Abstract The first part of the route is southward from Tucson to Nogales. The geology and tectonic fabric is variable and includes from north to south the Santa Catalina-Rincon mountains metamorphic core complex, Late Cretaceous and Early Tertiary volcanic rocks of the Tucson Mountains, several porphyry-copper deposits associated with Laramide siliceous intrusion, and Precambrian units, imbricate fault slices of Paleozoic formations, and the Early Cretaceous Bisbee Group in the Santa Rita Mountains. The route follows the valley of the Santa Cruz River in which abundant Plio-Pleistocene gravel deposits are located to the international line. Further south, in northernmost Sonora, the route continues along the valley of the upper reaches of Rio Magdalena. This valley is largely surrounded by Jurassic volcanic flows and intrusions as far south as Magdalena de Kino where another core complex is situated. The route then turns eastward across the Magdalena extensional basin where borate and gypsum deposits have been discovered in Miocene age continental deposits. After crossing a drainage divide at Puerto Cucurpe, where ignimbrites are exposed in Sierra Torreón, the route turns northward through the valley-fills deposits exposed by Arroyo Las Rastras to the Santa Gertrudis, Carlin-type, disseminated gold deposit.

Abstract The Santa Gertrudis mining district is located 80 km southeast of Douglas, Arizona. The open-pit mine first came into production in 1987. The host rocks are Mesozoic sedimentary rocks, correlatable with the Bisbee Group. They consist of conglomerate, sandstone, siltstone, and limestone units of the Morita Mural and Centura Formations of Early Cretaceous age, all striking northwest. The host rocks for the disseminated, micron-size gold deposits have been intruded by dikes and small bodies of intermediate and siliceous composition. During the Laramide orogeny, strata were folded and deformed by low-angle faults and later cut by high-angle faults with a northwest trend in Mid Tertiary time.

Abstract The route to the Lluvia de Oro disseminated gold deposits, hosted principally in Cretaceous clastic sedimentaryrocks along the Lluvia shear, allows examination of the Magdalena core complex detachment zone. There after, the route passes southward along the Magdalena River valley to the important road junction of Santa Ana. From there, and continuing in a southerly direction, the San Francisco gold deposit can be reached through Estaci6n Llano. This deposit is hosted by Precambrian gneiss.

Abstract The Lluvia de Oro mine is located 18 km northwest of Magdalena de Kino, Sonora. The project consists of drill-defined mineralization at the Creston deposit located in the Lluvia shear, and four other known areas of mineralization. The project area is covered by four claims totaling 590 hectares held by stage purchase contracts in favor of Cia. Minera Lluvia de Oro, S.A. de C.V. The geology of the project consists of a northeast-trending sedimentary basin, fault bounded by metamorphic rocks. The sedimentary unit consists of siltstones, sandstones, conglomerates, and carbonates with ranging degrees of calcareous cement. The Crestón deposit occurs at the intersection of the Lluvia and Este shears, which have provided the primary porosity and permeability to serve as a conduit for hydrothermal fluid movement and subsequent gold mineralization. The gold mineralization forms an elongate cymoidal-shaped body whose limbs parallel the host shear zone.

Abstract The San Francisco mine is located in Sonora, Mexico, near Estación Llano 20 km south of the town of Santa Ana. The Mexico highway 15 and the Nogales-Guadalajara Railroad pass through Estación Llano. San Francisco is a mesothermal gold deposit within a Precambrian metamorphic complex which is cut by the Mojave-Sonora megashear, a large left-lateral transform fault that trends southeasterly and has up to 800 km of displacement. A banded sequence of quartzo-feldspathic gneiss, greenschists, and amphibolite gneiss make up the San Francisco unit, the oldest rocks in the area. Most ofthe rocks have been altered to quartz-sericite-chlorite assemblages. All these rocks have been intruded by the younger Tertiary leucocratic granite, which hosts much of the gold. Quartz veinlets containing some tourmaline, in stockworks or disseminations, are found in the mineralized zone. In both, the foliation and mineralization strike is N30-70W and dip to the NE. The gold occurs principally as free gold, or more rarely as electrum, and is associated with goethite and pyrite. San Francisco has undergone at least three episodes of deformation and two episodes of metamorphism. The early episodes of deformation that may have occurred have been obscured by subsequent deformation. Associated hydrothermal alteration consists of sericite, coarse-grained pyrite, and some quartz.

