1. Introduction
Results
Conclusions
Abstract
Samples and Methods
Acknowledgements
The UGA Department of Geology provided microprobe time and support for
students to attend this meeting.
MINERALOGY OF A CARBONATE VEIN, BALSAM GAP OLIVINE MINE, NORTH CAROLINA
Cole, Meredith A. and Swanson, Samuel E., Dept. of Geology, University of Georgia, Athens, GA 30602
Carbonate-bearing veins occur in metadunites of the Blue Ridge of Georgia and North
Carolina. and cut a slightly hydrated noncarbonated assemblage of olivine (Ol) + chromite
(Chr) ± orthopyroxene + chlorite (Chl) ± tremolite (Tr). The purpose of this study is to
determine the mineralogy of the carbonate, and associated phases, and to relate their
petrogenesis to the host metadunite.
An exceptionally coarse-grained carbonate vein from the Balsam Gap olivine mine,
Macon County, NC, was selected for study. Very coarse-grained carbonate grains (up to 0.5
m) form the center of the vein. Included in the carbonate is coarse-grained talc (Tlc),
serpentine (Srp), (Ol), and (Chl). A zone of coarse-grained Ol+Chl+Tlc+Tr with lesser
amounts of carbonate marks the transition from the carboante-rich vein interior to the
enclosing metadunite.
The host metadunite is composed of a polygonal fabric of Ol (Fo 90-92) + Chr. Rare grains
of hydrous phases include Chl (mg# 93-96, Cr 0.2-0.6 apfu), Tlc (mg# 97-100) and Tr (mg#
95, IVAl 0.4-0.7) . A meshwork of fine-grained Srp (mg#95-97, Ni 0.01 apfu) outlines olivine
grains in some samples.
Magnesite (Mg 094-0.97, Fe 0.02-0.06 apfu) is the major carbonate phase, but small
amounts of dolomite (Ca 0.43-0.5, Mg 0.49-0.54 apfu) also occur in the vein. Nickle sulfides,
mainly pentlandite, also occur in the carbonate.
Compositions of minerals common to the metadunite and vein are generally similar
(e.g.metadunite Ol = Fo 90-92, vein Ol = Fo 94-95; metadunite Chl mg# 93-96, vein Chl mg#
94-95; metadunite Tr mg# 95, vein Tr mg# = 95-97 ), but the vein minerals range to higher
Mg contents. Trace element compositions of the silicate phases show some differences
(metadunite Chl Cr = 0.2- 0.6 apfu compared to vein Chl Cr = 0.4-0.6 apfu; metadunite Srp Al
= 0.1 - 0.3, Ni = 0.1 apfu; vein Srp Al = 0.0 - 0.1, Ni = 0-0.1 apfu).
This pattern suggests the veins represented pathways for H2O/CO2/S-bearing fluids
that altered the olivine of the metadunite. Fe from the altered olivine went into the
magnesite and, along with Ni, into the pentlandite. Differences in chlorite compositions are
related to the formation of Chl from Chr (elevated Al and Cr) in the metadunite. Fluid
interaction with the metadunite is probably responsible for the introduction of Tlc, and
possibly Tr, into the metadunite
Olivine-rich metadunites occur in the eastern Blue Ridge of Georgia and North Carolina.
These rocks record a protracted history of retrograde metamorphism and associated
hydration. The high grade assemblage is olivine (Ol) + chromite (Chr) ± orthopyroxene.
Additions of minor amounts of water results in chlorite (Chl) ± tremolite (Tr) added to the
assemblage. Later, lower grade phases include magnesiocummingtonite/anthophyllite, talc,
and serpentine. Many of these phases are often found in the same thin section, making it
very difficult to determine stable mineral assemblages. Detailed studies by Swanson (1981;
2001) suggest that carbonate-bearing veins that cut the olivine-rich fabric are associated
with the formation of antigorite serpentine, magnesiocummingtonite/anthophyllite, and talc.
The purpose of this study is to determine the mineralogy of the carbonate and associated
phases in the carbonate veins and to relate vein crystallization to minerals in the host
metadunite.
