Field Trip Report - Society of Economic Geologists

SEG Student Chapter of Geneva
Fieldtrip report of
Hercynian magmatism and associated
mineralization in the Northwestern Iberian
Peninsula of
Spain and Portugal
June 22nd to June 27th 2015
Participants
Students
Núria Bach Oller
MSc 2nd
Thomas Bovay
MSc 2nd
Julia Farré de Pablo
MSc 1st
Jonathan Lavoie
MSc 2nd
Merli Oliveras Segui
MSc 1st
Nuria Pujol Sola
MSc 1st
Samvel Hovakymyan
Post-Doc
Bertrand Rottier
PhD
Fieldtrip leader
Lluís Fontboté
Prof., University of Geneva
Andrea Dini
Prof., IGG Institute , Pisa
Program
Monday June 22
Visit of pegmatite outcrops and quarries close to Panasqueira.
Contact : Alexandra Carolino and António Almeida.
Night at Fundao/silvares, Rua Vasco Da Gama, 6230,375,
Fundao,tel.: +351275779930
Tuesday June 23
Visit of Panasqueira W-Sn (active) mine, Portugal.
Contact: Romeu Vieira
Night at Puerta del Sol, Rua del Sol 33, Ciudad de Rodrigo,
375000, Spain Tel.: +34923460671
Wednesday June 24
Visit of the Retortillo uranium project, Spain. If time
permitting, short visit to the Las Médulas Au paleo placer and a
World Unesco Heritage.
Contact: Enrique Martinez, Berkeley Resources.
Night at Hotel Monte Blanco, Cabana de Bergantinos, 15115,
Neano, Spain, Tel.: +34981714428
Thursday June 25
Corcoesto orogenic Au project and Arinteiro massive sulfide
deposit
Contact (Corcoesto): Lluís Boixet Martí, Geologist at Edgewater
Exploration Ltd.
Contact (Arinteiro) : Alberto Lopez Martin
Night at Hotel Las Cruces, Travesia la Veiga, 2, 33830 Belmonte
de Miranda, Spain. Tel.:+34985762319
Friday June 26
Boinás-El Valle Au-Skarn mine and processing plant, Spain.
Contact: Guadalup Collar Menéndez
Night at Hostal La Trucha, Calle de la Vina, 5, 49600
Benavente, Zamora, Spain, Tel. +34 980 63 42 70
Saturday 27
Flight to Geneva.
Leaving Porto to Geneva. Flight at 17h45.
Itinerary
June 22nd 2015 – The C-57 Lithium-rich pegmatite
The Northwestern Iberian Peninsula fieldtrip of the SEG student chapter of
Geneva has started at Pegmatitica PME head-office in Mangualde, 130 km southeast of
Porto in Portugal. We had a presentation on the C-57 pegmatite body by Geo. Alexandra
Carolino. C-57 pegmatite property has a superficy of 7 km2 with reserves of 2000 tons @
1.5 % Li. The main Li minerals are lepidolite ± zinwaldite, which are both Li-bearing
phyllosilicates. Until today, the pegmatite body has been exploited only for ceramic
industry regardless of its Li potential. Due to the increasing interest for lithium and the
Li-grade of the orebody, the mine is considering its extraction. Today, no metallurgical
technique exists to extract Li from Li-bearing phyllosilicates, even if similar process
plant is known for spodumene. Thus the exploitation of the pegmatite has stopped and the
company tries to find a way to solve this problem.
The C-57 pegmatite quarry is located near Gonçalo in the Zerere Valley. Several
Tin placer deposits were exploited in this valley during roman time. These deposits where
related to the alteration of cassiterite-bearing pegmatites. The C-57 pegmatite body
crosscuts a porphyritic granitic body which is part of the 320-290 Ma Beira batholith.
This hercynian igneous body forms the Cerro Estrella mountain belt and is affected by
numerous normal faults forming several horst and graben structures. It is also crosscut by
numerous aplitic-pegmatite dykes.
