Representative Petrographic Reports

Sample XT-1 Porphyritic Dacite:

Plagioclase Phenocrysts; Replacement Patches and Veinlets of Quartz-Calcite-Pyrite-Chalcopyrite

Phenocrysts of plagioclase are set in a groundmass dominated by plagioclase with lesser quartz, accessory biotite and pyrite, and minor sphene and apatite. Plagioclase phenocrysts are altered slightly to moderately to sericite, calcite, and K-feldspar; biotite is replaced moderately to strongly by chlorite; and sphene is replaced moderately to strongly by Ti-oxide-calcite. Abundant veinlets and replacement patches are of one or more of quartz, calcite, pyrite, chlorite, and chalcopyrite.

Plagioclase forms subhedral to euhedral phenocrysts and clusters of a few phenocrysts. They are replaced slightly to moderately by disseminated flakes of sericite from 0.01-0.05 mm in size, scattered patches of calcite, and dusty hematite. Some phenocrysts are replaced moderately by irregular patches of K-feldspar (see the stained offcut block for distribution of K-feldspar).

A few rectangular patches may represent hornblende phenocrysts. They are replaced completely by intimate intergrowths of biotite/chlorite and calcite (0.05-0.15 mm) with minor patches of Ti-oxide.

In the groundmass, plagioclase forms equant, slightly interlocking grains that are intergrown intimately with similar grains of quartz. Scattered grains of each mineral are coarser, and some coarser plagioclase grains grade texturally into fine phenocrysts. Plagioclase in the groundmass is altered as in the phenocrysts.

Biotite forms disseminated grains mainly from 0.03-0.05 mm in size in plagioclase-quartz aggregates. It also is concentrated in irregular, replacement patches from 0.3-1 mm in size. It is replaced strongly by pseudomorphic chlorite and minor patches and lenses of Ti-oxide. In some of these patches chlorite is intergrown with calcite.

Pyrite forms minor, ragged, dense to skeletal porphyroblasts. Some contain abundant silicate inclusions from 0.02-0.07 mm in size, and a few contain several inclusions of hematite-ilmenite/ magnetite from 0.03-0.06 mm in size.

Sphene forms a few anhedral patches that are altered in part to Ti-oxide, calcite, and chlorite. Apatite forms disseminated, anhedral to subhedral prismatic grains.

Quartz forms irregular replacement patches up to several mm across and veinlets up to 0.2 mm wide in which grains are anhedral and slightly interlocking. A few calcite-rich veinlets consist of porphyroblastic grains up to a few mm across. Pyrite forms patches and lenses of anhedral grains, in part alone and in part intergrown coarsely with quartz. Chlorite forms irregular patches of flakes intergrown with pyrite and quartz.

Chalcopyrite forms irregular replacement patches up to 1.5 mm in size replacing groundmass and plagioclase phenocrysts. It also occurs in a few veins. Most patches of chalcopyrite are not associated with pyrite. Chalcopyrite commonly has a thin rim of hematite.

Sample XT-1 Photo 1. Veinlet of calcite with patch of chalcopyrite-(chlorite) cuts plagioclase phenocrysts (altered slightly to sericite and calcite). crossed nicols for transmitted light, reflected light, Length of Photo: 1.6 mm.

Sample XT-1 Photo 2. Phenocrysts of plagioclase (altered slightly to sericite, calcite, and dusty hematite) in groundmass of plagioclase, quartz, and biotite (replaced strongly by chlorite-[Ti-oxide]). Quartz veinlets with trace calcite and chalcopyrite (with thin rim of hematite). Plane polarized light. Length of Photo: 1.6 mm.

Sample XT-1 Photo 3. Same field of view as photo 1. Crossed nicols.

Sample XT-2

Muscovite/Sericite-Dolomite-Chlorite-Pyrite Schist;

Early, Deformed Vein: Quartz-Dolomite-(Pyrite-Tetrahedrite-Chalcopyrite-Native Gold);

Late Veins: Quartz-Dolomite

The sample is a muscovite/sericite-dolomite-chlorite-pyrite schist that was cut by an early, deformed quartz-dolomite-(pyrite-tetrahedrite-chalcopyrite) vein and later fracture-filling veinlets of quartz-dolomite. The original rock may have been an intermediate volcanic rock, but no original texture is preserved. It contains porphyroblasts of pyrite and disseminated grains of arsenopyrite, rutile, and hematite(?).

