Why we map, #1: Artillery Mountains, Western Arizona

Geologic mapping is one of the primary functions of the AZGS. For the past 15 years, we have been aggressive participants in the USGS-run National Cooperative Geologic Mapping Program, especially in the Statemap component that matches state and federal funds. We just got notice of our funding award for FY12 so I thought it would be valuable to explain why we will be mapping the areas selected. This is part 1 of a 3-part series on next years plans, drawn from Jon Spencer's and Phil Pearthree's successful proposal to USGS. Our priority areas were recommended by the Geologic Mapping Advisory Committee (GMAC). The following is taken from the AZGS proposal written by Jon and Phil:

The Arizona GMAC recommended that the Arizona Geological Survey map the Artillery Mountains area because of its large, low-grade manganese and uranium deposits. These deposits are incompletely mapped and studied, and significant questions surround their genesis. New mapping is intended to clarify models for deposit genesis and estimates of deposit extent.

The Artillery Mountains are located in the low-elevation, dry desert region of western Arizona, which is very sparsely populated except along the Colorado River. The Artillery Peak 7.5' Quadrangle includes Lake Alamo, a water-storage reservoir and popular fishing area at the confluence of the Big Sandy and Santa Maria rivers. The surrounding area contains abundant manganese, uranium, and Cu-Pb-Zn-Ag-Au deposits.

The proposed map area includes part of the Harcuvar metamorphic core complex, which is the largest known terrestrial core complex on Earth (a larger core complex is present beneath the Philippine Sea). The Buckskin and Rawhide Mountains, located to the west of and within the map area, consist largely of Paleoproterozoic to Miocene granitic and gneissic rocks with a strong, middle Tertiary, mylonitic fabric. These rocks form the footwall to the Buckskin-Rawhide detachment fault, a subhorizontal to gently dipping normal fault that was active in Oligo-Miocene time and accommodated many tens of kilometers of top-ESE displacement. The three large antiforms that make up most of the Buckskin and Rawhide Mountains are enormous grooves in the detachment fault. Rocks above the fault include 27-12 Ma, syntectonic strata that were generally tilted to the southwest during faulting, with dips decreasing up-section. The proposed map area also includes the western end of Date Creek basin, a large, deep, dissected, roughly east-west-trending sedimentary basin that developed in conjunction with Oligo-Miocene detachment faulting. The proposed map area consists primarily of bedrock, with lesser amounts of dissected basin deposits and Quaternary surficial deposits.

Geologic background - Harcuvar core complex. The Harcuvar metamorphic core complex is associated with Earth’s greatest known concentration of mineral deposits related to detachment faults (Spencer and Welty, 1986, 1989). Core complexes consist of middle crustal rocks that were exhumed by large displacement below moderately to gently dipping normal faults. Exhumation and uplift of large core complexes was sufficiently rapid that, in some cases, the core complexes formed large thermal anomalies during and immediately after uplift (e.g., Scott et al., 1998). Detachment faults were conduits for ascent of hydrothermal fluids, with fault permeability maintained by repeated faulting and fault-zone crushing of hydrothermal minerals. Syntectonic ascent of hot (225-325°C) basin brines along detachment faults produced abundant specular hematite and common sulfide minerals along and adjacent to the Buckskin-Rawhide detachment fault and related faults (Wilkins and Heidrick, 1982; Wilkins et al., 1986). A few deposits related to detachment faults include substantial silica and economic concentrations of gold (Spencer et al., 1988). [right, five types of Oligo-Miocene mineralization and alteration phenomena that occurred in association with extensional detachment faulting and basin genesis in the Buckskin-Rawhide-Harcuvar Mountains area. Widespread potassium metasomatism, especially of mafic lava flows within basin sediments, is suspected to have been a source of base and precious metals that were deposited along detachment faults during extensional faulting (Hollocher et al., 1994). Chloritic breccias, characteristic of detachment-fault footwalls, are probably not geochemically related to the others, as indicated by oxygen isotope data (Smith et al., 1991).]

Geologic background - Artillery manganese deposits. The Artillery mining district is the most historically productive manganese district within the western Arizona manganese province (Spencer, 1991). Total historic manganese production in the province is about one hundred thousand metric tons, with 40% derived from the Artillery mining district. All of the deposits are Miocene in age, and most are vein deposits. The Artillery manganese district contains primarily stratabound manganese (Lasky and Webber, 1949), as do a few of the other districts in the province.

Tilted Oligo-Miocene strata of the Artillery Mountains area were deposited in a syntectonic basin directly above the gently northeast-dipping Buckskin-Rawhide detachment fault (Spencer et al., 1989). The large, low-grade, stratabound Artillery Mountains manganese deposits were deposited at Earth’s surface as very fine clastic material in the upper part of the tilted clastic sequence (Lasky and Webber, 1949). The geochemically similar elements iron and manganese are present in crustal rocks at ratios of roughly 50:1. Many manganese deposits are thought to have developed under conditions where iron- and manganese-bearing aqueous solutions gradually lost iron but retained manganese until manganese was deposited from solutions that had lost all of their iron. Under a significant range of ambient conditions, acidic solutions that react with carbonate to increase solution pH will precipitate iron and retain manganese (Krauskopf, 1957). Detachment-fault-related deposits typically contain abundant hematite or specular hematite (Fe2O3). Spencer and Welty (1989) suggested that mineralizing hydrothermal fluids that yielded iron, copper, and associated metals along detachment faults retained manganese until reaching Earth’s surface and deposited manganese in shallow veins or as clastic material.

