Maria dos Anjos Ribeiro

A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Pene- dono), together with a deposit linked to sub-vertical structures in silicified metasediments at Tres-Minas, and several intra- metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P-T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasedi- ments). They are: Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315-310 Ma) at P-T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphismOPa 300-350 MPa and Ta 500-5508CU: No clear evidence was found for gold deposition during this stage. Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P-T conditions between 100 and 300 MPa and 300 and 4508C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous- carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found. Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P-T conditions were ,100 MPa and ,3008C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb-Bi-Ag dominated). The fractures frequently contain chlorite (^ sericite) especially where they crosscut earlier sulphides (arsenopyrite). These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation. q 2000 Elsevier Science B.V. All rights reserved.

Petrographic Atlas: Characterisation of Aggregates Regarding Potential Reactivity to Alkalis

This RILEM AAR 1.2 Atlas is complementary to the petrographic method described in RILEM AAR 1.1. It is designed and intended to assist in the identification of alkali-reactive rock types in concrete aggregate by thin-section petrography. Additional issues include: optical thin-section petrography conforming to RILEM AAR 1.1 is considered the prime assessment method for aggregate materials, being effective regarding cost and time. Unequivocal identification of minerals in very-fine grained rock types may however require use of supplementary methods. the atlas adheres to internationally adopted schemes for rock classification and nomenclature, as recommended in AAR 1.1. Thus, rock types are classified as igneous, sedimentary or metamorphic based upon mineral content, microstructure and texture/fabric. in addition, the atlas identifies known alkali-reactive silica types in each rock type presented. It also identifies consistent coincidence between certain lithologies and silica types; however, it refrains from attributing alkali-reactivity to a specific silica property or quality. operator skill and experience remain essential for reliable assessment by thin-section petrography. aggregate materials must be classified according to local criteria, based on regional experiences with ASR-damaged field structures and geology. Access to additional data may be relevant for the assessment of imported materials. mere application of rock nomenclature does not provide any sort of warranty to the development of deleterious alkali-reaction. Such may result in either rejection of a suitable aggregate material, thus wasting a valuable resource, or acceptance of an unsuitable material leading to concrete damage, both of which are undesirable

Alkali–silica reactivity of some common rock types. A global petrographic atlas

The correct identification of potentially alkali–silica reactive aggregates is important for the prevention of alkali–silica reaction (ASR) in concrete. Although a number of standards for assessment of concrete aggregate by petrography are available, distinction of potentially deleterious from innocuous rock types can be problematic. The application of geological nomenclature alone is insufficient, as the geological history and hence mineralogical texture of a given rock type may strongly influence its performance in concrete. One of the goals of RILEM TC 219-ACS is to develop a worldwide photo atlas as a guide for petrographers in the identification of the mineral compositions and textures that are characteristic of alkali-reactive rocks. The atlas is based on micrographs of rock types recognized as potentially deleterious by field performance and/or laboratory expansion testing. It is intended to serve as an independent reference work and aims to unify rock terminology so as to improve the petrographic characterization of aggregates. The mission of RILEM, the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (Réunion Internationale des Laboratoires et Experts des Matériaux), is to advance scientific knowledge in the field of construction. Its Technical Committee TC 219-ACS focuses on alkali reactions and their effects on concrete.

Ferronigerite with dominant substitution TiSn −1 in muscovite+chlorite aggregate from massive quartz nodule associated with a petalite-rich aplite-pegmatite of the Barroso-Alvão pegmatite field, Northern Portugal

Strange ferronigerite with dominant substitution TiSn-1 in muscovite+chlorite aggregate from massive quartz nodule associated with a petalite-rich aplite-pegmatite of the Barroso-Alvao pegmatite field, Northern Portugal was examined including associated minerals. Such extensive substitution is not typical.

Study of Hungarian Rocks Regarding Potential Reactivity to Alkalis

The durability of concrete can be affected due to internal reactions between the alkalis of the cement paste and certain forms of silica present in the aggregates. Petrographic examination provides a useful tool to assess the alkali reactivity of aggregates. In the present work Hungarian stones that are used as aggregates were analyzed for alkali reactivity. Sandstone, two granitic rocks, one andesite and one diabase were studied using petrographic microscopy in order to identify the presence of potentially reactive forms of silica and to classify these rocks accordingly. Complementary methods such as scanning electron microscopy were also applied. Our results confirm that all studied rocks, except diabase, contain silica that has a potential of alkali reaction. Hence analyzed rocks are classified to Class II from the alkali reactivity point of view, while diabase is in Class I.

