GENERALIZED GEOLOGY AND ORE DEPOSITSOF THE WARREN MINING DISTRICT
Over the years, a number of very capable individuals and groups studied the geology of Bisbee beginning with Fredrick Ransome in 1904 whose excellent work served as the basis for subsequent efforts by Bonillas, Tenney, and Feuchere published in 1916, Carl Trischka’s work in 1938, as well as that of Bryant and Metz, in 1966. Because of these fine treatments of this topic by the aforementioned, only a brief overview will be presented here.
The rocks of the Bisbee Quadrangle consist of a basement of Precambrian quartz sericite schist overlain by 1,600 to 2,000 meters of predominantly calcareous sediments which were deposited during Paleozoic times. During the Jurassic, these rocks were intruded by stocks, dikes and sills and mineralized in several episodes of mineralization. Subsequently, erosion removed an unknown thickness of sediments and intrusives. It was also at this time that the first episode of supergene enrichment took place over the whole of the mineralized area.
Then, during late Cretaceous, some 1,500 meters of principally clastic sediments were deposited on this erosional surface, filling deep canyons and arresting the supergene activity. At some point, during basin and range faulting the whole area was uplifted and tilted about 30o to the east and erosion exposed the western portion of the mineralized to renewed supergene activity.
This unit is the basement for much of southern Arizona and, at Bisbee, is of an unknown thickness. It has been dated at 1.7 billion years old. Essentially, it is a fine grained, fissile, quartz-sericite schist that was most probably the result of regional metamorphism of thin-bedded, clastic sediments (Bryant & Metz, 1966). Locally, the Pinal has been heavily mineralized with abundant pyrite and very minor copper sulfides and, to this point, has never been a host to economic mineralization. This unit is most obvious in the hills north of Bisbee.
This unit is of middle Cambrian age (Bryant & Metz, 1966) and was deposited unconformably on the nearly level, eroded surface of the Pinal schist. The basal beds consist of a quartzite conglomerate, which grade upward into a pebbly phase, then into a gritty, largely argillaceous and calcareous phase, which in turn grades into the overlying Abrigo limestone. Ransome (1904) measure the Bolsa at 131 meters of thickness. While substantial pyrite occurs locally in the Bolsa, it has never been an ore-bearing horizon.
The Abrigo is of mid to late Cambrian age (Bryant & Metz, 1966) and achieved a total thickness of 235 meters (Ransome, 1904). It sits conformably on the Bolsa and is a thin-bedded, impure limestone with two distinct units. The lower unit is characteristically shaly while the upper one is crystalline. A thin quartzite unit terminates the Abrigo at the top and is known in the district as the Parting Quartzite.
The Abrigo was an important ore host for the underground mines at Bisbee even though its importance was not fully understood until the mid-1950’s. Substantial amounts of both primary sulfide and secondary oxide ores were mined from this unit.
Figure 1: Generalized geologic section, after Phelps Dodge (1938).
Upper Devonian in age and only 194 meters thick (Ransome, 1904), this unit lies unconformably on the Abrigo. The Martin is a dark, somewhat shaly to thick-bedded limestone that is often dolomitic. It was the host to many replacement deposits and was the most economically important unit for the underground mines. This unit was also host to many oxidation caves, which were closely associated with the thoroughly oxidized ores in the western part of the district.
This unit is of early to mid-Mississippian age and conformably overlies the Martin without a well-defined contact (Bryant & Metz, 1966). It is some 213 meters in average thickness and is generally light in color and thick-bedded. Impressive cliffs formed by the Escabrosa are evident on the south-facing slopes of the Mule Mountains near Bisbee. This unit was also an important source of ore for the mining operations, producing substantial amounts of both sulfide and oxide ores. Here, too, oxidation caves developed over some of the completely oxidized orebodies in the western part of the district.
Ransome (1904) named and measured this Pennsylvanian unit at 914 meters in thickness. Subsequently, Gilluly, et al. (1954) divided this unit into six new formations, of which only the lower three are present at Bisbee (Bryant & Metz, 1966). These are the Horquilla, which conformably overlies the Escabrosa, the Earp and lastly, the Colina. No younger Paleozoic rocks have been recognized in the Warren Mining District (Bryant & Metz, 1966). Only modest amounts of ore were ever discovered in these units, however, both sulfide and oxide ores were mined from the Naco, with oxidation caves associated with some of the oxides.
