Mineralogy of Amygdaloidal Mafic Flow Rocks of the Endako Group in the Kenney Dam area, northeastern Nechako River map area, central British Columbia.^1,2

E.M. Barnes³ and R.G. Anderson

GSC Pacific, Vancouver

Geological Survey of Canada Project 950036-04

Barnes, E.M. and Anderson, R.G., 1999: Mineralogy of amygdaloidal mafic flow rocks of the Endako Group in the Kenney Dam area, northeastern Nechako River map area, central British Columbia; in Current Research 1999E; Geological Survey of Canada, p. 9-20.

Eleven samples of Endako Group amygdaloidal, olivine basalt and basaltic andesite flows represent fresh and altered basal, medial and upper parts of an approximately 150 m thick sequence at Kenney Dam (NTS 93F/10SW). Petrographic, x-ray diffraction and scanning electron microscopic studies determined primary, secondary, and amygdaloidal minerals. Olivine, clinopyroxene (augite), orthopyroxene, andesine, magnetite and ilmenite phenocrysts in an opaque or feldspar microlite-rich groundmass characterize the flow rocks.

Limonite, goethite, hematite, siderite, chalcedony, opal, and calcite, in approximate order of crystallization, are common alteration products of the groundmass and clinopyroxene and/or vesicle fillings. Calcite, siderite, and iron hydroxides have several crystal habits, textures, and layering, suggesting multistage crystal growth. The abundance of later carbonate mineralization likely derived from alteration of local limestone units.

Modeling of the secondary mineral parageneses suggests they likely precipitated within a wide range of intermediate pH and low temperatures.

Résumé

Onze echantillons de coulees de basalte a olivine et d'andesite basaltique amygdo~des du Groupe d'Endako representent les parties basale, mediane et superieure fraiches et alterees d'une sequence d'environ 150m d'epaisseur au barrage Kenney (SNRC93 F/1OSW). Des etudes petrographiques, diffractometriques et de microscopie electronique a balayage ont penmis de detenniner les mineraux primaires, secondaires et des amygdales. Les roches des coulees sont caracterisees par des phenocristaux d'olivine, de clinopyroxene (augite), d' orthopyroxene, d' andesine, de magnetite et d' ilmenite dans une matrice opaque ou riche en microlite et feldspath.

La limonite, la goethite, I'hematite, la siderite, la calcedoine, l'opale et la calcite, en ordre approximatif de cristallisation, remplissent les vesicules ou sont des produits communs d'alteration de la matrice et du clinapyroxene. La calcite, la siderite et les hydroxydes de fer ont plusieurs formes cristallines et textures et se presentent en couches, ce qui laisse supposer qu'il y a eu une croissance en plusieurs etapes. L'abondance de mineraux carbonates tardifs serait attribuable a l'alteration d'unites de calcaire locales.

Une simulation de la paragenese des mineraux sec ondaires penmet de penser que ces mineraux ont probablement precipite a l'interieur d'une vaste gamme de pH intermediaires et de basses temperatures.

¹ Contribution to the Nechako NATMAP Project.
² This is a joint mapping project of the Geological Survey of Canada and British Columbia Geological Survey Branch.
³ Division of Earth Sciences, University of Glasgow, Glasgow, UK G12 8QQ

Sequences of Tertiary basaltic rocks in the Fort Fraser (NTS 93K) and Nechako River (NTS 93F) areas are widespread (e.g., Armstrong, 1941, 1949; Tipper, 1963; Kimura et al., 1980; Bellefontaine et al., 1995; Struik et al., 1997; Williams, 1997) (Fig. 1).

One of these, the Eocene Endako Group mafic rocks, is widely exposed in the south-central part of the Fort Fraser and the north-central part of the Nechako River map areas. The rocks display a variety of volcanological features and textures. The distinction of the mafic volcanic rocks of the Endako Group from those in the underlying Eocene Ootsa Lake Group (e.g., Anderson et al., 1999; Grainger and Anderson, 1999) and overlying Neogene mafic flows (formerly called the Chilcotin Group; e.g., Resnick et al. 1999; Fig. 1) is difficult, partly due to incomplete mineralogical descriptions of the rocks in the reference areas for the Endako Group.

