Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern-southeastern normal faults. These are associated with a 12-6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc-alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern-southeastern fault systems, which are analogous to those associated with the Cu-Co-Zn-Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5-15 m deep, 87°C), Fe-oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba-rich Mn oxides occur covered by silica-carbonate stromatolitic sinters. Some 10-30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between d 18 O-enriched meteoric and local marine water. The precipitation of Ba-rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn-deposits.
Authigenic carbonate crusts, surface muds and bivalve shell fragments have been recovered from inactive and active recently discovered cold seep sites in central Guaymas Basin.. In this study, for first time, redox conditions and fluid sources involved in mineral precipitation were investigated by analyzing the mineralogy and textures of surface samples, along with skeletal contents, and C, O and S isotopes variations. The δ 13 C values of aragonitic bivalve shells and non-skeletal carbonate
ABSTRACT.A multidisciplinary study that includes processing of Landsat ETM+ satellite images, chemistry of gas condensed, mineralogy and chemistry of fumarolic deposits, and fluid inclusion data from native sulphur deposits, has been carried out in the Lastarria Volcanic Complex (LVC) with the objective to determine the distribution and characteristics of hydrothermal alteration zones and to establish the relations between gas chemistry and fumarolic deposits. Satellite image processing shows the presence of four hydrothermal alteration zones, characterized by a mineral assemblage constituted mainly by clay minerals, alunite, iron oxides, and more subordinated ferrous minerals and goethite. Hydrothermal alteration zones present in the Lastarria sensu stricto volcano are directly related to the recent fumarolic activity. Geochemistry of fumarolic gas condensed, obtained from two fumaroles at temperatures between 328 and 320 °C, has allowed detecting 37 diverse elements corresponding to halogens, chalcophiles, siderophiles, alkali metals, alkali earth metals and Rare Earth Elements (REE), with concentrations that vary widely between 5,620 ppm (chlorine) and 0.01 ppm (Mo, Ag, Sn, Pb, Se, Mg and Cr). Logarithm of Enrichment Factor (log EF i ) for each element present values between 6.35 (iodine) and <1 (K, Na, Ca, Fe and Al). Those elements are originated primarily from a magmatic source, whereas at shallow level a hydrothermal source contributes typical rock-related elements, which are leached from the wall rock by a strong interaction with hyperacid fluids. Mostly of elements detected are transported to the surface in the fumarolic emissions as gaseous species, while very few elements (Mg, Ca and Al) are transported in silicate aerosols. A wide spectrum of minerals are present in the fumarolic deposits, which are constituted by sublimates and incrustations, and the main minerals phases are distributed in six mineral families, corresponding to sulphates, hydrated sulphates, sulphides, halides, carbonates, silicates and native element minerals. The sublimate/incrustation minerals are dominated by the presence of sulphate, sulphur, chlorine and diverse rock-related elements, which are formed by processes that include a. oxidation of gaseous phase; b. strong rock-fluid interaction; c. dissolution of silicate minerals and volcanic glass; d. gas-water interaction; e. deposition/precipitation of saline bearing minerals; f. oxidation of sublimates/incrustations to form secondary minerals and g. remobilization of sulphur deposits by meteoric water. Despite that sublimate/ incrustation minerals are dominated by rock-related elements, its chemistry shows high contents of high-volatile elements as As, Sb, Cd, among others. Fluid inclusions studies carried out in thin pseudobanded native sulphur from fumarolic deposits, by use of Raman and infrared spectroscopy combined with microthermometry analyses, provided evidence of H 2 O, CO 2 , H 2 S, SO 4 , COS bearing fluids, homogenization temperatures around 110 °C and salinities v...
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