RAMAN SPECTROSCOPIC EVIDENCE FOR THE PRESENCE OF CARBONATE AND SULFATE SPECIES IN FLUID INCLUSIONS IN THE SHEPHERD MOUNTAIN IRON OXIDE-APATITE DEPOSIT, SOUTHEAST MISSOURI: IMPLICATIONS FOR DEPOSIT GENESIS

The Shepherd Mountain deposit is a small but representative example of iron oxide-apatite deposits in the Southeast Missouri Iron Metallogenic Province. The deposit is hosted by early Mesoproterozoic rhyolites and consists of steeply dipping, northeast-trending veins of massive magnetite and specular hematite. The deposit was mined from 1815 to about 1861 and produced about 75,000 tons of ore. The Shepherd Mountain deposit has both hydrothermal and magmatic characteristics, similar to other deposits within the Southeast Missouri Iron Metallogenic Province. Because the igneous rocks in Southeast Missouri are predominantly silicic, the magmatic iron oxide mineralization has long been regarded to have formed through magma immiscibility where an iron-rich magma fraction separated from the bulk silicic magma. A possible trigger for this magma immiscibility is the assimilation of carbonate or sulfate-rich sedimentary rocks by silicic magmas. If so, hydrothermal fluids exsolved by the iron-rich magma and the magma itself may contain detectable enrichments of carbonate and sulfate inherited by this assimilation process, along with iron, which allowed the hydrothermal fluids to precipitate iron oxide, recording part of the mineralization process. Fluid inclusions in the Shepherd Mountain deposit may provide a record of such sulfate-carbonate enriched hydrothermal fluids.

This hypothesis was tested using Raman spectroscopy to look for the presence of aqueous carbonate and sulfate species in fluid inclusions in ore-stage minerals of the Shepherd Mountain deposit. Previous petrologic and microthermometric work on the Shepherd Mountain deposit has shown a complex fluid history with heterogeneous entrapment of four different types of fluid inclusions that include: (1) liquid-rich, (2) liquid-vapor in highly variable proportions, (3) liquid-rich with a daughter mineral, and (4) polycrystalline inclusions with multiple daughter minerals of varying types. So far, types 1 and 3 have been analyzed with Raman spectroscopy. Peak positions of ~980, 1017, and 1064 cm⁻¹ in the fluid inclusions analyzed indicate the presence of , , and , respectively. In addition, weak peaks were evident at 2570-2590 cm⁻¹, which may indicate the presence of H₂S and HS⁻.