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American Girl

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

State: California
County: Imperial
Elevation: 492 feet (150 meters)
Latitude: 32.8558
Longitude: -114.78720
Primary: Gold

American Girl MRDS Details

All data originates from the USGS Mineral Resource Data System.

Site Name

Commodity

Location

Primary: "American Girl"
Secondary: "Padre y Madre Mine"
Secondary: "American Boy Mine"
Secondary: "Oro Cruz Mine"
Secondary: "Cargo Muchacho Mine"
Primary: Gold
Secondary: Silver
Tertiary: Copper
Tertiary: Lead
Tertiary: Zinc
Tertiary: Mercury
Tertiary: Barium-Barite
Tertiary: Antimony
State: California
County: Imperial
District: Cargo Muchacho District
 

Land Status

Holdings

Workings

Land Ownership: BLM Administrative Area
Administrative Organization:
Land Ownership: Private
Administrative Organization: Imperial County Planning Dept.


Ownership

Production

Deposit

Owner Name: American Girl Mining Joint Venture
Company ID:
Percent:
Home Office: 60 East South Temple, Suite 2100 Salt Lake City, UT 84111 (801) 297-6900
Info Year:
Years: -
Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Plant Type:
Plant Subtype:
Operation Type: Surface-Underground
Mining Method:
Milling Method:
Year First Production:
Year Last Production:
Discovery Year: 1776
Discovery Method:
Years of Production:
Organization:
Significant: Y
Deposit Size: M


Physiography

Mineral Deposit Model

Orebody

Name:
Form: Tabular, lenticular


Structure

Alterations

Rocks

Type: L
Structure:
Description: American Girl Shear Zone
Type: R
Structure:
Description: San Andreas Fault, Chocolate Mountain Thrust Fault
Alteration Type: L
Alteration:
Alteration Text: Weak chloritization and clay alteration within ore zones. Occassional epidotization in host rocks. Metasomatic leaching and aluminous enrichment of country rocks.
Name: Gneiss
Role: Host
Description: Quartzofeldspathic
Age Type: Host Rock
Age in Years:
Dating Method:
Material Analyzed:
Age Young: Jurassic
Age Old:


