Abstract The Santa Rita mine is an epithermal precious metal deposit of the adularia-sericite type, and a new discovery within the San Dimas district. Ore is hosted by east-northeast to northeast and north-northwest-striking normal faults that cut andesitic and rhyolitic tuffs, agglomerates, and lava flows of Paleocene age. The mining project started in 1984 with surface geology, recognizing local structures during district-scale mapping, followed by a diamond drill program, and later with 2.5 km of adit to cut the veins. Total production, as of January 1999, was about 10.2 Moz of silver and 64,800 oz of gold from about 617,000 metric tons (t) of ore. The ore is sent to the Tayoltita mill, and represents 30 percent of the total production at Tayoltita. The total historical production of the San Dimas district is around 20.3 × 10 6 kg of silver and 284 × 10 3 kg of gold from about 19 Mt of ore. The stages of vein formation consist of an early quartz stage, an ore stage, and a late quartz stage. The ore stage is subdivided into quartz-chlorite-adularia, quartz-rhodonite, and quartz-calcite substages. Most of the ore consists of sulfide-bearing quartz veins with a variety of textures typical of this type of deposit. Silver occurs in acanthite, argentite, stromeyerite, jalpaite, pearceite-polybasite, and native silver. Gold occurs in electrum accompanied by minor amounts of base metal sulfides. The radiometric age of the deposit is around 38 Ma. Geochemical studies were done in two of the major veins of the deposit. One hundred five samples were analyzed for Ag, Au, Pb, Zn, Cu, As, Sb, Mn, Mo, and Hg. The data were analyzed using the total raw data and data subpopulations representing samples from above, within, and below the ore zone. Three major associations were determined from the geochemical data: (1) an Ag-Au association that defines the ore zone, (2) a Zn-Pb-Cu association, and (3) an As-Sb association. Microthermometric measurements of fluid inclusions were carried out to evaluate the role of temperature in controlling mineralization. Fluid inclusion studies suggest that the quartz-chlorite-adularia substage was deposited primarily around 223°C from fluids with salinities of 2.7 wt percent NaCl equiv, the quartz-rhodonite substage mineralization was formed primarily around 239°C from fluids with salinities of 3.2 wt percent NaCl equiv, and the quartz-calcite substage mineralization was deposited at 227°C from fluids with salinities of 0.7 wt percent NaCl equiv. The late quartz stage was deposited around 228°C from fluids with salinities of 3.3 wt percent NaCl equiv. Temperatures and salinities of the substages remained constant throughout ore deposition. Evidence of boiling, characterized by coexisting gas and liquid-rich fluid inclusions, is spatially sporadic. Ore deposition is inferred to have occurred mainly in response to boiling and fluid mixing mechanisms.

The sustained magmatic activity along the North American Cordillera during late Mesozoic and Paleogene times produced the emplacement of numerous porphyry copper deposits. This activity extended by most of western México, particularly along the northwestern part of the country. This region, along with Arizona and New Mexico in the United States, contains one of the most important centers of copper mineralization on Earth. Most of the Mexican deposits lie in the eastern part of the Laramide magmatic belt (90–40 Ma) and were formed predominantly between 75 and 50 Ma. The largest deposits occur in northeastern Sonora and are represented by Cananea (∼30 Mt Cu) and La Caridad (∼8 Mt Cu). The copper ores are locally accompanied by molybdenum, tungsten, gold, and other metals. However, the metal distribution is apparently coupled with major changes in the basement of emplacement, which can be roughly separated into three domains: a northern domain characterized by Proterozoic crystalline rocks of North American affinity; a central domain composed of Paleozoic deep-marine basin rocks underlain by the Proterozoic North American rocks; and a southern domain, represented by Mesozoic island-arc–related sequences of the Guerrero terrane. Sr and Nd isotopic data from Laramide plutons along these domains suggest that the basement modified the final composition of the Laramide magmas. Also, the basement seems to have partly controlled the metal commodities along the porphyry copper belt, with relatively larger deposits characterized by Cu-Mo-W mineralization in the northern and central domains, and smaller and more Cu-Au dominated systems in the southern (more oceanic) domain.

ABSTRACT We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.