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Panasqueira |
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Portugal |
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W
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Super Porphyry Cu and Au
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The Panasqueira wolframite mine is located at Barroca Grande, near the town of Fundao in central eastern Portugal (#Location: 40° 9' 14"N, 7° 44' 51"W).
Tin is believed to have been mined at Panasqueira by the Romans 2000 years ago, and later by the Moors. No mining was in progress however at the time of the discovery of the associated and more significant tungsten mineralisation in 1898. Sporadic small scale mining was carried out following the discovery until 1912, when a more organised operation was put together. In 1928, the current operating company, Beralt Tin and Wolfram Ltd was formed, and the annual production was lifted from 25 to 30 tonnes, to over 200 tonnes of concentrate. A reorganisation and injection of new capital in 1938 led to a steady and consistent growth in production, peaking at 2520 tonnes of concentrate in 1943. Output declined in the late 1940's only to pick up in the 1950's, and then fall again drastically following the 1962 tungsten price slump. Production has risen again since the mid 1960's and is currently at around 1600 tonnes of 75.4% WO3 concentrate per year.
In 1978 when the deposit was visited, the operation comprised an extensive underground mine at Barroca Grande, with an attached heavy media plant near the main adit portal. The pre-concentrate from this plant is transported some 4.15 km by an overhead cable way to the main mill at the nearby township of Rio.
At the time, the mine was owned by a Portugese incorporated company, Beralt Tin & Wolfram (Portugal) SARL which was 80.55% owned by Beralt Tin and Wolfram Ltd, the remaining 19.45% being held by Portugese banking interests. The major share holder of Beralt Tin and Wolfram Ltd is Charter Consolidated Ltd who hold 46.3% of the issued capital.
Geological Setting
The geology of the Panasqueira area is dominated by a thick sequence of pre-Ordovician metasediments to the west, cut by an equally extensive development of Hercynian granitic rocks to the east. The pre-Ordovician sediments are discordantly overlain by relatively small, elongate outcrops of Ordovician marine sediments. These sequences are overlain in turn by scattered outcrops of late Mesozoic and Tertiary continental deposits, predominantly arkoses.
The Panasqueira mineralisation is hosted by a series of quartz veins cutting the pre-Ordovician sediments. These sediments are probably of Cambrian or late Neoproterozoic age and comprise a monotonous sequence of slates, sandy-shale, sub-greywackes and fine grained quartzites. The succession has few marker horizons and particular beds cannot be traced for any great distance along strike. The individual facies have diffuse rather than abrupt contacts with each other. The proportion of each rock type within the area has not been determined. From the sequence observed at the mine, the sediments appeared to be evenly and finely bedded with graded bedding being the only readily recognisable sedimentary structure.
In the Panasqueira area this suite of sediments carries sparse but noticeable disseminations of fine pyrite and equally fine blebs of carbonate throughout.
The granites of the Panasqueira area are all Hercynian being mainly of Permo-Carboniferous age. They are usually discordant and comprise predominantly porphyritic biotitic granite grading into equigranular biotite and biotite-muscovite varieties. Large elongated inclusions of the pre-Ordovician sediments are occasionally found within the batholith.
Mine Geology, Structure and Mineralisation
Ore mineralisation at the Panasqueira Mine is contained within a series of parallel to sub-parallel quartz veins that form a subhorizontal vein swarm developed within and parallel to the margins of a planar zone, some 120 to 150 m in thickness. This zone is peripheral to a late-orogenic greisen cupola, and along its northern margin outcrops with an east-west strike and dip to the south at around 15°. It has an elliptical outline with known lateral dimensions of some 2000 x 1400 m in the north-south and east-west directions respectively. The host rocks are a sequence of pre-Ordovician sediments which strike at around 120° and have a variable dip of between 60 and 90°.
The vein swarm consists of hundreds of co-planar quartz veins that are overlapping and connected laterally over large distances. The pattern of distribution of the vein swarm indicate that they exploited a regional joint system. Detailed orientation analyses of the systematic joints reveals a geometrical relationship with the subvertical F2 fold generation, related to late-Variscan transpression. On both the regional and the outcrop scale, the joints are consistently orthogonal to the steeply plunging S0-S2 intersection lineation, and are therefore defined as cross-fold or ac-joints. This system of jointing is interpreted to have developed during the waning stages of Variscan orogenic activity, when it was at an upper-crustal level. Conditions of hydraulic overpressures and low differential stress accompanied the veining and reactivated these cross-fold joints. The consistent subperpendicular orientation of the veins relative to the non-cylindrical F2 hinge lines, also when having an inclined attitude, are interpreted to demonstrate that veining did not occur during far-field horizontal compression. Vein orientation is determined by local stress states variable on a metre-scale but with the minimum principal stress consistently subparallel to fold hinge lines. The conspicuous subhorizontal attitude of the Panasqueira vein swarm is the result of the geometry of late-orogenic folds, which developed synchronous with oroclinal buckling of the Ibero-Armorican arc (Jacques et al., 2018).
The sedimentary rocks in the mine area are predominantly finely laminated slates with lesser fine quartzites. Both facies are laminated on a 0.5 to 2 mm basis with rare bands up to 2 cm thick. Graded bedding is obvious and indicates a right-way-up facing in all examples. Bedding is marked by colour variations from pale to dark grey and brown. In some locations, abundant fine laminae of tourmaline are observable within the more quartzitic facies. These resemble the finely laminated tourmaline rich rocks often found in tin bearing sequences, although the tourmaline is interpreted to be related to the veining and declines away from vein margins. The main mineralised zone is underlain by a body of altered Hercynian granite. An apophysis of this body is found in the lowermost sections of the mineralised zone towards its northern limit. The veining is also more intense in this area. The upper margin of the granite falls away on all sides at an angle of around 30° or more.
