Petrographic Report: Polished Thin Section SLD-04-03M
Rock Type: clay-silica-carbonate altered albite-phyric rhyolite
The thin section displays an intensely clay-altered and carbonatized, very fine-grained feldspar-phyric
rhyolite with pervasive perlite texture (Fig. 1). Perlite texture develops when rhyolite has a high water content
and forms in silica-rich lava flows, domes, tuffs, plugs, or dykes. The higher water content of perlite is attributed
to the addition of “secondary” water late during cooling of the lava from external sources, such as groundwater
or surface water. Perlite development is contemporaneous with the earliest clay alteration event, which is
focused along perlite fractures. Early clay alteration with associated limonite after devitrification occurs along
the perlitic fractures. The primary mineral assemblage consists of very fine- to fine-grained quartz, chalcedony,
and feldspar (K-feldspar>albite), with approximately 5% albite phenocrysts ranging in size up to 1.5 mm.
Chalcedony occurs in plumose patches or as spherulites up to 1 mm wide. Albite phenocrysts are affected by
approximately 10-40% replacement by clays and calcite. Minor rutile also occurs as fine-grained disseminated
crystals and clumps of sub- to anhedral fine-grained crystals. The offcut has been stained to check for the
presence of K-feldspar, and a moderate, pale yellow colour was taken on, indicating some K-feldspar is present.
This composition would classify the original rock as an albite-phyric rhyolite. The sample is moderately to
strongly altered with approximately one third of the primary mineral assemblage remaining. The groundmass is
overprinted by at least three alteration events that are somewhat difficult to distinguish from one another.
These events include the early perlitization and associated moderate to strong clay alteration, a chlorite-sericite
event, and late carbonatization with associated quartz recrystallization and sulphide mineralization.
The first alteration event was a pervasive devitrification and clay alteration with associated
cryptocrystalline limonite. Clay alteration and limonite are especially concentrated around perlitic fractures (Fig.
1). Clay is generally too fine to identify its mineralogy, but it is likely dominated by illite-smectite. The
Mineral
Modal Percent Abundance
Primary rock
Quartz-1
11
Albite
7
Chalcedony
7
K-feldspar
6
Rutile
2
Early perlitization and clay alteration
Clay
16
Limonite
2
Chlorite-sericite
Chlorite
8
Sericite
7
Carbonatization
Calcite
24
Quartz-2
6
Smectite
3
Hematite
1
Chalcopyrite
Trace
Galena
Trace
Pyrite
Trace
Epidote
Trace
Cliffmont
Sample SLD-04-03M
Size Range (mm)
Up to 0.5 mm
Up to 1.5 mm
Up to 0.4 mm
Up to 70 microns
Up to 0.1 mm
cryptocrystalline
Up to 50 microns
Up to 0.4 mm
Up to 0.1 mm
Up to 2 mm
Up to 0.2 mm
Up to 20 microns
Up to 20 microns
Up to 0.1 mm
Up to 0.25 mm
Up to 20 microns
Up to 50 microns
Page 1
hand sample is not sticky to the tongue, indicating the dominant clay mineral is not kaolinite, although trace
amounts of it may be present.
The second alteration event consists of patches and clumps of mixed chlorite and sericite up to 1 mm
wide that are often associated with calcite and secondary quartz, and hence difficult to distinguish from the
third alteration event. These sheet silicates are not associated with any other particular element of the thin
section and are randomly distributed through the rock.
The third alteration event consists of massive patches of fine-grained calcite and wormy, forking calcite
veins (up to 0.05 mm wide) associated with secondary quartz recrystallization, minor smectite, minor hematite,
trace epidote, and trace sulphides (chalcopyrite, galena, and pyrite). Calcite alteration is commonly focused
along perlitic fractures as very fine grains of amorphous calcite, but mostly occurs in fine- to coarse-grained
patches up to 3 mm wide overprinting the host rock. Recrystallized quartz occurs in ovoid clumps within calcite
grains, as fine-grained rims around calcite (Fig. 2), and locally with chalcedony. Rare veins of secondary quartz
(up to 0.04 mm wide) trending NNE cut the groundmass. Minor coarse smectite occurs in clumps up to 0.1 mm
and is generally associated with secondary quartz and quartz veinlets. Minor hematite occurs as fine-grained,
bladed crystals, typically associated with secondary quartz or the groundmass. Trace amounts of galena,
chalcopyrite (Fig. 3), and pyrite occur in secondary quartz clusters. Irregular galena is also associated with
chlorite alteration disseminated through the host rock, and fine-grained chalcopyrite occurs within calcite with
minor sericite. Chalcopyrite has thin rims of covellite replacement where it occurs in secondary quartz (Fig. 3).
The alteration mineral assemblage here is most similar to the propylitic alteration assemblage, except for the
added secondary quartz and hematite.
Although moderately altered and having experienced some veining, the rock does not display much
evidence of deformation.
cal
ch
ab
qtz-2
hem
Figure 1: Photomicrograph of the concentric fractures
typical of perlitic texture of the thin section. Note the
strong clay alteration and limonite around perlitic
fractures. Photo taken in plane polarized transmitted
light.
Cliffmont
Sample SLD-04-03M
cal
Figure 2: Photomicrograph of typical textures of
calcite alteration as veins (left, cutting plumose
chalcedony) and patches (bottom of photo). Calcite
patches are rimmed by very fine grained
recrystallized quartz (qtz-2). A primary albite
phenocryst (ab) is weakly altered to clays and calcite.
Chalcedony (ch) rims the phenocryst. Photo taken in
cross polarized transmitted light.
Page 2
Figure 3: Photomicrograph of galena (gn) and
chalcopyrite (cpy) mineralization associated with a
cluster of recrystallized quartz (qtz-2). Calcite (cal)
occurs in small patches around the quartz, and minor
chlorite (chl) occurs above it. Photo taken in cross
polarized transmitted and reflected light.
chl
qtz-2
gn
cpy
cal
Cliffmont
Sample SLD-04-03M
Page 3