1. Monica M. McGrath NE-NC GSA Pittsburgh, PA 03.21.17
UNRAVELING THE PETROGENESIS OF ORE BODY TALC DEPOSITS: A GEOCHEMICAL AND PETROLOGICAL STUDY
Monica M. McGrath
NE-NC GSA
Pittsburgh, PA

2. Background
The talc deposits in Southwestern Montana are some of the highest purity talc observed in the world with three actively producing mines
Much of this high purity talc is used for cosmetic and pharmaceutical applications
Current Hypothesis – study area was subject to at least three stages of metamorphism and a later hydrothermal event
The main talc forming event was a volume by volume replacement of dolomitic marble to talc.
The large volumes of water needed for these reacts was provided by connate brine waters pushed up from old sedimentary basin as the Belt Basin opened
RJ Lee Group

3. Postulated model for talc formation
Connate brines fluids come from either bottom of the Belt Basin or from Proterozoic seawater
Gammons, C. H. and D. O. Matt (2002). "Using fluid inclusions to help unravel the origin of hydrothermal talc deposits in southwest Montana." Northwest Geology 31:

4. Goals
Use isotopic and geochemical approaches to define how hydrothermal talc mineralizes
Did the talc form as a volume for volume replacement of the dolomitic marble?
Does the talc share a similar isotopic signature as the marble units? If they are different then did metamorphism alter the talc unit or are the components coming from the fluid
Can we postulate anything about fluid source?
RJ Lee Group

5. Sample Site Location Treasure Mines Regal Mine Willow Creek
Need Legend
Yellowstone Mine
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6. View of Willow Creek Pit
Weathering and deformation have reclaimed most of the mine, and the lower benches are no longer visible. Upper benches have slumped as can be seen in the tree line. 13 samples were collected. Samples 2 and 3 were not analyzed because they contained mainly pure sepiolite by R.I. PLM.
RJ Lee Group

7. View of Willow Creek Pit
Weathering and deformation have reclaimed most of the mine, and the lower benches are no longer visible. Upper benches have slumped as can be seen in the tree line. 13 samples were collected. Samples 2 and 3 were not analyzed because they contained mainly pure sepiolite by R.I. PLM.
** WC-13 Not denoted on map
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8. Methods
POWDERED SAMPLES
50:1 8
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9. Overview of Mineralogy
Talc
Clinochlore
Chlorite
Calcite
Dolomite
Accessory Min
Mineralogy was based on XRF and thin section observation. Samples contained carbonates were selected to separate the talc from the carbonate and analyze each portion differently. Samples WC-7 and WC-8 contaned non-fibrous lizardite. Lizardite was restricted to these samples which were located outside of the talc ore. WC7 also still preserved forsterite olivine and with lizardite replacing the olivine rims. (FIND IMAGE)
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10. WC-7
WC-8
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11. RJ Lee Group

12. Major Element Reference Talc <10% Talc >10% Talc RJ Lee Group
Talc samples plotted against reference talc sample – IARC Monographs
Samples follow same relative concentration of major elements – this shows that the talc from this location is close to if not pure stoichiometric talc In WC1 and WC4 the trend is the same ase the over reference but the concentration are lower this could be due to the higher concentrations of clinochlore and chlorite.
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13. Whole Rock REE Patterns
<10% Carb.
>10% Carb.
Talc (green) show positive Eu anomaly
Carbonate (blue) show enrichment in light REE compared to heavy REE
WC-6 – is a almost pure chlorite sample that is very enriched in REEs – this sample is considered an outlier in the set and has to be invested as to what is causing this enrichment
RJ Lee Group

14. Weathering and deformation have reclaimed most of the mine, and the lower benches are no longer visible. Upper benches have slumped as can be seen in the tree line. 13 samples were collected. Samples 2 and 3 were not analyzed because they contained mainly pure sepiolite by R.I. PLM.
RJ Lee Group

15. REE Leachate compared to Whole Rock
Whole rock patterns show same general patterns with positive Eu anomaly and enrichment in light REEs.
RJ Lee Group

16. Weathering and deformation have reclaimed most of the mine, and the lower benches are no longer visible. Upper benches have slumped as can be seen in the tree line. 13 samples were collected. Samples 2 and 3 were not analyzed because they contained mainly pure sepiolite by R.I. PLM.
RJ Lee Group

17. Talc vs Potential Source Carbonate REE Patterns
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18. Conclusions
Multiple stages of metamorphism discussed in previous research evident in our samples
Including a late stage hydrothermal event
Talc from this area is very pure and most accessory minerals lie outside the ore body
Continuation of isotope work
Rb-Sr and Sm-Nd isotope studies will address
If the talc is distinctly different from the carbonate
If the talc mineralized directly from the fluid or as replacement of the carbonate
RJ Lee Group

19. THANK YOU TO RJ LEE GROUP, INC FOR FUNDING THIS PROJECT
Questions?