Multiple growth of zirconolite in marble (Mogok metamorphic belt, Myanmar): evidence for episodes of fluid metasomatism and Zr–Ti–U mineralization in metacarbonate systems
<p>Fluid infiltration into (meta-)carbonate rocks is an important petrologic process that induces metamorphic decarbonation and potential mineralization of metals or nonmetals. The determination of the infiltration time and the compositional features of reactive fluids is essential to understa...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-01-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/36/11/2024/ejm-36-11-2024.pdf |
| Summary: | <p>Fluid infiltration into (meta-)carbonate rocks is an important petrologic process that induces metamorphic decarbonation and potential mineralization of metals or nonmetals. The determination of the infiltration time and the compositional features of reactive fluids is essential to understand the mechanism and process of fluid–rock interactions. Zirconolite (ideal formula: CaZrTi<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>7</sub></span>) is an important U-bearing accessory mineral that can develop in metasomatized metacarbonate rocks. In this study, we investigate the occurrence, texture, composition, and chronology of various types of zirconolite from fluid-infiltrated reaction zones in dolomite marbles from the Mogok metamorphic belt, Myanmar. Three types of zirconolite are recognized: (1) the first type (Zrl-I) coexists with metasomatic silicate and oxide minerals (forsterite, spinel, phlogopite) and has a homogeneous composition with high contents of UO<span class="inline-formula"><sub>2</sub></span> (21.37 wt %–22.82 wt %) and ThO<span class="inline-formula"><sub>2</sub></span> (0.84 wt %–1.99 wt %). (2) The second type (Zrl-II) has textural characteristics similar to those of Zrl-I. However, Zrl-II shows a core–rim zonation with a slightly higher UO<span class="inline-formula"><sub>2</sub></span> content in the rims (average of 23.5 <span class="inline-formula">±</span> 0.4 wt % (<span class="inline-formula"><i>n</i>=8</span>)) than the cores (average of 22.1 <span class="inline-formula">±</span> 0.3 wt % (<span class="inline-formula"><i>n</i>=8</span>)). (3) The third type (Zrl-III) typically occurs as coronas around baddeleyite and coexists with polycrystalline quartz. Zrl-III has obviously lower contents of UO<span class="inline-formula"><sub>2</sub></span> (0.88 wt %–5.3 wt %) than those of Zrl-I and Zrl-II. All types of zirconolite have relatively low rare earth element (REE) contents (<span class="inline-formula"><i><</i></span> 480 <span class="inline-formula">µ</span>g g<span class="inline-formula"><sup>−1</sup></span> for <span class="inline-formula">Σ</span>REE). Microtextures and compositions of the three zirconolite types, in combination with in situ zirconolite U–Pb dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), reveal episodic fluid infiltration and element mobilization in the dolomite marbles. The first-stage infiltration occurred at <span class="inline-formula">∼</span> 35 Ma, leading to the formation of Mg-rich silicates and oxides and accessory minerals (Zrl-I, baddeleyite, and geikielite). The reactive fluid was characterized by high contents of Zr, Ti, U, and Th. After that, some Zrl-I grains underwent a local fluid-assisted dissolution–precipitation process, which produced a core–rim zonation (i.e., the Zrl-II type). The final stage of fluid infiltration, recorded by the growth of Zrl-III after baddeleyite, took place at <span class="inline-formula">∼</span> 19 Ma. The infiltrating fluid of this stage had relatively lower U contents and higher SiO<span class="inline-formula"><sub>2</sub></span> activities than the first-stage infiltrating fluid.</p>
<p>This study illustrates that zirconolite is a powerful mineral that can record repeated episodes (ranging from 35 to 19 Ma) of fluid influx, metasomatic reactions, and Zr–Ti–U mineralization in (meta-)carbonates. This mineral not only provides key information about the timing of fluid flow but also documents the chemical variation in reactive fluids. Thus, zirconolite is expected to play a more important role in characterizing the fluid–carbonate interaction, orogenic CO<span class="inline-formula"><sub>2</sub></span> release, and the transfer and deposition of rare metals.</p> |
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| ISSN: | 0935-1221 1617-4011 |