150 Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals (Posters)

Radiometric dating of ore deposits containing critical minerals. Studies discussing analytical aspects of methods such as U-Th-Pb, Ar-Ar, Rb-Sr, Sm-Nd, Re-Os and/or using radiometric ages for ore genesis research or critical mineral resource evaluation.

Monday, 20 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Petrology, geochemistry, and geochronology of the Tofty Ridge carbonatite: insights into petrogenesis

Paper number: 150-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Tofty Ridge carbonatite of Interior Alaska preserves a polyphase mineralization history of late-stage magmatic niobium mineralization and later hydrothermal alteration.  We present the results of petrography and whole rock x-ray diffraction analysis to determine carbonatite mineralogy, electron probe microanalysis to characterize niobium and REE-bearing mineral phases, and laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) to determine U-Pb ages of carbonatite emplacement and Nb-REE mineralization.

 

Niobium enrichment occurs in magnetite- and apatite-rich dolomitic carbonatite sills. Rutile and ilmenite with Nb-enriched cores and depleted rims are disseminated through the carbonatite. Monazite, although limited in extent and modal abundance, is disseminated through the carbonatite in breccias, and as a secondary product after apatite dissolution, as indicated by monazite overgrowths on the rims of and as inclusions in apatite. In-situ LASS-ICP-MS analysis of oscillatory-zoned and homogenous zircon grains together yield a 206Pb/238U weighted mean age of 215.4 ± 2.4 Ma, interpreted to reflect crystallization of zircon from the carbonatite magma. Monazite petrochronology of patchy and oscillatory zoned monazite yields a 208Pb/232Th weighted mean age of 118.1 ± 1.1 Ma, interpreted to reflect secondary hydrothermal alteration. REE-bearing monazite was thus post-magmatic, formed or remobilized in a regional hydrothermal event. Rutile and ilmenite cores, meanwhile, show partial replacement by rims of a Nb-poor TiO2 phase suggestive of an alteration event, but relict Nb-rich cores are interpreted to have crystallized directly from the carbonatite melt.

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

doi: 10.1130/abs/2025AM-414675

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