Analytical Data

Materials

Ore: Gold
Ore: Pyrite
Gangue: Quartz
Gangue: Gneiss
Gangue: Schist


Comments

Comment Type:
Development
Comment The Cargo Muchacho Mountains were first described by Spanish explorer Father Francisco Garcis who identified rich surface ores in 1776. By 1780, Spanish colonists had arrived and were working the placer deposits in Jackson Gulch and oxidized surface ores in Madre Valley (Van Wormer and Newland, 1996. This is believed to be the first gold mining in California. The district name, Cargo Muchacho, or Loaded Boy, refers to the legend that two Mexican boys returning to camp, arrived with their shirts laden with gold ore (Clark, 1970). American miners became interested in the area after the Mexican War of 1848. Gold was reportedly rediscovered in 1862 by members of a wagon train. The Cargo Muchacho Mining District was established that year, but was redefined several times in later years (Van Wormer and Newland, 1996). In the 1860s and 1870s small scale prospecting and mining flourished. The most important mines at this time were the Padre and Madre claims in the Madre Valley. The Padre y Madre deposits were first formally recorded in 1875 (Van Wormer and Newland, 1996). The completion of the Southern Pacific Railroad between Los Angeles and Yuma in 1877, brought an influx of American miners and additional strikes in the Madre Valley followed. Most of the early workings took place prior to 1890. The early Padre y Madre workings included several vertical shafts, the deepest of which was 325 feet. The extend of stoping is unknown but reportedly extended several hundred feet along strike. In 1880, a strike was made three miles further north in Gold Rock Canyon (later Tumco Wash). A small rush ensued and in 1884, the miners formally established the Ogilby Mining District and christened their settlement Gold Rock Camp (Van Warner and Newland, 1996). The principal claims were the Golden Cross, Golden Queen, and Golden Crown. The Gold Rock discoveries accelerated interest in the area. Rich ore samples were reported to grade as much as $9,000 to $12,000 a ton. In 1892, the Golden cross alone had produced $24,374 in gold from its small scale operation in which the ore was still hauled by wagon to Ogilby station and shipped by rail to the El Rio Mill. By the late 1880s, the rich surface ores were nearly depleted and many of the miners sold out, lacking the capital for underground mining. In April, 1893, having consolidated the Gold Rock claims and being adequately capitalized, William Hedges and Thomas Fuller formed the Golden Cross Mining and Milling Company. By October of the same year, the company had built a 20 stamp mill and was processing $15/ton ore from the Golden Queen shaft. The following year they installed a 12 mile pipeline to supply water from the Colorado River. By 1894, in anticipation of developing the Golden Cross and Golden Crown shafts, the company had added another 20 stamps and a Huntington Mill. The new mill crushed 100 tons a day and produced $1,000/day.
Comment Type:
Geology
Comment Mineralization developed along brittle-ductile shears that parallel foliation. In the Tumco Formation, orebodies were localized in areas enriched with biotite and magnetite, and occured as thin discreet shear zones rich in chlorite with disseminated sulfides and small quartz-pyrite veinlets. In the granite gneiss, the deposits occur as massive, ribboned, milky quartz veins and quartz stockworks with pyrite, chlorite, and sericite in and adjacent to the veins. Pyrite in both types of veins was associated with gold, and composed 3-5% of the veins, Chalcopyrite was locally associated with the gold. Tourmaline is an accessory (Tosdal, 1999, in press). Quartz-magnetite veins were associated with larger gold-bearing veins. Associated minerals included fluorite, scheelite, galena, sphalerite, and carbonates (Tosdal, 1999, in press). Fluid inclusion and stable isotope geothermometry from the quartz veins suggest deposition from magmatic fluids at a temperature of formation between 270- 300?C (Branham, 1988; Borrastero, 1990). Salinities were low, being less than 9 weight % NaCl. Some fluid inclusions contain CO2 which could indicate a contribution from metamorphic fluids. Timing of retrograde stage mineralization post dates formation of the aluminosilicate rocks and the initial stage of Jurassic gold mineralization Borrastero (1990) concluded it occurred during early Miocene extensional deformation due to similarities with retrograde metamorphics found under detachment faults of this age. Tosdal (1999, in press) suggested the mineralization was Paleocene since retrograde metamorphism and extensional faulting accompanied the Paleocene unroofing of the nearby Chocolate Mountains. The final stage of mineralization involved the brittle reactivation of older ductile and brittle shears zones and the remobilization of gold which locally upgraded existing orebodies. The brittle nature of deformation associated with the final stage of mineralization suggests a mid Tertiary age coinciding with regional brittle deformation or to the development of the San Andreas Fault system in the Pliocene (Dillon and Ehlig, 1993). Fractures and microfaults that are typically marked by red brown clay gouge. Remobilized gold rarely, by itself, reached ore grade. The small Sovereign Mine in Tumco wash was developed along one of the late brittle shear zones. The Padre y Madre Mine orebody was highly fractured and faulted with some of the gold associated with a red clay gouge horizon along a gently dipping shear zone. Also, the B Zone in the American girl Mine was cut by mineralized listric normal faults that soled into a breccia zone at the base of the B-Zone. However, Borrastero (1990) noted that the basal breccia zones were ore grade only where they contained abundant second stage auriferous quartz vein material.
Comment Type:
Geology
Comment Oro Cruz Orebodies 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 orebodies 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 orebodies 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 orebodies. Oxidation of the orebodies 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 occured 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 orebodies. 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 orebodies 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 orebody was geologically very similar to the Cross Pit orebody. 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 orebody, 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 orebodies 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).
Comment Type:
Environment
Comment 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.
Comment Type:
Workings
Comment 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.
Comment Type:
Geology