The C-57 pegmatite sill is a Li-bearing pegmatite of LCT (Lantane-CesiumTantale) type. It is up to 5 meters in thickness and its horizontal extension is superior to 7
km. The pegmatite sill was crosscut by several N-S normal faults creating a vertical
movement up to several meters. In the quarry, we could observe the sub-horizontal
contact between the granite and the pegmatite. The pegmatite body is strongly affected by
supergene alteration, and is now composed almost only of lepidolite (up to 50 volume
%), which is strongly resistant to alteration, kaolinite and quartz. At the upper border of
the pegmatite, big prismatic crystals up to 50 cm totally altered to kaolinite were visible
(Fig. 1 A,B). They presented a clear perpendicular cleavage and were interpreted as relics
of K-feldspars; later confirmed by observation of fresh part of the pegmatite. The bottom
part of the pegmatite was mainly composed by an alternation made of cm-thick layers
rich in lepidolite and K-felspars. At least two generations of lepidolites have been
observed. The first one is hosted in the layers of the pegmatite, and the second one
crosscuts the layering and the big K-feldspars.
Forty meters depth below the quarry, they found and excavated a fresh pegmatite
body. We were able to see some meter-sized blocs of this unaltered pegmatite. A basal
layering with succession of K-feldspar rich and Li-rich bands was visible (Fig. C,D).
Grain-size changes between the layers was visible, with bands more aplitic and others
more pegmatitic.
Figure 1: A) Altered pegmatite sills affected by a small normal fault; B) Altered K-feldspar (Kfs) transformed in
kaolinite and crosscut by lepidolite (Lpd); C-D) Fresh rocks of the pegmatite where layering texture is well visible.
The layers are mainly composed of an alternance of lepidolite and K-feldspar.
June 23rd 2015 – Mina da Panasqueira
The Panasqueira mine is located 40 km West of Fundao, and is a world famous
W-Sn vein-type deposit. The Company holds exploitation and exploration permits, and is
still active in both domains. The visit of the mine has started by an introduction of Geo.
Romeu Viera and the mining engineer. The morning has been spent underground using
the ramp access walking in order to see the room and pillar method for extraction, the
ore-shaft, and the underground train that brings the ore to the conveyor. The mining
technique is executed in three steps with pillars of 11x11 m, 3x11 m and finally 3x3
meters. The afternoon has been spent visiting the processing plant, the core shack and
ended in the mineral shop.
Underground visit
The mineralization occurs as continuous sub-horizontal veins up to 1m-thick.
These veins are hosted in metasediments presenting a vertical foliation and overlying the
western flank of a mica-rich granite. A greisen alteration area occurs in the upper part of
the granite and follows the contact between the igneous body and the metasediments. The
mineralization consists of 4 events: i) the first event contains mainly wolframite,
cassiterite, and silicates; ii) the second one is dominated by pyrrhotite, arsenopyrite, and
chalcopyrite; iii) the third one, also sulfide-dominated, contains mainly pyrite, marcasite
(replacing pyrrhotite), Sphalerite and galena; iv) the fourth one is formed by late
carbonates mainly siderite. Late barren quartz-carbonate sub-vertical veins crosscut the
ore bearing sub-horizontal veins.
During the underground visit we observed the big euhedral crystals of wolframite
(up to 30 cm) within quartz-dominated sub horizontal veins (Fig. 2 B,C). There were also
some cavities in the veins where generally euhedral crystals of quartz and other gemquality minerals like apatite were occurring. We were as well able to observe the subvertical barren vein crosscutting the mineralized vein. We unfortunately did not have
time to visit the greisen itself, but we had the opportunity to observe it later in the core
shack.
Panasqueira geological staff has a very unique way to control the ore grade. Indeed, they
cannot settle for sampling coupled with chemical analyses because of the important
nugget-effect resulting from pluridecimetric wolframite crystals in the ore bearing veins.