Haskin et al. (1998) and Anderson et al. (1998a, b) described four well-exposed stratigraphic sections of Endako Group rocks, established their regional setting, and summarized their geochemical characteristics. The sections near the Nautley River (see Struik et al., 1997), northeast of Bungalow Lake, and at Mount Greer span the contact between the Endako Group and the underlying Ootsa Lake Group. The study area near Kenney Dam northwest of Knewstubb Lake, the site of the present study, includes the thickest and most continuous sequence of Endako Group volcanic rocks preserved in the Nechako River area. The unit includes basaltic andesite and lesser basalt flows with minor associated hyaloclastite and tuffaceous sedimentary rocks; the flow rocks are aphyric to plagioclase- and pyroxene-, and rarely olivine-, phyric and are commonly amygdaloidal.

The regional setting (Anderson et al., 1998a), lithology, stratigraphy, and physical volcanology of the Endako Group rocks at the Kenney Dam locality described by Haskin et al. (1998) is supplemented in this study by petrographic, x-ray diffraction, and scanning electron microscope (energy dispersive) methods used to characterize samples, collected in early July 1998, from the base, middle and upper parts of the well-exposed sequence. The work is part of a baccalaureate study (Barnes, 1999) and represents the first detailed petrographic description of the Endako Group.

The study area is readily accessible via maintained and disused gravel roads including the Holy Cross, 500, and Kenney Dam forestry roads in the southwestern part of the Big Bend map area (NTS 93F/10; Fig. 1). A disused gravel road leads west and south from the Kenney Dam road to the basal part of the sequence northeast and at the foot of Kenney Dam.

At the Kenney Dam site (Haskin et al., 1998), the comparatively thick sequence of mafic flows is believed to mark the Nechako graben, part of a series of syn-volcanic extensional Eocene structures (Anderson et al., 1998a, b). The sequence at this locality comprises approximately 150 m of mafic-intermediate volcanic flows and minor hyaloclastite and clastic sedimentary rocks (base of section at UTM E370875, N5938430, zone 10; site A, Fig. 1). The base of the Endako Group at Kenney Dam is not exposed.

The section (Fig. 2a, b) consists of consecutively stacked 3-5 m thick flows whose finely-crystalline texture imparts a sparkly appearance. The rocks have an orange oxidizing rind on weathered surfaces. Microphenocrysts consist of subhedral, elongate plagioclase, up to 3 mm long, and minor pyroxene. The flows are subhorizontal, dipping gently (<20°) to the northwest. The flow tops are commonly brecciated.

Clinopyroxene- and plagioclase-phyric basaltic andesite and lesser basalt are variably vesicular ranging from >5% vesicles in the massive blocky interior of a flow, to 50% vesicles in the upper and lower portions (Haskin et al., 1998; and this study, Table 1a). The flow bases are thin (10-30 cm) and only sparsely vesicular. Vesicles are generally round, 0.5-6 cm in diameter, and evenly distributed throughout. The transition from flow interior to top is marked by a gradual increase in vesicle abundance and size range. The larger vesicles are flattened parallel to the flow contacts (Fig. 3a). Flow tops are 0.5-1 m thick and their porosity provided excellent sites for deposition of secondary minerals.

Representative samples of units were collected for petrographic and mineralogical studies. Magnetic susceptibility (m.s.) measurements were routinely obtained at most outcrops over a 5 minute period using a hand-held Exploranium KT-9 Kappameter. They are an average of at least 5 measurements, reported in 10^-3 SI units (Table 1a) and are typical of the magnetic susceptibility range for Endako Group rocks regionally.

Samples for analysis were selected to represent the regionally occurring secondary minerals typical of this unit. Five sites (sites 0101, 0102, 0103, 0104, and 0105) included horizons similar to but not identical to those established by Haskin et al. (1998) for their subsequent geochemical studies (e.g., Anderson et al., 1998b). At each site at least two samples were taken from the same flow. They included one fresh sample, usually from the more massive central flow area, and one or more altered samples from the vesicular and/or amygdaloidal, upper or lower parts of the flow, the number and sampling location dependent on the nature and abundance of the secondary minerals. The sites span an almost continuous cliff representing relief of at least 150 m.