Derivation of the Artillery stratabound manganese deposits from fluids that formerly yielded voluminous iron along detachment faults is appealing for a number of reasons, but has been difficult to confirm. Fluid-inclusion studies indicate that detachment-fault related deposits were derived from hydrothermal fluids with 12 to 24 wt. % NaCl equivalent (Wilkins and Heidrick, 1982; Wilkins et al., 1986) whereas four minor manganiferous vein deposits with associated calcite, barite, and quartz in the Artillery Mountains area were derived from fluids with 0-3 wt. % NaCl equivalent (Spencer et al., 1989). These four vein deposits are younger than the stratabound manganese in the Artillery Mountains and their genesis may have involved different processes. It is clear, however, that the relationship between manganese mineralization and detachment-fault mineralization is not well established, even though these deposits are the same approximate age and are near each other (or were before significant fault displacement).

Geologic background - Potassium metasomatism. We also propose geochemical analysis of common brick-red sandstones and associated altered basalts that might have been greatly modified by potassium metasomatism. The brick red color of sandstone that hosts the Artillery stratabound manganese deposits is typical of K-metasomatism, which can convert sandstone to an assemblage of quartz, adularia, and hematite (Chapin and Lindley, 1986; Roddy et al., 1988). This process is known to liberate manganese, copper, and zinc, probably during low-temperature diagenesis by alkaline, saline fluids (Roddy et al., 1988; Hollocher et al., 1994). Circulation of these metal-bearing brines during detachment faulting and core-complex exhumation is a possible source of both detachment-fault related mineral deposits and manganese deposits (Spencer and Welty, 1986, 1989).

Geologic background - Uranium deposits. Whereas manganese deposits in the Artillery Mountains are hosted by clastic sedimentary rocks near the top of the tilted Oligo-Miocene sedimentary sequence, uranium deposits are associated with lacustrine strata near the base of the sequence. Basal arkosic sandstone and minor conglomerate grade upward into finer grain sandstone, siltstone, and mudstone, which in turn are overlain by lacustrine mudstone and limestone with locally interbedded rock-avalanche breccia (Yarnold, 1994). In the Anderson Mine area, located at the northeastern upturned margin of Date Creek basin, lacustrine strata include carbonaceous mudstone with plant material, lignitic coal seams, and limestone and marl that contain freshwater gastropods, ostracods, and pelecypods (Otton et al., 1990). The lacustrine strata are overlain by sandstone, conglomerate, rock avalanche breccia, tuff, and basalt.

Calcareous and carbonaceous strata at the northern margin of Date Creek basin, which includes the Artillery Mountains area, commonly contain elevated uranium concentrations, typically within <3m thick zones in carbonaceous mudstone and associated marl, limestone, and tuffaceous mudstone. In areas of greatest uranium mineralization near Anderson Mine, uranium-bearing strata are up to 15 m thick, with unoxidized uranium in concentrations of 0.3-0.8% U3O8. Where oxidized in near-surface environments, uranium concentrations are up to 30% U3O8 (Sherborne et al., 1979; Mueller and Halbach, 1983). Uranium mineralization has been attributed to (1) alteration of uraniferous tuff beds (Sherborne et al., 1979), (2) derivation of uranium from surrounding bedrock and transport by groundwater to reducing environments in carbonaceous lacustrine strata (Mueller and Halbach, 1983), or (3) direct precipitation of uranium from anoxic lake-bottom water (Otton, 1981).

Justification for proposed mapping. The proposed map area (Rawhide Wash and Artillery Peak 7.5' Quadrangles) is centered on the Artillery manganese district, which contains the largest known manganese deposit in the United States. The only previous mapping study specifically directed at the manganese deposits was done by the U.S. Geological Survey during World War II as part of an assessment of strategic and critical minerals (Lasky and Webber, 1949). Manganese is necessary for steel production, and has a variety of other uses. We propose to map the Artillery Mountains primarily because of the potential economic significance of the manganese deposits. We especially hope that new mapping will identify manganiferous veins around the bedded deposits, and that we will be able to clarify the nature of mineralizing fluids by studying the veins. Ideally we will identify fluid conduits for suspected spring systems that delivered manganiferous hydrothermal fluids to Earth’s surface and produced the sedimentary manganese deposits. Better understanding of flow pathways for mineralizing solutions will potentially lead to improved mineral exploration strategies in this area.

Furthermore, we intend to map the eastern Rawhide Mountains to clarify the structural and lithologic setting of Fe-Cu-Pb-Zn-Ag-Au deposits. While this deposit type is moderately well understood (Spencer and Welty, 1989), these specific deposits in the Rawhide Mountains are not well mapped or studied. Some aspects, such as the presence of peripheral manganese mineralization that might reveal aqueous-solution pathways following iron deposition, will be carefully evaluated.

We propose to map facies in lacustrine and related strata to determine the setting of uranium mineralization. This will include data collection for radiation derived from uranium daughters, using a portable gamma-ray spectrometer, to identify uraniferous units and their distribution. Previous studies of the Anderson Mine area were not extended westward to the Artillery Mountains, even though many claims for uranium have been staked in the area and it is well known that the lacustrine strata are mildly to moderately uraniferous.

Finally, we will map exposed basin-fill deposits, surficial deposits, and river deposits. The sedimentology, clast composition, and ages of these various deposits may shed light on the timing and character of the development of the Bill Williams - Big Sandy - Santa Maria River system, which drains much of west-central Arizona and is a tributary to the Colorado River. In addition, we may find deposits along the river that were emplaced by very large early historical floods that have been reported for this river system (Pope et al, 1998).