The Bisbee group of Cretaceous sediments sits on a very uneven erosional surface of Precambrian schist, Paleozoic sediments and intrusives. The oldest unit in this post-ore group is the basal Glance conglomerate, which contains fragments of all pre-Cretaceous rocks including oxidized ore. The Glance is overlain by the Morita sandstone, then the cliff forming, Mural Limestone and lastly the Cintura sandstone/shale for a total thickness of at least 1,500 meters.
Juniper Flat granite
This granitic rock is most prominent to the north and west of the town of Bisbee where it forms impressive tan to pink cliffs. The rock which has course-grained to porphyritic textures is pink to purplish gray in color. Usually fresh and free of alteration, the Juniper Flat granite has been dated at 177 million years (Creasery and Kistler, 1962). Numerous associated dikes and sills, have intruded all of the pre-Cretaceous rocks as well.
Sacramento Stock complex
The term complex is used to describe the Sacramento Stock as it is actually composed of two distinct porphyry units as well as several breccias. The older of the intrusives is a highly altered, quartz porphyry, which was intensely silicified and pyritized with 15 to 18 percent sulfide (Bryant & Metz, 1966) by the first phase of mineralization and was almost totally devoid of hypogene ore minerals. It was effectively sealed during early alteration, precluding mineralization by later, ore-bearing fluids. This intrusion event also caused the development of a large intrusion breccia along the south side of the stock, which became very well mineralized, and an extremely important source of ore for the underground mining as well as the two open pit mines. This breccia is described in further detail below.
The younger intrusive is described as a quartz-feldspar porphyry (Bryant & Metz, 1966). It was moderately altered, first by hydrothermal fluids and then by supergene activity. This unit was reasonably well mineralized and was the principal source of ore for both open pit mines, due to the extensive supergene enrichment. This same unit is found in the underground mines as numerous dikes. A very large intrusive breccia followed this last intrusive event which is described in the next section.
Both of the intrusive units have been dated at 180 + 3 million years (Phelps Dodge, personal communication, 1972). Lowell and Guilbert have ascribed the younger age of 163 million years to these units. Never the less, they host Arizona’s oldest porphyry copper deposit.
Breccias are included here because of their abundance, obvious importance to ore deposition and their direct association with intrusive action. In depth information on the breccias at Bisbee and the evolution of understanding as to their geneses and role in mineralization can be found in reading Bonillas et al. (1916); Trischka, (1928), (1938); Bryant & Metz, (1966); Bryant, (1964), (1968), (1974). At Bisbee, the more important breccia types were referred to as intrusion, intrusive and silica breccias.
Early workers at Bisbee referred to this as the “contact” breccia as it occurred at the contact with the Sacramento Stock complex, particularly the older intrusive, and the sediments. It covered a large area and very much appears to have formed by the forcible intrusion of the older porphyry, which appears to have been dragged, or pushed in from depth (Bryant & Metz, 1966). It is composed of largely silicified angular to rounded fragments representing every pre-Cretaceous unit except the porphyries and typically grades into undisturbed wall rock at the margins. The matrix of this unit consists of a siliceous rock flour with small, brecciated fragments of the various rock units. As a source of ore, this breccia was extremely important for both the underground and open pit mines. Sulfides had locally replaced much of the matrix and some of the fragments, usually in the more siliceous parts (Bonillas et al. 1916). Locally, the copper content of this breccia exceeded 7% and was extensively mined.
This breccia type was found throughout the district in both the underground mines and as an important component of the Sacramento Stock complex. The size of the intrusive breccias was highly variable and could be as little as 5 mm thick to as much as 150 meters. They were a heterogeneous mixture of all pre-Cretaceous units, including the older porphyry in a matrix of rock flour, which locally contained appreciable sulfides. Contained fragments were angular to rounded and have been found to more than 30 meters in size. Bryant (1968) originally ascribed a fluid intrusion origin to these breccias and estimated that more that 80 million cubic meters of such breccias had been emplaced. Later, Bryant (1974) suggested that the sulfides were contemporaneous with emplacement of these breccias and that they well be considered “ore magmas.” An example of these breccias is shown in figure 2.
Just as the name suggest, these were highly siliceous units. They were composed of highly angular fragments of limestone, which had been completely replaced by silica in a matrix of cryptocrystalline quartz and specular hematite (see figure 3). Relict fossils were not uncommon and represent all of the fossiliferous units in the area of the breccia. These fossils indicate that movement has occurred both up and down as well as laterally.
These units were apparently restricted to the Paleozoic limestones in the Southwest, Shattuck and Higgins ore zones and were invariably pipelike and connections with porphyry at depth were common (Bonillas et al. 1916) see figure 4 below. Trischka (1932) estimated that 90 percent of the ore mined in these areas was in physical contact with silica breccias. The origin of these breccias remains unclear.