Comment During the late Jurassic or Cretaceous, basement and supracrustal rocks across southern California and southern Arizona were folded and thrusted northward and northeastward during the Cordilleran Orogeny. The Cordilleran Orogen developed as the principal effect of oblique northeastward subduction of the Farallon plate, and to some extent the Kula plate, along the western continental margin (Atwater, 1989). This produced a large belt of deformation from Canada to Mexico. As the plates converged, allochthonous accretionary terranes transported northeastward by these plates were scraped from the descending plates. In southern California, the allochthonous Baldy and the Santa Lucia-Orocopia terranes accreted to the continent between 60-40 Ma. Jurassic gneisses, schists and intrusive rocks were then thrust over the Pelona and Orocopia schists along a regional system of mylonitic thrusts including the Chocolate Mountains Thrust which underlies both the Cargo Muchacho and Chocolate mountains. In the southern Chocolate Mountains and Cargo Muchacho Mountains, the Orocopia schist forms the lower plate of the Chocolate Mountains Thrust Fault. Jurassic gneisses and schists and igneous plutonic rocks form the upper plate which host the Tertiary gold deposits. Thrusting has displaced the upper plate rocks as much as 30 miles to the northeast (Dillon, 1975). Regional studies indicate that metamorphism and thrusting were approximately coeval (Drobeck and others, 1986). During the early Tertiary, the Pacific Plate's relative motion slowed and became more northwesterly. Accordingly, convergence gave way to divergent plate motions with widespread volcanism and regional extension. Initial extension involved low angle detachment fault systems which accommodated much of the Oligocene-Miocene extension with an anatomizing network of low angle faults throughout southern California region (Frost and others, 1997). Important mineralization is associated with these detachment features at the Picacho Mine 12 miles to the northeast. Volcanism and normal faulting swept from east to west across the Basin and Range and into southern California. Basin and Range extension continues to this day; however, extensional accommodation shifted from Oligocene detachment faulting to high angle block faulting and strike-slip tectonics during the Miocene-Pliocene. By late Pliocene, the regional tectonic environment had become dominantly one of dextral strike-slip motion as represented by the Sand Hills Fault of the San Andreas Fault zone (approximately 10 miles west). Similar to much of the east margin of the Salton Trough and southern pediment of the Chocolate Mountains, the Cargo Muchacho range is cut by a series of northwest striking dextral strike-slip faults (Dillon, 1975) that are thought to be inactive strike-slip strands of the San Andreas fault system (Willis and Tosdal, 1992). Metallogeny The association of gold mineralization with the AGSZ suggests that similar deposits may be present to the east and west of the American Girl, Padre y Madre, and Oro Cruz mines. Regional trends suggest the zone is part of a regional feature that likely plunges below the thick cover of flanking alluvium to the east and west of the Cargo Muchacho Mountains. While significant economic ores have not been identified within the range and east of the mines, this does not preclude the presence of orebodies within the shear zone, perhaps at depth or fault separated. Further understanding of the shear zone kinematics, as well as the detachment faulting and strike-slip mechanics that control nearby Picacho and Mesquite mine mineralization, should advance our understanding of regional interactions and the complex relationships between tectonics and ore body mineralization. These advances will likely require a concerted effort employing sound geological interpretations, geochemical and geophysical studies, and exploratory drilling.
Comment Type:
Workings
Comment 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 orebodies. 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 orebodies. 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.
Comment Type:
Geology
Comment The AGSZ was subsequently impacted by younger Cenozoic brittle deformation. Dextral shear associated with the development of the San Andreas Fault system produced a regional fabric of right stepping, en-echelon northwesterly trending dextral strike-slip faults intersected by northerly striking normal faults. In the Cargo Muchacho Mountains, this deformation partitioned the range into several discreet blocks that may have undergone some counterclockwise rotation. This episode also reactivated parts of the AGSZ as shallow south dipping intrusive contacts and brittle low angle faults (Branham, 1988). Aluminosilicate mineral assemblages Feldspar, muscovite, and kyanite dominant mineral assemblages occur within the gneisses of the Tumco Formation. These assemblages form a zoned sequence that grades from the regional amphibolite facies quartzofeldspathic gneiss into locally occurring quartz-oligoclase-biotite-epidote-magnetite (feldspar zone), quartz-muscovite-biotite-magnetite-apatite-tourmaline (muscovite zone), and an aluminous quartz-kyanite-magnetite-rutile-apatite-tourmaline-lazulite (kyanite zone) assemblage (Owens and Hodder, 1993). The aluminosilicate assemblages are represented primarily by kyanite-quartz granofels and muscovite - biotite schists, that are laterally associated with all significant gold concentrations. The largest are about 0.5 - 1 mile west and on strike with the American Girl and Padre y Madre gold deposits along low angle faults. Henshaw (1942) concluded these rocks were products of regional metamorphism of sedimentary layers, but more recent interpretations favor a metasomatic origin (Wise, 1975; Dillon, 1976; Branham, 1988; Owens and Hodder, 1994). Owens and Hodder (1993) determined the zonation was consistent with hydrogen metasomatism by oxidizing slightly acidic magmatic fluids at 500-550? C and 400 kbar under mesozonal crustal conditions. Metasomatism was dominated by cation leaching and depletion, and the enrichment of immobile elements such as aluminum in the country rock. The oxidized fluids precluded gold deposition in the aluminosilicates, in favor of deposition in the more reduced peripheral chlorite and pyrite bearing assemblages (Owens and Hodder, 1993). Kyanite bearing inclusions within the intrusive rocks imply that metasomatism preceded or was coeval with the Jurassic intrusives The American Girl and Padre y Madre orebodies Most of the ore in the Cargo Muchacho Mountains has been produced from mineralized zones in the vicinity of the American Girl and Padre y Madre mines where orebodies are characteristically elongate southwest raking lenticular to sheet like bodies which are occasionally disrupted and offset by high angle faults. The primary deposits occurred within 1) sheared rock along 15?-45? south dipping faults within the AGSZ which served as conduits and host for mineralization (American Girl ore zone), 2) within fractured higher grade quartz veins along the these faults (B-Zone and Southwest Extension orebodies), or 3) within fractured and permeable gneisses of the Tumco Formation which allowed the spread of mineralization away from the fault zones. The low angle faults strike generally northeast-southwest in the American Girl Mine and northwest-southeast in the Padre y Madre Mine.