Abstract The route retraces the previous day's log as far as Santa Ana and then turns west on Mexico 2 highway towards Caborca. Subsequently at Km 43.5, the graded dirt road to the south leads to Trincheras. From this small community, situated on the railroad, a dirt road heads to the southwest and passes between El Boludo, a small rounded hill made up of Proterozoic dolomite and quartzite, and the Sierra La Salada that is largely underlain by biotitic granite, gneiss, mica schist, and epidote-bearing granite. This locality, and La Cienega area located 35 km further southwest, are part of the well-known gold placer deposits that have been exploited at various scales for more than 200 years. Nearby, on the western flank of Sierra La Salada, is the modern dry-washing plant at El Tiro. From there a brief visit to the Cerro Colorada gold prospect will be made before tracing the route to Santa Ana and Estacion Llano. Finally, the route follows near continuous outcrops of the Sonoran batholith southward to Hermosillo.

Abstract The Altar District, to the extreme northwest, is also very rich in gold, both quartz and placer. In this district are the Sierra Prieta mines, near the Gulf of California, which operate a narrow gauge railway to convey their ores to the mill on San Jorge Bay. The Quitovac mines, the Cuchilla properties, the Cajon de Amari lias, El Tiro and Cerro Colorado are quartz mines, with placers at La Cienega, Caborca, etc. In this district were conducted the earliest known placer mining operations. The output of gold from that district was extensive for many years, and is still considerable. Hampered by the want of an adequate supply of water, during long years the ground was worked by the most primitive processes. Shafts were sunk to bedrock, and drifts and tunnels thereon followed the gold channels. The auriferous earth was carried to the surface, and the gold skillfully separated by hand in boteas (the ordinary gold pan), without the use of water, in which art the Mexicans became very adept. Finally, there was invented a dry washer, a crude machine combining a blacksmith's bellows and an inclined oscillating table with cleats nailed across. Through a hopper the auriferous earth falls upon the oscillating table, and is blown and shaken down the incline, over the dump, the heavier particles of gold sinking and catching upon the cleats. Usually, the dirt is run through three or four times before all the gold possible is extracted though not over 60 percent is saved. In the placer fields of La Cienega, Cajon and other places, the ground is honeycombed in an area of many square miles by the shafts, drifts and tunnels from which the auriferous earth has been extracted.

Abstract Cerro Colorado lies near the center of Laramide Resources 5,346.4 hectare land package covering one of the historic mining districts of northern México. It is located at 31 °13'45″N, 111 °49'30″W some 60 km south-southeast of Caborca and 150 km north-northwest of Hermosillo in the state of Sonora, México (Fig. 1). Access is via a good dirt road through the town of Trincheras about 37 km to the northeast. The principal period of activity at Cerro Colorado occurred during the late 1800s to early 1900s. Both hard rock and dry placer operations worked during this period. Limited underground work is believed to have taken place again during the 1920's and 1930's. Within the district, small-scale, dry placer operations have continued intermittently to the present. The extent of past workings indicates that approximately 100,000 tons of high-grade, gold-bearing rock were mined at Cerro Colorado (MRDS MX00297). The production from underground mining and the placer workings is estimated to be approximately 50,000 ounces of gold.