Figure 2: Pyrite replaced fragments along with silicified limestone fragments and a large Bolsa quartzite boulder, in an intrusive breccia 1600 level, Campbell Mine.
Figure 3: Silica breccia 7th level, Southwest Mine, view- 10 cm.
Figure 4: Geologic cross section looking north east, showing silica breccia/porphry connection after Bonillas, et al. (1916).
Figure 5: General geology and projection of ores mined underground at Bisbee (Graeme, 1981)
Structurally, the Dividend fault zone is the most obvious and important feature in the district. It is an ancient structure, one that has apparently experienced numerous periods of activity, both before and following mineralization of the sediments. It is a normal fault, trending northeast with a southwesterly dip of from 60o to near vertical. At its eastern most exposure, nearly 1,500 meters of displacement can be measured, while more than 600 meters of movement has occurred at the west end of the fault. Underground, this fault zone ranges from just over 10 meters wide to nearly 80 meters. The Dividend Fault divides the Mule Mountains along their major axis from the mouth of Mule Gulch to the beginning of Tombstone Canyon were it appears to terminate against the Quarry fault.
Figure 6: Looking west at the Lavender pit with Bisbee in the background – 1974, general geologic features in approximate positions and mine sites indicated, P. Kresan photo.
The Quarry fault is the western limit for mineralization as well as the western-most fault in a series of north-northeast trending fault zones that are more or less perpendicular to the Dividend fault zone. From west to east, the more important of these structures are the Quarry, Escacado, Czar, Shattuck, Silver Bear, Mexican Canyon and Campbell. Ore occurrence was intimately associated with these structures (Bryant & Metz, 1966) as is so obvious in figure 5. Generally, these faults dip steeply to the west with a variable displacement from several tens of meters to more than 150 meters, as in the case of the Mexican Canyon fault.
Some four kilometers to the south and sub-parallel to the Dividend zone, is the Escabrosa fault zone. It is here that most of the north-northeast faults end and no ore has been identified south of this feature. A few sinuous structures with a generally northwest trend and largely parallel to the Dividend/Escabrosa faults lie between these structures.
From a mineral deposit perspective, it all began with a multi-stage intrusion and accompanying mineralization 180 million years ago (Anthony, et al. 1995). As noted above, an initial phase of silica/iron/sulfur mineralization followed the emplacement of the first of the several intrusive units that became the Sacramento Stock complex. The old porphyry unit was silicified as were large areas within the hosting sediments and meta-sediments and immense amounts of pyrite were deposited, both as huge replacement deposits in the sediments and the basement unit, as well as large amounts of disseminated pyrite in the intrusive unit. Bryant and Metz (1966) estimated that in excess of 500 million tons of pyrite were emplaced during this phase. A good deal of magnetite and hematite was deposited at this time as well, but largely in the sediments.
Subsequently, there was an iron/copper mineralizing event, which followed the dikes and other structures, creating the many copper replacement deposits in the limestones as well as depositing significant copper in the later intrusive unit of the Sacramento Stock complex. These were to become the ores mined for so many years at Bisbee. It is possible that this phase was coincidental with the emplacement of the intrusive breccias (Bryant, 1974), but this is not certain.
The third mineralizing episode was a lead/zinc phase, bringing with it large replacement-type deposits in the limestone. In many areas, these lead and/or zinc ores were emplaced adjacent to some of the preexisting pyrite/copper sulfide replacement deposits. Though minor in importance when compared to the copper deposits, the lead/zinc ores played an important role in the overall economic success of the mines at Bisbee. The lead/zinc minerals added an interesting dimension to the mineral suites found here.
Lastly, a multi-element phase was imprinted over many of the iron/copper-lead/zinc sulfide deposits (Graeme, 1993). While the total volume of mineralization associated with this apparent final phase was very small, it was relatively rich in precious metal and therefore economically significant. Anomalous amounts of tin, bismuth, vanadium, tungsten, arsenic, antimony and tellurium were also present in these mineralizing fluids, making the products of this episode mineralogically quite diverse.
The many sulfide replacement deposited in the limestones were invariably pyrite dominate, though other species were always present. Copper and/or lead/zinc sulfide minerals were typically present in much lesser, but economically important amounts. Most of the economic deposits were copper/iron/sulfur combinations with only modest amounts of lead or zinc. These latter two elements did indeed occur as the principal economic metals in a great number of replacement deposits, though typically with only modest copper present and often somewhat distant from the high copper deposits.
