Conditions for the crystallization of fluorite in the Mushgai-Khudag complex (Southern Mongolia): Evidence from trace element geochemistry and fluid inclusions

Highlights

• The fluorite mineralization is associated with carbonatite magmatism.

• Fluorites are extremely enriched in REE, especially light.

• Formation temperatures of fluorites reach more than 500 °C.

Abstract

The Mushgai-Khudag complex is part of the Late Mesozoic Central Asian carbonatite province. Fluorite mineralization is manifested throughout the province, including the Mushgai-Khudag complex. We have investigated the geochemical features and fluid inclusions of fluorites from different types of fluorite-bearing rocks. Fluorite from quartz-fluorite rocks has rare earth element (REE) concentrations in the range of 10500−144300 ppm and the highest light REE contents, with (La/Yb)_N = 56−960. Fluorite from the fluorite-apatite-celestine rocks has slightly lower REE enrichment, especially light REE content, with concentrations of 200−5900 ppm and (La/Yb)_N = 18−204. Fluorite from the fluorite-calcite rocks is characterized by REE contents of 22−1100 ppm and a variable (La/Yb)_N of 0.6−59. These variations in the fluorite REE composition from different types of rocks were probably caused by the fact that at elevated temperatures, fluorine-containing light REE complexes are more stable than fluorine-containing heavy REE complexes. The progressive enrichment of medium and heavy REEs in the latter fluorite is related to fluid evolution. The homogenization temperature and salinity values of fluid inclusions in the Mushgai-Khudag fluorites vary between 550 and 185 °C and from rather high to 2 wt.%, respectively. The parental fluids of the fluorite-bearing rocks evolved from quartz-fluorite rocks to fluorite-apatite-celestine rocks to fluorite-calcite rocks. The key component was changed from sulfate to carbonate-chloride along with the high to medium temperature decrease (∼500−245 °C).

Introduction

Fluorite is an important raw material for metallurgy and the chemical industry, and it is the most abundant fluoride mineral in the crust. It is stable over a broad range of thermodynamic conditions and has been found in a great diversity of rocks. Fluorite deposits have been classified into different categories based upon their host lithologies. However, their main feature is that they were formed by hydrothermal fluids from igneous, metamorphic or sedimentary sources (Öztűrk et al., 2019). Rock-forming fluids are genetically related to (1) amagmatic hydrothermal circulation, (2) felsic igneous rocks, and (3) carbonatites and alkaline igneous rocks (Dill, 2010; Sizaret et al., 2004). Deposits of fluorite with an unspecified relation to magmatism (the first type) are represented by extensional veins (Chaillac, France; Asturias, Spain; Jurassic veins of the Schwarzwald, Germany). Their formation is associated with the circulation of amagmatic fluids in the process of regional extension. Extensional vein deposits are the products of thermal anomalies that occurred at the beginning of rifting. Mixing between fluoride-rich basement-derived brines and low-salinity surficial or connate fluids has been accepted as the most important mineralizing process. Such deposits are frequently characterized by the presence of several different fluorite generations. Fluorite sediments are derived from slightly salty (<5 wt.% eq. NaCl) solutions with temperatures of about 130 °C and from salty fluids (≈20−25 wt.% eq. NaCl) with temperatures of up to 190 °C. The rare earth element (REE) distribution patterns of fluorite deposits are flat and have slight positive Eu and strong positive Y anomalies (Sizaret et al., 2004; Sanchez et al., 2009; Burisch et al., 2016).

Typical examples of the second type are fluorite deposits at St. Lawrence (Newfoundland), the Voznesenka ore field (Russia), the Manto fluorite deposit in Mexico, and the Rio dos Bugres fluorite mine in Brazil (Bulnaev, 2006; Flores et al., 2006; Kesler, 1977; Strong et al., 1984). Hydrothermal solutions that form fluorite mineralization of this category are characterized by variable salinity (0.2−20 wt.% eq. NaCl) and relatively low temperatures (80−280 °C). Fluorites associated with felsic magmatism are enriched in intermediate REEs rather than light REEs (LREEs) or heavy REEs (HREEs). Chondrite-normalized REE plots are characterized by negative Eu and strongly positive Y anomalies (Magotra et al., 2017).

Some of the largest deposits of fluorite are associated with carbonatites and are located at Okorusu (Namibia), Amba Dongar (India), and Mato Preto (Brazil). Carbonatites with associated fluorite mineralization form central parts of the zonal alkaline magmatic complexes, dikes, sills, breccias, and veins. Fluorite deposits are usually found at contacts with host rocks and have been interpreted as products of late-stage carbonatite-derived hydrothermal fluids. Such deposits are predominantly epithermal; according to the study of fluid inclusions, they have a homogenization temperature of <160 °C and a low salinity of <10 wt.% eq. NaCl (Bühn et al., 2002; Dill, 2010). In this lithological association, fluorite is usually strongly enriched in LREEs, and Eu negative or positive anomalies are typically absent from the chondrite-normalized REE spectra (Magotra et al., 2017).

Therefore, it appears that the absolute concentrations of REEs and the chondrite-normalized ratios of fluorite are good indicators of the rock association and environment in which the mineral was formed, whereas fluid inclusions provide further information about the crystallization temperature, the composition of the parental fluids, and the evolution of the mineralizing fluid. Here, we present a combination of a fluid inclusion study with new data on REEs and other trace element concentrations in the fluorite from the Mushgai-Khudag complex in Southern Mongolia. Fluorite is a ubiquitous mineral in Mushgai-Khudag, and the objective of this study was to better understand the genetic relationships (if any) between the fluorite occurrences in different rock types throughout the complex. A more general goal of the study was to provide insights into the source and composition of hydrothermal ore-forming fluids that operated in the complex.

The formation of carbonatite-bearing complexes in the Central Asian Orogenic Belt (CAOB) was related to early-late Mesozoic rifting (Doroshkevich et al., 2011). Nowadays, there are extensive studies on carbonatite- and fluorite-bearing complexes in Southern Mongolia (Lugiin Gol, Bayan Khoshu, Mushgai-Khudag), Western Transbaikalia (Yuzhnoe and Arshan), and Central Tuva (Karasug, Chailag-Khem, and Ulatai-Choza). During the past half-century, studies on the Mushgai-Khudag complex have focused mainly on the geology, geochemistry, and petrography of the silicate rock sequences (Baskina et al., 1978; Kynicky and Samec, 2005; Ontoev et al., 1979; Ripp et al., 2005; Samoilov and Kovalenko, 1983; Vladykin, 2013). Andreeva and Kovalenko (2003) carried out a melt and fluid inclusion study and demonstrated that the rocks and mineralized veins had been formed from silicate, silicate-salt, and diverse salt melts (carbonate-phosphate, phosphate-sulfate, fluoride-sulfate, chloride-sulfate, etc.). However, fluid inclusions in fluorite were studied only in fluorite-celestine veins. Here, we provide mineralogical and geochemical descriptions, present fluid inclusion data for all the types of fluorite-bearing rocks, and trace the evolution of the fluids from the magmatic to the hydrothermal stage.