WOLF CREEK PASS: COLLECTING MORDENITE AND HAULIN' CHICKENS

 

Me and Earl was haulin' chickens
On the flatbed out of Wiggins 
And we'd spent all night on the uphill side                                                                                                               
Of 37 miles of hell called Wolf Creek Pass
Which is up on the Great Divide

At one time, way back in the 1960s and 1970s, it seemed like the “whole world, or at least most of the U.S., had heard of Wolf Creek Pass near Pagosa Springs, Colorado, “up on the Great Divide.” The song was recorded/performed by Bill Fries under his nome de guer  of C.W. McCall and was a parody of “truck driven music” and CB radio chatter. The song reached #40 on the  U.S. Pop Top 40 in 1975 and followed on the heels of McCall’s  first successful commercial hit song: Old home filler-up an' keep on a-truckin' café. In case you are wondering, Fries had a previous successful gig in the upper Plains and Midwest hawking Old Home Bread.

Now we spied a sign, says eat gas now
We decided to whip in and pick up some chow
At the old home filler-up an' keep on a-truckin' café

But McCalls best known for his mega hit song Convoy described as a “first-person trucker spoken monologue, backed by a country arrangement.” Whatever, the song reached #1 in both the Country Western and Pop Charts for several weeks and made C.W. McCall a household name and started the CB Radio craze that swept through and over North America. Every kid, and many adults (mostly male), had a “handle” (CB Name) and practiced talking like nitwits: “Breaker breaker, one nine (meaning let me talk on CB channel 19).  Folks, we got us a fox in the henhouse” indicating someone had seen an unmarked law enforcement vehicle. The roadways were full of cars and vehicles with very tall radio antennas sprouting out of the back bumper.  It may seem rather simple but, you know, people were having fun and not worrying about tariffs, the destruction of National Parks, federal troops in the streets, or the Ukraine.

Breaker one-nine

This here’s the Rubber Duck

You got a copy on me Pig Pen, c’mon?

Mercy sakes alive, looks like we got us a convoy!

But, back to Wolf Creek Pass situated on one of the most scenic highways in the West, U.S 160 running from Walsenburg on the east side of the Colorado Front Range to Durango in the far southwest of the State. The scenery, geology, and historical sights along the 222 miles will fill your memory scrapbook. However, perhaps the best-known section of the road is from South Fork on the east over Wolf Creel Pass to Pagosa Springs on the west.

Wolf Creek Pass is located in the San Juan Mountains of southwestern Colorado, an area famous for voluminous mid-Tertiary volcanism. Geologists working for the USGS have written a number of scientific reports on the area based upon detailed mapping and skillful laboratory analyses of the rocks. They have mapped at least 22 major ignimbrite sheets and associated caldera structures active between 34 and 23 Ma. The scientists at USGS have described the volcanism as characterized by explosive ash flow eruptions resulting in collapse and the formation of calderas. Many of these calderas are associated with large scale ore deposits such as the Creede Caldera Complex. Probably the largest caldera, and perhaps the best known (at least to geologists) feature, is the massive La Garita Caldera formed when a “really large” supervolcano blew its stack and spewed out a guesstimate of 1,200 cubic miles of the Fish Creek Tuff  over at least 11,000 sq. miles around 28 Ma. The caldera itself is a oblong structure ~25 x ~50 miles in size.

I remain uncertain about the exact age of the volcanics at Wolf Creek Pass and their relationship to an extrusive caldera. The rocks are reasonably close to the Platoro Caldera near Summitville where the ejecta is generally referred to as the Treasure Mountain tuff; however, I cannot find evidence that the tuff extended to the Wolf Creek area. Perhaps the tuff is related to the La Garita eruption? Unfortunately, since moving to rural Wisconsin I do not have the resources of geology maps commonly found in Colorado research libraries.

Arnold Hampson wrote the definitive rockhound paper on Wolf Creek Pass collectable minerals and stated the host rock in the region is the Conejos Quartz Latite. However, over the past twenty years terminology has changed, and the Conejos is now listed as the Conejos Formation and is related to eruptions of the Platoro Caldera around 30 Ma. So, I am still confused about specifics and will place a call to the Acme Building looking for Guy Noir, Private Eye, to help answer one of life’s persistent questions (thanks to Garrison Keilor).

Treasure Falls, 105 feet of sheer beauty. Public Domain photo courtesy of Ahodges7.