Therefore, they developed an in-house method based on the measurements of the long
and short axis of the wolframite per square meter in order to determine the grade. The
actual cut-off grade is 0.06 wt. % while Sn and Cu are recovered as by-products.
Processing plant
The Panasqueira mine recovers principally tungsten, tin and copper from the ore. The ore
is first grinded by a jaw-crusher in the underground mine and then transported by
conveyor to the processing plant. The separation between gangue, sulfides, and
wolframite + cassiterite is done with a shaking table. The sulphide fraction is crushed and
then put in a flotation system where chalcopyrite is separated from pyrrhotite and
recuperated. The wolframite + cassiterite fraction still contains some sulphides. Thus a
second type of shaking table is used in order to separate the remaining sulphides from the
W- and S-bearing minerals. This second table uses a shaking system coupled with a
flotation fluid where only hydrophobic sulfide such as chalcopyrite stick on. The product
is then separated into wolframite and cassiterite+siderite according to their magnetic
properties.
Figure 2: A) End of two sub-horizontal veins crosscutting the metasediments; B-C-D) Typical vein mineralogy and
texture with W: wolframite, Qz: quartz, Sd: siderite, Py: pyrite, Asp: arsenopyrite, Cpy: Chalcopyrite, Po(Mrc):
Pyrrhotite replaced by marcasite.
June 24th 2015 – Retortillo Uranium project
On the third day we visited the Retortillo uranium project that pertains to the
Australian company Berkeley Resources in the province of Castilla y Leon in Spain. The
project geologist Enrique Martinez and his co-workers from the mining engineering,
metallurgist, and environmental departments gave us a presentation. The talk was
followed by the observation of different drill cores belonging to the project.
The Retortillo project evaluates the potential to open a mine. The total estimated
resources are around 88.2 Mlbs of U3O8 with a cut-off grade of 200 ppm, taking in
account all the different targets. The region was already known for its uranium
mineralization potential, because of the occurrence of an old important uranium mine:
Minas Fé. The Retortillo deposit has been found using geophysic tools such as airborne
and ground radiometric surveys and then performed an important drilling campaign.
More than 98% of the drills are reverse-circulation drillings.
The U mineralization is hosted in the SGC (Schist-greywacke complex) mainly
composed of black-shales, which are covered by tertiary sediments (several tens of
meters thick), which is the main outcropping unit. The black-shales are affected by folds
and large NE-SW shear zones related to the alpine orogeny. The U mineralization
consists of pitchblende (UO2), coffinite (USiO4), and Brannerite (TiUO6) as primary
minerals and Autunite, Meta-autunite and Torbernite as secondary minerals. The
mineralization is not directly visible in the drill cores (Fig. 3) and the company uses a
Geiger counter.
Uranium mineralization seems to be linked by the leaching/reprecipitation process
of uranium hosted within the black-shales. The uranium grade within the fresh black
shales ranges from 60 to 200 ppm. According to the mineralizing model, regional
transpressionnal deformation induced the formation of a large NE-SW dextral shear zone
and also created dilatation structures within the black-shales. The fluid migrates along the
fault, which is a weakness area and acts as a channel for oxidized meteoric fluid
circulation that leaches the uranium bearing black shale. When passing again through a
succession of black-shales layers, the fluid is focused in the dilatation cluster due to the
pressure gradient and the uranium precipitates because this lithology plays the role of a
reducing wall.
Environmentally, the company did a lot of radioactivity survey in the area,
looking at the soils and the water. The purpose of this activity is to assess the actual
regional uranium concentration and also to rehabilitate the exploitation afterward. During
this visit, we could notice that mining for an energy purpose like uranium is a lot
different than classical polymetallic or precious metals mining because most of the cost
goes to the extraction, treatment, storage, rehabilitation than the exploration and mining
itself.
Figure 3: A) altered mineralized black shales; B) non altered mineralized black shales; C) Sample very rich in
uranium, small black lines on the core parallel to the foliation are small veins rich in pitchblende.
