Gilles Levresse

Paleocene adakite Au-Fe bearing rocks, Mezcala, Mexico: evidence from geochemical characteristics

The Au–Fe mineralized granitoids at Mezcala district have a porphyry texture with a quartz+feldspar microcrystalline matrix and phenocrysts of plagioclase, quartz (with reaction rims), hornblende and biotite. The primary minerals are oligoclase–andesine, microcline and h-quartz. The accessory minerals are biotite, hornblende and, in minor amounts, apatite+zircon+sphene+titanomagnetite. Some intrusive rocks present abundant hornblende autoliths. Based on the petrography and bulk geochemistry of these granitoids, they are classified as monzonite, tonalite (the most abundant) and granodiorite with a strong calc-alkaline trend in potassium (K2O=3.8% average). The bulk and trace elements chemistry is SiO2=63.8%, Al2O3=15.83%, Fe2O3+MgO+MnO+TiO=6.52%; V=76.7 ppm, Cr=50.2 ppm, Ni=19.7 ppm, Sr=694 ppm. These granitoids show a strong depletion in heavy rare-earth elements (HREE), with average values of Yb=1 ppm and Y=13 ppm, this being the characteristic geochemical signature for adakite. The trace elements content suggests that the adakite granitoids from Mezcala were formed within a tectonic framework of volcanic arc related to the interaction between the Farallon and North America plates. This interaction occurred during the Paleocene after the Laramide Orogeny (post-collision zone) in a fast convergent thick continental crust (>50 km) subduction regime. The original magma is interpreted as being the product of partial melting of an amphibolite–eclogite transition zone source with little contribution of the mantle wedge. Along with the hydration processes, a metallic fertility also took place in the area. The geochemical signature of the adakites within the

Fluorite deposits at Encantada–Buenavista, Mexico: products of Mississippi Valley type processes

Petroleum and aqueous fluid inclusions from the Encantada–Buenavista fluorite mineralized zone in northern Mexico were analyzed by microthermometry, UV fluorescence, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and Confocal Scanning Laser Microscopy (CSLM) to evaluate the geochemical evolution of the mineralizing fluids. Two-phase (petroleum or brine+vapor) and three-phase (petroleum+brine+vapor) inclusions are described. Aqueous and petroleum-rich inclusions commonly occur in the same plane. Vapor-decrepitated and stretched fluid inclusions are present. A low-salinity methane-saturated fluid and a high salinity-fluid with highly variable methane contents are recognized. H2S is not quantified but is always detected in close association with methane. Petroleum inclusions are of two types: a low methane petroleum fluid (20 mol%) with low Th (60 jC) and a petroleum fluid with a methane of content near 30 mol% and a Th of 90 jC. Pressure and temperature diagrams for the aqueous and petroleum inclusions show three main intersects that allow P–T–X reconstruction of fluid evolution at La Encantada–Buenavista. A CH4- and H2S-rich low-salinity brine was mixed with oil that migrated under hydrostatic conditions with a thermal gradient of 70 jC/km. The arrival and mixing of a high-salinity aqueous fluid produced overpressure to 300 bars. A return to hydrostatic conditions was accompanied by an increase in the thermal gradient. The brine related to the fluorite orebodies appears to have a genetic relationship with the brines reported from the Jurassic petroleum basins located west of the fluorite bodies and similarities with reported fluids from Mississippi Valley type deposits. It is interpreted that the fluorine-rich fluids migrated toward the platform margins during the mid-Tertiary (30 to 32 Ma) using extension zones related to Basin and Range tectonism. Mixing of two different brines was responsible for precipitation and mineralization. Heat from magmas, related to tectonic extension, caused decrepitation and changes in the shape of fluid inclusions near the contact zones. D 2003 Elsevier Science B.V. All rights reserved.

Phytoavailability of antimony and heavy metals in arid regions: The case of the Wadley Sb district (San Luis, Potosí, Mexico)

This paper presents original results on the Sb and heavy metals contents in sediments and waste tailings, plants and water from the giant Wadley antimony mine district (San Luis Potosí State, Mexico). The dominant antimony phases in mining wastes are stibiconite, montroydite and minor hermimorphite. The waste tailings contain high concentrations of metals and metalloids (antimony, iron, zinc, arsenic, copper, and mercury). Manganese, copper, zinc, and antimony contents exceed the quality guidelines values for groundwater, plants and for waste tailings. Results indicate that peak accumulation is seasonal due to the concentration by high metabolism plants as Solanaceae Nicotiana. The metal phytoavailability in waste tailings is highly dependant on the metal speciation, its capability to be transported in water and, more particularly, the plant metabolism efficiency.