Sites 0101 (in the lowest accessible flow) and 0102 were taken from the lower 80 m of the cliff which lies north of Kenney Dam at the base of the dam wall (Fig. 2a). Sites 0103, 0104 and 0105 (in the highest flow) were taken from the upper 70 m of the sequence, exposed in a disused forestry road aggregate quarry and log loading site, 200 m south of the dam (Fig. 2b). No distinctive amygdaloidal or secondary minerals were observed at site 0105 and it will not be considered further. Four fresh rock samples and seven secondary mineral samples were studied in detail. Iron and carbonate secondary mineral phases were mainly studied in this project (Fig. 3). Common opal and chalcedony were not sampled due to their straightforward identification.

Standard petrographical techniques were used to identify minerals, alteration, and textures in covered thin sections of the rock-vesicle contact prepared from each of the seven altered samples. Composition of plagioclase phenocrysts were estimated by the Michel Levy method. The Munsell Rock-Color Chart was used to designate the colours of the secondary minerals as seen in hand sample (Table 1b).

X-ray powder diffraction methods were used to study samples of fresh rock from each site, as well as specific secondary mineral material (as highlighted in petrographic observations) or entire vesicle linings or amygdules of each mineralized sample. Each sample, comprising hand picked grains, was ground to a paste with acetone to provide 'smear mounts' for analysis.

A Philips X-ray diffractometer with vertical goniometer and computer controlled step scanner, with a standard Co tube were used. °2 theta was measured between 4 and 60 in 1 °2 theta steps, at a scanning speed of 2°2 theta/min. The data were compared with the Traces V4.0 database and the Division of Earth Sciences, University of Glasgow reference collection.

A Leica Electron Optical S360 Stereoscan scanning electron microscope, utilizing an electron beam accelerated at 20KV, was used to image and provide a semi-quantitative analyses of primary minerals from a polished thin section of sample 0102G to characterize the mineral compositions.

Geochemical modeling was undertaken via The Geochemist's Workbench^TM v3.0 (Bethke, 1996), a computer package that enables the modeling of aqueous geochemical reactions through the use of computational equilibrium thermodynamics using the dataset, thermo.dat 1994. The REACT program was used to test the reaction of fluid with minerals in an attempt to determine under what conditions the secondary minerals may have been produced. The graphing program GTPLOT was used to illustrate the reaction.

The four fresh samples, representative of the host volcanic flow rocks contain variable amounts of augite and andesine, lesser magnetite and ilmenite, and rare olivine and orthopyroxene phenocrysts set in an opaque or feldspar microlite-rich groundmass (cf. Table 1a; Fig. 4-7).

Pyroxene is subhedral to euhedral, <10 to 30% in abundance, 0.5 - 2 mm in size, and commonly shows an ophitic texture. Andesine is euhedral to subhedral, makes up 30-75% of the rock, is commonly 1 mm but up to 2 mm in length, and not aligned. Common albite and less common Carlsbad twinning are typical and oscillatory twinning is restricted to larger plagioclase phenocrysts. Magnetite and ilmenite comprise 5-10% of the rock, are euhedral to subhedral, or rarely occur as spherules entrained within an iron hydroxide vesicle lining, as in sample 0104D. Ilmenite occurs rarely in a dendritic form.

Confirmation of the presence of olivine was only available from SEM imaging and semi-quantitative analysis of polished slabs of sample 0102G.

A variable degree of alteration of the pyroxene is characteristic. Complete replacement of pyroxene by microcrystalline iron hydroxide or by calcite with minor silica-rich layers is common, with iron hydroxide commonly appearing as the first secondary mineral to be precipitated. Fresh pyroxene, however, is common, even at the rock/vesicle interface.

Intersertal glass (with dendritic ilmenite and plagioclase microlites constituting the rarely identifiable portion of the groundmass) is commonly altered, turning the plagioclase microlites green. With more extensive alteration the groundmass is opaque (e.g., Fig. 4a, 5a). Rare alignment of elongate acicular to prismatic rutile, spatially associated with andesine? and alteration product of the Fe-Ti oxides, occurs proximal to the vesicle margin (Fig. 6c).

Plagioclase commonly appears fresh throughout, with only minor alteration.

Analysis of the secondary minerals with X-ray powder diffraction identify chlorite, iron hydroxide, calcite, and siderite which corroborated petrographic observations (Table 1b; Fig. 4-7). The identities of clay minerals, found in the XRD analyses of the samples with the most extensive pyroxene alteration, were not further investigated. The abundance of clay increased with the intensity of pyroxene alteration in the samples (e.g., samples 0101E and 0102G).

