POST MINING MINERALS FOUND AT BISBEE, ARIZONA
Pyrite oxidation in wet areas usually generated copious amounts of various iron hydroxides. The precipitation of the gelatinous iron hydroxides was a problem caused by these low pH solutions. If left unattended, mine workings would fill to a depth of a meter or more with the troublesome slime. It is this iron laden, low pH water that is much a part of the environmental problems associated with pyrite degradation.
Ponded mine waters in sulfide areas collected behind rock falls or mine timber failures in abandoned areas. The dammed up solutions behind these obstructions often covered a thick layer of yellow mud, consisting of iron hydroxides, which had precipitated out of the high-iron solutions. Sometimes the mud would be waist-deep. If undisturbed, goethite rafts formed in still areas and eventually sank to the pond bottoms.
Figure 20: Acidic mine water depositing iron hydroxides in abandoned working, note abundant white to brown melanterite on ceiling and walls, 6 level, Southwest Mine, horizontal
view – 2.8 meters.
Figure 24: A one-gallon bucket coated by postmining aragonite. 6 level, Southwest Mine,
While some of these hydroxides were deposited as little more than a slimy mud in the workings, others often formed numerous long stalactites and massive stalagmites. Undoubtedly, some of the same iron compounds were deposited in both
Figure 21: Acidic mine water formed by the oxidation of pyrite. Mud-like iron hydroxides have precipitated for the water and partially filled the mine opening. Tan colored melanterite locally coats the mine walls. 7 level, Southwest Mine, horizontal view – 2.2 meters.
cases, but no effort was ever made on the part of the authors to determine either the chemistry or mineralogy of any, but rather to simply document the appearance of deposition and in some instances, note the ultimate apparent end minerals, which formed with dehydration
Figure 23: Acidic mine water with stalagmite forms and minor melanterite. 6 level, Southwest Mine, view – 38 cm..
Figure 22: Iron hydroxide stalactites and stalagmites deposited by the acidic water that have partially filled the mine opening. 7 level, Southwest Mine, vertical view – 2.3 meters.
In areas with substantial dripping water, which had passed through the oxidizing sulfides, impressive clusters of yellow to yellow/brown to dark brown stalactites would form. In several instances, the stalactite/stalagmite development was such that it nearly closed the mine opening.
For the most part, the stalactitic growths were delicate; not soft, but rather hollow and/or composed of successive layers of material that were poorly cemented together. Often straits of solution were in between the concentric, crustlike layers.
On drying over an extended period of time, the iron hydroxides became an impure mixture of several iron oxides, but were predominantly goethite and lepidocrocite. In any event, the collective term “limonite” best describes the appearance of the ultimate product.
The most common of the post-mining carbonates was aragonite. During the course of mining through the barren limestone host rock, it was common to intercept cracks, faults and solution channels that served as conduits for the usually small amounts of water within the limestone. Most often, the water in these openings was clean and metal free, but high in dissolved calcium and CO2. As the water entered the mine openings, it began to liberate the CO2 and deposit calcium carbonate, usually as aragonite, though calcite was deposited in some areas. The reason for the apparent significant preferential formation of aragonite over calcite is unclear, but may well be related to the relatively high kinetic energy associated with the deposition – cascading or fast flowing or long vertical drop of the depositing waters.
In limestone areas, post-mining aragonite was a common occurrence in most of the mines. Aragonite stalactites of several centimeters in length, often coated with small, acicular aragonite crystals, were locally abundant where the groundwater flow was modest. In mine openings that served as main ventilation ways, these stalactitelike formations would grow directed into the strong airflow almost parallel to the mine back (ceiling), but without the acicular crystal growth on their exterior. With moderate ground water available, the walls and back could be covered with parallel aragonite growth reaching ten centimeters and all pointed into the wind, where evaporation took place the most rapidly.
The apparent rapid rate of deposition of aragonite was noted is several places, but perhaps the most interesting in our view, is a single, one gallon, galvanized steel bucket left by lessees in late 1944 on the 6th level of the Southwest Mine. It had been placed under a steady drip of clean water to use for drinking, as suggested by the remains of a wooden water keg close by. It would have taken several hours for the bucket to fill, yet when first found some 30 years later, a full
A second, but very similar site was on the 2300 level of the Junction Mine where the waters from the Denn Mine poured down a timbered raise from the level above on their way to the 2200 level pump station. For the most part, this water came from the wide Dividend Fault zone, cut when sinking the Denn Shaft in the 1930s. Over time, the water quality appears to have change, no surprise as substantial mining was taking place. The change in water quality is reflected in the variable coloration of the deposited aragonite. As mining passed well below the 2000 level, the flow decreased and some coloration due to copper imparted. By the early 1970s, just a few hundred liters per minute were flowing down this raise, a fraction of what company records noted some 30 – 40 years before.
Near pure-white aragonite to 10 centimeters thick coated all of the timbers, in spite of the force of the hundreds of liters per minute falling down the raise. The receiving crosscut was flooded, as the 400 level station had collapsed sometime in the early 1950s, so the timber below the water level was somewhat less heavily coated by aragonite. As the rock damming up this significant flow was very course, the water passed through it easily, precluding the formation of aragonite/calcite rafts, so commonly seen covering still mine waters. Fortunately, good, natural ventilation kept the concentrations of liberated carbon dioxide low, allowing entry, as the water nearly filled the crosscut, forcing one to swim in the frigid water.