Comment Type:
Geology
Comment Ore Genesis and Mineralization Gold mineralization is thought to have occurred during three superposed events (Henshaw, 1942; Guthrie et al, 1987; Branham, 1988; Borrastero, 1990). The first two events consisted of ductile deformation with invasion of syn-metamorphic or magmatic ore fluids and produced the major economically important deposits in the district. The last event was limited to brittle reactivation of faults, remobilization of gold, and the enrichment of existing deposits. In general, the highest gold concentrations occur along faults within shear zones, whereas lower disseminated concentrations occur in the surrounding country rock. The gold is closely associated with pyrite and occurs as <1 to 20+ micron size grains that adhere to surfaces, occupy fractures, or are enclosed within pyrite. The auriferous pyrite occurs in quartz veins and veinlets, in fractures lacking quartz, and as disseminated grains. The initial stage of mineralization is characterized by widespread metasomatic iron enrichment, formation of the aluminous assemblage rocks, and gold deposition. Mineralization occurred during metamorphism and tectonism along the AGSZ in the late middle Jurassic. Highly oxidizing, sulfide poor, and iron rich hydrothermal ore fluids formed lenticular auriferous quartz-magnetite-biotite veins that are widespread in the Tumco Formation but best developed in Tumco Valley where they reach ore grade and were mined in the Queen and Cross pits of the Oro Cruz Mine. Initial mineralization of the American girl orebodies is also attributable to this stage. Gold was submicroscopic. Increased magnetite concentrations crudely correlated with higher gold grades with gold concentrations between 1 - 5.5 ppm occurring only in rocks with greater than 5 weight percent total iron. Copper, lead, and zinc also tended to be elevated when where gold was present. Trace amounts of scheelite, cinnabar, pyrite, chalcopyrite and fluorite were also associated with mineralization (Tosdal, 1999, in press). These orebodies were on strike with the metasomatized aluminous gneiss and were abundant along the basal contacts of the upper granite gneiss, but also occurred along discreet foliation parallel horizons in the Tumco Formation. The second phase of gold mineralization occurred during retrograde greenschist metamorphism. High metamorphic grade minerals of the upper greenschist-lower amphibolite facies were recrystallized to lower greenschist facies mineral assemblages forming chlorite-quartz-sulfide bodies and hydrolytically altering the wall rocks. In contrast to the first stage, gold deposition was accompanied by deposition of sulfide minerals, primarily pyrite, from less oxidized and sulfur bearing hydrothermal fluids. Locally gold deposition was superposed on the older iron oxide rich horizons. Most of the second stage orebodies were concentrated in American Girl and Padre y Madre mine area where they occur within the faults of the American Girl, B-Zone, and C-Zones.
Comment Type:
Geology
Comment 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 orebodies 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 orebody is in a shallow low angle fault in Tumco gneiss cut by high angle normal faulting exhibiting 100 feet of displacement. The orebodies 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 orebody was associated with the American Girl Fault zone which separates the overlaying granite gneiss sheet from the Tumco rocks. The orebody consisted of low-grade oxidized ore grading an average 0.051 opt and was mined via open pit. The American Girl B-Zone orebody 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 Madrte, 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 orebodies, 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?-60?E), north-south (N10?-20?E), and north-west (N40?-70?W). relative moveent 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 4range 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.
Comment Type:
Commodity
Comment Ore Materials: Native gold & auriferous pyrite
Comment Type:
Commodity
Comment Gangue Materials: Quartz, gneiss, schist
Comment Type:
Economic Factors
Comment Early production figures for the Cargo Muchacho District mines are sketchy at best. Hanks, (1886) reported that the "Cargo Muchacho mines" had worked 14,000 tons of ore and had produced $167,000 from discovery to June 17, 1882. The original American Girl Mine produced gold on a small scale between 1892-1916 and again between 1936-1939 (Branham, 1988). It produced 17, 750 ounces from 169,000 tons of ore at an average grade of 0.11 opt (Henshaw, 1942). Morton (1977) estimated 215,000 ounces had been produced from mines in the western and central parts of the range, largely from the mines in American Girl Valley, Madre Valley, and Tumco Wash. Commingled production from the American Girl Project mines between 1986 and their closure is estimated at slightly over 500,000 ounces yielding a total production estimate for the district of 700,000 ounces. While some of the early near surface oxidized ores and a select few quartz veins experienced higher grades, the average grade of ores mined in the open pits of the American Girl Project ranged 0.04-0.05 opt while richer underground ores averaged 0.2-0.5 opt.
Comment Type:
Development
Comment In 1894, the camp's name was changed to Hedges in honor William Hedges. By this time, some accounts place Hedge's population at several thousand. The same year, a new 100 stamp mill was added to supplement the 40 stamp mill whose operation had been curtailed for lack of tailings disposal space. By November 1895, 140 stamps were processing 500 tons a day of low grade $6 - $10/ton ore. Excessive expansion costs and lower than expected ore grades drove the company into debt. Within one year the company owed $125,822 and the owners agreed to a receivership with all mine proceeds going to retire the debt. However, the receivership also operated the mine at a loss. By 1897, the Golden Cross Mining Company sold their interests in the mines to the Free Gold Mining and Milling Company of Nevada. By 1898 Free Gold had the mill operating at full capacity and producing $43,000 a month. In 1901, a cyanide plant was added to process the accumulated mill tailings in. Cyanidization of tailings proved so successful that mining ceased and in 1902 the stamp mill was shut down. By 1905, all outstanding debts had been retired, the receivership was terminated, and the mine was closed. Shortly thereafter Hedges became a ghost town. The mines lay closed until 1909 when they were reopened by the United Mining