Las Medulas
In the afternoon, we travelled northwest toward Galicia and stopped at Las
Medulas. It is an Au paleo-placer exploited by the Celtics and then by romans. Las
Medulas is a typical continental deposit of high energy river. We could observe a clear
horizontal alternation of metric layers composed by an association of centimetric to
metric size rounded clasts (paleo-channel), and layers with finer material from paleoalluvial plains. Both layer types present the red-orange color characteristic of Fe3+ (Fig.
4).
To exploit the placer roman did numerous vertical and horizontal shafts inside the
hills (Fig. 1B). These shafts were filled with water in order to create water-induced
landslides. The water was transported from nearby mountains by a 50 km long water
channel system. We visited a part of the underground work realized by roman slaves.
Figure 4: A) Las Medulas landscape; B) Old roman gallery
June 25th 2015 – Corcoesto gold exploration project
The Corcoesto gold project is owned by the Canadian based exploration company
Edgewater Resources in the provincial of Galicia in Spain and the project geologist is
Lluis Boixet. The project is located in a dextral shear zone of the Malpica-Tuy gold-belt.
The reserves plus indicated resources are actually over 1 Moz Au with a cut-off grade of
0.5 g/T. The mining method will be open-pit mining with potential to go underground at
a later stage. The project is at the moment on hold pending the last authorization to move
forward with exploitation.
After a presentation we visited the core shack. The deposit is mainly hosted in the
orthogneiss with minor amount of micaschists and migmatites. The different
mineralization styles occur as discrete veins and sheeted veins (Fig. 5A). The gold
bearing veins are the antithetic riedel ones with a N70E orientation and contain from 1 to
150ppm Au. They are crosscutting the main foliation of the host rock. All other oriented
veins are barren.
A particular feature is that Corcoesto alteration is not typical of orogenic gold
where carbonate and chlorite minerals are often encountered. There is a potassicalteration halo around some veins but without magnetite. The highest grade occurred in
the pervasive silicification zone where there are thick greyish quartz veins.
Figure 5: A) Au-bearing qz-vein displaying high density occurence and mm-thickness; B) Secondary folds within
deformed host rock.
Arinteiro quarry
The second stop of the day was at Arinteiro, situated to the NE of Santiago de
Compostela, Galicia (Spain). Arinteiro is a
strongly metamorphosed
(amphibolite facies) volcanogenic hosted massive sulfide (VHMS) deposit, which has
been strongly deformed. Atlantic Copper has exploited the mine from 1972 to 1986 for
Cu, Au and Ag. The historical resources are estimated at 196 Mt @0.39% Cu (at 0.2%
Cu cut-off). Nowadays the mine is not being exploited anymore. The zone has been
rehabilitated and converted into a quarry where the amphibolite is exploited for aggregate
(Fig. 6A). Lundin Mining started a new exploration program in 2012, called “Touro
Copper Project”. According to new drillhole information the actual resources are
evaluated to 374 Mt @ 0.42% Cu, 0.03 g/t Au and 1 g/t Ag. Lundin Mining stopped
exploration in 2014.
The mineralization is formed by massive sulfide lenses from 10 to 60 m thick.
The sulfides are mainly pyrrhotite (> 95 volume %) resulting from the recrystallization of
pyrite during the metamorphism. The other sulfides are chalcopyrite, magnetite, pyrite,
and minor amounts of sphalerite and galena.
Figure 6: A)Old unexploited open pit from Arinteiro; B) Core shack
June 26th 2015 – El Valle Boinas
The Valle Boinas Au-Skarn Deposit is exploited by the company Orovalle and is
located in the southern part of the Río Narcea Gold Belt in the northwestern part of the
Hercynian Iberian Massif. The skarn is hosted by the Láncara formation, a series of
limestones and dolomites. This formation was intruded by granites, which then formed
the skarn by releasing hydrothermal fluids. The ore is mostly within the magnesium skarn
located at the bottom of the Láncara formation even though some part of the calcic skarn
is economically mineralized.
