Post by 1dave on Nov 22, 2020 22:28:29 GMT -7
A valuable source of information is:
geology.byu.edu/home/sites/default/files/burt_82.pdf
And
books.google.com/books?id=aWk_zML-3HAC&pg=RA2-PA35&lpg=RA2-PA35&dq=Blawn+Wash+Formation&source=bl&ots=CmHdckpt-z&sig=ESESQWlf33iq1KTW0WqKzqtM7TY&hl=en&sa=X&ved=0ahUKEwigvvCk4fXYAhUX7mMKHapuCqMQ6AEINDAC#v=onepage&q=Blawn%20Wash%20Formation&f=true
NOTE: Missing here is Texas Topaz! (Item #4)
Colorado Topaz
Utah Topaz in Topaz Rhyolite
Topaz Rhyolite in Mexico
geology.byu.edu/home/sites/default/files/burt_82.pdf
And
books.google.com/books?id=aWk_zML-3HAC&pg=RA2-PA35&lpg=RA2-PA35&dq=Blawn+Wash+Formation&source=bl&ots=CmHdckpt-z&sig=ESESQWlf33iq1KTW0WqKzqtM7TY&hl=en&sa=X&ved=0ahUKEwigvvCk4fXYAhUX7mMKHapuCqMQ6AEINDAC#v=onepage&q=Blawn%20Wash%20Formation&f=true
NOTE: Missing here is Texas Topaz! (Item #4)
Colorado Topaz
Utah Topaz in Topaz Rhyolite
Distribution and Age
The distribution of identified topaz rhyolites in the eastern 'United States could most easily be described as surrounding the Colorado Plateau (Fig, l)~Occurrences in New Mexico and Co1orado lie in or near the Rio Grande rift, and occurrences in Arizona, Utah, Nevada, and Idaho generally lie in the 'eastern part of the Basin and Range province. Otherwise similar young, high-silica, alkaline rhyolites in the Basin and Range province apparently lack topaz.
The distribution of topaz rhyolites roughly coincides with the distribution of abundant fluorite deposits and occurrences (Worl et aI., 1974; Shawe, 1976; Van Alstine, 1976; Van Alstine and Tooker, 1979). As mentioned, it also coincides with the distribution of Climax-type, Tertiary, topaz-bearing, high-grade porphyry molybdenum deposits in Colorado, New Mexico, Montana, Idaho, Nevada, and Utah (Shawe, 1976; Woodcock and Hollister, 1978; Keith, 1979, 1980; Mutschler et aI., 1981; White et aI., 1981; Westra and Keith, 1981).
The distribution of identified topaz rhyolites in the eastern 'United States could most easily be described as surrounding the Colorado Plateau (Fig, l)~Occurrences in New Mexico and Co1orado lie in or near the Rio Grande rift, and occurrences in Arizona, Utah, Nevada, and Idaho generally lie in the 'eastern part of the Basin and Range province. Otherwise similar young, high-silica, alkaline rhyolites in the Basin and Range province apparently lack topaz.
The distribution of topaz rhyolites roughly coincides with the distribution of abundant fluorite deposits and occurrences (Worl et aI., 1974; Shawe, 1976; Van Alstine, 1976; Van Alstine and Tooker, 1979). As mentioned, it also coincides with the distribution of Climax-type, Tertiary, topaz-bearing, high-grade porphyry molybdenum deposits in Colorado, New Mexico, Montana, Idaho, Nevada, and Utah (Shawe, 1976; Woodcock and Hollister, 1978; Keith, 1979, 1980; Mutschler et aI., 1981; White et aI., 1981; Westra and Keith, 1981).
Topaz Rhyolite in Mexico
Topaz Field Recognition
Numerous occurrences of topaz rhyolite lava undoubtedly remain to be discovered.
In the field, their defining feature is the presence of topaz, which is seen in miarolitic or lithophysal (concentrically layered) cavities in the lava.
Topaz is most easily distinguished from similarly appearing prismatic quartz by its perfect basal cleavage and orthorhombic symmetry. Transparent crystals in freshly broken cavities are typically yellowish to pinkish brown; this color gradually fades on exposure to sunlight. Where euhedral quartz and topaz occur together the quartz is normally present as small stubby crystals coating the walls of the cavity, whereas topaz is a large, single crystal growing into its center.
