19.7 - Turn left, southbound on US 395.

22.7 - Turn left at Airport Road.

23.2 - Turn right on Fish Hatchery Road (aka Hot Creek Road).

23.6 - Turn left into Fish Hatchery Complex.

23.7 - STOP 15 Mammoth Fish Hatchery. While not the most geologic of stops, my ten-year son old really liked it!  Near the hatchery, geothermal springs mix with the water from Mammoth Creek. The result is a series of ponds that maintain a year-round temperature of 52 to 62°F providing an ideal environment for incubating trout eggs and raising fish. The hatchery, run by California Department of Fish and Game has been in operation since the early 1930s.  You may feed the fish if you wish, (it costs a quarter) but stick your hand in the water at your own risk!

Return to Fish Hatchery Road, turn left.

24.4 - Driving over 1980 Fault Scarp. This linear, low scarp is one of several surface ruptures that occurred as a result of the 1980 earthquake swarm, this one possibly on a splay of the Hilton Creek fault. Extension cracks appeared on Hot Creek Road trending in a northwest-southeast direction at the base of the southwest-facing fault scarp. Extension varied from 0.5 to 3 inches with individual cracks 15 to 20 feet in length. Vertical displacement ranged from 6 inches to about one foot. Field mapping and fault plane solutions suggest extension, but the actual displacement mechanism is complex, possibly involving magma movement. (See previous discussion at McGee Creek)

26.5 - STOP 16 Hot Creek. Turn left into the parking area. From the interpretive overlook note the steam rising from fumaroles and hot springs along the creek. There are also hot springs discharging directly into Hot Creek near the remains of the bridge that formerly spanned the creek. The mingling of hot spring water with snow-melt fed stream water produces extreme temperature gradients in the creek. The wide range of temperatures has made this area popular for swimming year-round so the Forest Service has constructed change rooms for visitors.  During a recent visit (2003) the water on the southeast side of the creek had become so dirty and fouled with pollutants that even the idea of entering the creek was unappealing.  Let's hope the stream is able to cleanse itself!

Note the altered rhyolite in the gorge. Hydrothermal activity has kaolinized and opalized the rock producing the white, bleached appearance (we will see a better example at the next stop). The rhyolite has been dated 300,00 years. The northeast trend of Hot Creek is consistent with that of the Hilton Creek fault, so many geologists have theorized the main fault or a branch are the conduit for hydrothermal waters.  This hypothesis became more difficult to defend when recent strong earthquakes on the Hilton Creek fault (1998) had little impact on the thermal regime of Hot Creek. As you walk down the path to the creek also note the hummocky terrain on the northeast side of Hot Creek, reminiscent of the Owens River gorge.

Many of the current hot springs appeared suddenly on the evening of August 25, 1973. At least five hot springs formed, with the two largest starting as geysers that spouted water 10 feet into the air. Within weeks geyser activity had ceased, but the hot springs remain today.

The origin of the new hot springs remains unclear, but it has been noted that they appeared within hours of a relatively small (M=3.5) earthquake 25 miles southeast of Hot Creek. Presumably, seismic activity altered the subsurface plumbing system giving rise to the springs. Prior to the small earthquake, heated water was trapped below an impermeable horizon. The seismic event breached the impermeable strata and superheated water and steam rose rapidly initiating geysers at the surface. After the initial pulse of superheated water reached the surface, the heat flux decreased and the geysers became hot springs.

Turn left from parking onto Hot Creek Road from the parking area.

27.6 - Bear left at the intersection with Whitmore Tubs Road.

28.2 - Turn left on Owen's River Road.

28.9 - Turn left on Antelope Springs Road (3S05)

32.3 - Five-way intersection, turn right onto access road for the kaolinite mine you just passed.

32.5 - STOP 17 Huntley Kaolinite Pit. The Huntley Kaolinite Mine is an active mine currently owned by Standard Industrial Minerals of Reno, Nevada (formerly Standard Slag Company? of Youngstown, Ohio).  The mine does not operate on weekends, but mining equipment is parked at the site. Please stay off the equipment and exercise care on the property.  If the mine is operating, tailing piles can be accessed from Antelope Springs Road without entering the mine pit. The kaolinite has been formed by hydrothermal alteration of Pleistocene lakebed sediments and the underlying rhyolite. The rhyolite has been dated at 300,000 years making it correlative with that at Hot Creek.  The alteration appears to be controlled by a north trending fault system that created a graben.  Kaolinization is best developed along the east side of the graben.  Relict bedding from sedimentary layers and flow banding of the rhyolite can be seen in large boulders from the stockpiles and on outcrop in the pit.  Opal veins are common, while alunite is rare.  The mine is also listed as a pyrophyllite producer.  The genesis of the kaolinite and alunite will be discussed at the next stop, but both are common alteration products of acid-sulfate geothermal systems.

The Huntley Mine has been operating since 1952.  Standard Minerals has a patent on 180 acres of land.  The kaolinite is trucked from the mine site to a processing plant near Bishop, where it is crushed and bagged for shipment.  Kaolinite has been used as a filler in paint and paper as well as a whitening agent in cosmetics, ceramics and portland cement.

Turn around and return to five-way intersection.

32.7 - Five-way intersection.  Go straight through the intersection and continue southeast on the unnamed Forest Service Road (3S07).