Company (TUMCO) and Hedges was renamed Tumco. The venture was short lived and United Mining closed and abandoned the mines in 1911. Once again Tumco became a ghost town. All told, approximately 150,000 ounces were produced during the early years of operation of the Golden Queen, Golden Cross, and Golden Crown mines (Tucker, 1926; Morton, 1977). The original American Girl Mine in neighboring American Girl Valley was first opened in 1892 and was mined continuously until 1900, during which time an estimated 35,000 tons of $8/ton ore was produced. Little mining was done thereafter until the period 1913-1916 when 20,0000 tons of ore averaging $6.50/ton were milled. In 1920, the mine was patented. From 1916- 1936, the mine was idle but from 1936-1939 about 17, 750 ounces from 169,000 tons of ore grading 0.11 opt were produced (Henshaw, 1942). Limited intermittent activity occurred on the properties after WWII. Total estimated production was 205,000 tons valued at $1, 285,000. Development consisted of two single compartment inclined shafts 740 and 850 feet deep. The original American Girl working shaft was sunk in the footwall of a vein at an incline of 35? in the upper levels and at 25? in the lower levels. The Tybo Shaft, about 800 feet west of the first shaft was sunk at a similar inclination to 850 feet. Main levels were developed at 100 intervals to the 700 foot level, the lowest level at 740 feet. Total horizontal drifts exceed 8,700 feet. About 1 mile southeast of the Madre Valley, in Jackson Gulch where the earliest colonists had worked placer deposits, the Cargo Muchacho Mine was located in 1877, By 1882, 14,000 tons of ore averaging $12/ton had been produced by the Paymaster Company from an auriferous quartz vein. By the early 1890s, a 20 stamp mill and 12 mile pipeline from the Colorado River had been constructed. Unfortunately, a fault cut the vein and efforts to locate additional reserves resulted only low grade ore. By 1894, the mine was closed having produced only minor amounts of gold. Other periods of activity were 1936-1942, and 1949-1952(?). Cyanidization of previous tailings was conducted about 1940. Total production likely exceeded 25,700 ounces of gold valued at $852,000. The mine was developed by a 680 foot inclined shaft at the north end of the vein and a 200 foot vertical shaft about 1,100 feet south of the deep shaft Most of the stopes were below the third level. The area as far north as 800 feet from the main shaft was explored on the surface by several shallow shafts. consisted of a 550 foot main shaft and a 200 foot shaft south of the main shaft.
Comment Type:
Workings
Comment 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 orebody 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 orebody was 295,300 tons of average grade of 0.182 opt. The American Boy orebody 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 orebody 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 orebody rather than below it. Original estimated reserves were 182,000 tons of 0.182 opt ore.
Comment Type:
Location
Comment 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.
Comment Type:
Commodity
Comment Commodity Info: Natvie gold and auriferous pyrite generally occu in quartz veins and veinlets Disseminated gold and auriferous pyrite generally occur in host rocks.
Comment Type:
Geology
Comment LOCAL GEOLOGY The Cargo Muchacho Mountains are comprised of Jurassic metamorphic and intrusive igneous rocks. The AGSZ, an east-northeast trending low angle Mesozoic ductile shear zone, bisects the range in the vicinity of American Girl Valley. The shear zone divides the rocks into an upper plate of mid-crustal Jurassic granitic plutonic rocks and a lower plate of supracrustal rocks consisting of granite gneiss and quartzofeldspathic gneiss of the Jurassic Tumco Formation. The granite gneiss consists of metamorphosed granitic rocks that intruded the supracrustal rocks before and during regional metamorphism. The kinematics and direction of shearing has not been adequately resolved, however, the inverted section indicates contractional deformation was dominant. The shears also parallel regional foliation and the underlying Chocolate Mountains Thrust Fault. American Girl Shear Zone The AGSZ is characterized by a series of parallel, gently south dipping, ductile shears that, in cross section, interleave a wedge of the Tumco Formation between two large sheets of granite gneiss. The sheets dip to the south, paralleling both foliation and shears. The top of the AGSZ generally corresponds to the transition from deformed gneiss to diorite and granitic rocks in the upper plate. In most places this corresponds to the contact between the uppermost granite gneiss and the overlying Araz Wash diorite. The upper granite gneiss sheet metamorphosed the overlying diorite, locally converting to schist. The contact metamorphism and ductile shear fabrics in the upper plate indicate that granitic intrusion and shearing were contemporaneous. Movement on the AGSZ is interpreted to have occurred as early as lower Jurassic and possibly continued to the early Tertiary Ductile microstructures within the granite gneiss suggest down to the south motion of the upper plate (Branham, 1988). However, down dip shear would imply an extensional environment whereas the superposition of older mid-crustal rocks over younger supracrustal rocks requires contraction. Tosdal (1999, in press) suggested this disparity might be the result of post-metamorphic tilting of the range which modified the original dip of the shear zone. The Upper plate of the AGSZ consists of the Araz Wash diorite, a composite intrusive unit of commingled granitic rocks (Hayes, 1989, 1992; Murphy et al., 1990). Hornblende-biotite diorite, monzodiorite and porphyritic monzodoiorite to granite are the dominant rock types. Hornblende geobarometry from the mafic phases of the Araz Wash diorite indicate it was intruded at depths greater than 20 km (Hayes, 1989, 1992). U-Pb ages from zircons within the diorite indicate a Middle Jurassic age between 170-173 Ma (Dillon, 1975). The Tumco Formation and the granitic gneiss compose the lower plate. The Tumco Formation is primarily a gray fine grained, gray quartzofeldspathic gneiss derived from silicic volcanic and sedimentary protoliths. Petrologically, the Tumco Formation resembles biotite gneiss of the same age which hosts significant gold reserves in the Mesquite Mine of the southern Chocolate Mountains. Upper greenschist to lower amphibolite facies metamorphism is indicated by a quartz-microcline-plagioclase-biotite-epidote-magnetite mineral assemblage (Dillon, 1976). U-Pb isotopic data indicates the Jurassic age protolith. Syn-and post-kinematic pegmatite and granitic sills intrude the Tumco Formation. The sills merge with the main contemporaneous masses of granite gneiss. U-Pb dating of zircons within the granite gneiss yields a late middle Jurassic age between 160-175 Ma (Tosdal, 1999, in press). Dikes of pegmatite and granite also form northwest and southeast striking swarms. These dikes are mostly concentrated in the Tumco Wash where they intrude auriferous quartz-magnetite-biotite orebodies.
