GEO 381R Field Excursion to the Iberian Peninsula

In May 2001, graduate students Christine Fox, Karah Wertz, and James McGuire participated in a tour of Portugal and Spain, a region with an ancient mining history.  The focus of the trip was the Iberian Pyrite Belt (IPB) that contains numerous large volcanic-hosted massive sulfide (VMS) deposits, the ancient equivalents of modern sea-floor hydrothermal vent systems. The students spent Spring Semester studying the geology and mineral deposits of the Iberian Peninsula, including preparing a contribution for a guidebook on a topic of interest.  Over two weeks, the students toured many of the larger ore deposits in southern Spain and Portugal, as well as experiencing Iberian culture and history.  Thanks to generous funding from the Geology Foundation, the students were able to gain direct experience on the regional geologic framework and mineralization in Iberia that they had studied in class.

The group, sponsored in part by the Society of Economic Geologists, met in Lisbon, Portugal, and consisted of 22 students and professionals from the University of Utah, New Mexico Tech, and the University of Texas at Austin (for additional coverage, see ).  Fees collected from participating institutions and companies helped defray transportation and housing costs during the IPB tour.  Group leaders were Dr. Bill Chavez of New Mexico Tech and Dr. Erich Petersen of the University of Utah; local geologists provided a great deal of detailed information at each major site.

The first stop on the tour was the relatively new Neves-Corvo mine, outside of the small Portuguese hamlet of Castro Verde where the group enjoyed terrific seafood.  The Neves-Corvo deposit is a blind orebody that was discovered almost entirely with geophysics and is a very large VMS deposit characterized by high copper grades and high tin zones.  At Neves-Corvo, the group learned the frighteningly simple, but effective, approach used when logging core in the IPB: a basal metamorphic phyllite-quartzite (PQ), overlain by a volcano-sedimentary sequence (VS), capped by the flysch group.  Typically, VMS orebodies like those at Neves-Corvo are found at the top of the VS.

The next stop was the inactive Aljustrel VMS deposits that currently are undergoing a feasibility study for zinc production.  Students examined cores for orebodies at Aljustrel that averaged 1% Cu and 5% Zn, and learned an important lesson about how easily sphalerite can be overlooked when logging core.  The group moved on to Lousal, a site of Roman tunnels precariously perched on steep hillsides, where children were forced to labor in the gossan to provide gold for the Empire.  The Sao Domingo deposits, next on the itinerary, are VMSs initially operated as an open pit by the Romans, but more recently were used to provide pyrite for the production of sulfuric acid.  Acid mine drainage from the workings had a pH of 2 (!), and both Sao Domingo and Lousal have received attention recently as the Portuguese have begun to recognize the environmental legacy of abandoned VMS mines.

Nowhere was these effects more apparent than at the groups next stop, Spains historic Rio Tinto open pit mine.  At the mines museum, mining implements from the area believed to be 9,000 years old were displayed.  There has been almost continuous activity for the past 2,000 years, as the Romans and Phoenicians initially exploited its riches.  In excess of 500 million tons of massive sulfide ore have been removed from the main orebody.  Rio Tinto is so named because of the blood-red color of the river the captures all of the runoff from the area.  The Rio Tinto has a pH of 2, and biologists who specialize in extreme environments study its oxygen-generating bacteria, the only creatures hardy enough to live in it.

At Spains Aguas Teidas, Spanish for dyed waters, the group saw more small VMSs with acid mine drainage problems.  Due to these historic environmental problems, the Las Cruces mine, another Spanish VMS deposit visited by the group, has been developed with very strict environmental standards.  The Las Cruces orebody has no surface expression and was discovered by drilling a gravity anomaly.  Production will begin soon and Las Cruces will be one of Europes largest mines, meeting 6% of its copper demands.  After visiting the Las Cruces offices in Seville, students were given a day to experience the wonders of the city.

Los Frailes, the last VMS system visited, was perhaps the most infamous.  A tailings dam broke in 1998, spilling its contents down a river to a sanctuary that is the breeding grounds for many European waterfowl.  Poor evaluation of the properties of the underlying marl led to the dam failure.   The resulting environmental disaster is a primary reason that the mine is set to shut down in the next year, though significant reserves and resources remain.  Witnessing disasters like this made everyone acutely aware of the need for interaction between geologists and engineers in all phases of a project.

After Los Frailes, the group spent a day in Cordoba.  The days festivities included the Cordoba fair, visiting La Mezquita, a Moorish mosque built 1200 years ago, and viewing Roman ruins.  The remaining destinations would be out of the IPB, examining some of the other ore deposit types in central Spain and Portugal.

Almaden, a mercury deposit that is the worlds oldest continually active mine, has accounted for at least one-third of global mercury production. Regardless of the economic aspects, Almaden is a remarkable geochemical anomaly essentially without equal for any element on the planet.  These stratabound cinnabar deposits are monometallic and thought to be related to seafloor mafic volcanism.  Grades are as high as 20% mercury, but the cutoff grade is 3.5%.  Students descended to an active stope and broke open pieces of ore to find droplets of native mercury and shiny red crystals of cinnabar in the shale host.

The group explored the scenic mountain-top villa of Marvo, Portugal, on the way to Panasqueira, the last tour on the itinerary.  Panasqueira is a tin-tungsten mine located in north-central Portugal, and is known worldwide by mineral collectors for its assortment of large crystals of quartz, apatite, arsenopyrite, and wolframite.  The two- to five-ft wide quartz veins that host the tin-tungsten minerals formed during the Hercynian orogeny (~300 Ma).  After the mine tour, each member of the group was given a prize mineral specimen. 

Each member of the group returned to Lisbon having greatly increased their respective knowledge of Iberian geology and ore deposits.  Equally impressive were the increases in luggage weight, as despite my best efforts at being selective, my bag contained nearly 40 lbs of rocks!  The success of this trip, and previous GEO 381R trips to Chile in 1999 and Per in 2000, bodes well for opportunities for future international field courses.


Modified from story by Mac McGuire in the 2002 DGS Newsletter



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