Mineralizing fluids of the shallow epithermal Au–Ag deposits of the El Barqueño district, Jalisco, Mexico

La composición de elementos trazas y de isótopos de las secuencias magmáticas del terciario inferior en dos localidades de la Sierra Madre Occidental al norte de México, muestran una variación que manifiesta la composición y la edad del basamento por el cual fueron eyectadas. La corteza subyaciente en San Buenaventura corresponde a la parte asociada al basamento Norte Americano, en cambio en El Divisadero, la corteza subyaciente correspondería a los terranos alocthonos acrecionados durante el Mesozoico.En estas localidades las rocas volcánicas aparecen fuertemente diferenciadas, variando en su composición de basáltica a riolítica (SiO₂=50–76%). Las secuencias erupcionadas en los terranos de acreción presentan la más baja dispersión en su cociente isotópico, con un cociente mínimo inicial de Sr (>0.7044, corregido por la edad), y máximo de Nd (<0.5126) y de Pb (²⁰⁶Pb/²⁰⁴Pb 18.9).Los cocientes isotópicos correspondientes a la serie continental presentan una dispersión variable y una distribución fuera de la serie de los terranos hacia la composición más típica de la corteza antigua (⁸⁷Sr/⁸⁶Sr 0.710 y ¹⁴³Nd/¹⁴⁴Nd 0.5123). Las rocas volcánicas de la zona continental, muestran valores relativamente altos en elementos incompatibles en comparación con las rocas de los terranos acrecionados (Ce/Yb=25–45 vs. 13–33, respectivamente), y al mismo tiempo están empobrecidos en algunos elementos incompatibles como U y Rb (p.e. Th/U=3.8–7.5 vs. 2.5–4.0, respectivamente), lo cual indica una posición estratigráfica superior, con cocientes de ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, y Th/U más altos, y ¹⁴³Nd/¹⁴⁴Nd más bajos.Los resultados tienen implicaciones sobre la composición original y la petrogénesis de las rocas volcánicas. Los cocientes isotópicos de los dos lugares revelan una composición que se sitúa entre las rocas de composición máfica de la Sierra Madre Occidental y los xenolithos intermediarios y félsicos característicos del norte de México o del sud-oeste de los Estados Unidos de América. La relación entre los cocientes isotópicos de las secuencias y la edad del basamento, así como el hecho de que la totalidad de los resultados obtenidos presenten una disposición bien definida, demuestran la fuerte contribución de la corteza en la química de los magmas silicatados. En la serie continental los cocientes de isótopos covarían con los cocientes de Th/Pb y U/Pb, aproximándose a la composición observada en los xenolithos intermediarios y en las rocas de composicion félsica. Esto indica claramente que no se trata de una fusión anatéxica de la corteza inferior, sino más bien de una interacción del magma basáltico del manto y la corteza. La contribución de la corteza es del orden de 20–70%. La estrecha variación del cociente isotópico en la zona de la secuencia de terranos acrecionados refleja la interacción del magma basáltico con una corteza relativamente joven, cuya composición isotópica es similar a los magmas derivados del manto. Los altos cocientes de Th/U y de Th/Rb indican que la contaminación de la corteza tiene lugar en su parte inferior. Además, los cocientes menos radiogénicos de ²⁰⁶Pb/²⁰⁴Pb y ²⁰⁷Pb/²⁰⁴Pb en la serie continental indican que el empobrecimiento de elementos fuertemente incompatibles en la corteza inferior corresponde a un rasgo antiguo. Los cambios seculares de los cocientes isotópicos de la sección estratigráfica indica que la contribución de la corteza superior aumenta con el tiempo, predominando al principio una corteza inferior máfica y, más tarde, una corteza intermediaria de rocas félsicas. Analizando rocas volcánicas externas a las dos secciones fuertemente muestreadas, las diferencias obtenidas en su composición isotópica de las rocas volcánicas, los terranos acrecionados y el basamento antiguo permiten deducir la localización del límite corteza continental-corteza oceánica.

Fluid inclusion study of the Plomositas–Los Arcos polymetallic epithermal vein tract, Plomosas district, Sinaloa, Mexico

The most important deposit in the Plomosas–Rosario district, Sinaloa, is the vein tract named Plomositas–Plomosas–Los Arcos. These are NNW–SSE striking veins hosted in rocks of the Lower Volcanic Supergroup (LVS), and also in rocks at the bottom of the Upper Volcanic Supergroup (UVS). Both supergroups belong to the Sierra Madre Occidental. These veins evolved from an early intermediate sulfidation stage (1), rich in base metal sulfides, to a low sulfidation stage (2), rich in silver sulfides and sulfosalts. There is also a 45 m-wide stockwork with native silver and gold. Stage 1 is found in the deeper portion of the veins whereas stage 2 is found in the most shallow portion of the deposit. These stages record fluid inclusion salinities ranging from 7 to 12 wt.% NaCl equiv., and from 0.2 to 3.5 wt.% NaCl equiv., respectively. Homogenization temperatures range from 120 °C for surface samples to 200 °C at a depth of 320 m. The low homogenization temperatures recorded, and the dispersion of veins within host rocks as veinlets, suggest that this deposit formed at shallow depths and was probably blind.     

Fluid inclusion and stable isotope study of the Cobre–Babilonia polymetallic epithermal vein system, Taxco district, Guerrero, Mexico

The Cobre–Babilonia vein system formed during a single major hydrothermal stage and is part of the Taxco district in Guerrero, southern Mexico. Homogenization and ice melting temperatures range from 160 to 290 °C and from − 11.6 to − 0.5 °C, respectively. We determined an approximate thermal gradient of 17 to 20 °C per 100 m using fluid inclusions. A thermal peak marked by the 290 °C isotherm is interpreted as a major feeder channel to the veins. The highest content of Zn + Pb in ore coincides with the 220 and 240 °C isotherms. Salinities of mineralizing fluids range from 0.8 to 15.6 wt.% NaCl equiv, and are distributed in two populations that can be related with barren or ore-bearing vein sections, with 0.8 to 6 wt.% NaCl equiv and 7 to 15.6 wt.% NaCl equiv, respectively. δ¹³C and δ¹⁸O water values from calcite from the Cobre–Babilonia vein system and the Esperanza Vieja and Guadalupe mantos range − 5.4‰ to − 10.4‰ and 9.9‰ to 13.4‰, respectively. δ³⁴S values range from 0‰ to 3.2‰ and − 0.7‰ to − 4.3‰ in sphalerite, − 4‰ to 0.9‰ in pyrite, and − 1.4‰ to − 5.5‰ in galena. Both fluid inclusion and stable isotope data are compatible with magmatic and meteoric sources for mineralizing fluids. Also, sulfur isotope compositions suggest both magmatic and sedimentary sources for sulfur.     

Geochronological framework and Pb, Sr isotope geochemistry of the Qingchengzi Pb–Zn–Ag–Au orefield, Northeastern China

The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (²⁰⁶Pb/²⁰⁴Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (²⁰⁶Pb/²⁰⁴Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 17.12 to 17.41, ²⁰⁷Pb/²⁰⁴Pb = 15.47 to 15.54, ²⁰⁸Pb/²⁰⁴Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (²⁰⁶Pb/²⁰⁴Pb = 18.20 to 24.28 and 18.32 to 20.06, ²⁰⁷Pb/²⁰⁴Pb = 15.69 to 16.44 and 15.66 to 15.98, ²⁰⁸Pb/²⁰⁴Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.     

Hydrothermal alteration and fluid inclusion study of the Lower Cretaceous porphyry Cu–Au deposit of Tiámaro, Michoacán, Mexico

The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.