Abstract The first part of the route is to southeast from Hermosillo to La Colorada mine situated at the transition from the Buried Ranges province to the subprovince of Parallel Ranges and Valleys. Following the visit to this well known gold producing locality, that renewed production in late 1993, the route is retraced through Hermosillo and Santa Ana (Day 4) to the road leading to Trincheras (Day 3). From there, stops will be made to discuss the Cretaceous rocks and their structure in Cerros El Amol, Sierra El Chanate, Sierra El Batamote, and adjacent areas. Continuing to the westnorthwest, the final destination of Caborca is reached via Altar.

ABSTRACT La Colorada Mine is located 47 km southeast of Hermosillo, Sonora. Gold and silver mining activity dates back to 1740, with the period of greatest activity being from 1865 to 1916. The mine is currently operated by Exploraciones Eldorado S.A. de C.V. who commenced production in December of 1993. The current geological resource is 32,926,000 tons with an average Au grade of0.96 g/t, containing 1,020,226 ounces of gold. A mineable reserve of 7,753,261 tons grading 1.27 g/t Au is currently being exploited from El Creston open pit at a rate of 10,000 tons of ore per day. Rocks in the area are composed mainly of Paleozoic miogeoclinal and eugeosynclinal sedimentary sequences intruded by a series of Cretaceous plutons and overlain by Eocene-Oligocene calc-alkaline volcanic rocks. Laramide - contractional deformation and Tertiary Basin and Range extension led to the development of at least four orientations of regional faults, and imposed structural tilting on large fault blocks. Gold mineralization is localized along faults within both the Paleozoic sedimentary rocks and Cretaceous intrusions. Gold typically occurs as fine-grained particles and is primarily associated with silicification. Alteration includes quartz veining, stockwork and silicification, along with potassic and propylitic assemblages and abundant iron and manganese oxides. Fluid inclusion populations indicate a variety of intrusive bodies and quartz veins in the area. Ore deposition appears to be associated with low temperature, moderate salinity fluids of a deep epithermal to high level mesothermal environment. The mine is a conventional open-pit cyanide heap leach operation. At present time approximately 70% of ore is crushed in a two-stage crushing circuit to a nominal size of −1”, with the balance shipped to the leach pads as run-of mine ore. Combined ore and waste production of 10-12 million tons are mined with a contract fleet of Cat 992 loaders and Cat 7771773 trucks. The recovery process utilizes a Merrill-Crowe circuit.

ABSTRACT The western Cerros El Amol near Altar, northwest Sonora, Mexico, is underlain by a 5- km-thick clastic sedimentary sequence, of which, the upper part has been metamorphosed into greenschist facies. The stratigraphic column includes, in ascending order, the Bisbee Group, El Chanate Group and the Altar Formation. The first two groups are late Early and Late Cretaceous siliciclastic sequences, respectively, that represent the most northwestern deposition in the Bisbee Basin of Sonora. Several fining upward cycles with intercalated fossiliferous carbonate strata suggest deposition in a back-arc basin that experienced several transgression and regression cycles. A few volcanic and volcaniclastic horizons are supposedly related to the mid-Cretaceous activity of the Alisitos magmatic arc. The coarsening-upward cycles, the geometric pattern of conglomeratic wedges, their thicknesses and abundance in the Altar Formation suggest deposition at the foot of a range or a topographic prominence. A weak metamorphism is present in all units, but it is pervasive in the stratigraphically higher units which show a greenschist facies. A low-angle fault juxtaposes part of the Bisbee Group on El Amol member of the Altar Formation.

