Quartzite from The Tenmile Range, Colorado

Quartzite from the summit of Quandary Peak,

--- Tenmile Range, Colorado, U.S.A.

Figures 1-2. A white and green quartzite from the summit ridge of Quandary Peak, elevation 14,265 feet, the highest point in the Tenmile Range, south of Breckenridge. Here we see the setting, with a view (left) westwards from the summit, the sample location in the immediate foreground. At right is a view towards the east, and the summit, showing quartzite outcrop. This month's feature is dedicated to those fifty-odd cheerful souls who spent much of 21st September 2015 hiking up and down the mountain in perfect weather!

"Rock of the Month #174, posted for December 2015" ---

A quartzite from the heights of the Rockies,

in central Colorado. Professor W.H. Brewer of Yale was an expert of his day on the mountainous regions of the West. He noted that "near Mount Lincoln is a peak sometimes called Quandary Peak, sometimes McCollough's Peak, sometimes Ute Peak, and one man called it Hoosier Peak" (Brewer, 1872, p.203). The Tenmile Range is near Breckenridge, with 14,265' Quandary Peak, Fletcher Mountain and Crystal Peak: nearby is the Mosquito Range with Mount Lincoln, Mount Democrat and Mount Bross, all "14,000ers" of which the tallest are Mount Massive (14,421') and Mount Elbert (14,433'). Ormes (1979) was long a standard reference for the 54 Colorado summits 14,000 feet or more in height: now guidebooks abound. Quandary Peak, an east-west ridge on the north side of the valley of the Blue Lakes, slopes down toward the treeline and the road through Hoosier Pass to the east (National Geographic, 2013). Hikes up Quandary Peak are described by, e.g., Mary Ellen Gilliland (2006, pp.18-19) and Gerry Roach (1999, pp.54-59). The latter includes lists of the 54 peaks over 14,000 feet and of 109 over 13,800 feet.

The area of Quandary Peak, southwest of Breckenridge, is a Proterozoic terrane with granitic rocks in the 1400-1000 Ma age range, intruding 1800-1700 Ma metamorphic host rocks such as gneiss, schist, migmatite, minor quartzite and conglomerate. Cambrian quartzite (the Sawatch quartzite) occurs in the vicinity of Quandary Peak, and a short distance to the southwest, across the continental divide around Leadville (GTR Mapping, 2013).

Figure 3. A fresh broken surface through a sample from the summit ridge. Tough white, granular, massive quartzite is overlain along a sharp contact by a dull green layer of otherwise similar rock. Sample collected in situ on Monday, 21 September 2015.

So, how old is the quartzite around the summit? Such clastic sediments can be hard to date, since fossils are commonly absent. A maximum age can sometimes be set by direct dating of detrital zircon crystals, washed into the sandy protolith of the quartzite following erosion of the original host rock of the zircon, which is typically a granitic intrusion wherein the zircon crystallized. Lacking the resources for such advanced study, a literature search may offer correlations with similar strata elsewhere. Charles Doolittle Walcott to the rescue!

Walcott (1891) reviewed Cambrian strata and fossils across the USA and Canada. He noted that around Quandary Peak there is a unit 150-200 feet thick, with a few inches to one foot of conglomerate at the base. The lower 100 feet is `finely and rather thinly bedded white saccharoidal quartzites', the quartzite succeeded by more shaly layers. There are few or no fossils in the clastic sediment, but a bed of greenish chloritic slate occurs on the east side of the peak, about a mile above the Monte Cristo mine, and the slate contains Dikelocephalus, a trilobite akin to the D. minnesotensis found in the Potsdam Formation in Wisconsin. Thus the Quandary Peak quartzite is evidently of Cambrian age, and so an outlier atop the Precambrian basement.

Pending the preparation of a thin section (Figs. 4-5), we can speculate that the green colour seen in one or more layers of the quartzite is the mineral glauconite. This iron-potassium sheet-silicate mineral, related to the micas, is quite widely found in sediments of Cambrian to Recent age. It is decidely uncommon in older rocks, although numerous reports exist of its occurrence in the late Proterozoic Vindhyan sediments of central India. It is known in Cambrian strata across North America, e.g., in Quebec, Alberta, South Dakota and Texas, hence it would not be too surprising to find it in Colorado, if we accept that this unit is indeed of Cambrian age. Halka and Chronic (2014, p.145) show the ridge of Quandary as being composed largely Proterozoic metasediments, with older Paleozoic strata on the east flank, but they also note (ibid., p.215) that Paleozoic sediments cap the peak, "tiny remnants of the strata that once arched across the range".