The mineralised horizon lies within spotted slates immediately below the contact with overlying un-spotted slates. This contact parallels the upper boundary of the mineralised zone and is believed to be the expression of the outer limit of the underlying granite's metamorphic aureole. This contact cuts across the lithological boundaries and bears no relationship to the stratigraphy or sedimentary facies.
The spotting within the slates is due to 1 to 3 mm aggregates of cordierite and lesser andalusite developed in the cleavage planes of the slates.
Fine pyrite is dispersed throughout the sequence in both spotted and un-spotted slate. It occurs as fine crystals of 0.1 to 1 mm, accounting for less than 0.1% of the rock. Arsenopyrite is also found, but usually in the more quartzitic facies near known veining.
The density of veining drops off markedly both above, within un-spotted slates, and below the main mineralised zone in the spotted slate. The veins found outside of the economic zone are still mineralised but are usually too thin and sparse to extract.
Two generations of veins are recognised in the mine. The first is developed within the cleavage planes of the slates with dips in excess of 50°, cutting bedding at an angle of 30° or more. These are barren although they do carry disseminated pyrite and arsenopyrite. They can be distinguished from the later mineralised veins by their grey blue tinge, feldspar content and less compact nature. They are cut by the flat lying mineralised series of veins which are developed within joint planes. These veins range from 1 or 2 cm in thickness up to a maximum of around 1.5 m. The economically extractable veins average 30 cm in thickness, using a 20 cm cut off. Laterally they are made up of overlapping lenticules, each with lateral dimensions averaging 25 to 30 m. Adjacent lenticules usually touch at one point. Away from this line of intersection, they generally overlap by 1 to 3 m laterally and are separated vertically by up to 3 m, being developed in sub-parallel joints. Individual veins, made up of a series of these lenticules have average lateral dimensions of around 100 x 150 m. In general, at any one point within the 120 by 150 m thickness of the mineralised vein zone, there are from 5 to 10, but averaging 6, economically extractable veins (ie. >20 cm in thickness) each with lateral dimensions as outlined above.
The mineralised veins are compact with a milky colour, although coarse quartz crystal structures up to 5 x 2 cm being obvious in places. Coarse elongated wolframite crystals from 1 to 10 x 30 to 50 mm are developed within the outer 10 cm of the veins oriented normal to the direction of veining. These veins carry variable amounts of sulphides comprising in diminishing frequency; pyrite, arsenopyrite, chalcopyrite and sphalerite (marmatite). The sulphides are present in a number of modes ranging from 1 to 2 mm crystals accompanying the wolframite, to aggregates of 3 to 4 mm crystals up to 5 cm across developed, within the veins. In highly mineralised areas over intervals of up to 20 m, the veins comprise from 20 to 75% sulphides enclosing crystal shaped remnants of quartz and coarse wolframite crystals. These coarse wolframite crystals are up to 5 x 10 cm in dimensions and are sometimes found as aggregates up to 1 m x 20 cm on the vein margins.
There is a general zonation within the veins such that their outer margins are more highly mineralised and the centre is lower grade to barren. Laterally, individual vein systems and lenticules are poorer on their extremities and rich towards their central sections.
The veins often have sharp boundaries with only 2 to 3 mm selvages of mica rich quartz, while in other areas topazised margins up to 2 cm thick are observable.
Statistics
The following applied to the operation in 1977 when visited by the author
Ore treated by HMS plant in tonnes ..................... 407 605 t
Waste reject from HMS plant ................................ 373 280 t
Pre-concentrate to mill ............................................ 34 430 t
Pre-concentrate grade ...................................... 3.07% WO3
HMS plant recovery of WO3 ....................................... 87.7%
Mill recovery of WO3 ................................................... 91.8%
Concentrate grade WO3 ............................................. 75.5%
Total WO3 recovery % ................................................ 80.5%
Production of contained WO3 in tonnes ....................... 972 t
Production of contained W ........................................... 770 t
Production of contained Cu .......................................... 282 t
Grade of copper concentrate ......................................... 24%
Production of contained Sn ......................................... 43.5 t
Average head ............. 0.32% WO3, 0.014% Sn, 0.105% Cu
Cut off grade WO3 (approx) ........................................ 0.23%
Minimum mining height .............................................. 1.65 m
Average mining height ............................................... 2.00 m
Minimum vein width mined ......................................... 0.20 m
Average vein width mined .......................................... 0.30 m
Average mining dilution factor ..................................... 5.67:1
The most recent source geological information used to prepare this decription was dated: 2018.
Record last updated: 21/1/2018
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Panasqueira
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Selected References:
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Jacques, D., Vieira, R., Muchez, P. and Sintubin, M., 2018 - Transpressional folding and associated cross-fold jointing controlling the geometry of post-orogenic vein-type W-Sn mineralization: examples from Minas da Panasqueira, Portugal: in Mineralium Deposita v.53, pp. 171-194.
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Kelly W C and Rye R O, 1979 - Geologic, fluid inclusion and stabel isotope studies of the tin-tungsten deposirs of Panasqueira, Portugal: in Econ. Geol. v74 pp 1721-1822
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Polya D A 1989 - Chemistry of the main-stage ore-forming fluids of the Panasqueira W-Cu(Ag)-Sn deposit, Portugal: implications for models of ore genesis: in Econ. Geol. v84 pp 1134-1152
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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