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Petrography of the Country Rock Surrounding the Crabtree Pegmatite, Spruce Pine District, Mitchell County, North Carolin

Petrography of the Country Rock Surrounding the Crabtree Pegmatite, Spruce Pine District, Mitchell County, North Carolina. James A. Dockal and Michael S. Smith University of North Carolina Wilmington.

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Petrography of the Country Rock Surrounding the Crabtree Pegmatite, Spruce Pine District, Mitchell County, North Carolin

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  1. Petrography of the Country Rock Surrounding the Crabtree Pegmatite, Spruce Pine District, Mitchell County, North Carolina. James A. Dockal and Michael S. Smith University of North Carolina Wilmington

  2. The emerald bearing Crabtree pegmatite pre-dates the intrusion of the Spruce Pine Igneous Complex including the economically important pegmatite bodies. The Crabtree pegmatite and adjacent country rock have been altered by hydrothermal processes that post-date pegmatite formation.

  3. Crabtree Pegmatite

  4. 687. Hootowl mine 688. Chestnut Flats mine 690. Self (Waycaster) mine 691. Crabtree Mountain Emerald mine 705. McKinney mine Muscovite granodiorite Map from Lesure, 1968, USGS Prof. Paper 577

  5. 100 years ago (Kuntz, 1907, Plate X) As seen today.

  6. Crabtree pegmatite • Main minerals: plagioclase, quartz, beryl, tourmaline, garnet • Concordant to country rock, tabular shape. • Crystal size is not really that large. Bars = 0.5 mm

  7. Hornblende-plagioclase gneiss • Well foliated, lattice preferred orientation of hornblende. • Minerals: hornblende, plagioclase, quartz, garnet, clinozoisite, magnetite, zircon, carbonate (A) PPL (B) CPL bars = 0.5 mm (C) Garnet porphyroclast with carbonate inclusions

  8. Tappen, C.M., 1998, Beryl and tourmaline mineralization of the Crabtree pegmatite, Spruce Pine District, North Carolina: (masters thesis) University of North Carolina Wilmington, 115 p. (Figure 8)

  9. Granofels • Non-foliated • Minerals: plagioclase, cordierite, chlorite, actinolite (plus at least another dozen!) (A) PPL (B) CPL bars = 0.5 mm (C) Weathered outcrop surface

  10. Mica Schist • Well foliated • Minerals: Muscovite, biotite, plagioclase, quartz, garnet, clinozoisite, zircon, chlorite • PPL (B) CPL bars = 0.5 mm • (C) Typical outcrop

  11. Petrogenesis Stage 1: Protolith Formation (A) Calcite in granofels (CPL) (B) Zircon from hornblende-plagioclase gneiss (C) Zircon from granofels (D) Zircon from biotite schist 1 cm from pegmatite (E) Zircon from mica schist Bar = 0.5 mm Bars = 0.1 mm

  12. Petrogenesis – Stage 2: Pegmatite emplacement (A) Mica schist – pegmatite contact, parallels biotite LPO (PPL) (B) Tourmaline porphyroclast in mica schist (CPL) (C) Tourmaline porphyroclast in mica schist, note inclusions are foliated. (PPL) Bars = 0.5 mm

  13. Petrogenesis Stage 3: “Albitization” (A) Detail of plagioclase (albite) with inclusions of muscovite, biotite and a carbonate (CPL) (B) Typical plagioclase (albite) with abundant inclusions of a variety of minerals and cordierite (c) (CPL) (C) Same view a (B) in PPL, apatite (a) c Bars = 0.2 mm a

  14. (A), (B), and (C) sequence illustrating transition from biotite gneiss to granofels, each photo centered about 1 cm from the next. (D) Granofels margin just at the margin between the pegmatite and biotite schist. granofels granofels granofels T T pegmatite (E) “albitized’ pegmatite. Note the fracturing of the tourmaline (T) and how the traces of the fractures continue into the adjacent albite suggesting fracturing preceded albitization. biotite schist Bars = 0.5 mm

  15. Petrogenesis Stage 4: “Chloritization” (A) Chlorite replacing garnet in granofels (PPL) (B) Chlorite replacing interior of plagioclase in granofels (CPL) (C) Chlorite replacing cordierite in granofels (PPL) Bars = 0.4 mm cordierite

  16. Q (A) Chlorite (C) replacing biotite in mica schist (CPL) (B) Chlorite has replaced everything except tourmaline (T) and quartz (Q) in pegmatite (CPL) (C) Chlorite (C) and biotite (B) interleaved in granofels (PPL) T T T Bars = 0.4 mm B C C

  17. Petrogenesis Stage 5: actinolite-clinozoisite (A) Actinolite cross-cutting chlorite in the granofels (CPL) (B) Actinolite cross-cutting plagioclase in the granofels (CPL) (C) Clinozoisite cross-cutting chlorite in granofels (PPL) Bars = 0.4 mm

  18. Conclusions Crabtree pegmatite formed prior to the intrusion of the Spruce Pine Complex, possibly as early as Taconic. Some of the other pegmatite occurrences in the Spruce Pine District may also be temporally related to the Crabtree. The unusual mineralogy of the Crabtree may be caused in part by the mineralogy of the protolith of the granofels. The hydrothermal alteration observed in the country rock post-dates Crabtree pegmatite formation and is possibly related to the magmatism of the Spruce Pine Complex.

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