Petrographic analyses helped identify the nature, textures and parageneses of secondary minerals, in approximate order of crystallization in vesicles: limonite, goethite, hematite, siderite, chalcedony, opal, and calcite (cf. Table 1b; Fig. 4-7).

Goethite and hematite do occur together but goethite is more common in the two basal sites and hematite in the upper two sample sites. Limonite is ubiquitous throughout, microcrystalline, straw brown to dark brown colour in plane polarized light and exhibits strong absorption and isotropic character under cross polars. It exhibits a 'scaly' texture and 'shatter cracks'. Goethite resembles limonite in crystallinity and colour, but commonly crystallized as radially acicular grains with extinction parallel to elongate crystal axis. A sub-radial crystal habit occurs in microfibrous fringes on and ptygmatic bands found within limonite. Hematite is vibrant red brown and microcrystalline.

A geopetal structure is believed to be indicated by the asymmetrical but consistent distribution of the iron hydroxides and other secondary minerals within the vesicles. The structure is commonly characterized by a localized crystallization of goethite or limonite as the earliest mineral species in contact with the host rock along vesicle wall. This texture is accompanied by a proximal increase in the abundance of other secondary minerals in comparison to the remainder of the vesicle (Fig. 4 and 6).

Siderite is absent from the two basal samples but is abundant in all 0103 site samples and both 0104 site samples. It commonly occurs with iron-rich zones distinguishing growth bands, forms layers which overlie iron hydroxide and calcite, and also appears as distinct rhombs entrained within coarse calcite-rich areas (see Fig. 6 and 7). Where precipitated as a layer, siderite crystals commonly display chevron growth patterns (Fig. 7a).

Chalcedony appears in samples 0102G and 0103F and as a replacement for opal in sample 0103. Opal is also found in sample 0104D. However, as samples showing obviously silica-rich precipitates were not studied in detail, no correlation with stratigraphic height is implied.

Chalcedony occurs in finely crystalline radial spherules (e.g., Fig. 5) and layers and in mosaic textures as a replacement for opal. Colourless opal appears as a colloform layer with a rare, fine banding indicating periodic growth.

Extensive calcite precipitation occurred in a wide variety of crystal habits and is commonly the last secondary mineral precipitated. Crystal textures include: coarse, granular calcite as a vesicle lining; as the replacement mineral for pyroxene; microfracture filling; or as vesicle filling (Fig. 3b, c) exhibiting the fibrous textures in sample 0101E, the spherical growths of sample 0103F (Fig. 3b), and the discs of 0104G (Fig. 3c).

The calcite discs are unusual. In hand sample, the top surface is smooth, slightly concave and commonly pocked; in thin section they are characterized by entrainment of Fe-Ti oxides (?) and slight variations in crystal habit, which indicate a growth pattern originating distal from the rock surface (see Fig. 7c).

The late stage carbonate mineral precipitation may reflect interaction of meteoric water with scattered limestone units to the southeast of the Kenney Dam site (Tipper, 1963) now submerged beneath the Nechako reservoir (Knewstubb Lake).

From thin section observation, augite (Ca, Mg, Fe2+, Ti, Al)₂ [Si₂O₆], is commonly replaced by limonite, FeO(OH).nH₂O and goethite, FeO(OH), in the presence of opal SiO₂.nH₂O and/or chalcedony SiO₂.

This reaction provided the basis for a conceptual model to be tested on Geochemist's Workbench^TM v3.0 (GWB; Bethke, 1996):

(Ca, Mg, Fe²⁺, Ti, Al)₂ [ Si₂O₆] + H₂O = FeO(OH) + SiO₂ + Ca-nontronite

The initial system is then defined in terms of: mass and chemical composition of the fluid, amounts of minerals present, gas fugacities, temperature, Eh and pH.

Fresh water is the most probable fluid in contact with the rock, given that the area was inland during the Tertiary. Therefore salinity was assumed to be low, and the initial solute concentration was kept to a minimum in the modeling. A model 100 mg of iron-rich pyroxene end-member ferrosilite (representing augite which is not part of the GWB database) was reacted with 1 kg water at pH 7 at 25°C. The experiment was run to a temperature of 50°C. The gases, O₂ and CO₂, were given a normal atmospheric pressure at the start of the experiment. Hematite was suppressed from the products to allow goethite to be shown as a reaction product.

