A fundamental role of carbonate–sulfate melts in the formation of iron oxide–apatite deposits

Abstract

Genetic models for iron oxide–apatite deposits are controversial and span a spectrum from orthomagmatic to hydrothermal endmembers. This lack of consensus is rooted in uncertainties as to the nature and origin of ore-forming fluids in these systems. Here, we present a fluid-inclusion study of mineralizing fluids at two iron oxide–apatite deposits (Buena Vista, Nevada and Iron Springs, Utah). We found that the inclusions in both systems comprise both aqueous brine and ubiquitous iron-rich carbonate–sulfate melts. These melts were found throughout the paragenesis of both deposits and show a tremendous capacity to transport ferric iron. Hence, we argue that orthomagmatic fluids played a role in mineralization at both Buena Vista and Iron Springs, and that the main ore-forming fluid was an iron-rich carbonate–sulfate melt formed by the assimilation and anatexis of evaporite-bearing carbonate rocks. The geological conditions that give rise to carbonate–sulfate melts are also a common feature of other classic iron oxide–apatite systems worldwide. Hence, we argue that the process of assimilation, anatexis and immiscibility of carbonate–sulfate melts is fundamental to iron oxide–apatite formation and provides a common link between iron oxide–apatite systems in different geological settings.

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Fig. 1: Fluid and melt inclusions in apatite and actinolite from Buena Vista and Iron Springs.
Fig. 2: Phases within polycrystalline (melt) inclusions from Buena Vista.
Fig. 3: Melting of polycrystalline inclusions.
Fig. 4: Schematic cross-section showing IOA formation via anatectic carbonate melting.

Data availability

The authors declare that all data supporting the findings of this study are available within the paper and its Supplementary Information files. We are unable to make the data available in a publicly accessible repository at this time.

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Acknowledgements

We thank M. Barton and E. Seedorff for introducing us to the Buena Vista deposit. We thank N. Gerein for help with the SEM analysis, E. Soignard, E. Walton and Y. Klyukin for help with Raman analysis, J. Cline, S. Jowitt and D. Barkoff for help with microthermometry and A. Degrado for help with video editing. We thank A. Hofstra and J. Brenan for detailed and constructive comments that helped us improve the paper. This work was supported by NSERC through a Discovery grant to M.S.-M. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US government.

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W.M.B. conducted the analyses, interpreted the data and wrote the manuscript. M.S.-M. conceived and designed the study, interpreted the data and wrote the manuscript. K.L. and L.L. contributed to the isotopic analyses and interpretations. F.K.M. and E.M. contributed to the analyses, interpreted the data and wrote the manuscript.

Corresponding author

Correspondence to Matthew Steele-MacInnis.

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Supplementary information

Supplementary Information

Supplementary results, including Figs. 1–22 and Tables 1–6.

Supplementary Video 1

Video recording at room temperature, of the same polycrystalline inclusion shown in Figure S5, showing the movement of a vapor bubble in aqueous liquid in the inclusion. Movement can be observed in the area denoted by the red arrow and circle, and unambiguously demonstrates the presence of aqueous liquid and vapor in the polycrystalline inclusion.

Supplementary Video 2

Video recording of a polycrystalline inclusion from the Buena Vista system during heating from room temperature to >700 °C (temperature shown in lower left), showing the melting behavior. Notice that at T > 518 °C, several bubbles within the inclusion begin to deform in shape and gradually migrate. At T > 597 °C, a meniscus between solid and liquid migrates inward from the inclusion walls. The inclusion is mostly molten above 700 °C, although at least one small opaque grain is still evident in the upper left-hand corner of the inclusion at the highest temperature recorded in this video (731 °C).

Supplementary Video 3

Video recording of an aqueous inclusion from the Buena Vista system during low-temperature microthermometry. Notice the change in the size and position of the vapor bubble during heating above ~–70 °C; followed by increase in size of the vapor bubble heating from –50 to –43 °C. Obvious melting takes place from –43 through >0 °C, and in particular, a distinct melting event takes place at +9.2 to +10.2 °C, which is interpreted as melting of salt hydrate, likely a carbonate or sulfate.

Supplementary Video 4

Video recording of a polycrystalline inclusion from Iron Springs during heating from room temperature to 898 °C (temperature shown in upper right), showing the melting behavior. The onset of melting in this inclusion can be seen at ~190 °C and the majority of the inclusion abruptly melts at >750 °C, followed by the shrinking of the opaque phase which persists to higher temperatures.

Supplementary Video 5

Video recording of an aqueous inclusion from Iron Springs during low-temperature microthermometry. Notice the change in size and shape of the vapor bubble above ~ -82 °C with the onset of ice melting. At -24 °C, ice abruptly melts and the halite crystal begins to shrink with the formation of granular salt hydrate. At ~4 °C, another abrupt melting event occurs, interpreted as melting of salt hydrate, likely a carbonate or sulfate.

Supplementary Video 6

Video recording of an aqueous inclusion from Iron Springs during low-temperature microthermometry. Note the disappearance of the halite crystal at -36 °C and its sudden reappearance following hydrate melting at +12 °C.

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Bain, W.M., Steele-MacInnis, M., Li, K. et al. A fundamental role of carbonate–sulfate melts in the formation of iron oxide–apatite deposits. Nat. Geosci. 13, 751–757 (2020). https://doi.org/10.1038/s41561-020-0635-9

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