American Girl project, Obregon, Cargo Muchacho Mining District (Hedges Mining District; Ogilby Mining District), Cargo Muchacho Mountains, Imperial County, California, USAi

Regional Level Types
American Girl project	Project
Obregon	- not defined -
Cargo Muchacho Mining District (Hedges Mining District; Ogilby Mining District)	Mining District
Cargo Muchacho Mountains	Mountain Range
Imperial County	County
California	State
USA	Country

A large, open cast Au-Ag-Cu-Pb-Zn-Hg-Ba(baryte)-Sb occurrence/mine project located in secs. 1, 11 & 12, T15S, R20E and in secs. 6, 7, 16-20 & 29-30, T15S, R21E, SBM, on private (patented) land. Owned & operated by the American Girl Mining Joint Venture, 60 East South Temple, Suite 2100 Salt Lake City, Utah 84111. First production was in 1987. Ore processing was by the leach method (Hydromet). Milling method was heap leach-carbon column-Merrill-Crowe. MK Gold Company suspended mining operations at the American Girl Joint Venture gold mine on November 4, 1995, resulting in the lay off of 115 employees.

History and Status: The American Girl project involves three separate but adjacent mines operated by the American Girl Mining Joint Venture (AGMJV) during the 1980s and 1990's in the Cargo Muchacho Mountains of southeastern Imperial County. The individual mines include the American Girl, Padre y Madre, and Oro Cruz mines, all of which experienced significant historical mining during the 1800's. Modern operations involved a sequential phased approach to mining these properties, starting with the Padre y Madre and ending with the Oro Cruz operation. Due to topographic separation and different mining and development schedules, the mines were permitted and operated independently. Operations at all three mines were completed in late 1996, reclaimed in 1999, and are currently undergoing post-reclamation monitoring. The mines are within the historic Cargo Muchacho mining district, the oldest mining district in California, which includes those areas formerly known as the Ogilby, and Tumco/Hedges mining districts (Clark, 1970). The American Girl Mine was the largest and consisted of four interconnected open pits and four underground mining operations involving multiple ore bodies. Ore processing, milling, and heap leach facilities were maintained in American Girl Valley, with most of the ore from the Padre y Madre and all the ore from the Oro Cruz operations being transported to American Girl Valley. The Padre y Madre mine, less than 1 mile southeast of American Girl Valley, developed low-grade ores from two surface pits. Leaching of the Padre y Madre ores was conducted at both the Padre y Madre and American Girl mines. Like the American Girl operation, several ore bodies were mined in open pits and underground workings at the Oro Cruz operation (about 2 miles northwest). Generally, low grade oxidized ores were mined in the open pits and cyanide heap leached, whereas higher-grade ores requiring milling were mined in the underground workings.

The American Girl Project operations involved 809 acres on the western flank of the Cargo Muchacho Mountains 40 miles east-northeast of El Centro, California and 12 miles northwest of Yuma, Arizona. The American Girl and Padre y Madre permit areas consisted of 347 acres and 239 acres in American Girl Valley and Madre Valley respectively. Lands included patented and unpatented lode and placer mining claims, the later administered by the BLM. The Oro Cruz operation covered 191 acres of unpatented load and placer claims in Tumco Wash to the north. The project area included portions of unsurveyed sections 6, 7, 16-20, 29, 30, T15S-R21E and sections 1, 11, 12 , T15S-R20E, SBBM. The location point selected for latitude and longitude corresponds to the historic American Girl Mine symbol in the northwest quarter of unsurveyed section 17-T15S-R21E on the USGS Ogilby 7.5 minute quadrangle. The mines are reached by taking Interstate Highway 8 to the Ogilby exit, going 3.5 miles north to Ogilby on State Route 134, then traveling three miles northeast along American Girl Road.