Degassing as the Main Ore-forming process at the Giant Imiter Ag-Hg Vein Deposit in the Anti-Atlas Mountains, Morocco

The giant Imiter epithermal Ag–Hg vein deposit in the Anti-Atlas Mountains of southern Morocco formed during a major episode of mineralization linked with Ediacaran volcanism at ca. 550 Ma. Silver was deposited during two main epithermal mineralizing events referred to as epithermal-quartz (ESE-Qz) and epithermal-dolomite (ESE-Dol) stages under distinct stress fields (i.e., WNW-ESE and N-S shortening directions), and is confined to the late Neoproterozoic, N60-90° E-trending, transcrustal Imiter fault zone. Economic orebodies are aligned mainly along the interface between sedimentary and volcanic units of lower and upper Cryogenian age. The ore mineralogy consists principally of Ag–Hg amalgam, argentite, polybasite, pearceite, tetrahedrite-tennantite, proustite-pyrargyrite, imiterite, acanthite, arsenopyrite, pyrite, sphalerite, and galena. Gangue constituents are dominated by quartz (ESE-Qz stage) and dolomite (ESE-Dol stage). Wall-rock alteration is well developed and includes silicification and dolomitization, and minor propylitization and kaolinitization. Fluid inclusion data indicate that the mineralizing fluids evolved through time, from a mean temperature of ~180 °C and salinity of ~10 wt% NaCl during ESE-Qz stage I, to a mean temperature of ~165 °C and salinity of ~24 wt% NaCl equiv during ESE-Dol stage II. Calculated trapping pressures, in the range of 1.1–0.9 kbar, exclude fluid unmixing “effervescence” as a viable ore depositional mechanism. Conversely, halogen compositions suggest the involvement of magmatic brines and evolved seawater. Stable (C, O, S) and radiogenic (Pb, Re/Os) isotope data, together with noble gas isotope compositions, are consistent with various degrees of mixing between mantle and crustal sources along the fluid flow path. Collectively, these data suggest that degassing of CO2 and SO2 during epithermal mineralization and related fluid/rock interactions led to local redox-potential decreases and pH increases that resulted in preferential deposition of massive amounts of native Hg-rich silver instead of Ag and Hg sulphide minerals.

Is there a Grenvillian basement in the Guerrero-Morelos Platform of Mexico?

In the Guerrero-Morelos platform (Guerrero State, Mexico) the adakitic rocks of Early Tertiary age contain abundant hornblende-rich tonalite xenoliths. Zircon crystals have been concentrated from both adakites and xenoliths, and dated using in-situ U-Pb ion microprobe analyses. These analyses indicate the presence of inherited Grenvillian and early Triassic/late Permian crust ages, as well as a Paleocene age related with the adakite rock intrusion. This range of inherited ages is reported for the first time in the Guerrero-Morelos platform, suggesting that a continous Grenvillian crust exists between the Oaxacan complex,to the East, and the Guerrero-Morelos platform.

New geological and geochronological data of the Placer de Guadalupe uplift, Mexico: a new piece of the Late Triassic–Jurassic Nazas Arc?

Basement exposed in the Placer de Guadalupe–Plomosas uplift in northern Mexico provides important clues for the geologic evolution of the region. The stratigraphic units form stacked thrust sheets of psammitic and calcareous formations, interlayered with magmatic rock. The eastern calcareous and quartzite formations exhibit structures associated with ductile deformation, whereas the upper stratigraphic units only contain structures formed via younger brittle deformation. Porphyry interlayered in the upper Plomosas Formation has a U-Pb zircon age of 171 ± 1 Ma. This age is consistent with its stratigraphic position, interbedded quartzarenites with a maximum depositional age of ~168 Ma. Granite flakes within the Horquilla Formation are dated at 209 ± 3 Ma, and the La Viñata quartzite exhibits a maximum age of ~193 Ma. The Upper Plomosas Formation correlates well with the arc-related Middle Jurassic Nazas Formation of northeastern Mexico, constituting the first report of a Jurassic continental margin arc outcrop in the ‘Central Mexican Gap zone’. We document Late Norian to Bajocian ages for the stratigraphic units cropping out in the Placer de Guadalupe area. The Jurassic age cluster indicates that the Nazas Arc magmatism in the region occurred during the Late Triassic and ended in the Middle Jurassic times. Permian ages previously assigned to these rocks and the occurrence of a Permo–Triassic deformation event have to be dismissed.