The visit started in the morning with an introduction of the deposit and the mining
activity. We visited two representative outcrops of the two mineralization style observed
in the skarn body. The deposit is characterized by two mineralization stages. The first one
is associated to the skarn and occurred at 300 Ma. The sulfide paragenesis is composed of
chalcopyrite, bornite, pyrite, arsenopyrite, magnetite, and pyrhhotite. The observed
grades are 3.5 g/t Au, 15 g/t Ag and 0.7 wt. % Cu. The second stage occurred at 260 Ma
and is thought to be related to an epithermal overprint. The ore grades are 7 g/t Au, 15 g/t
Ag and 0.7 wt. % Cu. Higher grades are encountered where the epithermal system
crosscuts the skarns. Leaching and enrichment are common in the structural zones that
host epithermal mineralization.
We were also introduced to the two different mining method used depending on
the hardness of the ore-bearing rocks. Because of its hardness, the skarn is mined through
a blasting method that consist in digging a hole underneath the ore-bearing zone. Then
the product of the blast is recovered and sent to the processing plant. In the softer
oxidized part of the deposit the more cost effective cut and fill method is used.
During the afternoon, we visited the processing plant. They obtain, as a final
product, two types of bullions made of an association of Au, Ag, and Cu with different
specific concentrations depending on the method they apply. The gravimetric method
ends up with the following concentration: 60% Au, 30% Ag and 10% Cu. Whereas the
Carbon-in-leach method, where dissolved Au, Ag, and Cu is adsorbed onto fine carbon,
gives bullions with the following concentration: 40% Au, 40% Ag and 20% Cu (Fig. 7A).
Figure 7: A) Copper, silver and gold ingot. B) Separation of copper, silver and gold by flotation.
Stewart R. Wallace fund 2014 spending
The student chapter of Geneva was awarded 1 500USD from the SEG through the Stewart R. Wallace
grant.
The funds have been used to organize 2 fieldtrips: A one-day visit of Aurania resources’ prospects in
Wallis, Switzerland, and a 6-days fieldtrip in Northern Portugal and Northwestern Spain. More details
on the content of both excursions can be found in the annual report and the field excursions report.
Below is detailed how the Stewart R. Wallace grant has been used.
Aurania resources’ prospects – May 1st 2015
To this excursion, organized together with the SEG Student Chapters of Beauvais and ETH Zurich,
participated 17 persons (16 Students and one professor) among whom 9 were from Geneva. The
total budget for this one-day excursion costed 126.5 CHF (9-seat minibus rental 80 CHF and gas 46.5
CHF). A 3 CHF was asked to the students, the rest (100CHf) was covered using the awarded Stewart
Wallace grant.
Northern Portugal and Northwestern Spain fieldtrip – 22-27 June 2015
To this fieldtrip participated 8 students and 2 professors as fieldtrip leaders. For this fieldtrip
additional funding have been awarded to the student chapter by Amira international (800 USD).
Minibus rental
Gas
Accomodation 22/06/2015, Hotel Samasa
Accomodation 23/06/2015, Hotel Puerta del sol
Accomodation 24/06/2015, Hotel Monte Blanco
Accomodation 25/06/2015, Hotel Las Cruces
Accomodation 26/06/2015, Hotel La trucha
Field guides printing costs
Total
Euro
899.34
356.49
175
200
225
235
154
139.6
2384.43
USD
2704
Plane tickets and food during the excursion remained at the participants’ charge.
Thanks to the grants awarded by the SEG (Stewart R. Wallace) and by Amira international to the SEGSC of Geneva, a participation of 50CHF only was asked to the students.
The 2014-2015 SEG-SC of Geneva committee wishes to thank the SEG for their financial participation
allowing us to organize excursions affordable for students with no income.