The American Girl Project deposits consists of low grade (0.04-0.05? opt) oxidized surface ore bodies and higher grade (0.2-0.5?opt) underground ores which were mined via a combination of surface pits and underground mining methods. Generally, ore bodies were deposited in two main stages followed by a period of brittle deformation and gold remobilization during which earlier deposits were enriched. Most mineralization occurred within the American Girl Shear Zone (AGSZ), an east-west trending, southerly dipping, zone of Mesozoic ductile shear which bisects the Cargo Muchacho Mountains. The AGSZ separates an upper plate of diorite from a lower plate comprised of granite gneiss and Tumco Formation quartzofeldspathic gneiss and which contains the major ore bodies. During initial Jurassic mineralization, highly oxidizing, sulfide poor and iron rich ore fluids invaded the shear zone resulting in metasomatic auriferous quartz-magnetite-biotite veins which are best developed in Tumco Wash and the Oro Cruz Mine. Metasomatism also produced distinctive zonation from the mineralized suites to laterally associated unmineralized aluminosilicate assemblages. The largest deposits occur as stacked sub-parallel ore bodies in the American Girl and Padre y Madre mines where they are associated the southerly dipping American Girl, B Zone, and C Zone faults within the shear zone. These ore bodies were largely deposited during a second stage of mineralization during which retrograde greenschist metamorphism, accompanied by less oxidized sulfur bearing hydrothermal solutions, formed auriferous chlorite-quartz-sulfide ore bodies within the fault zones and hydrolytically altered adjacent wall rocks. Tertiary brittle deformation and reactivation of shears was accompanied by remobilization of gold and the enrichment of existing ores. Rarely did this stage form economic or in itself.

Environment: The mines are located in a barren region of the Colorado Desert portion of the Basin and Range physiographic province which is characterized by low mountain ranges, canyon washes, and alluvial fans. The landscape is dominated by the low relief Cargo Muchacho Mountains which rise from an otherwise featureless plain. The American Girl Project mines occur in adjacent valleys in a belt approximately 5 miles long and 2 miles wide along the west flank of the range. The bare rocky slopes, punctuated with southwesterly trending washes give way to an alluvium covered Piedmont (Pilot Knob Mesa) which slopes towards the Salton Trough and the Imperial Valley. Elevations range from approximately 500 feet at the foot of the range to 2,130 foot Stud Mountain, the highest point in the range. The American Girl Mine is centrally located in American Girl Valley, through which the ephemeral American Girl Wash drained to the southwest. The Padre y Madre Mine is about 1 mile to the south in Madre Valley. The two mines are separated by a low relief ridge (250-300'). The Oro Cruz Mine is 2 miles northwest of American Girl Valley in Tumco Wash and is separated from it by a ridge of considerable relief. Elevations within the project area range between 500 and 1,000 feet. The climate is arid low desert. Annual precipitation is approximately 2.4 inches per year which falls between late fall and spring. Winters are mild and summers hot. Average summer high temperatures generally exceed 100? between May and October, but can reach as high as 120?. Daily temperature fluctuations can be extreme. Vegetation is sparse and consists mostly of creosote bush, cholla cacti, and mesquite. A wide variety of small annuals appear when moisture is available in the spring months. Numerous historic gold mines and prospects exist throughout this uninhabited area of the Cargo Muchacho Mountains, some of which were significant historic gold producers. The area is marked by considerable surface disturbances, including derelict buildings, pits, and open shafts. The ghost town of Ogilby is 3 miles to the southwest. The nearest town of any importance is Yuma, Arizona (pop. 81,000) approximately 12 miles to the southeast.

Geology and Ore Bodies: Both the American Girl and Padre-Madre zones occur in shear zones. The American Girl Mine produced from several discreet zones within three stacked, sub-horizontal low angle fault zones within the AGSZ. The ore bodies occurred as shallowly dipping tabular to lenticular bodies sub-parallel to foliation. In the Padre y Madre Mine deposit, stacking was also present, but the ore bodies were thinner and lower grade. In the Padre Madre West Pit, the primary ore body is in a shallow low angle fault in Tumco gneiss cut by high angle normal faulting exhibiting 100 feet of displacement. The ore bodies are generally localized along fault zones between the two gneiss units or along fault planes entirely within the Tumco quartzofeldspathic gneiss. Two distinct styles of mineralization predominated including mesothermal sheared ribboned milky quartz veins that parallel foliation in granite gneiss and disseminated quartz stockworks in the Tumco Formation gneiss. The veins are surrounded by the lower greenschist facies minerals of chlorite, epidote, and quartz. The uppermost American Girl ore body was associated with the American Girl Fault zone which separates the overlaying granite gneiss sheet from the Tumco rocks. The ore body consisted of low-grade oxidized ore grading an average 0.051 opt and was mined via open pit. The American Girl B-Zone ore body dipped 20? to the south along the B-Zone fault entirely within Tumco Formation gneiss. The ore varied in thickness from 5-25 feet and was more sulfidic and higher grade (averaging 0.3 opt) than the overlying American Girl zone. The lowermost American Girl C-Zone parallels the C-Zone fault at the contact between the hanging wall Tumco Formation and lower granite gneiss sheet footwall. Much of the C-Zone ore was sub-economic. Generally along major shears where the Tumco Formation formed the hanging wall as at American Girl and Padre y Madre, gold generally was found within quartz veinlets and as disseminations.