Abstract The journey begins at Caborca and heads northwest to La Herradura mine located 30 km west of México 2 highway. This mine began gold production in April, 1998, and structurally controlled mineralization is hosted by Proterozoic gneisses. Thereafter, the route returns to the paved road to reach La Choya mine where mineralization is located in a granite complex. A short distance farther northwest is the Quitovac gold prospect followed by granitoid phases at Cerro La Silla and Sierra Cubabi. In the vicinity of Sonoyta a northwest-trending mylonite zone has been attributed to the Mojave-Sonora megashear. North of the international line the route passes through the Organ Pipe Cactus National Monument and then turns easterly across the Papago Indian Reservation and terrane. This terrane is characterized by autochthonous Early Jurassic volcanic and sedimentary rocks, and by allochthonous Proterozoic crystalline and Paleozoic sedimentary units. Back-arc rifting in which the Cretaceous Bisbee Group accumulated was deposited and preceded by magmatism in the Jurassic and succeeded by scarce Late Cretaceous-Early Tertiary volcanism and plutonism. The Papago terrane is also distinguished by peraluminous granite, regional metamorphism, and thrust faulting and later by Mid-Tertiary volcanism and extension, development of core complexes and detachment faulting.

Abstract La Herradura is a structurally controlled gold deposit located in northwest Sonora, México. Minera Penmont, S. A. de C. V., a joint venture between Servicios Industriales Peñoles, S. A. de C. V., and Newmont Gold Company manage the project. In 1989 regional exploration led the Penmont Venture to claim the area ofthe deposit. Geologically the deposit is situated on the southwest edge of the Sonora-Mojave Megashear, a left-lateral 300 ×3 40-km northwest-trending regional structure that hosts several gold occurrences. At La Herradura, Tertiary age reactivation of the Jurassic age Sonora-Mojave megashear faults juxtaposed gneissic rocks against greenschist-grade meta volcanics. The geology ofthe oxidized deposit is dominated by strongly sheared biotite and quartzofeldspathic gneisses that are present in northwest trending lithotectonic slices (20-50 m wide) bounded by high angle faults. Mineralization is hosted mostly in the quartzofeldspathic gneiss and consists of high-angle, gold-bearing, mesothermal style, quartz-iron oxide veins and stockworks that parallel the northwest trend. Each mineralized block generally consists of a principal quartz-iron oxide vein with a sericite-iron oxide-iron carbonate halo of alteration with quartz veinlets, silicified tectonic breccia and lower grade gold values. Exploration and development drilling using both core and reverse circulation, has delineated a gold deposit that will produce an average of 150,000 ounces of gold per year from an open-pit heap-leach operation for more than eight years. La Herradura is scheduled to start production in 1998 and will become the largest gold producer ofthe Sonora Desert region. This paper summarizes the discovery, geology and development of the deposit.

Abstract La Choya mine is located 55 km south of Sonoyta in northwestern Sonora, México. A crystalline granite complex hosts the mineralized zone. Primary mineralization occurred in a low-angle fault system, and was subsequently redistributed by supergene processes. The orebody is mined by open pit using wheel loaders and truck haulage. Gold and silver are recovered from low-grade ore by heap leaching followed by carbon adsorption. La Choya gold mine is Hecla's first operation in México. Purchase of La Choya mineral concessions was made in May of 1992. Design, permitting and construction was completed during 1993 and commercial production started in January, 1994

Abstract The Choya Sur Gold deposit, a 300,000 ounce deposit(Durgin and Teran, 1996), was placed into productionas an open-pit, heap leach mining operation in late November 1993, and was officially opened on February 10,1994. The deposit is part of La Choya property, which is controlled by Hecla Mining Company through its wholly-owned Mexican affiliate, Minera Hecla, S.A. de C.V.The mining operation has been described by Summers and Hufford (1997), and the geology summarized by Durginand Teran (1996). The purpose of this paper is to review the exploration history of the property, describe the geologic setting and principal characteristics of the deposit, and summarize the exploration results up until the time the propet1y was acquired by Hecla. This article is an adaptation of a paper (Thoms, 1993) presented at the 1993 Annual Meeting of the Society for Mining, Metallurgy and Exploration in Reno, Nevada, with permission of Hecla Mining Company.