Associated dark-colored minerals in the cavities can also be distinctive. The most common are anhedral to euhedral, red to black Mn-Fe garnet, black cubes of bixbyite, (Mn, Fe)20S, black acicular pseudobrookite, Fe2 TiO s , and black platy specular hematite (plus ilmenite?). Other common minerals are colorless crusts of sanidine, tabular to prismatic, pink to red hexagons of beryl, Colorless to purple fluorite, and ruby red, stubby to platy cassiterite (Lufkin, 1976). Most of these are illustrated by Holfert (1978) and Ream (1979).
Topaz and associated minerals are not present in all gas cavities or in all portions of a topaz rhyolite lava flow or dome. Nevertheless, if topaz is there, 15 to 30 minutes of diligent searching with a hand lens will generally reveal it.
Volcanology
Small intrusive or extrusive domes and lava flows of rhyolite containing topaz do not appear very different from those of other types of silicic magma. The crystalline topaz rhyolite lavas range from distinctly gray or chalky, phenocryst-rich, massive, miarolitic types to pinkish -gray to tan, phenocryst -Poor, flow- banded, lithophysal types. Both types of lava (and intermediate varieties) may develop a distinctive cavernous or honeycomb-like weathering pattern as in the Honeycomb Hills, Utah (Linqsey, 1977). The causes of this feature are complex, but silicified areas adjacent to fractures may be more resistant to weathering than the more friable fresh rock, which weathers to form cavities.
The effect of fluorine on rhyolitic magma is to lower its solidus temperature and viscosity (cf. Wyllie, 1979; Manning, 1981), as well as to expand the field of stability of quartz.
In this regard, fluorine has the same qualitative effect as water; the difference is that fluorine has a greater tendency to stay with the melt, rather than escaping explosively, on release of pressure (see reviews by Bailey, 1977, and Burnham, 1979). This feature, in conjunction with eruption temperature and water fugacity, permits some topaz rhyolite lavas (e.g., at Spar Mountain, Utah) to flow rather far from their vent areas.
Fluorine-rich extrusions (domes or flows) typically have black basal vitrophres that grade upward into thick devitrified zones.
The lava bodies commonly are underlain or bordered by various types of pyroclastic deposits. These range from near-vent explosion breccias and cross-stratified surge deposits (Sheridan and Updike, 1975) to minor, more distant plinian pumice fall deposits, and non welded to partly welded ash-flow deposits (cf. Sheridan, 1979; Self et aI., 1980). The well-sorted, layered, and even crossbedded nature of some of these pyroclastic units, particularly the surge deposits, has led to their mis-classification as water-laid tuffs (especially at Spor Mountain, Utah). This usage is incorrect and should be dropped.
Eruptions of topaz rhyolite lavas commonly are structurally controlled by earlier faults and fractures. On a local scale, as In the Spor Mountain district, Utah (Lindsey, 1977, 1982), vents occur along faults allegedly related to the collapse of earlier, unrelated calderas. On a more regional scale they can occur along linear mineral belts, such as the east-west~trending Deep Creek- Tintic and Wah Wah- Tushar mineral belts in eastern Utah ,(Hilpert and Roberts, i964; Stewart et al. 1977; Rowley et aI., 1978), On a still larger scale, they are commonly associated with rifts.
Eruptive volumes of topaz rhyolites from single vents are generally small (less than 10 km~; see Table 1), An exceptional Case is the Topaz Mountain Rhyolite, Thomas Range , Utah, that has an estimated minimum volume of 50 km 3 (Turley and Nash, 1980) that erupted from at least 12 separate vents (Lindsey, 1979), Two areas of thick topaz rhyolite lavas with cassiterite mineralization are multiple vent fields with higher eruptive volumes, 130 km 3 for the Taylor {Creek Rhyolite, Black Range, New Mexico (Rhodes. 1976) and 50 km2 for the Izenhood Ranch rhyolite Sheep Creek Range) Nevada (Fries, 1942; Stewart et' aI., 1977)
Numerous occurrences of topaz rhyolite lava undoubtedly remain to be discovered.
In the field, their defining feature is the presence of topaz, which is seen in miarolitic or lithophysal (concentrically layered) cavities in the lava.