The border of the vein is dominated by dolomite with patches of pyrite, lesser tetrahedrite, and minor chalcopyrite. The core of the vein is dominated by quartz with much less dolomite and minor pyrite. The vein was slightly cataclastically deformed and recrystallized. One strongly fractured pyrite grain contains abundant veinlets of tetrahedrite with scattered patches of native gold and chalcopyrite.

A few late veins up to 3 mm wide are of quartz and dolomite are present in the hand sample but are not obvious in the thin section.

Muscovite/sericite forms unoriented flakes intergrown finely with anhedral grains of dolomite. Chlorite is intergrown with muscovite/sericite and dolomite, and also is concentrated moderately in a few chlorite-rich patches and lenses of grains from 0.1-0.15 mm in size.

Pyrite forms disseminated, subhedral porphyroblasts. A few of these a partial overgrowth of chlorite on one or more sides of the crystal. Chlorite flakes in these overgrowths are mainly oriented perpendicular to pyrite crystal faces. A few large flakes are oriented parallel to pyrite crystal faces. Pyrite also forms disseminated grains and clusters of grains averaging 0.005-0.05 mm in size. A few pyrite porphyroblasts contain minor lensy patches and fracture-filling zones up to 0.02 mm wide of chalcopyrite.

Rutile forms disseminated grains and clusters of grains. Leucoxene is concentrated strongly in patches up to 0.2 mm in size in which anhedral leucoxene grains are intergrown with sericite, calcite, and chlorite. These patches may be secondary after original sphene. Hematite(?) forms disseminated, equant, commonly subrounded, isotropic grains.

The main vein is zoned.

Along the margin is a zone up to 3 mm wide dominated by dolomite with disseminated grains of pyrite. Dolomite grains are anhedral and slightly interlocking and commonly contain abundant dusty inclusions of iron oxide(?), giving it a turbid appearance. Against quartz grains, a zone up to 0.1 mm wide in the grains is recrystallized and clear, and is free of dusty inclusions. Pyrite forms subhedral grains and clusters of grains. Two patches up to 1.5 mm across along the border of the vein are of recrystallized dolomite (0.3-0.5 mm) with patches up to 0.5 mm in size of muscovite flakes (0.1-0.2 mm). Pyrite contains minor inclusions up to 0.03 mm in size of tetrahedrite and chalcopyrite, and one grain contains a lens of native gold 0.025 mm long. One strongly fractured pyrite grain contains abundant veinlets of tetrahedrite with scattered patches of native gold and chalcopyrite.

One patch 1.5 mm across and several smaller, irregular patches are of tetrahedrite with minor to moderately abundant chalcopyrite. Tetrahedrite also forms a few patches up to 0.2 mm in size associated with dolomite in the quartz-rich part of the vein.

The core of the vein is dominated by quartz with much less abundant dolomite and minor pyrite. Quartz forms anhedral grains that contain minor dusty opaque inclusions and show slight to moderate cataclastic deformation. Many were recrystallized slightly to moderately along grain borders to subgrain aggregates 0.01-0.015 mm in size that generally are free of dusty inclusions. Dolomite forms irregular patches up to 1.5 mm in size surrounded by quartz. Cores of grains are turbid and rims are clear as in the border zone. Pyrite forms minor disseminated subhedral to euhedral grains.

Some of the recrystallized zones of quartz and dolomite in the main vein may be associated with the late veins of quartz-dolomite.

Sample XT-2 Photo 1: Chlorite-muscovite-calcite rock; coarser grained lens of chlorite; minor disseminated pyrite (opaque). Crossed nicols. LOP: 1.6 mm.

Sample XT-2 Photo 4: Veinlets of tetrahedrite-(native gold-chalcopyrite) in strongly fractured pyrite grain. Reflected light. Length of photo: 0.4 mm.

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This page last updated Jan 15, 2006.