Petrographic Report: Polished Thin SectionSLD-03-01MA
Rock Type: altered feldspar-quartz-phyricrhyolite
The thin section displays a generally porphyritic texture of coarser euhedral feldspar laths and rounded quartz phenocrysts in a fine-grained groundmass. There are at least two overprinting generations of alteration. The sample is altered with approximately half of the primary mineral assemblage remaining. The primary mineral assemblage was dominantly a fine-grained groundmass of quartz and minor feldspar with approximately 20% phenocrysts of rounded quartz and euhedral laths of feldspar (albite > K-feldspar). Phenocrysts range in size up to 2 mm wide. This composition would classify the original rock as a feldspar-quartz-phyric rhyolite.The hand sample exhibits circular white patches up to 7 mm wide that react with HCl (indicating the presence of calcite), and a groundmass that is a greenish-grey colour from disseminated chlorite alteration. Green chlorite is more abundant in rims around the white calcite patches. The offcut has been stained to check for the presence of K-feldspar, and only a few small patches have taken on the stain, indicating very little K-feldspar is present.The two overprinting alteration events consist of an initial carbonatization(calcite) of the host rock associated with contemporaneous quartz-chlorite-sericite (±smectite?) alteration with trace rutile, epidote-clinozoisite, and pyrite. This was followed by a pervasive but weak, very fine-grained smectite (or zeolite?)-sericite-illitealteration of groundmass and feldspar minerals.
The first alteration event was a pervasive carbonatization represented by irregular and anhedral calcite overprinting fine-grained groundmass and some phenocrysts. Calcite in some placestimes occurs in short, discontinuous veinlets up to 0.7 mm wide that areis oriented parallel to crudely aligned feldspar phenocrysts and weakly foliated chlorite-calcite-sericite-rutile veins (Fig. 1) that all trend roughly NNE. Secondary quartz occurs in elongated patches of recrystallized medium-grained quartz grains parallel to this weak alignment. These quartz patches host trace pyrite, smectite, and epidote-clinozoisite. Rutileoften commonly occurs as fine-grained clusters of euhedral crystals within calcite patches (Fig. 2) and in anastomosing short calcite veinletscommonly edgingphenocryst
Mineral / Modal Percent Abundance / Size Range (mm)Primary rock
Quartz-1 / 25 / Up to 0.4 mm
Albite / 16 / Up to 2 mm
K-feldspar / 4 / Up to 1.5 mm
Earlycarbonatization with quartz-chlorite-sericite(±smectite) alteration
Calcite / 20 / Up to 2 mm
Quartz-2 / 14 / Up to 0.5 mm
Chlorite / 8 / Up to 0.1 mm
Sericite / 6 / Up to 50 microns
Smectite / 4 / Up to 50 microns
Rutile / 1 / Up to 0.1 mm
Epidote-clinozoisite / Trace / Up to 0.1 mm
Pyrite / Trace / Up to 20 microns
Late sericite-smectite-illite
Sericite / 1 / Up to 50 microns
Smectite / 1 / Up to 50 microns
Illite / Trace / Up to 10 microns
boundaries. This alteration mineralogy is consistent with propylitic alteration with limited albitization. The second alteration consists of very fine disseminated smectite (or zeolite?), sericite, and illite overprinting groundmass quartz and feldspar and feldspar phenocrysts, and forming rare, thin, rusty-coloured veins (less than 0.03 mm wide) that cut rutile-calcite veins (Fig. 3).
Although weakly altered and having experienced rare veining, the rock does not display much evidence of deformation. Calcite and to lesser extent quartz and hematite show a NE-SW alignment.
Figure1: Photomicrograph of weak alignment of calcite (cal) and clay minerals. The dashed white lines show the direction of alignment. Quartz (qtz) and albite (ab) phenocrysts are present. Photo taken in cross polarizedtransmitted light. / Figure2: Photomicrograph ofa sub-rounded calcite (cal) patch with associated subhedralrutile (rt) and epidote-clinozoisite (ep). Very fine-grained sericite is disseminated around the right side of the calcite. Photo taken in cross polarized transmitted light./ Figure3: Photomicrograph of a rusty-coloured sericite-smectite (srct-sm) veinlet cutting across a weakly developed calcite-rutile (cal-rt) vein. Pervasive chlorite (chl) is also present throughout the photo. Photo taken in plane polarized transmitted light.
Cliffmont Sample SLD-03-01MAPage 1