The small number of samples in my collection came from near Treasure Falls, a 105-foot gorgeous stream of water tumbling through and over the volcanics and situated along U.S. 160 about 15 miles east of Pagosa Springs—over the crest of Wolf Creek Pass heading west. The Falls are a popular visiting site for travelers and therefore parking is at a premium for RVs and almost every time I motored over the Pass I was unable to park my bumper pull RV and missed out on collecting. However, there was one instance when I was able to pull over and scurry to pound on the rocks a couple of times. The best collecting site is about ¾ of a mile north of the Falls in exposures along the roadway.

MinDat lists 11 valid minerals and 12 other varietal mineral names that have been extracted from the four small collecting localities referred to as Wolf Creek Pass. Most of the attractive collected minerals are zeolites, and mordenite [(Na₂,Ca,K₂)₄(Al₈Si₄₀)O₉₆ · 28H₂O] is the most abundant and spectacular of the group. The mineral occurs as thin, white tufts and sprays of delicate acicular crystals that are silky to vitreous, and transparent to translucent. They are usually associated with amygdaloidal cavities in basalt where it commonly is “associated with black nontronite, green celadonite, and orange heulandite” (Hampson, 2004). Interestingly, Hanner (1976) described the Treasure Falls mordenite crystals as “some of the finest in North America.”

Width FOV ~12 mm.

Width of spray ~1.5 mm.

Width FOV ~5 mm.

Above photomicrographs of acicular mordenite sprays on dark green/black nontronite. The patch of green “spherules” to the right on lower is perhaps celadonite or, perhaps an unknown/listed copper mineral?  The orangish base of some sprays may be due to heulandite-Na.

Nontronite [Na_0.3Fe₂((Si,Al)₄O₁₀)(OH)₂ · nH₂O] is also a common mineral in the Wolf Creek basalts but is not a zeolite. It is the ferric iron-rich end member of the Smectite Group of clay minerals. It commonly mixes with celadonite to form dark green to dark brown aggregates in the basalt vugs. At other times it occurs as almost black (?darkest green) spheric aggregates. It is quite soft at ~2.0 Mohs and the aggregates are waxy to resinous and appear opaque. Their color and size make my photography difficult. Inherently, most specimens of nontronite at other localities are much lighter in color---yellow green, yellow orange, brown, and green.

Vug filled with black nontronite with a narrow “rind” of celadonite and a few acicular crystals of mordenite. Maximum length of vug ~2 mm.

The upper photomicrograph is a small (length ~15 mm), vug-filling, “concretion-like” specimen that “popped out” of the vesicle and was damaged on both ends. The inside the “concretion” is filled with spherical aggregates of darkest green nontronite. A second unknown appears as a silver colored, metallic luster mineral (see lower photo). The reflection from the light source is intense. My only guess is pyrite.

Another somewhat uncommon mineral, at least in Colorado, is the Mica Group mineral celadonite [K(MgFe³⁺◻)(Si₄O₁₀)(OH)₂}. In the Wolf Creek basalts it often mixes with nontronite as described above. At other times it is composed of waxy, dull, blue-green to olive green spherules lining the vugs. Black nontronite spheres and acicular mordenite crystals are often associated with the lined cavities.

Vug filled with darker nontronite and celadonite with a greenish-gray patch of celadonite on top. Width vug .75 mm.

 

Vugs filled with celadonite. Large vug ~ 1 mm.

The tiny saponite crystals appear as dust-like flakes in mordenite-filled vug (width ~5 mm).

A surprise mineral, at least to me, is saponite Ca_0.25(Mg,Fe)₃((Si,Al)₄O₁₀)(OH)₂·nH₂O], an aluminosilicate member of the Smectite Group of clay minerals. Although  it occurs in many colors and habits (from small subhedral crystals to massive to granular and points in-between), the specimens from Wolf Creek are quite tiny, greenish-brown, subhedral to “flakes” and difficult to photograph details with my equipment. But interestingly, at Wolf Creek the minute saponite crystals are suspended from the acicular mordenite crystals.

REFERENCES CITED

Hampson, A.G., 2004, Zeolites and associated minerals of the Wolf Creek Pass area: Rocks & Minerals, vol. 79. No. 5.

Hanner, M. E., 1976, Mordenite at Wolf Creek Pass: Mineralogical Record, vol. 7, no. 6.

Went down and around and around and down
We run outta ground at the edge of town
Bashed into the side of a feed store
In downtown Pagosa Springs