Fluid characteristics of the world-class, carbonate-hosted Las Cuevas fluorite deposit (San Luis Potosi´, Mexico)

Abstract Las Cuevas is a world-class high-grade fluorite district that accounts for over 7% of the world total fluorite production. This district is mainly hosted in the Cretaceous limestones of the El Doctor Formation, and is in fault contact with Tertiary rhyolites. This preliminary study is focused on the “G” orebody, a mass of fine-grained fluorite, with abundant cavities lined up by fluorite, sometimes stalactitic, late calcite and clays. Fluid inclusions in cavity filling minerals have salinities up to 0.18 wt.% NaCl eq. with homogenization temperatures ranging from 60 to 110 °C, with the T h decreasing consistently from early cavity filling fluorite to late calcite. δ 18 O and δ 13 C values suggest that both an organic matter maturation and a decarbonation process might have occurred during the formation of the deposit. All the characteristics of the deposit suggest an MVT related origin rather than a magmatic-hydrothermal one as previously proposed by other authors.

Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapán, Mexico

Three tectonic and physiographic provinces are present in the study area: (1) the Sierra Madre Oriental (SMOR; Suter, 1987; Carrillo-Martinez and Suter, 1982) formed by the succession of anticlines and synclines with a consistent NNW– SSE trend; (2) the Basin and Range (BR), formed by horst and grabens oriented NE–SW and NW– SE; and (3) the Trans-Mexican Volcanic Belt (TMBV; Aranda-Gomez et al., 2000), a continental volcanic arc on the southwest margin of the NorthAmerican plate resulting from the subduction of the Rivera and Cocos plates along the Acapulco trench (Fig. 1). The SMOR hosts a huge variety of ore deposits: Skarn Pb +Zn +Ag+(Hg–Sb) type ore deposits of Paleocene–Eocene age are found mostly at the El Pinon anticline; low sulfidation epithermal Au–Ag

Paleo-fluid and actual-fluid in the Los AzufresGeothermal Field, Central Mexico

Abstract After 20 years of exploitation of the Los Azufres field, paleo-fluids and actual brines show the same thermodynamic conditions and spatial distribution of temperature, pressure, fluids and non-condensate gases (NCG) composition. The deepest brines have temperature, salinity and pressure ranges of 300–340 °C, 0.2–0.8 eq. wt.% NaCl and 180 bar, respectively. A pressure drop induced a boiling zone characterized by a temperature of ca. 300 °C, apparent salinity range of 1.2–4.9 wt.% NaCl and a pressure of 85 bar. The upper zone presents a “clathrate CO 2 cap” with a temperature range of 120–150 °C. This steam-dominated zone is principally composed of CO 2 .

Fluid inclusion evolution at the La Verde porphyry copper deposit, Michocan, Mexico

Abstract La Verde porphyry copper deposit presents Ca-Na-K hydrothermal alteration, which is typical of porphyry copperdeposits. The temperature of homogenization (Th) in quartz from the potassic zone are Th=480-780 °C and melting temperatures (Tm) of the NaCl range from 430 to 570 °C, which correspond to salinities between 50 and 65 wt.% NaCl. In the stockwork zone, both subsaturated and saturated fluids are present Th range from 180 to 480 °C and salinitie group around 11–14 eq. wt.% CaCl2 and 22–54 wt.% NaCl. It is possible that these conditions reflect a plastic-brittle phase transition that characterizes porphyry copper deposit formation. In the propylitic alteration zone, the fluids are undersaturated, the salinities are 9–10 eq. wt.% CaCl2 and 5–23 eq. wt.% NaCl with Th range from 185 to 395 °C. Calcite in late-stage veinlets has salinities of 12.8–22.3 eq. wt.% NaCl. with Th range from 150 to 265 °C. La Verde porphyry copper deposit data present boiling in the mineralized zone caused by a drop in pressure from 1200 bars (equivalent to ca. 4.8 km) to 345 bars. Late-stage fluids show dilution by non-magmatic waters. 6345 values range from + l.15 % to +2.50 %o and 613C in calcite from −6 %o to −2the features are characteristic of magmatic fluids.