36-2 Pressure-Temperature Gradients of Biotite in the Lights Creek Stock

Session: Cordilleran Mineral Systems: Interdisciplinary Perspectives on Critical Metal Ore Deposits (Posters)

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Abstract:

The Lights Creek Stock is a pluton that hosts three Iron Oxide Copper Gold deposits located within the Plumas County Copper Belt. The three ore deposits are Superior, Engels, and Moonlight. While the Engels and Superior deposits have already been historically mined,the magmatic and hydrothermal history of the Lights Creek Stock are still being investigated. Our current study focuses on the alteration of biotite across the altered but unmineralized portion of the stock. It is important to view the stock as a whole, rather than only the individual ore deposits, to put together a complete story of its formation. 

Analysis from EPMA data of biotite crystals have shown a gradient of pressures (Uchida et al., 2007) and temperatures (Wu and Chen, 2015) of formation across the stock. Samples closer to the Moonlight deposit preserve some of the few remaining primary biotites and record higher temperatures with lower pressures. Interestingly, many of the hydrothermally altered biotites in the altered but unmineralized stock show lower temperatures with higher pressures. This may be due to chemical changes during alteration causing a relative increase in Al content irrespective of P, or it may reflect changing conditions during alteration by the hydrothermal fluid. The decrease in temperature with increase of pressure being more distal from the mineralized portions may suggest cooling of potassic hydrothermal fluids as they move further away from the ore body. Understanding these gradients and what they tell us about hydrothermal activity provides further insight to the formation of IOCGs, which are valuable economic deposits.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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