Petrographic Report: Polished Thin Section SLD-06-02M
Rock Type: altered albite-phyric rhyolite flow
The thin section displays a felty-textured, cryptocrystalline to fine-grained albite-phyric rhyolite with
weak clay-carbonate alteration overprinted by quartz-chlorite-sericite-pyrite veining. The sample is weakly
altered with almost 70% of the primary mineral assemblage remaining. The primary mineral assemblage consists
of cryptocrystalline to fine-grained feldspar blades and laths (too fine to accurately identify, but probably Kfeldspar > albite), euhedral albite phenocrysts, anhedral quartz grains, and minor rutile. The offcut has been
stained to identify the presence of K-feldspar, and the offcut has taken on a weak, disseminated pale yellow
colour indicating the presence of minor K-feldspar. Albite phenocrysts range in size up to 2.5 mm, and are
weakly to heavily altered (15 to 90%) by carbonate-clay and chlorite-sericite. Quartz is generally very finegrained except in patches (up to 0.5 mm wide) of coarser material. Rutile occurs as euhedral to anhedral
rounded grains and as aggregates of fine, granular crystals up to 0.4 mm wide. Rutile rarely is replaced by
hematite. This composition would classify the original rock as an altered albite-phyric rhyolite. The hand sample
groundmass is a mottled pink-green-grey colour with patches of burgundy hematite alteration up to 3 cm wide.
Phenocrysts are visible as white laths and cubes. Despite some carbonate alteration in the thin section, the hand
sample does not react to HCl, indicating the carbonate alteration mineral is not calcite. The thin section has
been affected by at least two alteration events, consisting of an initial carbonatization and clay alteration of the
host rock followed closely by a quartz-chlorite-sericite-pyrite alteration in patches and veins.
The first alteration event was a pervasive clay alteration and accompanying carbonatization represented
by very fine-grained, felty-textured illite-smectite alteration of the groundmass feldspar minerals and
overprinting carbonate minerals. The clay alteration has an overall mottling effect across the thin section, where
clay alteration is generally wispy and in elongate strands roughly oriented NS across the section. The
overprinting carbonate alteration consists of sub- to euhedral, rhombic crystals and amorphous, indistinct
patches (up to 0.5 mm) of very fine-grained carbonate. Carbonate, and lesser illite-smectite, alteration of albite
phenocrysts is weak to strong, and may replace up to 90% of a phenocryst.
Mineral
Modal Percent Abundance
Primary rock
Feldspar
15
Albite phenocrysts
10
Quartz-1
8
Rutile
2
Early carbonatization and clay alteration
Carbonate
10
Illite-smectite
10
Quartz-sericite-chlorite-pyrite alteration and veining
Quartz-2
11
Chlorite
13
Sericite
10
Pyrite
8
Hematite
3
Limonite
Trace
Epidote-clinozoisite
Trace
Cliffmont
Sample SLD-06-02M
Size Range (mm)
Up to 0.08 mm
Up to 2.5 mm
Up to 0.7 mm
Up to 0.4 mm
Up to 0.8 mm
Up to 30 microns
Up to 0.4 mm
Up to 0.4 mm
Up to 0.15 mm
Up to 1 mm
Up to 0.1 mm
Up to 40 microns
Up to 0.15 mm
Page 1
The second alteration event closely follows the first, and consists of veins of quartz-sericite-chlorite with
minor pyrite and associated massive aggregates or clumps of knotty chlorite and sericite. Veins range from 5
microns to 0.5 mm wide, are randomly oriented across the section, bifurcate, and cross-cut each other. The
veins are usually quite straight, but where quartz is absent from sections of these veins, and sericite is the
dominant mineral, they are wormy in shape. Vein density is almost 15% in the section. In the wider veins, quartz
is the dominant mineral, with chlorite occurring as knots and clumps in the center, and fan- to plumose-shaped
sericite occurring more along vein selvages. Minor euhedral cubic pyrite is associated with these veins and
overprints the wall rock (Fig. 1). Fine- to coarse-grained euhedral pyrite occurs disseminated throughout the wall
rock distal to the quartz veins. The pyrite locally forms trails trending roughly NE, and these trails are associated
with chlorite. Pyrite is locally zoned with the zonation defined by rounded quartz inclusions. Chlorite also occurs
as patches or knotty clumps (up to 2 mm wide) within the wall rock, where it usually is replacing carbonate, and
is associated with fan-shaped sericite. Chlorite commonly replaces carbonate rhombs and rarely occurs as nettextured patches overprinting albite phenocrysts. Finally, chlorite also occurs in rare, thin veinlets up to 40
microns wide and 1 mm long that are oriented perpendicular to the dominant quartz-sericite-chlorite-pyrite
veins. Sericite also occurs as patches, mostly associated with chlorite and small patches of secondary quartz, and
as replacement of albite phenocrysts, overprinting illite-smectite replacement. Minor, fine-grained, granular to
lath-shaped hematite is disseminated throughout the wall rock, is especially associated with chlorite, and occurs
in thin (up to 5 microns wide) veinlets up to 1 mm long. Hematite appears to replace rutile in places. Trace
limonite also occurs in aggregates of cryptocrystalline granules up to 0.5 mm wide. Trace epidote-clinozoisite is
associated with secondary quartz. The mineral assemblage of the first and second alteration events is indicative
of propylitic-style alteration.
carb
qtz
rt
py
Figure 1: Photomicrograph of a subhedral rutile (rt)
grain with finer, granular crystals at the top. Photo
taken in cross polarized transmitted light.
Cliffmont
Sample SLD-06-02M
Figure 2: Photomicrograph of a quartz (qtz) vein
hosting cubic pyrite (py) that cuts across the vein into
wall rock. Photo taken in cross polarized transmitted
and reflected light.
Page 2