Jordi Tritlla

Fluid mixing and recycling during Pyrenean thrusting: evidence from fluid inclusion halogen ratios

Abstract Syntectonic fluids have been sampled through fluid inclusion microthermometry and crush-leach analyses (cations and halogens) from a 50 km N-S transect through the central-southern Pyrenees. The fluid inclusions are contained in syntectonic quartz veins in Triassic redbeds, Cretaceous carbonates and Hercynian basement rocks, with some calcite and dolomite data from limestones and evaporites in more external parts of the belt. The main datasets come from (1) Alpine shear zones cutting the Neouvielle granodiorite in the Hercynian Axial Zone at the north end of the transect; (2) An imbricate zone beneath the Alpine Gavarnie Thrust at the Pic de Port Vieux; (3) Several localities in the footwall and hangingwall of the Gavarnie Thrust on the southern margin of the Axial Zone. The inclusion fluids generally decrease in salinity from 27–35% at the northern end of the transect to 7–22% on the southern margin of the Axial Zone. The majority of the inclusions have Cl/Br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper Trias. This interpretation is supported by Cl-Br-Na systematics, which are consistent with a change from halite to halite + sylvite precipitation with progressive evaporation. Fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations, although it is possible that final concentration of brines in the Neouvielle Massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals. The fluids are also very similar in salinity and halogen chemistry to those found in veins associated with Mesozoic Pb-Zn-F deposits which predate the thrusting. The lower salinities seen at the southern margin of the Axial Zone are interpreted to reflect mixing of the brines with a higher level fluid (connate or meteoric water) circulating within the Mesozoic carbonates of the higher thrust sheets. At one locality where Triassic evaporites are still present, high Cl/Br ratios at relatively low salinities are present in inclusions within the underlying Triassic redbeds, but low Cl/Br ratios at higher salinities are seen lower in the sequence. This is consistent with dissolution of halite by a dilute fluid, but with limited penetration downwards. We suggest that the fluid history of the Pyrenees evolved through a series of stages: (1) Upper Triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks; (2) Circulation of brines with formation of Pb-Zn deposits along faults at some time between the Triassic and the Upper Cretaceous; (3) Renewed extension with erosion of Triassic rocks in many areas and further drawing down of Triassic brines into the basement; (4) Deposition of U. Cretaceous and Palaeocene carbonates containing connate waters of marine origin; (5) Formation of the Pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults; (6) Creation of topography with a high-level circulation system in the Mesozoic thrust sheets driven largely by topography. At the southern margin of the Axial Zone there was limited mixing of the deeper, overpressured brines with these more dilute, hydrostatically pressured fluids. An important point is that because of their density, hypersaline brines are difficult to expel from the upper crust, and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years.

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.

A newly dated Cretaceous hydrothermal event in the Iberian Ranges (Eastern Spain) and its significance within the Mesozoic thermal history in the Iberian Peninsula

Abstract The Iberian Peninsula hosts the world-class Hg mining district of Almaden. Besides pre-Hercynian ore bodies, alpine-cycle Hg-bearing veins are also present in the eastern Iberian Ranges (Espadan deposits). We present both the first absolute ages (84±4 and 85±3 Ma) for a post-Hercynian Hg deposit in Spain, obtained from primary muscovites, and a complete compilation of published Mesozoic radiometric absolute ages of the Iberian Peninsula. We deduce that there are three main thermal episodes that affected the Iberian Peninsula, which have been revealed after the comparison among the magmatic, metamorphic and hydrothermal ages. Moreover, the Espadan hydrothermal system was active when both the Cretaceous alkalic magmatism took place in Southern Portugal and in the Pyrenees, and during the opening of the Bay of Biscay.

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.