The limestone replacement deposits that resulted from the mineralizing events are generally arranged around the Sacramento Stock complex in semicircular fashion with offshoots radiating outward like the spokes of a wheel. This arrangement is the result of orebody concentrations in fractured and fault zones (Bryant & Metz, 1966). This cresentlike arrangement of the ores is a reflection of the shattered zone in the limestones, which surround the southern portion of the stock complex while the “spokes” reflect the fault zones and fractured areas (Bonilla, et al. 1916), (Bryant & Metz, 1966). This arrangement is clearly visible in figure 5.
The economic copper orebodies of 3.5% Cu or greater, were almost always associated with large, low-grade bodies of siliceous pyrite and were commonly peripheral to the siliceous pyrite. These generally occurred along the footwall or keel of the pyrite deposits and as scattered orebodies within the mass as well (Bryant & Metz, 1966). For the most part, the economic portion of the replacement deposits was small, averaging about 25,000 tons of mineable ore (Bryant & Metz, 1966) even though the total sulfide mass would be several, if not many, times larger. There were some exceptions however, with a very few copper orebodies exceeding one million tons of minable ore (Bryant & Metz, 1966).
The several intrusive units in the Sacramento Stock Complex were mineralized differently. The older unit was heavily pyritized, silicified, and effectively rendered impermeable to the subsequent copper bearing fluids. Thus, it contained few hypogene ore minerals except for a small, highly fractured area in the southwestern part of the Lavender Pit where a few high-grade spots were mined (Bryant & Metz, 1966). Much of Sacramento Hill was composed of this older porphyry unit and remains as a largely unmined knob along the northern side of the Lavender Pit as shown in Figure 7.
Figure 7: Remnants of Sacramento Hill in 1975 showing high sulfide mineralization of older porphyry and overlying gossen, note the trace of the Dividend Fault zone (L).
In the younger porphyry unit, ore grade mineralization was erratic and consisted of minor amounts of chalcopyrite and bornite associated with abundant pyrite in a disseminated fashion, very typical of copper porphyries. The intrusive breccias associated with this unit were mineralized in a more or less disseminated manner with chalcopyrite and bornite and tiny amounts of sphalerite and galena present. Bryant, (1974) has suggested that this breccia may well have been sulfide bearing at the time of intrusion.
Figure 8: Pyrite and chalcopyrite coated by chalcocite in quartz, Lavender pit mine - 8 cm.
In many of the limestone areas surrounding the older porphyry unit, the peripheral contact, or intrusion breccias were locally, heavily mineralized to the point that they were frequently the source of very high-grade ores. Both the earlier underground mining and the Lavender Pit exploited the high-grade zones in the intrusion breccias.
In addition to the copper, lead, zinc and precious metal mineralization noted above, the limestones at Bisbee also host a number of small, isolated manganese deposits. Without exception, these are near surface, totally oxidized deposits containing manganese as one of several oxides and/or silicates. While mineralogically interesting, these deposits were of sporadic economic importance for a very short period of time from 1914 to 1919.
The time and nature of the emplacement of these deposits is uncertain, but they were probably closely related in time to one of the last phase of mineralization and emplaced as limestone replacement deposits. Factors suggesting a late deposition include the low copper content (Ransome, 1920); the presence of vanadium (Taber and Schaller, 1930) and arsenic (Palache and Shannon, 1920), (Williams, 1970) as well as their common though not absolute association with silica breccias.
All of these deposits occur partially or completely in limestone in a manner that is highly suggestive of limestone replacement. However, there are no known unoxidized equivalents in the district, thus it is not possible to say with all certainty that these too are replacement type deposits. Taber and Schaller (1930) suggested these were replacement in nature and Bonillas, et al. (1916) were of the same opinion. Bonillas, et al. (1916) also noted that the manganese oxides in these deposits are markedly different from those so commonly associated with the oxidized copper replacement deposits throughout the district.
Figure 9: Near-surface manganese stope with the ore zone indicated, Twilight Claim, Higgins Mine – 1967.
Little can be reconstructed of the Precambrian other than to note that at some point after the regional metamorphism was completed, the resultant schist was intruded by several basic dikes and eventually peneplained. During mid-Cambrian times, deposition began during which nearly 400 meters of sediments were laid down, rapidly at first, then at a much slower rate.