Mineralized bodies had dimensions as much as 70 feet thick. Where granite formed the hanging wall, as in the B-Zone, gold frequently accompanied a massive quartz vein with grades that exceed 0.5 opt. West of the American Girl and Padre y Madre ore bodies, the granite gneiss and Tumco rocks grade into the aluminosilicate-kyanite dominated rocks produced by metasomatic leaching event (Branham, 1988). Three principal normal fault sets are present. From oldest to youngest, they are: northeast (N45-60E), north-south (N10-20E), and north-west (N40-70W). relative movement on these has been down to the eastern or northern sides, however, due to the overall southerly dip of the structural fabric, the northern portion of the Cargo Muchacho range is structurally deeper than the southern portion. For these reasons, the Padre y Madre zone and the American Girl Zone could be equivalent shears (Guthrie, et al, 1987). Younger brittle deformation reactivated most parts of the shear zones as shallow south dipping brittle faults (Branham, 1988; Borrastero, 1990). . These faults were localized along shallow dipping intrusive contacts or other planes of structural weakness.

Oro Cruz Ore bodies: The Oro Cruz Mine is entirely within a section of Tumco Formation gneiss interfoliated with 10-30 foot thick biotite schist horizons. Localized conglomeratic lenses in the Oro Cruz area indicate a sedimentary origin. Generally, the larger ore bodies were 50-60 feet thick rod-like lenses of highly deformed quartz veinlets high in iron oxides (predominantly magnetite with minor specularite) that extended over 2,000 feet downdip at shallow angles. Veins were flattened, boudinaged, folded, recrystallized, and demonstrably older than syn-kinematic pegmatites which intruded across the ore bodies suggesting multiple phases of intrusion. Trace amounts of scheelite, chalcopyrite, galena, pyrite, stibnite, cinnabar, fluorite, and barite were present. Elevated levels of copper are also associated with the gold. Biotite was visibly enriched, and epidote was common in the ore bodies. Oxidation of the ore bodies extended to a depth of about 150 feet (Tosdal, 1999, in press). In the Cross Pit area, the gold mineralization occurred in three massive quartz-magnetite lenses are localized along stacked, sub-parallel zones dipping 20?-35? south and displaying a total thickness of 150-200 feet. Deposits were typically narrow (up to 200 feet wide) and lenticular with a marked down-dip elongation that contained irregular high-grade (0.25 opt) massive magnetite lenses from 5-30 feet wide and up to several feet thick. A halo of lower grade mineralization occurred in the surrounding wall rock. The deposits pinched and swelled, and typically only one or two of the horizons was present in any particular location. Several near vertical faults striking N15?W cut and offset the ore bodies. These structures are thought to have a complex history and may have acted as original conduits for the hydrothermal mineralizing fluids. The highest grades and thickest ores occur in proximity to the north-northwest trending high angle dextral faults. Individual mineralized zones were generally 20-60 feet but often merged near these faults into zones of higher-grade ore over 100 feet thick. The Cross Pit ore zone extended 500-600 feet along strike and over 1,400 feet down dip. The ore zone continued down dip from the pit floor where higher-grade portions were mined underground. Portions of these ore bodies were exploited in the historic Golden Queen Mine where the ores were mined to a depth of 1200 feet on the incline and in the glory hole. Alteration accompanying the mineralization is weak and consists of chloritization and weak clay alteration within the ore zone and occasional surrounding zones of epidotization surrounding the ore zone. Although it represents a separate and distinct ore body, the Queen Pit ore body was geologically very similar to the Cross Pit ore body. Mineralization occurred along stacked south-dipping lenticular to rod-like quartz-magnetite masses extended 300 feet along strike and 600 feet downdip. The deeper, higher-grade portions of the Queen zone ores were extensively mined in the past. In the historic Golden Queen Mine, the ore body, grading upwards of 0.5 opt, was mined to 700 feet before being cut off by a high angle normal fault. Only the shallow lower-grade ores were mined in the modern Queen Pit. The Queen and Cross ore bodies were separated by a high angle fault and considered to be dismembered pieces of the same original deposit. The reconstructed ore body is estimated to have been 200-500 feet long and at least 2,100 feet in dip length (Frost et all, 1986).Local rocks include Mesozoic granitic rocks, unit 3 (Sierra Nevada, Death Valley area, Northern Mojave Desert and Transverse Ranges).

Local geologic structures include the American Girl shear zone. Regional features include the San Andreas fault and the Chocolate Mountain thrust fault.

