Petrographic Report: Polished Thin Section SLD-05-03M
Rock Type: albite-phyric lapilli tuff
The thin section displays abundant lapilli of granular, interlocking quartz with two overprinting
generations of alteration. The sample is weakly altered with almost 80% of the primary mineral assemblage
remaining. The primary mineral assemblage was dominantly quartz lapilli, feldspar (albite > K-feldspar), quartz,
albite phenocrysts, and K-feldspar spherulites. Lapilli make up about 40% of the thin section, range in size up to
3 mm wide, and commonly contain radiating fans of secondary biotite, epidote, and hematite, and rarely
contain albite phenocrysts. The lapilli are locally rimmed by outward-radiating K-feldspar that is reminiscent of
K-feldspar spherulites (Fig. 1). Rare K-feldspar spherulites up to 1.2 mm wide also occur within the section,
typically with lapilli, and may contain inclusions of anhedral quartz and feldspar. Lath-shaped euhedral albite
phenocrysts range in size up to 3 mm. The groundmass is felty-textured and composed of cryptocrystalline to
fine-grained quartz and feldspar, likely with more albite than K-feldspar, although the grains are too small to
accurately identify. This composition classifies the rock as an albite-phyric lapilli tuff. The hand sample is
composed of rounded grey fused lapilli with white centres that are slightly flattened in one direction. Pink Kfeldspar spherulites are rare between lapilli. Small patches of rusty alteration occur throughout the hand
sample. The offcut has been stained to check for the presence of K-feldspar, and it has taken on a slight pale
yellow colour, indicating trace to minor amounts of K-feldspar. The two alteration events include an initial
biotite- hematite-epidote alteration event followed by clay alteration of feldspar minerals, and lastly, an iron
oxide alteration event.
The first alteration event was a selective alteration of quartz lapilli by biotite, hematite, and epidote,
accompanied by biotite veining in albite phenocrysts and groundmass, and hematite dissemination throughout
the thin section. Biotite, hematite, and epidote overprint lapilli cores (Fig. 2) or rarely rims the cores, and rarely
occur in clusters within the groundmass. Biotite in lapilli occurs as radiating fans and acicular aggregates that are
rusty in places, and are rarely rhombic in shape. Hematite in lapilli occurs as bladed grains between biotite fans,
while epidote occurs as subhedral prisms between biotite and overprinted by hematite. Within the groundmass,
Mineral
Modal Percent Abundance
Primary rock
Quartz
38
Feldspar
22
Albite phenocrysts
10
K-feldspar spherulites
3
Early biotite-clay-hematite-epidote alteration
Biotite
12
Hematite
4
Epidote
1
Sericite
Trace
Clay alteration
Illite-smectite
10
Iron oxide alteration
Limonite
Trace
Lepidocrocite
Trace
Size Range (mm)
Up to 0.4 mm
Up to 0.1 mm
Up to 3 mm
Up to 1.2 mm
Up to 0.15 microns
Up to 1.2 mm
Up to 0.2 mm
Up to 0.1 mm
Up to 10 microns
Up to 30 microns
Up to 0.5 mm
biotite occurs as thin, wormy veinlets up to 0.03 mm wide and 1 mm long, which also rarely cut across albite
phenocrysts. Hematite occurs as fine-grained disseminated anhedral grains and clusters up to 1 mm wide. More
Cliffmont
Sample SLD-05-03M
Page 1
often than not, euhedral epidote is associated with hematite clusters. Hematite rarely fills fractures in albite, but
does not occur as replacement of the phenocrysts. The mineral assemblage of the first alteration event is not
typical of epithermal-style deposits.
The second alteration event consists of very fine-grained felty-textured illite-smectite overprinting
feldspar phenocrysts, in the groundmass, and spherulites. Illite-smectite also rarely occurs as patches within
lapilli, where it may replace biotite. Replacement of albite phenocrysts and K-feldspar in spherulites by illitesmectite is weak, very fine-grained and generally replaces less than 5% of the crystal. The mineral assemblage of
the second alteration event is indicative of argillic-style alteration.
The last and final event for this rock was a late, weak iron oxide alteration event characterized by the
overprinting of granular limonite on the groundmass and occasionally replacing biotite, accompanied by thin
veinlets cutting the groundmass. The limonite veinlets are up to 0.02 mm wide and 2 mm long, and generally
oriented SE. Trace lepidocrocite replaces hematite in places and occurs as needles or blades.
Although weakly altered, the rock does not display much evidence of deformation.
qtz
K-fsp
qtz
py
bt
Figure 1: Photomicrograph of a lapilli composed of
interlocking, granular quartz (qtz) and secondary fanshaped biotite (bt). K-feldspar (K-fsp) encircles the
lapilli and radiates outward from it. Photo taken in
cross polarized transmitted light.
Cliffmont
Sample SLD-05-03M
ep
bt
hem
Figure 2: Photomicrograph of hematite (hem) blades
associated with fan-shaped biotite (bt) and euhedral
epidote (ep) within a quartz (qtz) lapilli. Photo taken
in cross polarized transmitted light.
Page 2