mixed with the less saline connate fluids in sediments of Triassic and predominantly Jurassic ages. The less saline connate brines were assumed to be the fluoride-bearing solutions, as suggested by the high F contents of the rocks containing them and REE patterns.