Petrographic Report: Polished Thin Section SLD-03-01MB
Rock Type: feldspar-quartz-phyric rhyolitic crystal tuff
The thin section displays a thinly laminated texture with beds defined by alternating finer and coarser
grains. Beds are on the order of 0.2 to 1 mm thick and bedding is slightly deformed and facing upward toward
the northeast corner of the thin section. Bedding wraps around phenocrystic albite, K-feldspar, and quartz
crystals (Fig. 1). Coarser grained quartz and feldspar exist in pressure shadows along the sides of the
phenocrysts. The sample is weakly altered with approximately three-quarters of the primary mineral assemblage
remaining. The primary mineral assemblage was dominantly very fine- to fine-grained albite, K-feldspar, and
quartz aligned parallel to bedding, with approximately 10% phenocrysts of angular quartz and euhedral lath-like
feldspar (albite > K-feldspar) ranging in size up to 1.5 mm wide. This original rock would be classified as a
feldspar-quartz-phyric rhyolitic crystal tuff. Trace amounts of a prismatic apatite occur. The apatite is pleochroic
honey yellow, displays moderate relief and commonly display idiomorphic hexagonal cross sections. The hand
sample exhibits ovoid white patches up to 1 cm wide that react with HCl (indicating the presence of calcite), and
a groundmass that is purplish-red in colour. The offcut has been stained to check for the presence of K-feldspar,
and a very weak, pale yellow colour was taken on, indicating a very little K-feldspar is present throughout the
section. There are at least two overprinting generations of alteration. The early alteration event is characterized
by an initial carbonatization (calcite) of the host rock associated with contemporaneous secondary quartz, thin
chlorite-sericite-carbonate-hematite veins (up to 0.3 mm wide), and sericite-chlorite-kaolinite alteration of
feldspar phenocrysts. This was followed by the development of thin sericite veinlets that trend NE-SW,
perpendicular to bedding.
The first alteration event was a pervasive carbonatization represented by irregular and rhombic calcite
overprinting fine-grained groundmass and some phenocrysts. Calcite rarely occurs in short, discontinuous
Mineral
Primary rock
Quartz-1
Albite
K-feldspar
Apatite
Modal Percent Abundance
Size Range (mm)
19
28
5
Trace
Up to 0.3 mm
Up to 1.5 mm
Up to 1.5 mm
Up to 0.25 mm
Early carbonatization with secondary quartz and chlorite-sericite-calcite±hematite veining
Sericite
12
Up to 40 microns
Calcite
10
Up to 1 cm
Chlorite
7
Up to 0.5 mm
Hematite
7
Up to 0.6 mm
Illite
5
Up to 20 microns
Quartz-2
5
Up to 0.3 mm
Epidote-clinozoisite
1
Up to 80 microns
Limonite
Trace
Up to 20 microns
Pyrite
Trace
Up to 80 microns
Sphalerite
Trace
Up to 0.2 mm
Late sericite veinlets
Sericite
Cliffmont
1
Sample SLD-03-01MB
Up to 50 microns
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veinlets up to 0.5 mm wide that are oriented perpendicular to bedding. Carbonate also occurs in ovoid patches
of coarse calcite up to 1 cm wide. Calcite is in some places rimmed by contemporaneous coarse-grained
secondary quartz and occasionally has chlorite-sericite-calcite-hematite veins wrapping between the calcite and
secondary quartz (Fig. 2). Chlorite-sericite-calcite-hematite veins are also randomly oriented across the thin
section with a vein density of about 5%. Chlorite dominates the mineralogy of these veins. The veins intersect
themselves and have an enterolithic texture due to N-S direction flattening of the host rock (Fig. 1). In some
places, these veins dissipate into thin, straight hematite veinlets trending NE-SW. Albite and K-feldspar
phenocrysts are heavily altered (between 40 to 100% of the crystals) to sericite (dominantly), calcite, chlorite,
and minor hematite and illite. Trace amounts of epidote-clinozoisite, pyrite, and sphalerite (Fig. 3) also occur in
heavily altered phenocrysts, and are particularly associated with chlorite. A single grain of sphalerite found in a
calcite-sericite altered albite phenocryst exhibits moderate chalcopyrite disease. This phenocryst replacement is
likely contemporaneous with carbonatization and veining. Hematite also occurs as fine, anhedral grains and
masses/knots disseminated throughout the groundmass of the thin section, and is coarser in coarser-grained
beds. Pyrite is closely associated with hematite knots, and is probably replaced by hematite. The alteration
mineral assemblage here is consistent with the propylitic alteration assemblage.
The second alteration consists of very thin, straight sericite veinlets trending NE-SW overprinting
groundmass quartz and feldspar and chlorite-calcite-sericite±hematite veins. The sericite veins post-date the
chlorite-dominant veins since they are undeformed (Fig. 1).
vn
chl-srct
cal
srct-2
ab
qtz
cal
ab
hem
Figure 1: Photomicrograph of deformed bedding
wrapping around an albite (ab) phenocryst. Bedding
faces upward towards the northeast. Albite is heavily
altered to chlorite, sericite, and carbonate. Very thin
veinlets of sericite (srct-2) are undeformed and
discordant to bedding. Photo taken in plane polarized
transmitted light.
Cliffmont
Sample SLD-03-01MB
hem
Figure 2: Photomicrograph of a patch of irregular
calcite (cal) rimmed by secondary quartz (qtz). A thin
chlorite-sericite-calcite-hematite vein (vn) winds its
way between the calcite and quartz. An albite (ab)
phenocryst is heavily replaced by calcite and sericite.
Photo taken in cross polarized transmitted light.
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Figure 3: Photomicrograph of a heavily calcite-sericite
altered albite phenocryst containing a single grain of
sphalerite (sph) with chalcopyrite disease associated
with a small, subrounded pyrite. Photo taken in cross
polarized transmitted and reflected light.
py
sph
cal
cal-srct
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Sample SLD-03-01MB
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