Topaz is most easily distinguished from similarly appearing prismatic quartz by its perfect basal cleavage and orthorhombic symmetry. Transparent crystals in freshly broken cavities are typically yellowish to pinkish brown; this color gradually fades on exposure to sunlight. Where euhedral quartz and topaz occur together the quartz is normally present as small stubby crystals coating the walls of the cavity, whereas topaz is a large, single crystal growing into its center.
Associated dark-colored minerals in the cavities can also be distinctive. The most common are anhedral to euhedral, red to black Mn-Fe garnet, black cubes of bixbyite, (Mn, Fe)20S, black acicular pseudobrookite, Fe2 TiO s , and black platy specular hematite (plus ilmenite?). Other common minerals are colorless crusts of sanidine, tabular to prismatic, pink to red hexagons of beryl, Colorless to purple fluorite, and ruby red, stubby to platy cassiterite (Lufkin, 1976). Most of these are illustrated by Holfert (1978) and Ream (1979).
Topaz and associated minerals are not present in all gas cavities or in all portions of a topaz rhyolite lava flow or dome. Nevertheless, if topaz is there, 15 to 30 minutes of diligent searching with a hand lens will generally reveal it.
Volcanology
Small intrusive or extrusive domes and lava flows of rhyolite containing topaz do not appear very different from those of other types of silicic magma. The crystalline topaz rhyolite lavas range from distinctly gray or chalky, phenocryst-rich, massive, miarolitic types to pinkish -gray to tan, phenocryst -Poor, flow- banded, lithophysal types. Both types of lava (and intermediate varieties) may develop a distinctive cavernous or honeycomb-like weathering pattern as in the Honeycomb Hills, Utah (Linqsey, 1977). The causes of this feature are complex, but silicified areas adjacent to fractures may be more resistant to weathering than the more friable fresh rock, which weathers to form cavities.
The effect of fluorine on rhyolitic magma is to lower its solidus temperature and viscosity (cf. Wyllie, 1979; Manning, 1981), as well as to expand the field of stability of quartz.
In this regard, fluorine has the same qualitative effect as water; the difference is that fluorine has a greater tendency to stay with the melt, rather than escaping explosively, on release of pressure (see reviews by Bailey, 1977, and Burnham, 1979). This feature, in conjunction with eruption temperature and water fugacity, permits some topaz rhyolite lavas (e.g., at Spar Mountain, Utah) to flow rather far from their vent areas.
Fluorine-rich extrusions (domes or flows) typically have black basal vitrophres that grade upward into thick devitrified zones.
The lava bodies commonly are underlain or bordered by various types of pyroclastic deposits. These range from near-vent explosion breccias and cross-stratified surge deposits (Sheridan and Updike, 1975) to minor, more distant plinian pumice fall deposits, and non welded to partly welded ash-flow deposits (cf. Sheridan, 1979; Self et aI., 1980). The well-sorted, layered, and even crossbedded nature of some of these pyroclastic units, particularly the surge deposits, has led to their mis-classification as water-laid tuffs (especially at Spor Mountain, Utah). This usage is incorrect and should be dropped.
Eruptions of topaz rhyolite lavas commonly are structurally controlled by earlier faults and fractures. On a local scale, as In the Spor Mountain district, Utah (Lindsey, 1977, 1982), vents occur along faults allegedly related to the collapse of earlier, unrelated calderas. On a more regional scale they can occur along linear mineral belts, such as the east-west~trending Deep Creek- Tintic and Wah Wah- Tushar mineral belts in eastern Utah ,(Hilpert and Roberts, i964; Stewart et al. 1977; Rowley et aI., 1978), On a still larger scale, they are commonly associated with rifts.
Eruptive volumes of topaz rhyolites from single vents are generally small (less than 10 km~; see Table 1), An exceptional Case is the Topaz Mountain Rhyolite, Thomas Range , Utah, that has an estimated minimum volume of 50 km 3 (Turley and Nash, 1980) that erupted from at least 12 separate vents (Lindsey, 1979), Two areas of thick topaz rhyolite lavas with cassiterite mineralization are multiple vent fields with higher eruptive volumes, 130 km 3 for the Taylor {Creek Rhyolite, Black Range, New Mexico (Rhodes. 1976) and 50 km2 for the Izenhood Ranch rhyolite Sheep Creek Range) Nevada (Fries, 1942; Stewart et' aI., 1977)
Can You Read my mind 😘 Spring is approaching fast my friend an I’m chomping at the bits to get back to my 2nd home!! See ya soon