The test phase of underground development at the American Girl Mine was initiated in April 1987 with the sinking a 2,500 foot incline into the B-Zone orebody and 6,000 feet of drifts to recover a 50, 000 ton bulk ore sample for metallurgical testing. Due to the weakness of the ore body rocks, the southwesterly decline was driven parallel to and about 50 feet below the orebody in more competent rock. Since vehicles had could not operate on the 20? dip of the orebody, the decline had to spiral back and forth under the orebody at an average angle of about 9.5?. A straight southwesterly ventilation shaft was also driven. It was originally proposed to remove 534,000 tons of ore from this orebody. Mining was conducted with rubber tired excavators using a "drift and fill" method in which drifts were driven, then backfilled until the entire orebody had been replaced with cemented backfill. Due to the inherent weakness of the orebody, it was feared open drifts would collapse if left alone. The ore body was developed into "stoping zones" with independent raises off the main decline. Five stoping zones were required to fully extract the ore. The access raises off the main decline were centrally positioned in each zone. In each stoping zone, an initial horizontal tunnel was driven down dip along the footwall contact of the ore body in both directions to the orebody's limits. This tunnel was not backfilled until stoping had been completed. Equally spaced cross drifts were driven from the footwall tunnel to the hanging wall contact of the orebody, keeping the hanging wall contact in the roof of the cross drifts. The roof was supported by roof bolting and chain link mesh where required. Four cross drifts were initially mined in each zone, each drift being mined almost to the boundary of the next stoping block. Drifts were mined to a height of 7 feet and 8 feet wide using a one cubic yard load-haul-dump vehicle. Where the ore was thicker than the tunnel, the floor will be drilled and blasted down to the footwall contact. On completion of a drift and extraction of the full depth of ore, the drift was backfilled back to the footwall tunnel. Backfill consisted of waste rock and cement in appropriate proportions to attain a compressive strength sufficient to support the overlying rock load. Once the backfill had set, a new parallel cross drift was driven next to the backfilled drift. The cycles of ore extraction and backfilling was then be repeated until the full width and depth of the ore within each stoping zone was extracted. Ultimately, only the last drift in a series of parallel drifts remained open with the opening being surrounded on both sides by two more backfilled drifts. The Southwest Extension ore body was developed after the B Zone had been exhausted. Development was conducted through the B Zone underground workings with the B Zone decline being extended further southwest to reach the ore. The ore was mined using the same "drift and fill" method used in B Zone. Original projected reserves for the Southwest Extension ore body was 295,300 tons of average grade of 0.182 opt. The American Boy ore body was developed after completion of the Southwest Extension and concurrently with development of the C Zone. Using the same surface facilities as B Zone, the ore body was reached by an easterly decline driven from a point 350 feet down the B Zone decline. The historic American Boy shaft was refurbished as a ventilation shaft. Since the C Zone was considerably west of the other underground workings, it could not be accessed by existing workings. It was developed by its own twin southwesterly trending development and ventilation drifts. Mining was conducted using the same "drift and fill" method with the exception that the development access was driven within the actual ore body rather than below it. Original estimated reserves were 182,000 tons of 0.182 opt ore.

Initial underground operations at Oro Cruz started in 1994 with development and ventilation declines driven from a portal along the main haul road southeast of the Cross Pit. Full scale mining commenced in 1995 and was completed in 1996. The workings exploited deeper portions of the Cross Ore Zone developed in the Cross open pit. Approximately 500,000 tons of ore were initially identified. Mining methods were similar to the drift and fill method. However, mining methods were altered to accommodate differences in the ore bodies. Unlike the American Girl deposits, the Oro Cruz deposits consisted of 3 discontinuous, steeply dipping (20?-40?) parallel structural zones of considerable thickness (20-40 feet). Hence, much of the mining took place in the adjacent country rock outside the ore bodies. Three hundred thousand tons of backfill for the Oro Cruz underground workings were obtained from an abandoned borrow pit west of the Padre y Madre Mine. The last drifts, ventilation decline and main development drift were left open. Processing The heap leaching process involved stacking crushed ore in lifts on a prepared pad, percolating a cyanide solution through the material to dissolve the gold and silver, and recovering the metals from the leachate. The leach pad at the American Girl Mine covered approximately 50 acres and was designed to accommodate 5.8 million tons of ore. It was later expanded by approximately 11 acres and increased in height to accommodate ore from the Oro Cruz Mine. Pads were prepared by grading and compacting the ground surface, then placing a thin finely screened layer of compacted base material over the ground surface. The base grade was sloped toward peripheral PVC lined interceptor channels and leachate collection and storage basins located next to the pad. Impermeable 40 and 60 mil high density polyethylene (HDPE) leach pad liners were placed over the ground surface and overlain with a geotextile fabric for added protection. A network of perforated leachate collection/drainage pipes were placed on the liner and connected to the interceptor channels. The Padre y Madre leach pad site consisted of 37 acres adjacent to the West Pit near the mouth of Madre Valley. It was constructed in 2 phases, the first in 1986-87 and consisting of a pilot 200,000 ton pad. The second stage was designed to accommodate 3.5 million tons of ore stacked to 70 feet. Two ponds, a barren and pregnant pond, were located south of and adjacent to the pad. From late 1987 through 1990, approximately 21 million tons of ore covering 22 acres of the pad were processed before ore processing was shifted to the American Girl Mine. The remaining 15 acres of the pad were never completed.

