AN OVERVIEW OF THE POST MINING MINERALS FOUND AT BISBEE, ARIZONA
POST-MINING IRON HYDROXIDES AND MELANTERITE COATING MINE TIMBER ON THE 2833 LEVEL OF THE CAMPBELL MINE
As the name suggest, this is an overview of those minerals, which formed after mining had opened up new, previously buried areas. When mining occurred, it introduced near-surface atmospheric conditions to the newly-opened areas, bringing oxygen, humidity and bacteria into contact with the previously buried minerals, thereby allowing the process of oxidation to take begin with pyrite. Pyrite (FeS2), was the most common hypogene (primary) sulfide mineral deposited at Bisbee and is thermodynamically unstable and oxidizes, often quickly, as documented by the voluminous literature on the subject.
Pyrite is the species that usually begins the process of post-mining mineral development and is caused and accelerated by the presence of oxygen and the water in the newly introduced atmosphere, while the bacteria, Thiobacillus ferrooxidans and Thiobacillus sp are usually present and involved. If conditions are ideal, this oxidation can proceed at a very rapid pace.
Figure 1: Mining fresh sulfide ores - 2833 level, Campbell Mine.
Peter Kresan photo
Below is the typical chemical reaction and products:
(1) FeS2(s) + 7/2O2(g) + H2O(l) → Fe2+(aq)+2H+(aq) + 2SO42(aq) - continuing to
(2) Fe2+(aq) + 1/4O2(g) + H+(aq) → Fe3+(aq) + 1/2H2O(l)
A wide variety of sulfide minerals were deposited at Bisbee during the multiple phases of mineralization (Graeme, 1993). Large amounts of iron/copper/lead/zinc/sulfur bearing minerals replaced the hosting limestones and were deposited in the rock units comprising the Sacramento Stock complex. However, pyrite was always the dominant mineral in the hypogene sulfide deposits at Bisbee; both in the porphyries of the open pits and the many, scattered limestonehosted replacement deposits mined underground. Bryant and Metz, (1966) estimated that more than 500 million tons of pyrite had been deposited in the rock units hosting and surrounding the copper deposits at Bisbee.
Post-mining mineral deposition occurs in several ways. At Bisbee, most such minerals are sulfates and deposited as the natural processes of sulfide oxidation continued during and after miners made their openings for either exploration or exploitation. Minerals other than sulfates formed as postmining deposits as well, but were relatively minor in nature. In some cases, the non-sulfate, postmining minerals were also a direct result of the sulfide oxidation, but in reactive environments, such as limestone, which typically caused carbonates to form or elemental copper to replace iron. While Bisbee suffered a number of sulfide mine fires, most were extinguished using water or a water-based slurry of mill tailings. This would have dissolved most sulfates. However, a few fire created minerals have been identified, as will be discussed later.
Figure 15: (right) ponded low pH waters depositing iron sulfates due to evaporation. 2833 level, Campbell Mine.
For scale; the mine workings are approximately two meters wide.
More detail on the post-mining mineral occurrences at Bisbee can be found in Anthony, et al., (1995), Graeme (1981, 1993), (Graeme, et al., 2015), Merwin and Posnjak (1937) and Mitchel (1921). Unless otherwise noted, all photographs are by the authors and the specimens are from the Graeme collection
Many of the post-mining sulfate minerals found here have been recognized only in this type of depositional environment, while others are known to form under typical supergene conditions as well. It is, however, reasonable to assume that the majority of these species also occurred as a part of normal supergene processes, but were not noted. This lack of recognition was probably a function of their nondescript appearance and typically high solubility. A list of those species recognized as forming under post-mining conditions is presented below:
Table 1: Post-mining mineral species recognized at Bisbee
The vast majority of the recognized post-mining species are water-soluble, hydrated sulfates. Postmining sulfates were widely distributed throughout the mines and locally abundant. These sulfates develop as a result of the decomposition of the sulfides, hypogene or supergene, in the moist, now oxygenated mine environment. Pyrite is the first of the sulfides to be impacted by the newly introduced conditions. The decomposition of pyrite under damp conditions, generates small amounts of low pH, iron sulfate solutions that migrate along fractures and crystal boundaries to the mine openings. When present, other sulfides react with these iron sulfate solutions liberating other elements, particularly copper or zinc, albite more slowly.
Figure 2: In situ, massive pyrite decomposing and generating low pH solutions, which are forming iron hydroxides (brown) and have begun to mobilize copper - copper iron sulfates from other sulfide minerals. View-30 cm, 6 level Southwest Mine.