Brine and hydrocarbon evolution during the filling of the Cantarell Oil Field (Gulf of Mexico)

The main oil reservoir in the Cantarell Field, offshore Campeche, consists of a dolomitized carbonate breccia with an ejecta seal on top, considered to have been formed during the Chicxulub impact event. Two different dolomitization events have been identified associated with the reservoir. The first generation (D1) is a bright-red luminescent saddle dolomite while the second generation is a minute, non-luminescent dolomite (D2). Brine fluid inclusions show an evolution from D1 to D2 to higher temperatures (from 80–120 to 100–120 jC) and salinities (from 2–8 to 6–8 wt.% eq. NaCl). Hydrocarbon-bearing fluid inclusions evolved from heavy oils to light oils in D1 (from core to rim), while in D2, all inclusions appear to be formed by heavy oil with an jAPI similar to the oil contained in the present reservoir. These facts suggest that the end of the dolomitization process was closely related with the beginning of the main accumulation of oil into the reservoir, displacing the aqueous fluids and precluding the precipitation of carbonates. D 2003 Elsevier Science B.V. All rights reserved.

Fluid flow patterns during Pyrenean thrusting

Geochemical data for fluid circulation patterns during thrusting in the Pyrenees are reviewed. New halogen data from fluid inclusions suggests that brines responsible for metasomatic alteration in shear zones, and expelled along the Gavarnie Thrust, were of evaporitic origin. These brines mixed with more dilute formation waters in a topographically driven flow system at higher levels. The brines probably formed in the Triassic and were also involved in the formation of Mesozoic Pb‐Zn deposits. Dense fluids are hard to completely expel from upper crustal rocks, and recycling of such fluids through several metasomatic events is probably a common process. q 2000 Elsevier Science B.V. All rights reserved.

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.

Palynologic evidence for iron-oxide ash fall at La Perla, an Oligocene Kiruna-type iron ore deposit in northern Mexico

La Perla is an Oligocene deposit of apatite iron ore located in northern Mexico. The main ore types are massive ore, ore breccia and powdery ore. The latter is of special genetic interest because it contains well-preserved palynomorphs; fossil pollen representing several plant families growing in the region of investigation during the late Paleogene-Neogene; the assemblages include angiosperm and gymnosperm pollen grains, and also fossil fungal spores from two genera, Frasnacritetrus and Dyctiosporites, indicative of Eocene to Miocene age. The beds of powdery ore are stratified and size-sorted, but in some places there is no discernible stratification. The ore consists of a friable open framework of anhedral to euhedral hematite plates, or less commonly, martitized magnetite octahedra. Locally, the ore is even unconsolidated. The ore minerals show no abrasive rounding or other epiclastic features, and the high porosity of the iron-oxide crystal aggregate embedding the palynomorphs rules out formation by hydrothermal deposition or replacement. The exines of the palynomorphs have a light yellow color which demonstrates that they are unaffected by thermal alteration. This shows that the pollen-bearing powdery ore was deposited at a temperature below 150°C, probably as volcanic ash that captured wind-blown pollen.

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

Fluid origin of the Ixtacamaxtitlan hydrothermal deposits, Puebla State, Mexico

The hydrothermal deposit of Ixtacamaxtitlan (Puebla, Mexico) is made up by a succession, from bottom to top, of quartz veins and stockwork enclosed in a porphyritic subvolcanic body, a kaolinitized rhyolitic tuff and a layered opal deposit. This vertical arrangement coupled with the distribution of the different alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit. The fluid inclusion study carried on the veins and the stockwork along with the stable isotopic analyses performed on the kaolinitized bodies helped us to propose two major hydrothermal events that occurred in the area: an early event, characterized by hot, hypersaline fluids (up to 280 jC and 36 wt.% NaCl eq.) closely associated with a potassic alteration episode; and a late event, distinguished by cooler and dilute fluids (up to 150 jC and 4 wt.% NaCl eq.), associated with propylitic and quartz-sericite alterations at depth and acid-sulfate alteration close to the paleosurface due to steam-heated phreatic waters. Post-trapping changes found affecting the primary fluid inclusions (hook-shaped morphologies) suggest that there was an uplifting period between the two stages. D 2003 Elsevier Science B.V. All rights reserved.