Article by Paul Martz
Geology – December 1998 – Colorado Central Magazine
HOT SPRINGS, as simple as they may seem on the surface, have a wide variety of mechanisms that drive them from the crust of the earth.
The most common type of hot spring in Central Colorado, or for that matter worldwide, is caused by hot rock, sometimes even melted rock, at a depth where it heats ground water and drives it back to the surface. The greater the amount of heating, the more rapidly the water rises to the surface.
By producing large amounts of heat from friction, faulting can also serve as a heat source for hot springs. And faults may also provide a pathway for the heated water to migrate to the surface.
The temperature of the earth’s crust increases naturally with depth, and that heat can also produce hot springs. When water either finds a way to migrate downward, or has been trapped within sediments that have been buried in a subsiding active basin, that water can subsequently find its way back to the surface as hot or warm springs.
Not surprisingly, Central Colorado contains examples of all of those types of hot springs.
But an additional, although uncommon source of hot water can also occur. Gulf Oil discovered that springs caused by natural radioactive decay weren’t just a hypothetical possibility, but were an actuality at the Mt. Taylor project near Grants, N.M.
In addition to many active hot springs, our region contains numerous fossil hot springs systems as well.
The 64 square miles of the Leadville Mining District contain not only ore-grade sulfide minerals, but a surrounding altered zone. Exposed to weathering, either by mining or natural forces, the minerals that now cause environmental concern in the Arkansas River are the result of what were — once upon a time — enormous hot springs. That particular hot springs system operated over both a large geographical area, and a lengthy time period.
However, the prize winner for the oldest preserved hot spring system in the area goes to the Sedalia mine just outside of Salida — which is the eroded remnant of a sub-marine hot spring that operated more than two billion years ago.
Most mineral districts in Colorado are the product of hot springs that were heated by magma and finally cooled off. The pathways of those mineral-bearing waters have since been eroded, exposed by nature, and mined by humans.
However, when most of us think of hot springs, we have actual warm water in mind, not some fossil system that is now frozen in time. Our position astride the axis of the Rio Grande rift guarantees that we will have hot water rising to the surface somewhere nearby. That’s because rift systems thin the base of the earth’s crust, bring molten rock to the surface, and have many significant faults.
The Poncha Hot Springs that provide hot water to the Salida Hot Springs Pool are being heated by a body of hot, if not molten, rock at depth. This type of heat source is indicated not only by the temperature of the water but also by the heavy load of dissolved constituents that plugged the old collection galleries and required the replacement of the supply line to the pool.
MINERALS THAT WOULD BE QUITE HAPPY to stay dissolved at depths of 5,000 to 8,000 feet (where the water is being heated), become saturated and crystallize as both the pressure and temperature of the water decrease. That the system was once much hotter than it is today is demonstrated by the fluorite deposits that occur just to the east of where the springs presently emerge.
About a half mile west of the Poncha hot springs there’s another warm spring that probably results from the same source, although its flow rate and temperature are significantly lower. The warm spring to the west of the main springs demonstrates the influence of cooler surface waters and the quenching action that occurs as a result. Much of the dissolved mineral matter is rapidly precipitated out as the temperature drops.
The “black smokers,” featured in numerous oceanographic films, which support strange and wonderful life forms at great depth on the sea floor, are the result of such quenching. The “smoke” is actually tiny sulfide grains formed instantaneously as the transporting hot water is cooled from several hundred degrees to 32° F.
Conversely, I’ve heard it said that in places in Iceland, you can catch a fish at depth and cook him at the same time. That is, you can boil him if you bring him up slowly through the volcanically heated water that “floats” on the cooler, more dense water beneath. I’ve heard this fish story a number of times, but have always wondered what the fisherman was standing on while he was doing his “cooking.”
The Valley View Hot Springs probably result from friction along a fault. This is indicated by their relatively low temperature, small total dissolved solids (minerals), and comparatively low flow rate. If there were large deposits of travertine and siliceous sinter present, then one could interpret the heat source as a cooled magma that is nearly in equilibrium with its surrounding rocks.
The other major clue that they result from fault friction is their location right atop the structures that raise the Sangre de Christo Range above the floor of the San Luis Valley.
The nearby Mineral Hot Spring may have a similar source — but then again, it might not.
What makes the source of the Mineral Hot Spring problematical is that it has many of the characteristics of a magma-related system, but it just happens to lie astride one of the major faults along the axis of the rift system. There are large mounds of siliceous sinter present, which along with high temperatures and a large load of dissolved minerals, seems to imply a volcanic source.
If one continues on U.S. 285 towards Saguache, the highway cuts through a basalt flow that is not very old in a geologic time frame. Just south of Saguache is an even younger flow with an even more impressive road cut. So a volcanic heat source is not by any means out of the question.
However, there is a geologic hook in those apparent worms and that is the artesian basin beneath the valley. Wells that pierce the confined aquifer do not produce water as hot as one might expect if there were magma near the surface.
About a mile north of Mineral Hot Spring is evidence of very recent movement on what appears to be the same structure that the spring flows up along. By very recent I mean a few years, or tens of years at the most; a shallow wash of the type that can be formed by a single torrential thunderstorm shows displacement that has not yet been corrected by subsequent run-off. In between those two points is an occurrence of sinter in the form of a low mound.
The implications here are that the present spring is on an active fault, and that the location of surface emanation of the water has migrated with time.
While it may be that the heat source is magmatic, there is also the possibility that Mineral Hot Spring results from fresh water deeply buried by the rapid subsidence and filling of the San Luis Valley, that’s warmed by crustal heat rising along the rift axis, then subsequently given a path to the surface as a result of recent faulting. This question could be resolved by isotope data from the water and the sinter mounds.
The Mount Princeton Hot Spring undoubtedly has a magmatic source that is on the wane. The adjacent Chalk Cliffs, which are actually a type of granite that has been chemically leached and altered, result from the action of a much larger hot springs system in the geologic past.
Evidence of even larger systems occurs to the west in the eroded remnants of mineralized areas that supported mining around Saint Elmo. The iron-stained outcrops of the northeast flank of Grizzly Peak are probably the core of an earlier system that could have rivaled Yellowstone’s Old Faithful.
Every year when I make the pilgrimage up the mountain to hunt aquamarines, I stop and marvel at the once seething cauldron whose roots are recorded in those flashy red, orange, and yellow outcrops.
If you take your out-of-town guests to visit Sante Fé via U.S. 285, you will pass through Ojo Caliente — which means coincidentally, hot spring. The waters of this spa are probably heated by a volcanic source — but they also rise along a major rift structure and may have a history more complex than just taking a blob of hot rock, adding some aqua, allowing them to mix and presto — getting an historic bathing location.
Each hot spring system has its own geology and in an active rift system like Central Colorado, that hot water on the surface may have a surprisingly complex origin beneath it. Just about everything that can create a hot spring — thin crust, deep faults, near-surface volcanism — is present hereabouts, and considering the numerous rich ore deposits in our area, these factors have been present here for millions of years.
Paul Martz holds a master’s degree in geology. A chronic malcontent, he has been terminated as an exploration geologist by several Fortune 500 companies, and now maintains a consulting geologic practice in Poncha Springs.