Apatite in carbonatitic rocks: Compositional variation, zoning, element partitioning and petrogenetic significance

Abstract

Apatite-group phosphates are nearly ubiquitous in carbonatites, but our understanding of these minerals is inadequate, particularly in the areas of element partitioning and petrogenetic interpretation of their compositional variation among spatially associated rocks and within individual crystals. In the present work, the mode of occurrence, and major-and trace-element chemistry of apatite (sensu lato) from calcite and dolomite carbonatites, their associated cumulate rocks (including phoscorites) and hydrothermal parageneses were studied using a set of 80 samples from 50 localities worldwide. The majority of this set represents material for which no analytical data are available in the literature. Electron-microprobe and laser-ablation mass-spectrometry data (~600 and 400 analyses, respectively), accompanied by back-scattered-electron and cathodoluminescence images and Raman spectra, were used to identify the key compositional characteristics and zoning patterns of carbonatitic apatite. These data are placed in the context of phosphorus geochemistry in carbonatitic systems and carbonatite evolution, and compared to the models proposed by previous workers. The documented variations in apatite morphology and zoning represent a detailed record of a wide range of evolutionary processes, both magmatic and fluid-driven. The majority of igneous apatite from the examined rocks is Cl-poor fluorapatite or F-rich hydroxylapatite ( 0.3 apfu F) with 0.2-2.7 wt.% SrO, 0-4.5 wt.% LREE 2 O 3 , 0-0.8 wt.% Na 2 O, and low levels of other cations accommodated in the Ca site (up to 1000 ppm Mn, 2300 ppm Fe, 200 ppm Ba, 150 ppm Pb, 700 ppm Th and 150 ppm U), none of which show meaningful correlation with the host-rock type. Silicate, (SO 4) 2and (VO 4) 3anions, substituting for (PO 4) 3-, tend to occur in greater abundance in crystals from calcite carbonatites (up to 4.2 wt.% SiO 2 , 1.5 wt.% SO 3 and 660 ppm V). Although (CO 3) 2groups are very likely present in some samples, Raman micro-spectroscopy proved inconclusive for apatites with small P-site deficiencies and other substituent elements in this site. Indicator REE ratios sensitive to redox conditions (Ce, Eu) and hydrothermal overprint (Y) form a fairly tight cluster of values (0.8-1.3, 0.8-1.1 and 0.6-0.9, respectively) and may be used in combination with trace-element abundances for the development of geochemical exploration tools. Hydrothermal apatite forms in carbonatites as the product of replacement of primary apatite, or is deposited in fractures and interstices as euhedral crystals and aggregates associated with typical late-stage minerals (e.g., quartz and chlorite). Hydrothermal apatite is typically depleted in Sr, REE, Mn and Th, but enriched in F (up to 4.8 wt.%) relative to its igneous precursor, and also differs from the latter in at least some of key REE ratios [e.g., shows (La/Yb) cn  25, or a negative Ce anomaly]. The only significant exception is Sr(REE,Na)-rich replacement zones and overgrowths on igneous apatite from some dolomite(-bearing) carbonati...

Composition and U—Pb ages of apatite in the Amba Dongar carbonatite–alkaline complex, India

“…The apatite REE budget in the Amba Dongar carbonatites is low compared to others reported worldwide (e.g., Chakhmouradian et al, ). Carbonatitic fluorapatites sampled from different localities worldwide have REE contents that generally exceed 1 wt% (Bühn, Wall, & Le Bas, ).…”

Chemical Composition and Petrogenetic Implications of Apatite in the Khibiny Apatite-Nepheline Deposits (Kola Peninsula)

“…In the apatite ore, there are zones of large late apatite of dark color. Previous studies [24][25][26][27] have established that Khibiny apatite corresponds to fluorapatite according to new nomenclature [28]. In this paper we investigated in detail the trace element composition of Khibiny apatite from several deposits and various sections through ore bodies.…”

“…Khibiny apatite contains significantly lower concentrations of barium on average 363 ppm. Relative to apatites from several other localities, including Pilansberg [29] and Lovozero [25], apatite from Khibiny is depleted in rare earths and radioactive elements ( Table 2). The concentrations of Sr in the Khibiny apatite are comparable to those from Lovozero, Pilansberg and Ilimaussaq.…”

Rare Earth Elements (La, Ce, Pr, Nd, and Sm) from a Carbonatite Deposit: Mineralogical Characterization and Geochemical Behavior

“…REE ores are often contaminated with actinides such as thorium, uranium, and other contaminants (e.g., Ba, Sr, Mn, Ta) [11,12]. Social acceptability of REE production is an important issue for REE investors.…”

“…The least problematic coarse fraction could be sent to waste rock piles with suitable and specific environmental management and waste disposal strategies. REE ores are often contaminated with actinides such as thorium, uranium, and other contaminants (e.g., Ba, Sr, Mn, Ta) [11,12]. Social acceptability of REE production is an important issue for REE investors.…”