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PLATINUM GROUP ELEMENT MINERALISATION IN TWO BASIC-ULTRABASIC COMPLEXES IN N. PORTUGAL: - THE PLATINUM GROUP ELEMENTS: MINERALOGY AND ANALYSIS: - STRUCTURAL AND LITHOLOGICAL CONTROL ON MINERALISATION.

Objective

THE AIM OF THIS JOINT PROJECT IS:

- TO IDENTIFY AND UNDERSTAND THE DISTRIBUTION, ABUNDANCE AND ORIGIN OF PLATINUM GROUP OF ELEMENTS (PGE) CONCENTRATIONS IN THE BRAGANCA AND MORAIS BASIC/ULTRABASIC COMPLEXES USING INTEGRATED METHODOLOGIES;

- TO EVALUATE THE ECONOMIC POTENTIAL IN PGE OF THESE COMPLEXES;

- THESE METHODOLOGIES COULD BE APPLIED TO SEVERAL SIMILAR COMPLEXES IN GALICIA, SPAIN.
Particular attention was given to ultramafic rocks in the Upper Allochthonous Thrust Complex (UATC) because they host most platinum group elements. The peridotites studied so far in the UATC can be divided in terms of whole rock composition into an assemblage corresponding to depleted mantle, common for example in Braganca peridotites, and nondepleted peridotites as seen in the UATC (Vinhas and Caminho Vellio) of the Morais complex. The depleted peridotite consists of harzburgite with lenses of diorite sometimes containing chromite mineralisation. Pyroxenite layers are common. The nondepleted assemblage includes cumulate troctolite. The depleted assemblage is considered to be a relict mantle. The nondepleted assemblage cumulates may have originated at a higher stratigraphic position than the mantle assemblage.

The geochemistry of the chromites from Braganca polycyclic displays a podiform nature. Within the podiform chromite classification the Braganca samples fall in the SSZ type formed in a subarc environment where a subduction zone is present beneath a continent. This is consistent with a large degree of partial melting, associated with the fluids from the subducting slab, producing vesidual harzburgite and with the presence of podiform chromite itself.

The significant platinum group elememts are restricted to chromite rich samples within the Braganca massif. The distribution of platinum group minerals within these samples indicates early crystallisation of osmium, iridium and ruthenium bearing platinum group minerals within chromite grains, followed by later formation of platinum bearing platinum group minerals in the interstitial silicate matrix. This minerology is similar to that described in the Shetland chromite rich samples. Palladium bearing minerals are associated with sulphides and appear to be concentrated by different sulphide related processes.

The geochemistry of the platinum group elements represented by chondrite normalised plots is complex . Positive slopes, typical of stratiform complexes, more recently described as fractionated assemblages, and negative slopes, typical of ophiolite complexes, more recently described as less fractionated assemblages, are present. There are also unusual chondrite patterns with irregular slopes. Palladium in particular often does not lie on a straight line with other platinum groups elements, perhaps reflecting a different process of concentration.

The significant grades of platinum group elements are widely distributed between chromite quarries, but there is an interesting geographical distribution of samples with different platinum to palladium ratios. In the area nearest Rio Tuela some ratios are high (over 2.0), elsewhere the ratio is low (less than 2.0).

New structural mapping within this project has shown that the Rio Tuela area belongs to a less deformed tectonic thrust slice different from tectonic thrusts slices occupying the area to the south and east. This division is clearly shown by the chromium and platinum group elements data.

The distinction between the polydeformed and polymetamorphic tectonic unit of the Upper Allochthonous Thrust Complex (UATC) and the monocyclic lower techtonic unit of the Ophiolite Thrust Complex (OTC) is confirmed. The age and geodynamic significance of this lower unit is now certain. The age and significance of the upper unit is still debated. The Ophiolite Thrust Complex shows all the characteristics of a typical ophiolite sequence with the recognised association of rock types and geochemistry with a distinct oceanic affinity. The age of metamorphism of this unit is known to be 390 million years by argon-40/argon-39 dating methods, but the age of the protalith is less certain, being probably of Upper Silurian to Lower Devonian age.

The age and nature of the Upper Allochthonous Thrust Complex is still debated, but progress was made in the elucidation of the internal structure of these tectonic units. A major distinction has been established between a supercrustal segment comprising cover (Lagoa mica schists) and upper granite crust (Lagoa augen gneiss) and an infracrustal segment comprising mafic granulites and peridotites. The internal structure of the infracrustal segment is very complex. Repetition is caused mainly by thrusting.