Comment Type:
Identification
Comment The American Girl Project involves three separate but adjacent mines operated by the American Girl Mining Joint Venture (AGMJV) during the 1980s and 1990s 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 1800s. 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 orebodies. 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 orebodies 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.
Comment Type:
Geology
Comment INTRODUCTION The Cargo Muchacho Mountains are an isolated northwest trending range approximately 8 miles long and 3 miles wide located 10 miles south of the Chocolate Mountains. The range consists of Jurassic metamorphic and crystalline rocks within the upper plate of the Chocolate Mountains Thrust Fault. In addition to the American Girl mines, other important hydrothermal gold deposits including those of the Mesquite and Picacho mines are located within similar rock sequences along the southern flank of the Chocolate Mountains. REGIONAL GEOLOGY Crystalline basement units Regionally significant basement lithologies are the late Mesozoic Pelona, Orocopia, and Rand Schists (collectively referred to as the POR schists) and older Jurassic gneisses and schists. The POR schists are units of highly metamorphosed and deformed greywacke, basalt, chert, limestone, and ultramafic rock stretching across southern California into Arizona, whose protoliths are considered to represent Triassic- Jurassic accretionary wedge deposits. These deposits were regionally metamorphosed during the Cordilleran Orogeny. Suprajacent rocks Suprajacent rocks in the region consist of Tertiary volcanics and conglomerates which, where preserved, unconformably overlie the basement metamorphic. The earliest volcanics were basalt flows portions of which are preserved as basalt caps on the more conspicuous mesas in the Chocolate Mountains. Fanglomerates, alluvial fan deposits, overlie the basalts and are in turn followed by several hundred feet of agglomerates, flows and breccias of the Oligocene Quechan Volcanics. Deposition of alluvium on low land and pediment surfaces followed a period of extensive erosion. The youngest deposits occupy the washes that have dissected the older alluvium and cut into the older erosion surfaces. Regional Structure and Tectonics Regionally, the Colorado Desert area has undergone a complex history of metamorphism, intrusion, volcanism and faulting. At least four important tectonic episodes have contributed to the structural complexity of the area: Jurassic-Cretaceous thrusting and metamorphism, Oligocene-Miocene extension with detachment and strike-slip faulting, Miocene-Pliocene Basin and Range normal faulting, and Pliocene and younger dextral strike-slip faulting associated with the evolving San Andreas Fault system. Structural ambiguities are many due to overprinting and fault reactivation. Late Cenozoic structural features overlap and sometimes obscure earlier Tertiary features, which in turn overprint Mesozoic features.
Comment Type:
Development
Comment In the late 1930s and early 1940s, In Tumco Wash, the Sovereign Mine was developed along a shallow dipping quartz vein (Sampson & Tucker, 1942). The vein was followed to a depth of 200 feet on the incline and along a 350 foot strike length (Tosdal, 1999, in press). Exploration from 1941-1942 delineated a low grade resource in the hanging wall of the Golden Cross Mine (Calvocoresses, 1942). By 1950, an estimated 215,000 ounces had been produced from mines in the western and central parts of the range, largely from the mines in American Girl Valley, Madre Valley, and Tumco Wash (Morton, 1977). In 1979, Newmont Exploration, Ltd. began exploring the area of the old American Girl and Madre Valley mines. Exploration delineated nine economic zones (Guthrie et al., 1987) and placed original aggregate open pit mineable reserves at 6.4 million short tons grading 0.051 opt. Underground reserves in the American Girl Valley were originally pegged at 1.2 million short tons containing 0.232 opt (Tosdal, 1999, in press). In May 1986, Eastmaque Gold Mines Ltd., through their American Girl Mining Corp subsidiary, purchased the American Girl and Padre y Madre properties from Newmont Exploration Ltd. In 1989, Eastmaque Gold sold a 50% interest to Morris-Knudsen Gold Corp and formed the American Girl Mining Joint Venture (AGMJV) to operate the mines. Mining was implemented in a phased approach, starting with the Padre y Madre Mine and ending with the Oro Cruz operation. Due to topographic separation and different mining and development schedules, the mines were permitted and operated independently. By 1987, the pilot phase of the Padre y Madre operation was permitted for heap leaching of 200,000 tons of ore from the West Pit and an incline to access the underground B Zone had been driven at the American Girl Mine. After a successful pilot run, full scale development work in the West and East Pits was permitted later that year for 3.5 million tons of ore and 12.5 million tons of waste rock. Surface mining commenced at the American Girl Valley Mine in 1989 which was permitted for 8.5 million tons of ore and 400,000 tons of waste rock. Underground Mining commenced in 1990. While the American Girl Mine was originally planned to be the final phase of the American Girl project, in 1990, the AGMJV acquired the neighboring Oro Cruz properties. Previous exploration and historical mining had identified both surface and subsurface ores. In 1993 permits were approved to mine 2.5 million tons of ore and 8.5 million tons of waste rock from the Golden Queen and Golden Cross pits and underground orebodies as a third phase of the American Girl Project. At the start of Oro Cruz surface mining, open pit mining at the American Girl and Padre y Madre operations was phased out. The operation was confined to mining with all processing done at the American Girl Mine. Operations at the Oro Cruz mine ceased in November, 1996. Mining operations at all three mines was completed by 1996, with reclamation completed in 1999. Currently, the mines are undergoing post-reclamation monitoring.
Comment Type:
Workings