Evaporation of the solutions from the mine walls and floors typically forms multi-mineral crusts with the lower pH phases at the wall surface grading to higher pH phases at the outer part of the crust, reflecting evaporation and, consequently, changing of the pH, as discussed by Merwin and Posnjak (1937). Non-reactive wall rocks such as porphyry, massive sulfides or silicified units are important to the development of these crust growths. While these post-mining crusts often contained multiple species, it was not uncommon for one species to be decidedly more abundant, with many of the others little more than accessory species.
In some cases, large areas of non-reactive wall rock were covered by post-mining mineral crust. In moist, high pyrite areas, the authors have seen tens of square meters covered by post-mining sulfates.
Figure 4: A post-mining, multi- mineral crust containing coquimbite, Fe23+(SO4)3.9H2O: (purple), rhomboclase, HFe3+(SO4)2.4H2O: (yellow-white), botryogen, MgFe3+(SO4)2(OH)3: (orange) 1600 level, Campbell Mine,
view – 3.2 cm.
Figure 3: A post-mining, multi-mineral crust continuing at least - copiapite Fe2+Fe3+(SO4)6(OH)2.20H2O: (yellow), melanterite (cuprian) Fe2+SO4.7H2O: (Green), kornelite Fe3+(SO4).7H2O: (very light purple). In situ, 7 level Southwest Mine, view - 16 cm.
Figure 5: Post-mining crust of coquimbite, Fe23+(SO4)3.9H2O: (light purple), paracoquimbite, Fe23+(SO4)3.9H2O:(light purple), copiapite, Fe2+Fe3+(SO4)6(OH)2 .20H2O: (yellow), melanterite Fe2+SO4.7H2O: (white), voltaite, K2Fe52+Fe43+(SO4)12.18H2O: (black). Lavender Pit Mine, view – 8 cm.
Single species development along mine openings was quite common as well. Typically, the areas that hosted a single or very few post-mining species were somewhat less moist and/or contained only modest amounts of sulfide minerals. Thus, the oxidation and the development of the postmining mineral/minerals was both slower and more limited. Other factors such as a single sulfide species present could also contribute to the development of just one post mining species or one dominate species.
Figure 6: Melanterite, Fe2+SO4.7H2O, coating high-pyrite wall rock. 6 level, Southwest Mine.
Figure 7: In situ halotrichite, Fe2+Al2(SO4)4.22H2O:, 1600 level, Campbell mine:, View – 50 cm.
Figure 8: White epsomite, MgSO4.7H2O:, effervescing from Martin limestone, 6 level, Southwest Mine:
view – 1.35 meters.
Under conditions where more water is available, other types of growths develop with distinct crystals uncommon. Stalactitic and less commonly stalagmite forms were common where dripping solutions were present. These forms are generally composed of a single mineral species that is dominant in the area and reflecting the source mineralogy; due to a single source for the solutions and a relatively consistent pH, as opposed to multi-mineral crust in moist areas.
Figure 9: In situ, blue ransomite, Cu2+Fe23+(SO4)4.6H2O:, with minor white melanterite , 1800 level, Campbell mine: view - 35 cm.
However, it was not particularly uncommon to find copper, zinc, or iron-containing varieties of the dominant species locally intermixed with the more prevalent species. An example is the presence of pisanite (cuprian melanterite) stalactites was noted with similar stalactitic forms of melanterite and/or chalcanthite. (See Figure 11.
Figure 10: Chalcanthite, Cu2+SO4. 5H2O: as stalactites and stalagmites. In a mined out stope. 2300 level, Junction Mine
Figure 11: Chalcanthite, Cu2+SO4. 5H2O: (blue), with cuprian melanterite, Fe2+SO4.7H2O: (near - black green) on copiapite, Fe2+Fe3+(SO4)6(OH)2 .20H2O: (yellow) coated mine timber. 2833 level, Campbell Mine, vertical view - 61 cm.
Figure 12: Goslarite, ZnSO4.7H2O:, as stalactites on mine timber. 1500 level, Junction Mine. Vertical view - 1.25 meters.
Broken sulfides scraps left underground in damp, non-reactive environments, often developed sulfate coatings. These ranged from thin veneers to layers a centimeter thick. Lumps of massive pyrite frequently completely decrepitated, while relatively pure massive bornite, chalcopyrite and chalcocite had only a surficial alteration to brochantite and/or antlerite, devilline or chalcanthite, respectively. Galena acquired a thin coat of anglesite and the typically iron-rich sphalerite formed goslarite and a few other sulfates, depending on the relative abundance of pyrite. However, even modest amounts of intermixed limestone with the broken sulfides either inhibited or prevented sulfate crust formation altogether through buffering the acidic formation.
