Abstract
Monazite [(Ce,Th)PO4] from the Llallagua tin ore deposit in Bolivia is characterized by low radiogenic element contents. Previously reported field evidence and mineral associations suggest the mineral formed via direct precipitation from hydrothermal fluids. Monazite compositions thus may provide insight into characteristics of the fluids from which it formed. Chemical compositions of three Llallagua monazite grains were obtained using Electron Probe Microanalysis (EPMA), n = 64] and laser ablation mass spectrometer (LA-ICP-MS, n = 56). The mineral has higher amounts of U (123 ± 17 ppm) than Th (39 ± 20 ppm) (LA-ICP-MS, ±1σ). Grains have the highest amounts of fluorine ever reported for monazite (0.88 ± 0.10 wt %, EPMA, ±1σ), and F-rich fluids are effective mobilizers of rare earth elements (REEs), Y, and Th. The monazite has high Eu contents and positive Eu anomalies, consistent with formation in a highly-reducing back-arc environment. We speculate that F, Ca, Si and REE may have been supplied via dissolution of pre-existing fluorapatite. Llallagua monazite oscillatory zoning is controlled by an interplay of low (P + Ca + Si + Y) and high atomic number (REE) elements. We suggest monazite compositions provide insight into fluid geochemistry, mineral reactions, and tectonic settings of ore deposits that contain the mineral.
Highlights
The Llallagua ore deposit is part of the tin belt that extends from southern Peru to northernArgentina (Figure 1) (e.g., [1,2])
Pb reported for the Llallagua monazite is likely due to the small amounts of radiogenic common Pb reported for the Llallagua monazite is likely due to the small amounts of radiogenic elements elementspresent presentin inthe themineral, mineral,and andthus thusaalower loweramount amountofofdetectable detectableradiogenic radiogenicPb
We speculate that the Llallagua monazite precipitated directly from a fluid [15] or via the fluid-mediated replacement of apatite [49]
Summary
The Llallagua ore deposit is part of the tin belt that extends from southern Peru to northern. The richest ore deposits are located within a zone where the belt makes a sharp curve (Figure 1) and are commonly found in association with intrusive igneous rocks [1,2,4,5,6,7,8,9,10]. The. Llallagua monazite is known for oscillatory zoning in backscattered electrons (BSE) (Figure 2) [7,11]. Llallagua monazite is known for oscillatory zoning in backscattered electrons (BSE) (Figure 2) [7,11] This type of zoning is considered rare for the mineral, which is more commonly seen as sector zoned in BSE [12,13,14].
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