Comment Surface mining operations were conducted similarly at all three operations. Size and shape of the pits was determined by geometry of mineralized zone, economics, geological/geothechnical characteristics of pit areas, equipment limitations, and safety. The Padre y Madre Mine involved only surface mining in two open pits (West and East pits) approximately 1,000 feet apart on the floor of Madre Valley. The East Pit measured up to 1,100 feet wide by approximately 1,500 feet long in a northwesterly direction. The West Pit lay about 1,000 feet to the northwest and measured approximately 1,600 by 1,100 feet. The overall strip ratio for the Padre y Madre open pits was 4:1. The Padre y Madre leach pad was located immediately west of the West Pit at the mouth of the valley. The more extensive American Girl Mine involved separate orebodies occurring in a relatively continuous area about 400 feet wide by 3,000 feet long. Hence, they were mined in four interconnected pits. The four pits, the Tybo, West, Main, and American Boy pits were mined progressively up canyon with mining progressing from the west to east. The American Girl leach pads were west of the pits. The Oro Cruz Mine included 2 open pits, the larger Cross Pit of about 25 acres and the Queen Pit of about 16 acres. Oro Cruz operations commenced at the Queen Pit (northwest) where ore was available with minimal waste rock removal. Initial stripping of rock from the upper levels of the Cross Pit was conducted concurrently with mining of the Queen Pit. After depleting the Queen Pit, operations were moved to the Cross Pit and the Queen Pit was backfilled with Cross Pit waste rock. At the other surface pits, waste rock was used to partially backfill the pits. A small percentage of high grade surface ore produced at the Padre y Madre Mine and high grade underground and surface ore from the Oro Cruz Mine were hauled to the American Girl crusher mill and processed in the existing cyanide mill. The lower grade Oro Cruz ore was hauled to the American Girl crusher and leach pads. Open pits were developed on 40 foot benches with pit walls maintained at approximately 1:1 slopes. Ground water, where encountered, exhibited very low flow rates (<1GPM) which readily evaporated. Open pit ores were excavated using a blast hole grid loaded with ANFO explosives and by mechanical ripping. Blast hole cuttings assays were used to delineate ore-waste boundaries according to assayed grade. The ore was loaded with front end loaders into a portable crusher and crushed to minus 2.5 inches. Bulk lime and water was added during crushing for agglomeration, pH control, and to minimize dusting. The crushed ore was trucked to the heap leach pads or mill processing facilities depending on grade. To minimize haulage, overburden stockpiles and waste rock dumps were located adjacent to the open pits. Waste dumps were developed by end dumping over the active dump face. Dumps were developed on 40 foot lifts separated by 20 foot catch benches. Underground mining at the American Girl Mine involved the systematic development of four discreet orebodies commencing with the B-Zone followed by the Southwest Extension Zone, after which the American Boy and C-Zone orebodies were concurrently developed. Based on their proximity, the B, Southwest Extension, and American Boy zones were mined using the same surface facilities using the B-Zone surface portal. The C-Zone orebody and surface facilities were located 0.5 miles down valley to the west.
Comment Type:
Workings
Comment Mining Placer gold deposits in the Cargo Muchacho Mountains were first worked in 1780 by Spanish colonists using primitive dry washing methods. Winnowing of the dry deposits was done with small bellow washers and with blankets (Clark, 1970). By the late nineteenth century, most placer deposits had been exhausted, and mining turned to rudimentary underground workings. Most notable were the Golden Queen, Golden Cross, and Golden Crown mines in Tumco Wash. Details of early mine workings are sketchy. The Golden Queen Mine reportedly followed a linear orebody extending 700 feet before being lost to faulting. The Golden Cross orebody was mined to a depth of 1,200 feet on the incline and in a glory hole along a productive strike length up to 500 feet long. The Golden Crown orebody was mined along its 1,100 foot length along a 20? incline (Frost et all, 1986). The neighboring American Girl Mine is reported to have followed an orebody along a 200 foot incline, from which drifts at several levels were driven. Details of later historic workings are unavailable. The modern American Girl Project involved three adjacent mines that were developed in phased stages by the American Girl Mining Joint Venture (AGMJV). This allowed a systematic and methodical approach to mining the several discrete orebodies while minimizing the expense of duplicative equipment and facilities and minimizing surface disturbances. The mines involved both surface mining and cyanide heap leaching of lower grade oxidized surface ores and underground mining, milling, and cyanide recovery from higher grade orebodies. With the exception of a pilot leach pad at the Padre y Madre Mine, all heap leach pads, milling equipment, processing and recovery facilities were maintained at the larger and more central American Girl Mine with ores from the neighboring mines being trucked in. Pilot scale operations commenced in 1987 at the Padre y Madre Mine. Mining at American Girl commenced in 1989, and the short-lived Oro Cruz Mine commenced operations in 1995. The Padre y Madre pilot operation involved surface mining and heap leaching of 200,000 tons of ore. Full-scale development was permitted for 3.5 million tons of ore and 12.5 million tons of waste rock from two pits within a 239 acre site. The leach pad facilities occupied 37 acres. The American Girl Valley Mine was permitted for 8.5 million tons of ore and 400,000 tons of waste rock (on a 347 acre site) and used both milling and heap leaching to process ores. Eight orebodies were mined; 4 in surface pits and 4 in underground workings. Lower grade oxidized surface ores were heap leached while the higher-grade underground ores and a small percentage of high grade surface ore was processed at the American Girl mill facility. While not originally part of the American Girl Project, the need for additional reserves to operate the American Girl Mine lead to the acquisition, exploration and development of the Oro Cruz Mine. The Oro Cruz Mine consisted of two open pits and underground workings permitted for 2.5 million tons of ore and 8.5 million tons of waste rock on the 191 acre site. Exploration and previous historical mining identified both surface and subsurface ores. This operation was confined to mining only with all processing and recovery done at the neighboring American Girl Mine. Like the American Girl Mine, it involved mining of different orebodies including low grade oxidized surface ores and higher grade underground ores.
Comment Type:
Deposit
Comment The American Girl Project deposits consists of low grade (0.04-0.05? opt) oxidized surface orebodies and higher grade (0.2-0.5?opt) underground ores which were mined via a combination of surface pits and underground mining methods. Generally, orebodies 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 orebodies. 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 orebodies 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 orebodies 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 orebodies 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.


