Petrographic Report: Polished Thin SectionSLD-01-02M

Rock Type: spherulitic rhyolite flow

The thin section displays abundant spherulites of radiating feldspar with three overprinting generations of alteration. The sample is weakly altered with almost 70% of the primary mineral assemblage remaining. The primary mineral assemblage was dominantly feldsparspherulites with approximately 20% anhedral quartz, 25% feldspar (K-feldspar > albite), and trace cubic pyrite. Spherulites range in size up to 2.5 mm wide, and often contain inclusions of anhedral quartz and feldspar. Phenocrysticeuhedral feldspar minerals range in size up to 2.5 mm. This composition would classify the rock as a spherulitic rhyolite.The hand sample groundmass is a pinkish-red colour from disseminated hematite alteration, spherulites are visible as grey circles, calcite is visible as white patches that react with HCl, and K-feldspar phenocrysts occur as euhedral pink laths and cubes. The three alteration events consist of an initial carbonatization(calcite) of the host rock followed closely by a chlorite-sericite-hematite-quartz alteration in masses and veins, and lastly by a very weak illitization of spherulites and feldspar minerals.

The first alteration event was a pervasive carbonatization represented by irregular and anhedral calcite overprinting spherulites (Fig. 1) and fine-grained feldspar and quartz groundmass.Calcite replacement of spherulites occurs as fine-grained rims around cores and as massive calcite across the spherulite. Calcite replacement of the host rock occurs as fine-grained masses and coarse, anhedral, irregular masses, especially over fine-grained quartz and feldspar groundmass.

The second alteration event consists of massive aggregates or clumps (up to 2 mm wide) of knotty chlorite and sericite with minor hematite, and thin discontinuous veins of chlorite-hematite-quartz. Fine-grained hematite is disseminated throughout the thin section, but is coarser grained and bladed wheren associated with chlorite-sericite aggregates, and less abundant near calcite. Chlorite-sericite-hematite aggregates overprint

Mineral / Modal Percent Abundance / Size Range (mm)
Primary rock
Feldspar spherulites / 25 / Up to 2.5 mm
Quartz-1 / 20 / Up to 0.5 mm
K-feldspar / 17 / Up to 2.5 mm
Albite / 6 / Up to 1.8 mm
Pyrite / Trace / Up to 60 microns
Earlycarbonatization
Calcite / 16 / Up to 2 mm
Earlychlorite-sericite-hematite alteration and veining
Chlorite / 6 / Up to 0.1 mm
Sericite / 4 / Up to 80 microns
Hematite / 4 / Up to 0.6 mm
Quartz-2 / 2 / Up to 0.2 mm
Epidote-clinozoisite / Trace / Up to 0.3 mm
Limonite / Trace / Up to 10 microns
Late illitization
Illite / Trace / Up to 10 microns

fine-grained feldspar and quartz groundmass and are intergrown with or overprint calcite alteration. Trace amounts of epidote-clinozoisite are associated with these aggregates. Trace pyrite occurs with secondary rims of hematite and chlorite, and is also pseudomorphed by hematite associated with chlorite-sericite (Fig. 2). Trace limonite is associated with chlorite-sericite-hematite aggregates overprinting calcite. Thin (up to 50 microns wide) discontinuous veins of chlorite-hematite-quartz cut spherulites, groundmass, and calcite. The veins trend NNE-SSW, and hematite is largely confined to vein centres. The mineral assemblage of the first and second alteration events is indicative of propylitic-style alteration.

The last and final event for this rock was the late, weak illitization of spherulites and feldspar minerals. The illite is very fine- grained and alters less than 1% of all the spherulites and feldspar minerals.

Figure1: Photomicrograph of feldsparspherulites (spher) with replacement by fine-grained calcite (cal) around a core. The spherulite is cut by a chlorite (chl)-hematite (hem)-quartz veinlet. Coarse, anhedral calcite (cal; top right) overprints quartz and feldspar minerals in the groundmass. Hematite (hem) overprints spherulites and calcite. Photo taken in cross polarizedtransmitted light. / Figure2: Photomicrograph ofhematite (hem) pseudomorphing aftercubic pyrite (py) associated with bladed hematite (upper left) in chlorite-sericite aggregate. Photo taken in plane polarized reflected light.

Cliffmont Sample SLD-01-02MPage 1