34.0 - Turn left on unimproved dirt road (look for the water storage tanks.

34.1 - STOP 18 Blue Chert Mine.  The term mine is something of a misnomer for this outcrop.  This small hill appears to be the top of a fossil hot springs system.  Numerous other small knolls occur throughout this area. (Remember those we could see looking to the north from Hot Creek.)  Each hill represents the locus of a hot springs or fumerole.  As hot water rises in the vent conduit and cools it deposits microcrystalline silica along the walls of the fissure.  This often seals the vent system which remains dormant for decades until seismic activity reopens the fissures.  When that happens, the trapped geothermal waters rise rapidly to the surface and flash to steam.  The explosiveness of the erupting geyser often overcomes the tensile strength of the rock shattering it and creating a breccia pipe.  The rapid temperature drop deposits a silica sinter blanket around the vent opening.

The chert from this locality has been dated at approximately 275,000 years B.P.  This is consistent with ages for the rhyolite at the Huntley Kaolinite Mine and that at Hot Creek.  Presumably, the chert formed during a major period of hot springs activity that developed following volcanism associated with western "moat" rhyolite emplacement.  One intriguing question is the source of the blue color. It appears restricted to this hill, and has not been explained adequately. The source of all color in cherts, according to many websites, is impurities incorporated during chemical precipitation.  The most common impurity is amorphous iron oxide resulting in the red variety of chert know as jasper.  The nature of the impurities in other colors of chert is uncertain and the subject of ongoing research.

Vista Gold Inc. of Littleton, Colorado holds a lease on the Blue Chert property.  Through a purchase agreement with Standard Minerals, owner of the Huntley Kaolinite Mine, they currently control approximately 1800 acres.  An extensive drilling project was completed in 1998 which outlined 68,000,000 tons of ore grading 0.018 oz/ton gold.  The company green-lighted property development in 1998, but the drop in world gold prices coupled with environmental opposition has delayed plans.  During a visit in 2003 no evidence of recent activity was noted.  It would be a shame if this mining venture is not allowed to proceed.  One of the missing pieces of the educational puzzle for a Long Valley field trip is a view of the caldera as it appeared after eruption of the Bishop Tuff.  Today we see the caldera only after it has been infilled with 700,000 years of sediment and volcanics.  Imagine being able to drive to the rim of the proposed pit, over one mile wide and five hundred feet deep, and peer inside.  It would be like looking into the caldera at the time of its inception.  Perhaps the mining company could even be persuaded to pile their leach dumps around the periphery of the open pit, simulating the raised caldera rim!  The resulting tourist attraction would significantly enrich the economy of Mono County.  Geologists should initiate a letter writing campaign similar to the one that saved "The Big Pumice Cut", advocating creation of the "Blue Chert Open Pit".  Further generations will applaud our efforts in forthcoming Long Valley field guides!

The association of gold with fossil hot springs systems has been recognized for over half a century.  What was missing was a model to explain this association.  To the neophyte this may not seem difficult, but remember gold is one of the most insoluble and inert of commodities.  To get enough gold into solution to make an ore deposit and then concentrate it is a perplexing problem.  In the 1970s academics came up with an explanation for the solubility problem, complexes.  It seems that complexes increase the solubility of gold dramatically, cyanide is the best known example.  Of course natural cyanide solutions are unknown.  However, bisulfide (HS-) can also increase the solubility of gold significantly and this is a common constituent of reducing environments.  So we now had the mechanism to dissolve gold, all that was missing was a model for deposition.  That was provided in the early 1980s by exploration and academic geologists.  This model, called the acid-sulfate gold model, was based largely on research conducted in the Nevada gold belt.  The illustration to the right is from a model proposed for the Round Valley Gold Mine, a major Nevada  gold producer.

This model envisions the gold being carried as a bisulfide complex.  Bisulfide is only stable under reducing conditions.  As the hydrothermal solutions rise along fracture systems they are oxidized and the bisulfide becomes sulfate (SO4).  The gold can no longer remain in solution since the complex has broken down and it is deposited.  The sulfate combines with the free hydrogen released when HS- breaks down, generating H2SO4 (sulfuric acid) a powerful acid.  This explains the observation that many hot springs are highly acidic.  The newly formed acidic water cools as it flows laterally, and convectively percolates downward into the subsurface.  The powerful acid leaches the rock leaving only the most insoluble of compounds, kaolinite and the characteristic alunite.  The water heats as it circulates downward and is reduced.   It recharges the aquifer system and is recycled upward through the breccia pipe conduit.  So the characteristic elements of this model are a fossil hot springs system i.e., chert/silica sinter, brecciation, argillic alteration (kaolinite), and alunite.  It seems the Blue Chert property was a potential "gold mine" waiting to be found by anyone working with this model.  Remember the Blue Chert Mine is a fossil hot springs system, what might be happening in the subsurface along Hot Creek today?  Perhaps, some of you younger geologists will get the opportunity to examine that question in a hundred-thousand years or so.

Return to to Antelope Spring Road (3S07).

35.5 - Turn left on Antelope Springs Road.

41.0 - Turn southeast (left) at the dead-end; continue on the paved road running parallel to Highway 395.

41.7 - Turn right on Substation Road (Highway 203).

41.8 - Turn right onto the northbound onramp for U.S. 395.

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