Petrographic Report: Polished Thin Section SLD-08-01M
Rock Type: carbonate-altered crystal-bearing lithic tuff
The thin section is composed of a very fine-grained, felty-textured groundmass of anhedral to
subrounded quartz, feldspar, and ash with two kinds of lithic fragments. The sample is intensely overprinted by
at least one alteration event with less than one-third of the primary mineral assemblage remaining. The hand
sample is light grey coloured with tiny (up to 1 mm) black fragments throughout and larger (up to 2 cm) rounded
and irregular burgundy-red fragments with green rims.
The primary mineral assemblage was dominated by anhedral to subrounded quartz and ash-sized
particles (too small to accurately identify). Quartz also occurs as coarser, subrounded grains up to 0.6 mm wide.
A significant proportion of the original rock was composed of two kinds of lithic fragments (Fig. 1). The first type
of fragments are composed of dominantly fine-grained, flattened quartz with occasional pressure solution
textures, with minor lath-like feldspar and sericite. Many of these fragments are partially replaced by carbonate.
The second type of fragments is composed mostly of recrystallized, fine to coarse grained carbonate with minor
chlorite and trace quartz. These fragments are somewhat larger than fragment type 1. Chlorite occurs as clumps
within the middle of these fragments or as rims around the edges of the fragments. The rest of the primary
mineral assemblage is composed of minor anhedral, blocky albite and rare blocky to lath-shaped albite
phenocrysts. Minor rounded, prismatic rutile also occurs within the section. This composition would classify the
rock as a crystal-bearing lithic tuff.
At least one carbonate alteration event overprints this sample. The alteration is composed mostly of
overprinting, amorphous to coarse-grained carbonate and carbonate veins associated with minor chlorite,
pyrite, sericite, and epidote. Carbonate occurs as amorphous to fine-grained, pervasive alteration of the
groundmass, with some denser patches, as minor replacement in type 1 fragments, and recrystallization of type
2 fragments. Carbonate alteration also occurs as randomly oriented, discontinuous, forking veins up to 0.3 mm
wide (Fig. 1). Where these veins are less than 10 microns wide, they appear rusty. Vein density is roughly 5% of
the thin section. Chlorite occurs as net-textured and very fine-grained clumps within carbonate fragments and
alteration patches within the groundmass, and also as rims (up to 0.1 mm wide) on type 2 carbonate-dominated
Mineral
Modal Percent Abundance
Size Range (mm)
Primary rock
Quartz
18
Up to 0.6 mm
Lithic fragments
- quartz >> carbonate > albite,
8
Up to 2 mm
sericite
- carbonate > chlorite > quartz
5
Up to 2 cm
Albite
2
Up to 0.25 mm
Rutile
1
Up to 80 microns
Carbonate alteration and veining
Carbonate
51
Up to 0.9 mm
Chlorite
12
Up to 0.1 mm
Pyrite
3
Up to 0.15 mm
Sericite
Trace
Up to 0.1 mm
Epidote
Trace
Up to 0.15 mm
fragments. Chlorite rarely occurs as thin, wormy veins up to 50 microns wide and 2 mm long. Euhedral cubic and
rounded pyrite is associated with these chlorite veinlets (Fig. 2). Pyrite also occurs as fine, rounded grains
Cliffmont
Sample SLD-08-01M
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disseminated throughout the groundmass and overprinting only the carbonate-altered type 1 quartz-dominated
fragments. Sericite occurs in trace amounts in the sample and occurs as very fine-grained laths associated with
carbonate. Epidote also is a trace mineral and occurs as rounded grains associated with carbonate alteration.
This alteration assemblage is most similar to the propylitic alteration, except that it lacks secondary albite and
abundant epidote.
frag1
frag2
py
vein
Figure 1: Photomicrograph of the two types of lithic
fragments in this sample: fragment type 1 (frag1) are
those fragments dominated by flattened quartz grains
with minor feldspar and sericite, and minor carbonate
alteration. Fragment type 2 (frag2) are those
composed of recrystallized carbonate with chlorite
rims and minor quartz. This image also shows a weakly
developed carbonate vein (vein). Photo taken in cross
polarized transmitted light.
Cliffmont
Sample SLD-08-01M
chl
rt
Figure 2: Photomicrograph of a thin, wormy chlorite
(chl) veinlet hosting minor pyrite (py). Minor primary
rutile (rt) is also present. Photo taken in plane
polarized reflected light.
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