Abstract Similarities between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico and in the Inyo Mountains-Death Valley region, California led Silver to attribute the distribution of these units to left-lateral displacement along a Jurassic transform fault postulated to extend from California to the Gulf of Mexico. Although the fault trace is commonlyobscured by later deformation, intrusions and cover, a well-preserved exposure of the postulated fault occurs within Sierra de Los Tanques, southwest of the village of Sonoita, Sonora, Mexico. Along Route 8, 13 km southwest of Sonoita, a tripartite assemblage of: 1) Jurassic volcanic and volcaniclastic strata to the northeast, 2) an axial zone of mylonitic rocks, and 3) masses of granitic rocks and compositionally banded quartzofeldspathic gneiss of Proterozoic(?) and Late Triassic age may be recognized. Steep mylonitic rocks, which coincide with the fault, form a discontinuous band (commonly 1 to 3 km wide) extending for 25 km amidst Cretaceous intrusive rocks. Structural and kinematic data record both left-lateral strike-slip and contractional deformation attributed to transpression.

Abstract The Mojave Sonora megashear hypothesis proposes that the Caborca terrane, northwest Sonora, arrived at its present position with respect to the North America craton via left-lateral (southeastward) displacement along a strike slip fault system. Nonetheless, clear stratigraphic and paleomagnetic links exist between rocksof the Sonoran segment of the Jurassic Cordilleran arc (JCA), and lower Mesozoic strata of the Caborca and Antimonio terranes supporting an alternative Jurassic paleogeography for northwest Mexico. The characteristic “J” magnetizations in Jurassic rocks of the JCA givea (tilt corrected) mean of D=15.0°, 1=4.0° (a95=14.3°;k=12.4; N=10 sites). Magnetizations pass fold, conglomerate, and reversal tests and are interpreted to be primary in origin. The age of these rocks is roughly bracketed between about 190 and 160 Ma. Secondary “J*” magnetizations in Neoproterozoic and Jurassic rocks southof the f - megashear give an overall (in situ) mean of D=15.0°, 1=10.0° (a95=5.8°; k=23.0; N=28 sites). “J*” magnetizations fail a fold test but timing of acquisition is bracketed between about 120 and 190 Ma, based on the youngest age of remagnetized Jurassic strata andthe fact that acquisition must predate the Cretaceousnormal polarity superchron. The overall means of rocks south and north of the MSM are statistically indistin uishable, arguing against the existencer of a crustal discontinuity along the proposed locat on of the MSM. For the interval that brackets acquisition of secondar “J*” and primary “J*” magnetizations, rotation of both the JCA and the Caborca terrane with respect to North America is 12° to 50° clockwise, depending on the age assumed. Estimates oflatitudinal displacement vary from as little as l-250 km southward, for a Sinemurian age of the magnetization (195 Ma), toas much as -800 km northward for a 1' Callovian-Oxfordian age (155 Ma). If the Sonoran results are compared with high-latitude Middle Jurassic poles for North America, larger estimates of northward displacement(>1400 km) result. Although the timing of magnetization acquisition is based on reasonable geological arguments, an Early Cretaceous age for “J*” magnetizations is permissible. Such an interpretation would indicate significantly larger northward displacement (>2000 km) with respect to cratonic [North America. Together, the lack of clear evidence for large southward displacement, the observed clock-wise rotation, and the similarity of the Jurassic magnetizations in the Cordilleran arc with those ofthe Caborca block are not consistent with the Mojave-Sonora megashear model of significant Late Jurassic southeast motion of northern Mexico along a left-lateral strike-slip fault system.

Abstract During the last eight years several new gold mines have been developed in northern Sonora. To date, six_of the deposits are open-pit, hard rock producers whereas the seventh is a modern, dry-washing placer operation. The geographic distribution of these economic mineral deposits extends from northwestern Sonora, southeastwards through Caborca, Altar, Santa Ana to La Colorada, situated 45 km southeast of Hermosillo and beyond. Along and adjacent to this trend, some 450 km in length, over 60 gold prospects are distributed in a zone that has an average width of approximately 100 km. Within this, 45,000 km 3 area exploration continues for these low-grade, disseminated type deposits.