Assessment of the Stability of a Rock Slope by Using Different Methodologies

The action of the geodynamic agents (external and/or internal), or even the human activities, can cause changes in the structure of a slope that may result in an imbalance between the internal and the external forces acting on the ground. The aim of this study is the analysis of the stability of a natural rock slope located along the Tavora river, Northern Portugal. The geological mapping of the area and the use of stereonet to determine the main sets of joints were the starting point for the performed analysis. It was complemented with the study of different factors regarding the stability of the rock mass. The geomechanical characterization of the rock mass was also used, with the application of Bieniawski and Romana ratings. The methodologies applied proved to be a simple but effective way to describe the potential behavior of the rock mass with respect to the probability of occurrence of slope movements.

Assessment of Concrete Aggregate for ASR Potential by Petrography. The Work Developed by RILEM TC-ACS (2007–2013)

Alkali-silica reaction is a mechanism of deterioration of concrete structures which is associated with the composition of the rocks used as aggregates. The assessment of aggregates to evaluate their potential reactivity to alkalis is made both by visual methods and laboratory expansion tests. There are a number of national standards regarding the petrographic examination of new aggregates but the identification of the features that are related with the reactivity is not always easy. Supplementary to the recommendation of the petrographic method, RILEM has developed in recent years a worldwide petrographic atlas aiming at providing information regarding the features typical of each rock type that might be involved in alkali-silica reactions. In this work, a presentation is made on the results of the petrographic study of about 70 samples received from 25 countries in all continents. The samples include rock fragments, sand and gravel, concrete from structures and from prims subjected to expansion tests.

Building up of a nested granite intrusion: magnetic fabric, gravity modelling and fluid inclusion planes studies in Santa Eulália Plutonic Complex (Ossa Morena Zone, Portugal)

The Santa Eulalia Plutonic Complex (SEPC), located in the Ossa Morena Zone (south Portugal), is composed of a medium- to coarse-grained pink granite (G0-type) and a central grey medium-grained biotite granite (G1-type). Available Rb–Sr data indicates an age of 290 Ma. An emplacement model for the SEPC is proposed, taking into account magnetic fabric, 2D gravity modelling and fluid inclusion planes studies. The G0 and G1 types demonstrate different magnetic behaviour: G0 is considered a magnetite-type granite and G1 is an ilmenite-type granite. The formation of G0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma. The 2D gravity modelling and subvertical magnetic lineations show that the feeder zone of the SEPC is located in the eastern part of the pluton, confirming the role of the Assumar and Messejana Variscan faults in the process of ascent and emplacement. The magma emplacement was controlled by ENE–WSW planar anisotropies related to the final brittle stages of the Variscan Orogeny. The emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field. The magnetic fabric however suggests emplacement of the G0-type first, closely followed by emplacement of the G1-type, pushing the G0 laterally which becomes more anisotropic towards the margin. The G1-type became flattened, acquiring a dome-like structure. The SEPC is a nested pluton with G0-type granite assuming a tabular flat shape and G1-type forming a rooted dome-like structure. After emplacement, SEPC recorded increments of the late Variscan stress field documented by fluid inclusion planes in quartz.

Characterization of Deleterious Expansive Reactions in Fagilde Dam

A monitoring system has been detecting progressive displacements in Fagilde dam, completed in 1984. Site inspection revealed the existence of random cracking, surface discoloration, and superficial dissolution of cement in the spillway surfaces and in the upstream face of the dam. The macroscopic signs of deterioration, which could be due to different mechanisms, showed an uneven distribution in the exposed surfaces of the structure. Crushed limestone and alluvial siliceous sand were used as aggregates. Alkali–aggregate reaction was considered to be the likely cause of concrete distress. In order to investigate the causes of cracking and of the movements registered in the wall, drill core sampling was performed and the concrete investigated for the occurrence of expansive reactions. Concrete petrography was complemented by other tests in order to assess the probable evolution of the reactions in the structure in the near future. Residual expansion tests, soluble alkalis evaluation, and unconfined compression tests were carried out. Alkali–aggregate reactions and internal sulfate reaction were considered to be the main causes of damage in the concrete. In this study, microscopy showed to be an essential tool in the identification of the main causes of deterioration.