One other post-mining sulfate worth noting was a single occurrence of post-mining alunite, which was an actively oozing from a fracture on the 1300 level station of the Cole Mine in the 1960s. It was light blue in color and frequently “harvested” by the miners as it grew in size. However, once on the surface for a few days, it would craze upon dehydration, becoming quite fragile and much lighter in color.
Figure 13: Chalcanthite, Cu2+SO4. 5H2O: (blue) on chalcocite and brochantite, Cu42+(SO4)(OH)6: (green) on bornite as alteration coatings on broken ore left behind. In situ, 6 level, Southwest Mine, view – 1.20 meters.
Figure 14: (left) ponded, low pH water with iron hydroxides forming on the surface and edges. Tunnel level, Higgins Mine.
Where acidic solutions formed stagnant ponds, it was common to find crystals, often large, of several of the sulfates intermixed. Chalcanthite, melanterite and large, colorless epsomite crystals were the most common species in the ponds as strikingly beautiful, but ephemeral specimens.
A change in the environment to warmer, dryer conditions often occurred as the available water drained from the rock, and continued sulfide decomposition, an exothermic process, warmed the air. Modifications in mine ventilation also altered the nature of the environment as well by changing the airflow and bringing in drier or increased volumes of air. These changes frequently caused dehydration of the original sulfates, with new species such as hexahydrite replacing epsomite and rozenite or siderotil replacing melanterite
Figure 16: Collecting cuprian melanterite and epsomite crystals from ponded mine waters on the 2700 level of the Junction mine.
Figure 17: Römerite, Fe2+Fe23+(SO4)4.14H2O: growing on pyrite-rich porphyry in the Lavender Pit Mine, View-80 cm.
The exposed sulfides in remaining porphyries and the heavily pyritized Pinal schist of the Lavender Pit Mine develop an interesting, albeit typically short-lived, suite of post-mining minerals as well. The periodic rains filter through the broken wall rock and form spotty effervescent growths of usually iron sulfate species. However, bluish blossoms of ilsemannite can be seen on occasion, betraying the highly localized and uncommon presence of molybdenum. For the most part, these sulfate growths only last until the next rainfall, when they are dissolved or the more typical dryness of the Arizona air returns, which causes their dehydration and decrepitating.
This tendency for the rapid dehydration of some of the sulfates has complicated their study. When collecting these minerals, one must go prepared to preserve, as much as possible, the humid conditions before removal from the location of deposition by using airtight containers. Then, storage, once removed from the mine, must be considered. Even with the best of planning and efforts, most of these species will still dehydrate with time. Indeed, few truly remain unaltered.
Very few examples of the hydrate sulfates from Bisbee have been preserved, as most quickly dehydrate on exposure to the dry, surface atmosphere and either alter to other, less hydrated species or completely decrepitate due to the loss of water from the mineral structure. Many were the miners or mine visitors who collected brilliant blue samples of the abundant chalcanthite or cuprian melanterite, only to have it turn into an uninspiring white, chalklike mass within a few days in the dry surface atmosphere as it lost its water of hydration and altered to another mineral, usually white or yellow/white.
Many hundreds of post-mining gypsum specimens were collected at Bisbee and frequently seen in collections. Gypsum, as the variety selenite, was common in stagnate ponded, low to moderate pH mine waters as well, but never in association with an abundance of other sulfate species, as the necessary calcium carbonate partially neutralized the solutions. Selenite often occurred as a growth on limestone walls of flooded drifts or on the bottoms of the ponds, but always on or very near limestone.
Fortunately, a few sulfate species do survive. Excellent examples of römerite, voltaite, copiopite, coquimbite, paracoquimbite, botryogen, szomolnokite, and zincobotryogen are in several collections. Post-mining antlerite, brochantite, devilline and langite are also well represented as interesting, but usually uninspiring specimens.
Figure 18: Ponded acidic waters to 25 cm. deep, in a limestone crosscut, 6 level ,Southwest Mine, vertical view - 2.30 meters.
Abundant yellow/white, post-mining selenite had/was forming at the limestone acid water interface as thick masses of crystals, often exceeding seven centimeters in length.
Figure 19: Gypsum CaSO4.2H2O:, as selenite on post-mining copper, 2200 level, Campbell mine, specimen - 7cm.
This specimen formed in a very small pond of low pH-high copper solutions in a limestone crosscut bottom. Little to no iron was present in the solutions, something not too common.