A very stable reaction producing goethite (also representative of the observed limonite and hematite), quartz (also representative of the observed opal and chalcedony) and Ca-nontronite (smectite), was modeled. In the simulations, the reaction was shown to be stable between the starting temperature and at least 50°C and between pH 5 and 8. The reaction produced a nominal increase in acidity of 0.1 pH unit.

Secondary minerals found by petrography and X-ray powder diffraction were successfully modeled in this simplified system. Smectite has not been positively identified in this project, but fine grained silicates were present in the X-ray powder diffraction. The relative stability of the model reaction suggests that the secondary mineral assemblages and textures found in the Endako Group in the Kenney Dam area might have regional significance.

Eleven samples of amygdaloidal olivine basalt and basaltic andesite flow rocks represent basal, medial and upper parts of an approximately 150 m thick sequence of thin mafic flows of the Eocene Endako Group at Kenney Dam. A variety of techniques confirm the essential mafic and feldspar mineralogy of the flow rocks and provide a case study of the variety and range of parageneses of limonite, goethite, hematite, siderite, chalcedony, opal, and calcite mineral species that make up the amygdules which characterize the flow rocks of the Endako Group.

The senior author would like to express her sincere thanks to the GSC and especially NATMAP project leader Bert Struik; her participation was under the auspices of the GSC's Volunteer Program, with the approval of Brian Bell of the University of Glasgow, and supported by the financial assistance of the estate of the late Jo Wylie; their backing is deeply appreciated. A sincere thank you to Jonah Resnick for his excellent field assistance, and to Lori Snyder for sharing her field experience and unpublished petrographic descriptions.

Support of the Division of Earth Sciences through the expertise of Robert McDonald (SEM technician), Bill Higgison (XRD technician), and John Gilleece (thin section preparation), critical to the data acquisition, is gratefully acknowledged. The assistance, guidance and support of Dr. Allan Hall is greatly appreciated.

We appreciate the time spent by Glenn Woodsworth, whose constructive review of an earlier version of the manuscript hopefully led to a better final version. Bev Vanlier is thanked for her digital preparation of the pre-press version of the manuscript.

Anderson, R.G., Snyder, L.D., Resnick, J., and Barnes, E.
1998a: Geology of the Big Bend Creek map area, central British Columbia; in Current Research 1998-A; Geological Survey of Canada, p. 145-154.

Anderson, R.G., Snyder, L.D., Wetherup, S., Struik, L.C., Villeneuve, M.E., and Haskin, M.
1998b: Mesozoic to Tertiary volcanism and plutonism in southern Nechako NATMAP area: Part 1: Influence of Eocene tectonics and magmatism on the Mesozoic arc and orogenic collapse: New developments in the Nechako River map area; in L.C. Struik and D.G. MacIntyre (ed.), New Geological Constraints on Mesozoic to Tertiary Metallogenesis and on Mineral Exploration in central British Columbia: Nechako NATMAP Project; Geological Association of Canada, Cordilleran Section, March 27, 1998, Short Course Notes, 26 pages.

Anderson, R.G., Snyder, L.D., Resnick, J., Grainger, N., and Barnes, E.M.
1999: Bedrock geology of the Knapp Lake map area, central British Columbia; in Current Research 1999-A; Geological Survey of Canada, p. 109-118.

Armstrong, J.E.
1941: Fort Fraser (west half); Geological Survey of Canada, Map 631A.

1949: Fort St. James map-area, Cassiar and Coast Districts, British Columbia; Geological Survey of Canada, Memoir 252, 210 p.

Bailey, D.G., Jakobsen, D.E., and Lane, R.
1995: MINFILE 093F Nechako River mineral occurrence map; British Columbia Ministry of Energy, Mines and Petroleum Resources, MINFILE, revised March 1995.

Barnes, E.M.
1999: The Eocene Endako Group basalts, central British Columbia. Alteration and secondary minerals; B.Sc. thesis, University of Glasgow, Glasgow, Scotland, 64 p.

Bellefontaine, K.A., Legun, A., Massey, N., and Desjardins, P.
1995: Digital Geological Compilation of Northeast B.C. -Southern Half (NTS 83D, E, 93F, G, H, I, J, K, N, O, P); British Columbia Ministry of Energy, Mines and Petroleum Resources, Open File 1995-24.

Bethke, C.M.
1996: Geochemical Reaction Modeling, Concepts and Applications; Oxford University Press, New York, 397 p.

