150-6 Petrology, geochemistry, and geochronology of the Tofty Ridge carbonatite: insights into petrogenesis

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

Poster Booth No.: 283

Presenting Author:

Lily Norwood

Authors:

Norwood, Lily¹, Regan, Sean P.², Acosta, Marisa D.³, Broman, Bonnie⁴, Van Nieuwenhuyse, Rick⁵, Kylander-Clark, Andrew Robert Cooper⁶

(1) Geoscience, University of Alaska, Fairbanks, Fairbanks, AK, USA, (2) Geophysical Institute-University of Alaska Fairbanks, Fairbanks, AK, USA, (3) University of Alaska Fairbanks, Fairbanks, AK, USA, (4) Contango Ore, Fairbanks, AK, USA, (5) Contango Ore, Fairbanks, AK, USA, (6) UC Santa Barbara, Santa Barbara, CA, USA,

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 Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10896

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