There was a hiatus in deposition from late Cambrian until upper Devonian times, or at least no evidence remains if any occurred. The record resumes with the Devonian seas deepening and the deposition of shaly to dolomitic limestone. During Mississippian times, the seas were much shallower as evidenced by the numerous reef formations present in the Escabrosa limestone deposited during this period. The deposition of limestone lasted through Pennsylvanian and into Permian times when uplift occurred. The Paleozoic sediments had, by then, reached a total thickness of perhaps more than 2,000 meters and were undergoing erosion. At some time during the late Triassic or early Jurassic times, while still relatively flat lying, the sediments underwent extensive faulting, and activity along the previously existing Dividend zone occurred once again.
A large granitic mass, the Juniper Flat granite was emplaced into the center of what would become the Mule Mountains. With this intrusion came numerous dikes that penetrated the Precambrian rocks and the Paleozoic section for kilometers from the intrusion center. Little to no alteration followed this intrusion, thus the rocks remain much as emplaced.
Some seven kilometers away, a second intrusion that was probably comagmatic in origin (Bryant & Metz, 1966) formed the earlier phase of what is known as the Sacramento Stock complex. How quickly this intrusion followed the emplacement of the Juniper Flat granite is unknown, but about 180 million years ago, a quartz porphyry followed the course of the Dividend fault cutting up through the schist and into the overlying sediments. Extensive peripheral breccias were formed along the contact with the limestone wall rocks.
Next came a period of intense silicification of the existing parts of the Sacramento stock complex and to a lesser degree, the hosting sediments. Silica rich solutions followed numerous structures for kilometers into the Paleozoic rocks resulting in local, but intense silicification of the sediments. Silicification was quickly followed by heavy pyritization in the porphyry, schist and the silicified portions of the sediments. Huge masses of pyrite were deposited in the schist and numerous, often very large replacement bodies of pyrite were scattered throughout the limestones.
Following the same path along the Dividend fault and close in time came another intrusive, pushing its way alongside the first and spreading throughout the sediments for great distances as dikes. Soon thereafter, intrusive breccia dikes and sills also invaded the sediments while an irregular pipelike mass of breccia more 150 meters in diameter pushed its way into the stock. It is probable that this was contemporaneous with copper mineralization (Bryant, 1974). The silica breccias most probably formed at this time as well.
As a result of the intrusive and breccia complex more than 1,500 meters across in their midst, the adjacent sediments within a short distance of the complex were irregularly metamorphosed. Replacement by garnet, diopside, wollastonite and vesuvianite near the stock complex quickly gave way to a tremolite, actinolite and edenite assemblage indicating only minor effects of high temperatures immediately adjacent to the stock complex. This assemblage, in turn, soon graded into recrystallized limestone followed by unaltered rock. In all, a contact metamorphic halo of little more than 500 meters developed around the stock complex.
Metamorphic effects are also noted in the limestone along many, but not all of the porphyry dikes. Generally, quartz is the most abundant alteration mineral followed by epidote and garnet. No truly definitive pattern is obvious in these instances because of the overlapping nature of the aureoles as well as the very irregular and erratic development of alteration minerals.
Before the Cretaceous period, regional uplift occurred, and the sediments apparently remained reasonably flat. Erosion stripped an undetermined thickness of the Paleozoic sediments during this time, cutting deep canyons in the limestones. Oxidation occurred to variable depths in the limestone replacement deposits with the permeability of the controlling structures an important factor. A typical, supergene chalcocite blanket formed in the younger porphyry unit of the Sacramento Stock complex that was more or less parallel to the pre-Cretaceous topography.
During early Cretaceous times, rejuvenation of the Dividend fault dropped the southern block 500 to 700 meters relative to the north side (Bryant & Metz, 1966). To the south of the Dividend fault, the deep canyons were filled and the severe topography covered by angular material derived from the northern side as it was leveled by erosion, forming the Glance Conglomerate. Subsequently, the area was covered by shallow seas and the clastic Morita formation was deposited, followed by the rest of the sediments that make up the Bisbee group.
With the close of the Mesozoic Era, the whole of southern Arizona was subjected to the intense compression with thrust faulting of the Laramide orogeny. The Bisbee district acted as a single block and was singularly unaffected by this series of events (Bryant & Metz, 1966). Minor movement along the Dividend fault at this time displaced the Cretaceous sediments.
With Pliocene times came the region-wide normal faulting of the basin and range type, which formed the major topographic features so evident today. During the development of the basin-and-range features, the entire Mule Mountains were tilted to the northeast some 30o followed by uplift. Uninterrupted erosion since that event has stripped the Cretaceous sediments from the western portion of the mineral deposits and allowed supergene alteration to resume in this area. .