Grainger, N.C. and Anderson, R.G.
1999: Geology of the Eocene Ootsa Lake Group in northern Nechako River and southern Fort Fraser map areas, central British Columbia; in Current Research 1999-A; Geological Survey of Canada, p. 139-148.

Haskin, M.L., Snyder, L.D., and Anderson, R.G.
1998: Tertiary Endako Group volcanic and sedimentary rocks at four sites in the Nechako River and Fort Fraser map areas, central British Columbia; in Current Research 1998-A; Geological Survey of Canada, p. 155-164.

Kimura, E.T., Bysouth, G.D., Cyr, J., Buckley, P., Peters, J., Boyce, R., and Nilsson, J.
1980: Geology of parts of southeast Fort Fraser and northern Nechako River map areas, central British Columbia; Placer Dome Incorporated, Internal Report and Maps, Vancouver, British Columbia.

Resnick, J., Anderson, R.G., Russell, J.K., Edwards, B.R., and Grainger, N.
1999: Neogene basaltic flow rocks, xenoliths, and related diabase, northern Nechako River map area, central British Columbia; in Current Research 1999-A; Geological Survey of Canada, p. 157-167.

Struik, L.C., Whalen, J.B., Letwin, J., and L'Heureux, R.
1997: General geology of southeast Fort Fraser map area, British Columbia; in Current Research 1997-A; Geological Survey of Canada, p. 65-76.

Tipper, H.W.
1963: Nechako River map area, British Columbia; Geological Survey of Canada, Memoir 324.

Williams, S.P.
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Figure 1. - Geological map of Big Bend area (after Anderson et al., 1998a) and location of sampling areas A (UTM E370875, N5938430, zone 10) and B (UTM E370746, N5937806, zone 10); inset map shows study area location within Nechako NATMAP project area.

Figure 2. - Location of samples: a) Lower part of Endako Group sequence north of and at base of Kenney Dam; figure (see arrow) is 1.8 m tall. Samples 0101 and 0102 locations shown; b) Upper part of Endako Group sequence, south of Kenney Dam; cliff is ca. 805 m tall. Locations of samples 0103, 0104, and 0105 shown. No distinctive amygdaloidal minerals were observed in sample 0105.

Figure 3. - Typical mineral habits for amygdules in hand sample for samples: a) 0101E: flattened vesicle contains black iron hydroxides with shatter cracks and inner zone of chlorite(?); b) 0103F: calcite spherules (greyish orange, 5YR 4/4; field of view 5.5 cm wide); and, c) 0104G: zoned calcite disk-like amygdules (greyish orange, 10YR 7/4; disks 2 cm in diameter).

Figure 4. - Camera lucida sketch (a) and photomicrograph (plane polarized light) of distinctive petrographic textures for sample 0101E. Note limonite geopetal structure characterized by shatter cracks as initial vesicle filling and later iron hydroxide spherules overgrown by calcite crystal. Magnification in sketch is 10x; width of photomicrograph image is 5.6 mm.

Figure 5. - Camera lucida sketch (a) and photomicrograph (plane polarized light) of distinctive petrographic textures for sample 0102G. Note goethite and geopetal structures characterized by shatter cracks as initial vesicle fillings. Iron hydroxides are overgrown by chalcedony and calcite. Magnification in sketch is 10x; width of photomicrograph image is 1.4 mm.

Figure 6. - Camera lucida sketch (a) and photomicrographs (plane polarized light) of distinctive petrographic textures for sample 0103E (a, b) and 0103F (c). Note sequential vesicle filling by limonite geopetal structure, rhombic siderite, several periods of alternation of siderite and iron-rich zones and late siderite growth in (a) and (b). Magnification in sketch is 10x; width of photomicrograph image is 5.6 mm. Photomicrograph of sample 0103F (c) shows alignment of rutile in volcanic flow host subparallel to vesicle margin coated sequentially by calcite and siderite (width of photomicrograph image is 1.4 mm).

Figure 7. - Camera lucida sketches ((a) and (b)) and photomicrograph (plane polarized light) of distinctive petrographic textures for sample 0104D (a) and 0104G ((b) and (c)). Opal is sole vesicle filling for sample 0104D. Magnification in sketches is 10x; width of photomicrograph image is 1.4 mm.

Table 1a. - Mineralogy of basaltic andesite flow rocks.

Table 1b. - Mineralogy of secondary minerals.