Thick, massive post-mining aragonite was abundant in several places where large volumes of mine water were transferred from level to level down 100-foot raises. Ladders and other timber were often coated with 5-10 cm of aragonite by the forceful, cascading waters. The most memorable are a raise between the 300 and 400 levels of the Czar Mine that channels water from under Queen Hill. These clean waters were kept separate from other mine water and collected for use on the surface in boilers, showers, etc., during the operation of the mine.
three centimeters of semi-pearlescent aragonite had been deposited where the water flowed from the vessel. Today, more than five centimeters of aragonite have encrusted the lip of one side and the whole of the bucket is coated, representing some two centimeters of additional deposition in less than 40 years.
As exploration east of the Campbell Mine progressed in the early 1940s, substantial amounts of water were hit, as expected. While every reasonable precaution had been taken, the Campbell Mine was partially flooded in 1941 by a flow from 12 crosscut on the 2700 level, estimated at more than 200,000 liters per minute (Mills, 1958).
Figure 2 5: Two views of an 8 cm. aragonite stalactite-like form that grew over broken mine timber in about 25 years. The color variation in the top view, reflects changing water quality from turbid to clear. 2300 level, Junction Mine.
The diminished flow allowed for the formation of colorful blue/green rafts along the still margins of the ponded water. Similarly colored cave pearls to two centimeters in size formed where a small flow of water fell from a displaced mine timber into a small, otherwise still pool.
Figure 26: Aragonite “cave pearls” to 2 cm, 2300 level, Junction Mine.
After months of pumping, access was regained, but substantial water still flowed from a series of subparallel faults in the mine wall. By 1960, thick, travertinelike aragonite as much as three centimeters thick coated the crosscut floor and the water was still flowing 25 centimeters deep across the whole of the two meter wide crosscut floor. A match lit a meter above the flowing water would quickly be extinguished lowered a bit by the high CO2 concentrations due to the rapid degassing of the ground water. This degassing, of course, caused the relatively rapid deposition of aragonite.
Figure 28: Post-mining aragonite from the floor of 12 XC, 2700 level, Campbell Mine. Specimen – 7 cm.
Specimens of the tan to yellowish to gray, flowstonelike aragonite collected from the fast-flowing water often contain angular fragments of fresh limestone or sulfides as well as anything else thrown aside, such as wood, nails and broken light bulb pieces. Even pieces of forgotten native copper, which were placed in the water for cleaning (a common practices), have been covered by this aragonite.
Figure 27: Water door on 12 XC, 2700 level, Campbell Mine in 1962. Note the abundant post-mining botryoidal aragonite between the mine rails. An example of this material is shown in Figure 28, below.
On the 2200 level of the Campbell Mine, an unusual blue colored postmining aragonite/calcite mixture formed in small amounts on and under a leaky wooden flume. The flume was constructed to capture water pouring from a two-inch diamond drill hole and carry it over a gobbed stope to a water ditch. This aragonite clearly shows ripple-like features and is occasionally stalactitic when it formed on the bottom of the flume. The reason for its striking blue color is unknown, but it isreasonable to assume a high degree of copper substituting for calcium in the crystal lattice. More than a few have thought this material was azurite.
Figure 29: Aragonite/calcite – Front and back views of a 13 cm. post-mining aragonite and calcite mixture. This formed on a leaky, timber flume used to conduct water over broken ground. The pattern of the wood is clearly visible on the back side. 2200 level, Campbell Mine.
The intense blue color of this material caused it to be confused with azurite, but it is largely aragonite more or less altered to calcite.
Miners were known to find a developing “bird’s nest” and hid it by placing old mine timber to protect it from view and/or prevent someone from stepping on the delicate treasure. They also often began the task of preparing for removal by driving large nails around the area to develop cracks and would use strips of canvas to control the splash area into the area of interest. More than one miner would add a small bit of chalcanthite on occasion to enhance the color. However, no matter the care and planning taken, it was usually luck that allowed a successful recovery. Over our many years of viewing Bisbee minerals, we have seen fewer than ten such specimens that are of any real aesthetic interest.
Figure 31: Aragonite “Bird’s nest,” with splinters of mine timber, 2200 level, Campbell Mine, specimen - 9 cm
From a mineral collector’s point of view, the postmining aragonite forms the miners call “bird’s nest” are a much coveted prize. These interesting forms developed in inactive, limestone areas of all of the mines, in still-air environments and from slow dripping solutions. These were flat deposits, cementing small areas of the loose rock of the mine floor, with a depression developed where the constant water drops hit. Small rock fragments within the depression would be coated with successive layers of aragonite, resulting in rounded, growths, much like cave pearls that resembled small eggs. Thus the local name.
These interesting forms were usually white, but bluish, copper-tinted, and red to brown; iron-colored examples were not uncommon. While “bird’s nest” were not truly rare in the mines, successfully collecting one was a real challenge. Thus, few good examples exist. It took a number of years, often decades, for the slow deposition of aragonite from a single dripping source, to become thick enough to withstand the force needed to remove one; even though they formed on broken rock or compacted fill on the mine floor.
Figure 30: Aragonite “Bird’s nest,” forming adjacent to a pool of water. 6 level, Southwest Mine, view - 25 cm.
Figure 32: An uncommonly fine aragonite “Bird’s nest,” 2200 level, Campbell Mine, specimen - 27 cm
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