Significance of apatite REE depletion and monazite inclusions in the brecciated Se–Chahun iron oxide–apatite deposit, Bafq district, Iran: Insights from paragenesis and geochemistry

Abstract

The Se–Chahun magnetite–apatite deposit (Bafq district, central Iran) comprises several orebodies with large-scale replacement and brecciation textures, and a sodic–calcic alteration envelope. Such members of the iron-oxide copper–gold (IOCG) deposit family commonly exhibit REE enrichment, but the source and evolution of the REE component is arguable. In the Anomaly X orebody, semi-massive ilmenite-rich magnetite and coarse REE-rich fluorapatite formed at 510 ± 8 Ma (U–Pb LA-ICPMS age), at the end of the main regional sodic magmatic event (525 ± 7 Ma). Prior to a major brecciation, a metasomatic event removed Ti and Al from magnetite grain boundaries, and leached primary fluorapatite (BSE-bright) of LREE, Y, Na, Cl, Mg, Mn and Fe, leaving BSE-dark apatite that contained LREE-rich monazite and co-existing vapor- and liquid-dominated fluid inclusions. Subsequent brecciation and hydrothermal infill produced a matrix dominated by actinolite, K-feldspar, biotite, chlorite, calcite, hematite, rutile, and titanite, but apatite was not further metasomatized. It is concluded that P, REE, Ti, Al and Fe were readily transported in the early sodic–calcic fluids, but P and the REE had very restricted mobility (mm's to cm's) in the CO₂–K–Cl-rich fluids responsible for apatite metasomatism. A role for fluid over-pressuring as a means of increasing the efficacy of apatite REE leaching is circumstantially suggested by the observation that the metasomatic event was terminated by tensile failure. Overall, a case is emerging for fluid-induced nucleation of monazite, and accompanying apatite REE depletion, to be included as a typical but subtle paragenetic feature within the iron oxide–apatite deposit model.

Introduction

The iron oxide-(Cu–Au) deposit style (IOCG) is consistently associated with rare earth element (REE) enrichment, and consequently it is an ideal venue to study hydrothermal REE behavior during and following the emplacement of REE-bearing minerals. One of the most REE-enriched IOCG varieties are the Kiruna-type deposits (Hitzman et al., 1992, Hitzman, 2000), also recently referred to as “IOA” (iron oxide–apatite; Williams, 2010), or P-rich iron oxide deposits (Groves et al., 2010). The classic IOA districts (Kiruna (Sweden), Bafq Mineral Province (BMP; Iran), Chilean Iron Belt, El Laco (Chile)) are attributed to Andean margin processes, and are physically and temporally distinct (with some important exceptions) from economic IOCG mineralization that occurs in some of these regions (Groves et al., 2010). They consist of massive to brecciated, high temperature, magnetite–fluorapatite–calc–silicate bodies that may transgress or be transitional to regional alteration aprons of albite–actinolite ± K-feldpsar ± phyllic alteration. They show a common association with evolved felsic intrusions, although the nature of the relationship between intrusions and IOA deposits is an area of substantial argument (Parak, 1975, Hildebrand, 1986, Nyström and Henriquez, 1994, Frietsch and Perdahl, 1995, Barton and Johnson, 1996, Barton and Johnson, 2004, Treloar and Colley, 1996, Broman et al., 1999, Hitzman, 2000, Sillitoe and Burrows, 2002, Williams et al., 2005).

The question of a genetic relationship between IOA and economic IOCG deposits is important to understand for exploration models. A link is supported by the fact that some REE-rich IOCG deposits contain textural evidence of an early IOA stage (e.g., Gawler Province, South Australia: Olympic Dam (Oreskes and Einaudi, 1990) and Oak Dam (Davidson et al., 2007); Carajas district: Sossego deposit (Monteiro et al., 2008)). In most IOCG and IOA deposits, REE enrichment is strongly tied to the formation of P minerals, although allanite, and REE carbonates such as bastnaesite and parisite are also important at some sites (e.g., Olympic Dam; Oreskes and Einaudi, 1990). In an example that illustrates some of the complexity in the P-REE relationship, in the Kiruna district, Edfelt (2007) found that REE compositions of IOCG-associated apatite differ from that of IOA apatite. These aspects reinforce the need to better understand the relationship between REE and P-minerals in the iron oxide-associated clan of Groves et al. (2010).

IOA deposits, with their high formation temperatures, are a logical place to explore the P-REE relationship. Apatites from these deposits typically contain 2000–6000 ppm REE (Bafq province apatites contain up to 1.75 wt.% REE; Daliran, 2002), and are strongly LREE-enriched, with moderate to strong negative Eu anomalies (Parak, 1975, Frietsch, 1982, Frietsch and Perdahl, 1995). They contrast with the flat REE patterns of anorthosite-related apatites in ‘Nelsonites’ (apatite–magnetite bodies, e.g., Darling and Florence, 1995). The combined use of back scattered electron imaging (BSE) and LA-ICPMS-based analysis has more recently revealed that apatite can be subject to significant post-depositional REE leaching (Roedder et al., 1987, Harlov et al., 2002), so that it is important to evaluate this possibility before making comparisons to well known apatite end-members (e.g., Belousova et al., 2002). Where REE leaching has occurred in IOA apatite, it is common to observe monazite, and/or xenotime inclusions within the leached zone, or attached to the margins of the leached apatite, and/or within the adjacent calc–silicate-dominated matrix (Harlov et al., 2002, Torab and Lehmann, 2007, Stosch et al., 2010). Experimental studies prove that these monazite and xenotime inclusions typically nucleate after internal redistribution of the apatite REE by a Na–Ca-poor fluid (Harlov and Förster, 2003, Harlov et al., 2005). While this may occur as a part of the typically extensive hydrothermal history of IOA deposits, it may also accompany hydrous metamorphism (Darling and Florence, 1995, Harlov et al., 2002, Edfelt, 2007). Stosch et al. (2010) have used U–Pb dates from apatite-hosted monazite inclusions to contend that the REE redistribution in apatite may even occur during hydrothermal overprinting tens to hundreds of millions of years after IOA formation. Thus it is now uncertain whether apatite REE depletion and consequent formation of monazite inclusions has paragenetic significance within IOA formation models.

Our contribution describes the geology and geochemistry (with emphasis on REE, U and Th) of an IOA deposit, Se–Chahun, from the BMP, Central Iran. The deposit has received little previous attention, but is economically important, and is of interest in the context of the above brief review of REE-P relationships in IOA deposits, because it provides new information on the timing of apatite metasomatism within an IOA paragenesis, and in particular its relationship to brecciation. Our samples were collected from mined outcrops and from drillcore, and were analyzed using transmitted light microscopy, BSE imaging, mineral liberation analysis (MLA), electron microprobe analysis (EMPA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) techniques. Numerous samples were used to construct the paragenesis, and our detailed geochemistry was undertaken on 6 of these.