An exceptionally coarse-grained carbonate vein from the Balsam Gap olivine mine, Macon
County, NC, was collected in 1994 and used to model carbonate veins in this study (Fig. 1). The
vein includes a very coarse-grained carbonate core with individual carbonate grains up to 0.5 m
in diameter. Included in the carbonate core are coarse-grained (Tlc), (Srp), (Ol), (Chl) and
sulfide grains. A zone of coarse-grained Ol+Chl+Tlc+Tr with lesser amounts of carbonate forms
a transition zone between the carbonate-rich vein interior and the enclosing metadunite. The
host metadunite is composed of a polygonal fabric of Ol + Chr. Rare grains of hydrous phases
include Chl, Tlc, and Tr. A meshwork of fine-grained Srp outlines olivine grains in some
samples.
Samples from the vein core and transition zone were selected for microprobe analysis.
Polished thin sections were prepared and examined with a petrographic microscope. Samples
were analyzed on the JEOL 8600 electron microprobe (EMPA) in the Department of Geology at
UGA. Ordinary machine conditions for silicate analyses were: 15KeV accelerating voltage, 5nA
beam current. A 2 um beam diameter (10 um for carbonate minerals) and 10 sec counting times.
Standards were natural minerals and phi-rho-z techniques were used in data reduction.
Figure 1. Sketch and photo of carbonate vein.
Petrography Coarse carbonate grains (up to 0.5 m diameter) dominate the core of the vein (Fig.
1). Olivine associated with the vein (core and transition zone) is coarse-grained, several cm in
diameter. Alteration of Ol to Srp occurs along partings in the Ol (Fig. 2) produces a linear fabric.
Some Ol is completely replaced by Srp. Serpentine also replaces some of the carbonate along
cleavages (Fig. 3). Coarse grains of Tlc in the carbonate appear as elongate grains, perhaps as a
replacement of amphibole. Coarse-grained sulfide minerals occur within the carbonate (Fig. 4).
The transition zone between the carbonate core and metadunite is marked by a blackwall
assemblage of olivine being replaced by talc and chlorite with isolated grains of carbonate (Fig.
5).
Mineral Compositions Silicate and carbonate minerals within the carbonate vein
are Mg-rich reflecting the composition of the host metadunite. The carbonate
phase is mostly magnesite with minor amounts of dolomite (Table 1). Carbonate
compositions are near end-member, but small amounts of Fe occur in the
magnesite. Coarse-grained Ol in the vein is more Mg-rich than the fine-grained
Ol in the host metadunite (Fig. 6). Vein and metadunite compositions of
chromian clinochlore, tremolite - magnesiohornblende, and Tlc are similar.
Serpentine in the metadunite is higher in Al than Srp in the vein (Fig. 7). Sulfide
minerals are mainly pentlandite (Table 2 ), but a few grains of heazlewoodite and
millerite were identified.
Carbonate veins represent infiltration of the metadunite by a CO2 - rich fluid
along fractures. Recrystallization of the metadunite adjacent to the fracture
produced coarse-grained Ol that is more Mg-rich than the host dunite.
Amphibole, chlorite and talc in the vein and metadunite have the same
composition. Crystallization of these phases in the metadunite was associated
with fluid infiltration. Components for other vein phases (Fe & Ni for sulfides;
Mg for magnesite) came from olivine alteration. Later infiltration of H2O - rich
fluids produced the serpentine. Al-bearing phases in the metadunite and Chl
grains in the vein contributed Al to the serpentine.
Figure 2. Serpentine replacing olivine. Figure 3. Serpentine replacing magnesite.
Figure 4. Pentlandite in reflected light.
Figure 5. Blackwall assemblage of
magnesite, olivine, talc, and chlorite .
FeO 1.01 3.20 0.42 2.44
MgO 22.14 46.75 21.88 47.62
CaO 29.43 0.31 29.71 0.31
CO22 48.07 53.27 47.51 53.61
total 100.65 103.53 99.52 103.98
Dol Mgs Dol Mgs
Table 1. Composition of magnesite and dolomite. Table 2. Composition of sulfide minerals
Fe 0.73 29.11 32.05 0.15
Ni 71.45 35.27 32.95 62.48
Co 0 0.33 1.13 0.06
S 26.72 32.55 33.84 33.26
total 98.90 97.26 99.97 95.95
Haz Pn Pn Mil
Haz = hazelwoodite, Pn = pentlandite,
Mil = millerite
Figure 6. Olivine compositions. Figure 7. Serpentine compositions.