References

Reference Category:
Deposit
Pages:
Reference: Van Wormer, S. R., and Newland, J. D., 1996, The history of Hedges and the Cargo Muchacho Mining District, Part I: A case study of the lives of Mexican miners in a company town of the southern California desert, The Journal of San Diego History, V. 42, no. 3, 16 p.
URL:
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Deposit
Pages:
Reference: Wise, W. S., 1975, the origin of the assemblage: quartz + Al-silicate + rutile + Al-Phosphate, Fortschritte Mineralogie, v. 52, p. 151-159.
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Deposit
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Reference: Willis, G. F. and Tosdal, R. M., 1992, Formation of gold veins and Breccias during dextral strike-slip faulting in the Mesquite mining district, southeastern California, Economic Geology, vol. 87, pp. 2002-2022.
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Deposit
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Reference: Miscellaneous information on the Cargo Muchacho mines are is contained in File Numbers 322-5644 (American Girl) and 330-8048 (Oro Cruz) (CGS Mineral Resources Files, Sacramento).
URL:
Reference Category:
Deposit
Pages:
Reference: Atwater, T., 1989, Plate tectonic history of the northeast Pacific and western North America., in Winterer, E. L., Hussong, D. M., Decker, R. W., editors, The geology of North America: The eastern Pacific Ocean and Hawaii, Geological Society of America, p. 21-72.
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Reference: Borrastero, Raul H. 1990, Gold mineralization at the American Girl B-zone Mine, Cargo Muchacho Mountains, southeasternmost California, unpublished masters thesis, University of Montana, Missoula, MT, 112 p.,
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Deposit
Pages:
Reference: Frost, W., Drobeck, P., Hillemeyer, B., 1986, Geologic setting of gold and silver mineralization in southeastern California and southwestern Arizona, in, Cenozoic stratigraphy and structure, and mineralization in the Mojave Desert, Geological Society of America, Field trip guidebook, Trips 5 and 6, p. 71-119.
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Reference: Frost, E. G. and others, 1997, Emerging perspectives of the Salton Trough region with an emphasis on extensional faulting and its implications for later San Andreas deformation: in Baldwin, J. and others, editors, Southern San Andreas Fault- Whitewater to Bombay Beach, Salton Trough, California, South Coast Geological Society Field Trip Guidebook N. 25, p. 57-98.
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Deposit
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Reference: Henshaw, P. C., 1942, Geology and mineral deposits of the Cargo Muchacho Mountains, Imperial County, California, California Journal of Mines and Geology,
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Deposit
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Reference: Morton, P. K., 1977, Geology and mineral resources of Imperial County, California: California Division of Mines and Geology County Report No. 7, p. 46-61.
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Reference: Morris, R. S., 1986a, Base of the Orocopia Schist as imaged on seismic reflection data in the Chocolate and Cargo Muchacho Mountains region of southeastern California and the Sierra Pelona region near Palmdale, California: Geological Society of America, Abstracts with programs, v. 18, p. 160.
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Reference Category:
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Reference: Guthrie, J. O., Cockle, A.R., and Branham, A. D., 1987, Geology of the American Girl- Padre y Madre gold deposits, Imperial County, California, Society of Mining Engineers, Preprint 87-87, 4 p.
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Reference Category:
Deposit
Pages:
Reference: Hayes, E. M., 1989, Mid-crustal Mesozoic plutonism in the Cargo Muchacho Mountains, southeasternmost California, geological Society of America Abstracts with programs, v.21, p. 92.
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Reference Category:
Deposit
Pages:
Reference: Hanks, Henry, G., 1886, San Diego County, Sixth Report of the State Mineralogist, California State Mining Bureau, p. 81.
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Reference Category:
Deposit
Pages:
Reference: Hayes, E. M., 1992, Petrology of Jurassic pluton and older crystalline units in the Cargo Muchacho Mountains, southern California, Unpublished Masters thesis, University of Southern California, Los Angeles.
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Deposit
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Reference: Crawford, J. J., 1896, Gold - San Diego County, Thirteenth Report of the State Mineralogist, California State Mining Bureau, p. 331-338.
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Deposit
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Reference: Dillon, J. T., 1975, Geology of the Chocolate and Cargo Muchacho mountains, southeasternmost California: unpublished doctoral thesis, University of California Santa Barbara, 405 p.
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Reference: Dillon, J. T., and Ehlig, P. L., 1993, Displacement on the southern San Andreas Fault, in, Powell, R. E., Weldon, R. J., and Matti, J. C. eds., The San Andreas Fault system: Displacement, palinspastic reconstruction, and geologic evolution, Geological Society of America Memoir 178, p. 199-216.
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Reference Category:
Deposit
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Reference: Dillon, J. T., Haxel G.B., and Tosdal, R.M., 1990, Structural evidence for northeastward movement on the Chocolate Mountains Thrust, southeasternmost California: Journal of Geophysical Research, v. 95, p. 19,953-19,971.
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Reference Category:
Deposit
Pages:
Reference: Drobeck, P. A., Frost, E. G., Hillemeyer, F. L., and Liebler, G. S., 1986, The Picacho mine: A gold mineralized detachment in southeastern California, in Beatty, B., and Wilkinson, P. A. K., editors, Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. 16, p. 187-221.
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Reference Category:
Deposit
Pages:
Reference: Branham, A. D., 1988, Gold mineralization in low angle faults, American Girl Valley, Cargo Muchacho Mountains, California, unpublished Masters thesis, Washington State University, 144 p.
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Reference Category:
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Reference: Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Tectonic overview of the Cordilleran orogen in the western United States: in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., editors, The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-3. p. 407-479.
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Reference: Clark, W. B., 1970 Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 153-154.
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Reference: Crawford, J. J., 1894, Gold - San Diego County, Twelfth Report of the State Mineralogist, California State Mining Bureau, p. 238-239.
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Reference: Murphy, G. P., Tosdal, R. M., Wooden, J. L., Kent, J., Vaugh, R. B., and Hayes, E. M., 1990, Chemical and isotopic character of Jurassic granitoids, Cargo Muchacho Mountains, southeast California, Geological Society of America Abstarcts with Programs, v. 22, p. 71.
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Reference: Tucker, W. B., 1926, Imperial County, Twenty second report of the State Mineralogist, California State Mining Bureau, p. 248-285.
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Reference: Owens, Eric O., Osborne, M., Kennedy, Lawrence P., 1988, Metasomatism of the Tumco Formation, Cargo Muchacho Mountains, southeastern California, in geological Society of America, Cordilleran Section, 84th annual meeting abstracts with programs, Geological Society of America, 219 p.
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Reference: Owens, Eric O., 1992, Magmatism, deformation and mesothermal metasomatism; interpretation of aluminosilicate mineral assemblages in the Cargo Muchacho Mountains, southeastern California, unpublished doctoral thesis, University of Western Ontario, London, Ontario, 342 p.
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Reference: Owens, Eric O. and Hodder, R. W., 1994, Aluminosilicate assemblages in the Cargo Muchacho Mountains, southern California; metasomatism and gold concentration associated with magmatism and deformation in mesozonal environments in Canadian Journal of earth sciences, pp. 310-322.
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Reference: Sampson, R. J., and Tucker, W. B., 1942, Mineral resources of Imperial County, California, California Division of Mines and Geology Report 38, p. 105-146.
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Reference: Tosdal, R. M., 1990, Jurassic low angle shear zones, southeast California and southwest Arizona: thrust faults, extensional faults, or rotated high angle faults?, Geological Society of America Abstracts with programs, v. 22, p. 89.
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Reference: Van Wormer, S. R., and Newland, J. D., 1996, The history of Hedges and the Cargo Muchacho Mining District, Part I: A case study of the lives of Mexican miners in a company town of the southern California desert, The Journal of San Diego History, V. 42, no. 2, 20 p.
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