Petrographic Report: Polished Thin Section SLD-08-02P
Rock Type: quartz-sulphide vein in clay-altered lithic tuff
The thin section is dominated by a milky white quartz-sulphide vein greater than 2 cm wide that covers
three quarters of the thin section. Quartz and sulphides are coarse-grained and sub- to euhedral. The host rock
is preserved along the right and top sides of the thin section, and is an intensely clay-altered lithic tuff that has
undergone some quartz flooding proximal to vein selvages. In thin section, the host rock is also preserved as a
few elongate rounded fragments (up to 4 mm wide) close to vein selvages. The host rock is composed of angular
to sub-angular fragments (up to 2 mm wide) in ash that are completely replaced by clay and sericite. Host rock
mineralogy is dominated by very fine-grained illite-smectite, rounded to sub-angular primary quartz, and finegrained fan-shaped sericite. Illite-smectite alteration is pervasive and intense, whereas sericite mostly occurs in
patches, with secondary quartz veinlets, and along main vein selvages. A very minor amount of fine-grained,
disseminated rutile (about 2% of the host rock) and trace secondary granular epidote are present. Secondary
euhedral quartz occurs as bladed and euhedral grains flooding into the host rock up to 1 cm from vein selvages,
and as discontinuous cross-cutting veins (up to 0.2 mm wide) with minor galena, sphalerite, and chalcopyrite.
Secondary quartz in the host rock is contemporaneous with the much wider quartz vein. The contact between
host rock and the main vein is sharp to gradational in places. Euhedral cubic to pyritohedral pyrite also occurs
within the host rock, and is syngenetic with silicification and pre- to syn-genetic with the other sulphides. At
least two, possibly three, generations of alteration occurred within this rock. The first is characterized by the
intense clay-alteration of the host rock (described above), and the second is characterized by quartz-sulphide
Mineral
Modal Percent Abundance
Host rock
Illite-Smectite
18
Quartz-1
5
Sericite
4
Rutile
Trace
Epidote
Trace
Silicification and quartz-sulphide veining
Quartz-2
22
Pyrite
20
Sphalerite
17
Galena
7
Smithsonite
3
Chalcopyrite
3
Covellite
1
Bornite
Trace
Replacement of sulphides
Digenite?
Trace
Chalcocite
Trace
Anglesite
Trace
Native gold
Trace
Size Range (mm)
Up to 0.1 mm
Up to 0.3 mm
Up to 0.1 mm
Up to 80 microns
Up to 50 microns
Up to 2.2 mm
Up to 4 mm
Up to 3.5 mm
Up to 2.5 mm
Up to 0.5 mm
Up to 0.6 mm
Up to 0.1 mm
Up to 0.2 mm
Up to 0.1 mm
Up to 50 microns
Up to 0.5 mm
Up to 50 microns
veining and silicification. The third may be syngenetic to immediately post-genetic with the second generation,
and is comprised of carbonate, anglesite, and sulphosalt alteration of the sulphide minerals.
Cliffmont
Sample SLD-08-02P
Page 1
The second generation of alteration is composed of silicification of the host rock and contemporaneous
quartz-sulphide veining. The main vein is vuggy in places in the hand sample and composed of coarse-grained
sub- to euhedral hexagonal, zoned quartz with abundant sulphides. It is contemporaneous with the thinner
secondary quartz veins within the host rock. Pyrite is the dominant sulphide and occurs as coarse, sub- to
euhedral cubes and pyritohedra between quartz crystals. Pyrite is commonly fractured, and fractures are
generally filled with other sulphide minerals, especially galena and chalcopyrite, indicating that pyrite
crystallization just barely precedes other sulphides in the paragenetic sequence of vein mineralization. Pyrite
grains are disseminated randomly throughout the vein and finer grained along vein selvages. Minor amounts of
droplet shaped chalcopyrite and rare galena occur as inclusions in pyrite. Sphalerite is the second most
abundant sulphide in the thin section, and occurs as coarse anhedral grains in the main vein, with ragged edges
and fractures commonly replaced by covellite, and abundant chalcopyrite disease. There appears to be two
intergrown sphalerite generations (Fig. 1): sphalerite 1 contains fewer, but generally coarser, inclusions of
chalcopyrite and is pale brown in colour; and sphalerite 2 contains more abundant, finer grained chalcopyrite
and is colourless. Galena occurs as anhedral, and rarely euhedral, irregular grains mostly within interstices
between pyrite and sphalerite, and within fractures in pyrite. Galena has curved crystal faces in this section,
which may indicate contamination by trace amounts of silver. Chalcopyrite occurs as abundant chalcopyrite
disease in both sphalerites, as rounded inclusions in pyrite, and also as irregular grains interstitial to quartz.
Rarely, chalcopyrite exhibits replacement by covellite around its rims, and this occurs mostly in chalcopyrite
grains within quartz. Trace amounts of bornite occur within fractures in quartz and as inclusions in sphalerite. A
single grain of native silver occurs along the edge of a sphalerite grain associated with galena.
The possible third alteration event (which may actually be syngenetic with the veining event) is
characterized by smithsonite-covellite and anglesite-sulphosalt alteration of sphalerite and galena. A dirty blackbrown, crustiform carbonate mineral occurs along grain boundaries between the two sphalerites and along
fractures in sphalerite. Fine-grained covellite commonly replaces both sphalerite generations around fractures
and
sph-2
carb
ang
gn
qtz
sph-1
sph-1
Figure 1: Photomicrograph of the two intergrown
sphalerites in this thin section: pale brown sphalerite 1
(sph-1), chalcopyrite-rich, colourless sphalerite 2 (sph2). A dark black-brown carbonate mineral occurs along
fractures and grain boundaries of the two sphalerites.
Photo taken in plane polarized transmitted light.
Cliffmont
Sample SLD-08-02P
py
Figure 2: Photomicrograph of galena (gn) in pyrite (py)
being replaced by trace amounts of anglesite (ang)
that cuts across the galena and pyrite. Photo taken in
plane polarized reflected light.
Page 2
grain edges, particularly where sphalerite is in contact with the carbonate. In places galena is replaced by trace
amounts of anglesite that cuts across the galena and fractures (Fig. 2). Chalcocite and possibly digenite and rim
galena, and rarely sphalerite, as amorphous, crustiform replacement from the outside of the grains inward.
Native gold (Fig. 3) may be related to this possible third alteration event, and occurs with smithsonite and nettextured chalcocite intergrown with covellite where they replace sphalerite.
Figure 3: Photomicrograph of a single grain of native
gold (au) within smithsonite (sm) alteration of
sphalerite (sph). Thin rims of fine-grained blue
covellite (cv) and and intergrown chalcocite occur
around relict sphalerite grains and disseminated
through the carbonate. Photo taken in plane polarized
reflected light.
qtz
py
sm
sph
au
cv
Cliffmont
Sample SLD-08-02P
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