The role of sulfate-rich fluids in heavy rare earth enrichment at the Dashigou carbonatite deposit, Huanglongpu, China

Delia Cangelosi,Bruce Yardley,Martin Smith,David Banks

doi:10.1180/mgm.2019.78

Delia Cangelosi, Bruce Yardley + Show 2 more

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https://doi.org/10.1180/mgm.2019.78

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Abstract

AbstractThe Huanglongpu carbonatites are located in the north-western part of the Qinling orogenic belt in central China. Calcite carbonatite dykes at the Dashigou open pit are unusual due to their enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE), leading to a flatREEpattern, and in that the majority of dykes have a quartz core. They also host economic concentrations of molybdenite. The calcite carbonatite dykes show two styles of mineralogy according to the degree of hydrothermal reworking, and these are reflected inREEdistribution and concentration. TheREEin the little-altered calcite carbonatite occur mostly in magmaticREEminerals, mainly monazite-(Ce), and typically have ΣLREE/(HREE+Y) ratios from 9.9 to 17. In hydrothermally altered calcite carbonatites, magmatic monazite-(Ce) is partially replaced to fully replaced byHREE-enriched secondary phases and the rocks have ΣLREE/(HREE+Y) ratios from 1.1 to 3.8. The fluid responsible for hydrothermalREEredistribution is preserved in fluid inclusions in the quartz lenses. The bulk of the quartz lacks fluid inclusions but is cut by two later hydrothermal quartz generations, both containing sulfate-rich fluid inclusions with sulfate typically present as multiple trapped solids, as well as in solution. The estimated total sulfate content of fluid inclusions ranges from 6 to >33 wt.% K2SO4equivalent. We interpret these heterogeneous fluid inclusions to be the result of reaction of sulfate-rich fluids with the calcite carbonatite dykes. The finalHREEenrichment is due to a combination of factors: (1) the progressiveHREEenrichment of later magmatic calcite created aHREE-enriched source; (2)REE–SO42–complexing allowed theREEto be redistributed without fractionation; and (3) secondaryREEmineralisation was dominated by minerals such asHREE-enriched fluorocarbonates, xenotime-(Y) and churchite-(Y) whose crystal structures tends to favourHREE.

Highlights

The light rare earth elements (LREE; La to Sm) and the heavy rare earth elements (HREE; Eu to Lu) are classified as critical raw materials by the European Commission (European Commission, 2017) due to the growing demand for these strategic elements
Mineral and fluid-inclusion parageneses were established using optical petrography followed by scanning electron microscopy (SEM) at the University of Leeds using a FEI Quanta 650 field-emission gun scanning electron microscope operated at 20 kV employing back-scattered electron (BSE) and cathodoluminescence (SEM-CL) imaging
Smith et al (2018) described the REE mineral paragenesis, demonstrating that the hydrothermal REE mineralisation becomes more HREE enriched with each subsequent hydrothermal event

Summary

Introduction

The light rare earth elements (LREE; La to Sm) and the heavy rare earth elements (HREE; Eu to Lu) are classified as critical raw materials by the European Commission (European Commission, 2017) due to the growing demand for these strategic elements. Carbonatite deposits are commonly LREE-enriched (Woolley and Kempe, 1989; Rankin, 2005; Verplanck et al, 2014), but a few carbonatite deposits, such as Huanglongpu (central Shanxi Province, China), the Lofdal complex (north-western Namibia) This investigation follows from the recent work of Smith et al (2018) on the Huanglongpu calcite carbonatites. This is associated with a shift to more silica-rich hydrothermal conditions leading to quartz growth, and with sulfate mineralisation. Hydrothermal alteration of magmatic REE phases, notably monazite-(Ce), was accompanied by growth of secondary REE minerals, in particular britholite-(Ce) and Ca–REE fluorocarbonates, which show relative HREE enrichment compared to magmatic phases

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