Petrographic Report: Polished Thin Section SLD-13-02M
Rock Type: carbonatized sub-arkose sandstone
The thin section is a well sorted sandstone with less than 2% ovoid to rounded fragments. The hand
sample is greenish-tan coloured and has parallel layering. In hand sample, the fragments are up to 6 mm long,
ovoid to elongate, and burgundy-red in colour with thin green rims. Thin white veins cut the section in random
orientations. Fine-grained pyrite is disseminated throughout the sample. In thin section, the primary mineral
assemblage is composed of well sorted and fine-grained (up to 0.25 mm) detrital quartz, lesser K-feldspar, and
minor albite, muscovite, and rutile. Quartz grains are subangular to subrounded, and minor pressure solution
structures are occasionally present between grains. K-feldspar and albite grains are commonly blocky and
subrounded, and rarely weakly altered to clay. Rutile grains are fine-grained, sub-rounded, and disseminated
through the section. Detrital muscovite occurs in plates and laths. The few rounded fragments (less than 2% of
the rock) are composed of medium-grained recrystallized carbonate and minor rounded, fine-grained quartz.
These few fragments are composed of 80% carbonate, 10% quartz, and 10% pyrite. The original detrital
composition classifies the original rock as a fine-grained sub-arkose sandstone (using the Folk 1965 sandstone
classification scheme). The sample is overprinted by at least two generations of alteration. The two events
consist of an initial, weak carbonate alteration with accompanying chlorite, pyrite, and epidote, and a later weak
clay alteration dominated by illite-smectite and minor sericite. Alteration has not reduced grain size, and is
largely confined to interstices between detrital minerals.
The first alteration event was a pervasive but weak carbonate alteration of the host rock and
recrystallization of fragments. Carbonate is fine-grained and largely found interstitially between detrital grains,
but in some places overprints detrital muscovite. It also occurs in denser patches in places up to 1 mm wide.
Fragments with coarser carbonate (up to 0.5 mm wide) are rimmed by anhedral, net-textured chlorite (rims up
to 0.4 mm wide). Carbonate alteration was accompanied by pyrite crystallization. Pyrite is fine-grained,
euhedral, cubic, and disseminated throughout the section. Pyrite occasionally bears inclusions of detrital rutile
and quartz, and trace secondary chalcopyrite. Chalcopyrite also occurs as rare interstitial grains in carbonate.
Mineral
Primary rock
Quartz
K-feldspar
Muscovite
Albite
Rutile
Early carbonate alteration
Carbonate
Pyrite
Chlorite
Epidote
Limonite
Chalcopyrite
Sphalerite
Native gold?
Late clay alteration
Illite-smectite
Sericite
Cliffmont
Modal Percent Abundance
Size Range (mm)
57
6
1
1
2
Up to 0.25 mm
Up to 0.1 mm
Up to 0.1 mm
Up to 0.05 mm
Up to 0.15 mm
15
5
2
1
Trace
Trace
Trace
Trace
Up to 0.5 mm
Up to 0.8 mm
Up to 80 microns
Up to 50 microns
Up to 30 microns
Up to 50 microns
Up to 20 microns
Up to 10 microns
9
1
Up to 80 microns
Up to 25 microns
Sample SLD-13-02M
Page 1
Trace sphalerite also occurs as rare interstitial grains in carbonate. A single grain of native gold (Fig. 1) occurs
associated with a pyrite grain in the groundmass. This alteration mineral assemblage is consistent with the
propylitic alteration style.
The second alteration event is very weak and comprised of very fine-grained, felty-textured illitesmectite and minor associated sericite overprinting carbonate. What appear as distinct white veins in hand
sample are barely distinguishable from the rest of the sample in thin section (Fig. 2), and are identified as
ribbons of material up to 0.5 mm wide with more abundant clay alteration than the surrounding rock. This
alteration mineral assemblage is similar to the argillic alteration style.
au
rt
clay vein
py
Figure 1: Photomicrograph of a grain of native gold
(au) associated with secondary pyrite (py) formed
during the carbonate alteration event. A grain of
detrital rutile (rt) is also observed here. Photo taken in
plane polarized reflected light.
Cliffmont
Sample SLD-13-02M
Figure 2: Photomicrograph of the general finegrained, equigranular texture of angular to euhedral
crystals and common mineral phases in this thin
section. Photo taken in cross polarized transmitted
light.
Page 2