Crushed ore was end dumped and spread in 10 foot lifts. Successive lifts were smaller in order to shape the pile to resemble a truncated pyramid 50-60 feet high with 2:1 grade slopes. The top of each lift was graded level and berms were constructed to segment each lift into 20 cells to provide for metallurgical accounting and control. A 400ppm sodium cyanide solution, prepared to a pH of 10-11, was pumped from a barren solution pond to the appropriate section and applied by drip irrigation grid. As the solution percolated through the lifts, it became pregnant with dissolved gold and silver. At the bottom of the pile, it was captured by the collection system and drained by gravity to the interceptor ditches and into the pregnant solution pond. Each cell was leached based on optimum leach cycles determined through column leach tests. Between lifts, residual solution continued to dissolve gold which was recovered when the subsequent lift was placed and leached. Leaching of a cell continued until the mineral values were stripped, then the leaching area was moved to another cell. As each leach pile was spent, it was decommissioned by draining and flushing with circulating fresh water for about 60 days or until analytical tests indicated acceptable cyanide levels. Leach pile slopes were then regraded to reduce slope gradients and seeded. Pregnant solution was pumped to the recovery plant and through a series of three activated charcoal columns where the gold was adsorbed onto the carbon. The barren effluent was recycled back to the barren solution pond and make-up chemicals added to maintain the proper pH and cyanide levels. A caustic solution of sodium cyanide and sodium hydroxide was used to strip the adsorbed gold from the carbon. Gold was recovered from the strip solution by electroplating onto stainless steel cathodes and then smelted onsite, poured into dore bars and shipped to an outside refinery for purification and marketing. Ore processing included both cyanide heap leaching of the low grade oxidized ores and milling of higher grade sulfidic ores. Since both types of ore were suitable to recovery involving carbon adsorption, heap leach and milling solutions were combined in a single pregnant solution. The American Girl Project Mines used a milling process to treat the underground ores and selected pockets of high grade open pit ore. The process used two stages involving a coarse crushing plant rated at 100 tons/hour which discharged to a ball mill grinding circuit in which lime and water were added to create a coarse pulp. An overflow ball mill and cyclone unit fed sized product to an aerated agitated leaching circuit where sodium cyanide solution was agitated with the ground ore slurry and air was pumped through the pulp for about 48 hours. This slurry was then thickened and the solids separated from the gold bearing solution using a vacuum filtration dewatering process to separate the pulp or tailings and the gold laden solution was pumped to the pregnant solution pond to be combined with the leach pile pregnant solution. Solids from the separation process are washed to remove additional dissolved gold and to remove sodium cyanide from the filter cake. Mill tailings were temporarily retained in a lined tailings pond, after which they were dewatered and disposed of with waste rock from the open pits and on spent leach pad.

Production data: Production statistics: Year: 1993: Au production: 71,789 Troy ounces/year; Ag production: 36,291 Troy ounces/year. Recovery percentage: Au: 2,035,182 grams; Ag: 1,028,831 grams. Year: 1994: Au production: 64,980 Troy ounces/year; Ag production: 39,002 Troy ounces/year. Recovery percentage: Au: 1,842,150 grams; Ag: 1,105,687 grams. Year: 1995: Au production: 45,722 Troy ounces/year; Ag production: 27,772 Troy ounces/year. Recovery percentage: Au 1,296,195 grams; Ag: 787,322 grams.

Reserves and resources: Type: In-situ (estimate year = 1996): demonstrated reserves: 6,076,000 metric tons of ore; total resources: 6,076,000 metric tons of ore. Commodity: Au: 1.71 grams/metric ton.

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