Progress has been made on the internal structure and differentiation of the Ophiolite Thrust Complex. There is a definite geochemical difference between Morais and Braganca with more typical oceanic affinities in the geochemistry of Morais and a more transitional character in the Braganca tectonic slices of Zoio, Nogueira and Soeira. The internal structure of the Ophiolite Thrust Complex in Morais is different in northern and southern segments, which are separated by the Morais fault. Repetition occurs by thrusting in the north and recumbent folding occurs in the south.

The purpose of the project was to determine the grade and distribution of platinum group elements (PGE) within the Braganca and Morais complexes of north eastern Portugal. The aim was to utilize the relationship between the PGE and their host lithologies in terms of primary igneous rock type, metamorphic grade, deformation state and alteration to understand the processes which are important in concentrating and redistributing the PGE.

Mapping and structural work has confirmed the distinction between the polydeformed and polymetamorphic tectonic unit Upper Allochthonous Thrust Complex (UATC) and the monocyclic lower tectonic unit (Ophiolite Thrust Complex (OTC)). The OTC shows all the characteristics of a typical ophiolite sequence, including a diagnostic sheeted dyke complex, and geochemistry with a distinct oceanic affinity. Progress has been made in the elucidation of the internal structure of these tectonic units.
Some progress was made concerning the internal structure and differentiation of the OTC. There is a definite geochemical difference between Morals and Braganca, with more typical oceanic affinities in the geochemistry of Morals and a more transitional character in the Bragnanca tectonic slices of Zoio, Nogueira and Soeira.

The field relations, textures and geochemistry of the chromites from the Braganca polycyclic unit suggest a podiform rather that a stratiform nature. Significant PGE concentrations are restricted to chromite rich samples within the Braganca massif. The distribution of the platinum group minerals (PGM) within these sample indicates early crystallization of osmium bearing PGM, iridium bearing PGM, and ruthenium bearing PGM within chromite grains followed by later formation of platinum bearing PGM on the edges of grains. Palladium bearing minerals are associated with sulphides. The geochemistry of the PGE represented by chondrite normalized plots is complex. Present are both postive slopes and negative slopes. In addition there ar e unusual chondrite normalized patterns with irregular slopes. In particular palladium often does not lie on a straight line with other PGE perhaps reflecting a different process of concentration of this element.

This collaborative project has shown geographical variations in the platinum/palladium and chromite chromium/(chromium plus aluminium) ratios both with high values in the north west which can be placed structually in context in the light of the newly mapped units in the Braganca complex.
IN THE FRAMEWORK OF THIS JOINT PROJECT THE LISBON UNIVERSITY AND THE GEOLOGICAL SURVEY OF PORTUGAL WILL PRODUCE MAPS OF THE BRAGANCA AND MORAIS COMPLEXES ON A SCALE OF 1:25,000 AND MORE DETAILED STUDIES AT 1:5,000. THIS MAPPING WILL BE ACCOMPANIED BY PETROLOGICAL AND GEOCHEMICAL STUDIES, STRUCTURAL INTERPRETATION OF THE TECTONICS OF THE THRUST SLICES AND CHARACTERISATION OF THE METAMORPHISM. IN ORDER TO UNDERSTAND THE ORIGIN OF THE PGE AND THEIR CONCENTRATION DURING EACH STAGE OF THE EVOLUTION OF THE AREA THE DIFFERENT TYPES OF PGM ASSOCIATIONS AND THEIR GENETIC PROCESSES MUST BE SEEN WITHIN THE GENERAL GEODYNAMIC HISTORY OF THE AREA.

IN THE FRAMEWORK OF THIS JOINT PROJECT,THE OPEN UNIVERSITY WILL FOCUS ITS RESEARCH ON THE STUDY PLATINUM GROUP ELEMENTS (PGE) CONCENTRATION PROCESSES IN THE BRAGANCA AND MORAIS BASIC/ULTRABASIC COMPLEXES ON NORTHERN PORTUGAL. THE MINERALOGY AND DISTRIBUTION OF THE PGM WILL BE STUDIED USING ORE MICROSCOPY, BETA-AUTORADIOGRAPHY, ELECTRON MICROPROBE AND MORE CONVENTIONAL TECHNIQUES FOR ASSOCIATED ELEMENTS WHICH COULD BE USED AS PATHFINDERS FOR PGM CONCENTRATIONS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Open University
Address
Walton Hall
MK7 6AA Milton Keynes
United Kingdom

Participants (1)

UNIVERSIDADE DE LISBOA
Portugal
Address
Campo Grande
1699 Lisboa