Figures 4-5. Two photomicrographs of the green layer in the quartzite. Both images in crossed-polarized transmitted light, nominal magnification 50X, long-axis field of view 1.7 mm. The thin section reveals a rock that is largely (circa 94-96%) fine-grained quartz, with moderately rounded grains mostly 0.4-0.8 mm, or as large as 1.2 mm in diameter. The quartz is partially cemented by a carbonate (4%). Rare grains of detrital green tourmaline, and of equant pyrite, are present. Thin veinlets lined by muscovite mica cut the rock (Fig. 4). In the green layer thin films of a fine-grained flakey sheet silicate form a rind around quartz grains, and occasionally discrete rounded pellets darkened by fine-grained iron oxides (limonite, hematite). This silicate, 3% of the green layer, appears to lend its colour to the bulk rock, and may indeed be glauconite, but the identification is tentative. In the photomicrographs the quartz is seen in variable degrees of extinction, while the fine-grained phases at grain boundaries appear dark. The rock is a mature, moderately sorted quartzite.

The old Monte Cristo mine, mentioned above, is situated below tree line, low on the east flank of the mountain. It is said to have yielded ore with a modest grade of silver (15 ounces/ton) in argentiferous galena with sphalerite, hosted by Paleozoic (Silurian) quartzite (Lakes, 1888, pp.104-105).

In the past half-century, by far the most famous mine on the east side of the divide separating Breckenridge from Leadville would be a mineral specimen mine, the Sweet Home mine west of Alma, on the east side of the northern Mosquito Range, which is world-famous for rhodochrosite specimens. The mine and its minerals are documented in the July 1998 issue of Mineralogical Record, a multi-author review that goes far beyond the frequent appearance of exquisite, shocking-pink carbonate rhombs in the "vanity press" of minerals! The range is a gently east-dipping succession to Paleozoic sediments of Cambrian to Carboniferous age, intruded by a range of Laramide to Tertiary stocks, dykes and sills (Misantoni et al., 1998). The Sweet Home workings lie below the level of the Paleozoic sequence, in Precambrian granitoids cut by Tertiary intrusives.

In passing, we may note that the very place names of Colorado attest to the importance of rocks, minerals and mines in local history. Here are 25 examples, mostly in the mountainous western half of the state: Agate, Agate Creek, Bedrock, Carbonate, Coalbed Canyon, Coal Creek, Copper Mountain, Dinosaur, Fossil Ridge, Gem Village, Gold Hill, Gypsum, Gypsum Gap, Hot Sulphur Springs, La Plata, La Veta Pass, Leadville, Marble, Mineral County, Piedra, Placerville, Platoro, Silt, Silver Cliff, and Telluride. And incidentally, I have read somewhere that the name Quandary Peak has its origins in Earth science. When prospectors found traces of mineralization nearby they were puzzled by a blue metallic mineral. Not silver, not galena - so, what was it? The answer turned out to be molybdenite (MoS₂, principal ore mineral of molybdenum). The puzzle (quandary) is immortalized in the name of this mountain.

References

Brewer,WH (1872) Explorations in the Rocky Mountains and the High Peaks of Colorado. Journal of the American Geographical Society of New York 3, 193-215.

Gilliland,ME (2006) The New Summit Hiker and Ski Touring Guide. Alpenrose Press, Silverthorne, CO, 8th printing of revised edition, 118pp. plus fold-out map.

GTR Mapping (2013) Colorado Geologic Highway Map and Shaded Elevation Map, with 14,000 ft. peaks, selected mining districts, & dinosaur localities. GTR Mapping, Canon City, CO, 2013 edition of 2-sided colour map, 1:1,000,000 scale.

Lakes,A (1888) Geology of Colorado Ore Deposits. News Printing Company, Denver, CO, 162pp.

Misantoni,D, Silberman,ML and Lees,BK (1998) Geology of the Sweet Home mine and Alma district. Mineral.Record 29 no.4, 101-113.

National Geographic (2013) Breckenridge and Tennessee Pass. National Geographic topographic map 109, 1:40,680 scale, revised.

Ormes,RM (1979) Guide to the Colorado Mountains. Colorado Mountain Club / World Press Inc., 7th edition, 344pp.

Roach,G (1999) Colorado's Fourteeners, from Hikes to Climbs. Fulcrum Publishing, Golden, CO, 2nd edition, 308pp.

Walcott,CD (1891) Correlation Papers: Cambrian. USGS Bull. 81, 447pp.

Williams,F and Chronic,H (2014) Roadside Geology of Colorado. Mountain Press Publishing Company, Missoula, MT, 3rd edition, 399pp., pp.145,215.

Graham Wilson, 16,17,25 October 2015, 05,19,20,23 November 2015, 22,26 December 2015, wee update 15 April 2022

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