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The Montaña de Manganeso is a manganese vein-type deposit spatially associated with back-arc basin remnants of the Guerrero tectonostratigraphic terrane. The study of major- and trace-element geochemical characteristics of the deposit provides insight into the controls on ore-forming processes within the area. The deposit is characterized by low Co, Cu and Ni abundances, and high Ba (>10,000 ppm) contents and Mn/Fe ratios (<500), typical of hydrothermal Mn deposits. In addition, the low ∑REE abundances (18.7 to 103 ppm), negative Ce anomalies (0.2 to 0.6), and positive Y (1.00 to 2.34) and Eu anomalies (0.6 to 4.4) also suggest a hydrothermal source for the deposit. Discrimination plots involving ∑REE and Zr vs. (Cu + Ni + Co) and Ce/Ce* vs. Nd and Y/Ho further indicate a hydrothermal source in an oxidizing depositional environment. The Mn-Fe-(Ni + Cu + Co), MnO 2 -MgO-Fe 2 O 3 , (Cu/Zn)/Fe 2 O 3 vs. (Zn/Ni)/MnO 2 and Na/Mg diagrams display intermediate signatures between marine and terrestrial environments. This suggest that the Montaña de Manganeso deposit is the result of two metallogenic stages: (I) the earliest stage, which involved the formation of Mn oxides by hydrothermal/diagenetic processes in the Arperos back-arc basin during the Cretaceous; and (II) the latest stage took place subsequent to accretion the Guerrero tectonostratigraphic terrane onto the continent and involved the remobilization of the Cretaceous submarine Mn oxides (and associated trace elements) and subsequent redepositation by Tertiary continental hydrothermal activity. • The Montaña de Manganeso deposit is associated with the composite Guerrero tectonostratigraphic terrane. • Manganese deposition occurred initially in the back arc Arperos basin. • Mn was remobilized by Tertiary volcanism of the Central Plateau. • The deposit formed in a two-stage hydrothermal process. [ABSTRACT FROM AUTHOR]

The El Barqueño epithermal vein district is located in the southwestern part of the Sierra Madre Occidental of Mexico. It is made up of several polymetallic (Au-Ag-Cu-Zn-Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rock s, granites, and granodiorites, which belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental, within the Jalisco block. The veins trend from 45° to 100°, dipping to the southeast and south, and steepening with depth. The maximum strike length of the veins at El Barqueño is about 5 kin, and the maximum vein thickness is about 10 m (avg 2-3 m), narrowing with depth. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base metal sulfides and virtually no gold or silver. Stage II consists of crustiform quartz bands that form symmetrical vein fillings and contain the precious metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs after bladed calcite ar e widespread in stage II, providing evidence for boiling. Gold- and silver-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Conductive cooling and m ixing also may have been important. The temperature and salinity of the mineralizing fluids ranged from 101° to 252°C and 0.2 to 19.1 wt percent NaCl equiv, respectively. Whereas temperatures cluster in the range of 120° to 160°C, salinities have a bimodal distribution, mos t of them falling below 3 or higher than 10 wt percent NaCl equiv. These two arrays of salinity values only locally reflect mixing and may represent different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respective ly. An adularia separate from crustiform quartz bands of stage II was dated by 40Ar/39Ar geochronology at 57.91 ± 0.44 Ma (isochron age). Thus, El Barqueño has the oldest absolute age of any epithermal deposit in Mexico an d suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico associated with several types of ore deposits, including iron oxide-copper-gold, porphyry-type skarn and epithermal deposits. This further suggests that older rocks beneath the Upper Volcanic Supergroup of the Sierra Madre Occidental may be prospects for other epithermal veins and that exploration for epithermal deposits and related mineral deposit types should be focused in the oldest outcrop porti ons of the Sierra Madre Occidental. [ABSTRACT FROM AUTHOR]

Geochemical mass balances, supported by mineral characterization and whole-rock elemental analyses, were used to decipher the alteration paragenesis and to understand the elemental mobility related to the interaction of hydrothermal fluids with volcanic rocks (ignimbrites and lavas ranging from rhyolite to dacite) from the Acoculco geothermal system, in the eastern portion of the Trans-Mexican Volcanic Belt. Three extensive events of hydrothermal alteration were distinguished, from earliest to latest: (1) selective silicification and argillization, (2) hydraulic brecciation by silica-rich fluid, and (3) acid-sulfate (vuggy and argillic advanced) alteration. According to the Gresens-Grant mass-balance method, the altered volcanic rocks of Acoculco are generally enriched in Ti and Si and depleted in the rest of major elements; this chemical behavior is striking in the case of vuggy and silicic altered rocks. Small changes in some element concentrations calculated by the Gresens-Grant method may be due to chemical heterogeneity between the rock chosen to be the reference ("fresh rocks") and the protolith of the altered ones. The chondrite-normalized REE gave different patterns for distinct types of alteration; vuggy and silicified rocks present low LREE and high HREE in opposition to the advanced argillic alteration. Boiling, primarily related to the last alteration event, may have happened at variable depths (825, 325 and 102 m below present surface); it produced changes fluid composition, resulting in acidic pH surface alterations. [ABSTRACT FROM AUTHOR]