113-17 Pyromorphite-hosted rare earth element enrichment at Bunker Hill Mine, Idaho

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

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Abstract:

Pyromorphite is lead chlorophosphate Pb₅(PO₄)₃Cl. It is not familiar to most geoscientists, except those with experience in the fine mineral trade where it is desired by collectors who celebrate reniform, botryoidal specimens encrusted with vivid green to orange prismatic crystals. Scientific interest to-date has been exclusive to the environmental sciences, which exploit its ability to instantaneously immobilize aqueous Pb. It is used as an amendment for the remediation of Pb-contaminated soils. Although it has received little academic study as an ore mineral, pyromorphite is associated with Pb-rich, polymetallic deposits where it precipitates when Pb-rich waters flow into the oxidized supergene zone. In a series of experiments Sordyl (2025) demonstrated pyromorphite also immobilizes REE via coupled substitution on the Pb²⁺ crystallographic site. These experiments show pyromorphite gangue may be enriched in REE and could represent a new revenue stream that intersects with national security interests connected to critical minerals. To test this hypothesis in the context of the domestic production of critical minerals, we analyzed pyromorphite from the Ag-Pd-Zn deposits at Bunker Hill Mine in the Coeur d’Alene Mining District in north Idaho. Bunker Hill pyromorphite is ~85 wt.% Pb, 5 to 10 wt.% P, 1 to 6 wt.% As, and 1 wt.% Cl. Metals are rare to absent, but the pyromorphite hosts mineral inclusions of an unknown Pb-rich oxide that contain up to 30 wt.% Sb, 3 wt.% Fe, and 1 wt.% Ag. Pyromorphite is commonly zoned in As and P. P increases from core to rim, whereas As decreases from core to rim. LREE are enriched relative to HREE, with La and Nd occurring in the highest concentrations (up to 600 and 1500 ppm, respectively). HREE concentrations range from <1 to 25 ppm. Because pyromorphite is insoluble and immobilizes Pb + REE, its composition may represent a space-averaged signature of the Bunker Hill ore bodies provided by scavenging hydrothermal waters. Crystal zoning demonstrates fluid compositions evolved with time towards lower As concentrations. Low HREE contents suggests they are not available anywhere in the system. Elevated LREE concentrations indicate they may have an enriched, prospective source in the metal-rich hypogene zone at Bunker Hill.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10636

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