Crabtree Pegmatite Petrography

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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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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.
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Crabtree Pegmatite
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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
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100 years ago (Kuntz, 1907, Plate X)
As seen today.
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• 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
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• 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
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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)
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• 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
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• Mica Schist
• Well foliated
• Minerals Muscovite, biotite, plagioclase,
  quartz, garnet, clinozoisite, zircon, chlorite

• PPL (B) CPL bars 0.5 mm
• (C) Typical outcrop

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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
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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
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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
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(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
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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
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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
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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
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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.