Petrographic Report: Polished Thin Section CA-01P
Rock Type: quartz-sulphide-iron oxide vein in clay- and iron oxide-altered crystal tuff
The thin section is dominated by a 3 cm wide vein that covers 85% of the thin section and consists of
milky white vuggy quartz bearing sulphides and iron oxides. The vugs in the hand sample are 1 to 4.5 cm wide
and host euhedral hexagonal quartz needles coated with blue-black lepidocrocite. The host rock is preserved in
thin edges around the vein selvages, and is a clay- and iron oxide-altered crystal tuff that has undergone some
silicification proximal to vein selvages. In thin section, the host rock is also preserved as small, rounded clasts
(0.5 to 2 mm wide) close to vein selvages. The host rock comprises rounded to sub-angular primary quartz and
fine-grained smectite (± minor kaolinite) and sericite overprinted by cryptocrystalline limonite. Sericite occurs in
greater abundant where smectite is absent, and vice versa. Secondary quartz occurs in multi-crystalline blades
and short, discontinuous cross-cutting veins (up to 0.1 mm wide) with minor iron oxides, and overprints
limonite. Secondary quartz in the host rock is contemporaneous with the much wider quartz vein. Thin wormy
hematite-lepidocrocite veins (up to 0.04 mm wide, greater than 0.5 mm long) overprint and offset limonite and
quartz veins. Intergrown hematite and lepidocrocite also occurs as sub-rounded cubes that are likely
pseudomorphs after pyrite.
The vein is vuggy and composed of coarse-grained subhedral bladed to euhedral zoned hexagonal
quartz with minor sulphides and iron oxides, and is contemporaneous with the secondary quartz veins within
the host rock. Pyrite is the dominant sulphide and occurs as very fine anhedral grains between quartz crystals,
and as coarser, rounded to euhedral cubes and pyritohedra, gernerally fractured and/or rimmed by later
hematite and lepidocrocite. Pyrite grains are disseminated randomly throughout the vein, but locally aligned
with the vein selvages, outlining growth zones in the vein. Trace amounts of droplet-shaped native gold (Fig. 1)
and galena (Fig. 2) occur as inclusions in pyrite and chalcopyrite. Native gold and galena were not observed as
inclusions in any other mineral. The reflectivity of the gold is less than normally observed, suggesting it is
probably the typical electrum found in epithermal mineralization and comprised of approximately one third
silver by weight.
Mineral
Host rock (as clasts)
Quartz-1
Limonite
Smectite (/kaolinite)
Quartz-2
Hematite
Lepidocrocite
Sericite
Quartz-sulphide-hematite vein
Quartz-2
Pyrite
Hematite
Lepidocrocite
Smectite
Chalcopyrite
Covellite
Native gold
Cliffmont
Sample CA-01P
Modal Percent Abundance
Size Range (mm)
3
3
2
2
2
2
1
Up to 0.1 mm
Cryptocrystalline
Up to 20 microns
Up to 0.4 mm
Up to 0.4 mm
Up to 0.5 mm
Up to 60 microns
43
17
12
9
3
1
trace
trace
Up to 5 mm
Up to 1.8 mm
Up to 1.5 mm
Up to 80 microns
Up to 80 microns
Up to 0.5 mm
Up to 0.3 mm
Up to 50 microns
Page 1
Pyrite is typically completely or partly replaced by hematite and minor lepidocrocite (Fig. 3).
Lepidocrocite also occurs as thin veins (up to 0.08 mm wide) filling fractures and between grain boundaries, and
in some places rims the inside of vugs. Small patches (up to 0.07 mm wide) of rusty brown, fine smectite occur
towards the ends of some of these lepidocrocite veins. Minor chalcopyrite occurs as anhedral grains filling grain
boundaries between quartz. It is commonly associated with pyrite, and generally exhibits replacement by
covellite at grain boundaries. Covellite rims on chalcopyrite range in thickness from less than a micron to 40
microns wide. Locally the percentage of replacement may vary up to 100% covellite after chalcopyite.
The rock is consistent with an epithermal gold deposit type ore deposit. Although severely altered and
veined, the rock only displays minor evidence of deformation in the form of offset secondary quartz veins along
lepidocrocite veins.
qtz
gn
au
py
py
cov
qtz
cpy
Figure 1: Photomicrograph of a rounded inclusion of
native gold (au) hosted in pyrite (py) within the quartz
vein (qtz). In the lower part of the photo, an anhedral
grain of chalcopyrite (cpy) replaced by covellite (cov)
around its rim sits between quartz grains. Photo taken
in plane polarized reflected light.
Figure 2: Photomicrograph of an inclusion of galena
(gn) in pyrite (py) within coarse vein quartz (qtz).
Photo taken in plane polarized reflected light.
Figure 3: Photomicrograph of the remnants of a
fractured pyrite (py) grain nearly completely replaced
by hematite (hem) and lepidocrocite (lpd). Photo
taken in plane polarized reflected light.
py
lpd
hem
Cliffmont
Sample CA-01P
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