1. Guidebook
Field Mapping in Porphyry Copper Environments
Cerro Verde Mine
Arequipa, Peru
August 13-16, 2000
William X. Chavez, Jr.
Erich U. Petersen
2. Field Mapping in Porphyry Copper Environments
Participants
1. Brooks, William 105525.1436@compuserve.com
2. Carrizales L., Hector, Minera Teck Peru teckgeo@terra.com.pe
3. Tejada L., Walter, Minera Teck Peru teckgeo@terra.com.pe
4. Escobar B., Daniel, Minera Teck Peru teckgeo@terra.com.pe
5. Araya, Erick, Rio Algom Chile exploranto@rioalgom.cl
6. Alva, Roly, AngloGold Peru apardo@anglogold.com.pe
7. Sanchez, Carlos, AngloGold Peru
8. Waddell, Alistair, AngloGold Peru awadell@anglogold.com.pe
9. Palacios, Celso, AngloGold Peru
10. Anyosa, Nazario, AngloGold Peru apardo@anglogold.com.pe
11. Miranda, Juan, AngloGold Peru
12. Riquelme, Santiago, AngloGold Peru
13. Cortes Y., Jaime, Rio Tinto Chile jaimecortes@compuserve.com
14. Carrasco M., Pedro, Rio Tinto Chile rioanto@entelchile.net
15. Salazar S., Percy, Rio Tinto Peru mansa@rtzlima.com.pe
16. Palpan R., Ewald, Rio Tinto Peru mansa@rtzlima.com.pe
17. Paredes C., Angel, Rio Tinto Peru mansa@rtzlima.com.pe
18. Ferreira Espada, Eugenio, Rio Tinto Peru eugenio@rtzlima.com.pe
19. Pritting, Jack, Consultant/BHP Chile jrpritting@hotmail.com
20. Carrera N. Alex, student carreraalfa@hotmail.com
21. Nicolson, Diane, Noranda Peru dnicol@attglobal.net
22. Fava, Luis, Noranda Peru
23. Krischev, Yassen, Rio Tinto Europe yassen_khrischev@yahoo.com
24. Minaya Pastor, Henry, CEDIMIN geologia@cedimin.com.pe
3. Itinerary
13 August, Sunday
6:00 PM Assemble at Hotel Jarusalen
14 August, Monday
6:00 AM Breakfast
7:00 AM Depart from Hotel Jarusalen for Cerro Verde
8:00 AM Check in at Cerro Verde, Mine Safety
9:00 AM Mapping I: Cerro Verde Pit
1:00 PM Lunch
1:30 PM Mapping II: Cerro Verde Pit
5:00 PM Depart Cerro Verde for Hotel
Evening Session
15 August, Tuesday
6:00 AM Breakfast
7:00 AM Depart from Hotel Jarusalen for Cerro Verde
8:00 AM Mapping III: Santa Rosa Pit
12:00 PM Lunch
1:00 PM Mapping IV: Santa Rosa Pit
5:00 PM Depart Cerro Verde for Hotel
Evening Session
16 August, Wednesday
6:00 AM Breakfast
7:00 AM Depart from Hotel Jarusalen for Cerro Verde
8:00 AM Mapping V: Tour of Porphyry copper special features
1:00 PM Lunch
1:30 PM Mapping VI: Cerro Negro Tour
3:30 PM Depart for Airport
4. Acknowledgements
We wish to acknowledge the many individuals and organizations that made this
course possible. We thank Phelps Dodge and Sociedad Minera Cerro Verde S.A.A. for
granting generous access to the Cerro Verde and Santa Rosa deposits. Jim Jones and
David P. Braxton provided invaluable help in organizing the course. The Society of
Economic Geologists sponsored the mapping course.
Erich U. Petersen William X. Chavez, Jr.
Department of Geology and Geophysics Minerals & Environmental Engineering
The University of Utah Department
135 S. 1460 E., Room 719 New Mexico School of Mines
Salt Lake City, UT 84105 Socorro, NM 87801
801-581-7238 505-835-5252
Eupeter@mines.utah.edu wxchavez@nmt.edu
http://www.mines.utah.edu/pyrite
5. Geology of Cerro Verde
Cerro Verde y Santa Rosa, ubicados a 30 km al SO de Arequipa, son yacimientos
del tipo pórfido de cobre y molibdeno, emplazados en el sgmento sur del Batolito de la
Costa, Segmento Arequipa, Superunidades Tiabaya y Yarabamba. Localmente se tiene
rocas plutónicas intruidas por rocas subvolcánicas porfiríticas, volánicas y sedimentarias,
con edades que van desde el Precámbrico hasta el Terciario inferior. Remanentes de
ignimbritas, cenizas y arenas volcánica, corresponden a los últimos eventos ocurridos in
el área.
Las rocas plutónicas que engloban al os pórfidos subvolcánicos, genéticamente
relacionados a los depósitos de cobre porfirítico de Cerro Verde y Santa Rosa, conforman
el complejo intrusivo conocido como "La Caldera", estos pórfidos están alineados según
una dirección NO-SE, que es la misma de la Cordillera de los Andes. Fisiográficamente,
presentan una topografía madura con cerros redondeados de baja altura, quebradas secas
y drenaje dendrítico. Geológicamente estos depósitos se extienden más allá de sus
limites conocidos, tanto lateral como vericalmente, pero económicamente y en términos
operativos comprometen sólo parte de la extensión anterior. Dentro del área de estudio,
los principales tipos de rocas son: gneis, granodiorta, pórfidos de dacita-monzonita y
brechas de cuarzo-tourmalina, curazo-dumotierita y silíceas, estas brechas contienen
fragmentos de distintas litologías antes mencionadas.
El origen de estos depósitos está estrechamente vinculado al emplazamiento de
los stocks porfiríticos dacíticos-monzoníticos, de origen hipabisal. La edad asignada a
estas rocas es de 60 m.a. La alteración hipógena producida por estas intursiones tiene
una dstribución algo simétrica y concéntrica, con una zona potásica al centro, seguida de
una fílica, que es la más difundida y finalmente la propilítica envolviendo casi eteramente
a las anteriores. De forma asimétrica y sólo en algunas partes del depósito se presentan la
alteración argílica avanzada y la silicificación; la primera está muy relacionada con la
6. zona de enriquecimiento supergénico por calcosita y covelita; la mieralización dentro de
la zona de óxidos por brochantita, crisocola, malaquita y "pitch de cobre", pro encima de
estas zonas se encontraba una potente cubierta de minerales lixiviados y limonitas.
El fallamiento premineral sigue la dirección NO-SE del levantamiento Andino,
los cuerpos de pórdidos y brechas se alinean sigiendo esta misma drección; el fallamiento
tensional post-mineral revela una importante componente E-O también los sistemas NO-
SE y NE-SO.
Departamento de Geologia MCVSAA
7.
8. Cerro Verde Rocks
A suite of rocks collected in May 2000 in the Cerro Verde and Santa Rosa open
pits are described below. Many of the rocks were examined petrographically and
analyzed by X-ray diffraction. The plates can also be seen in the Mapping Course
website. The URL is http://www.mines.utah.edu/pyrite/mappingcourse.
Rock Plate A
A CV 0+0: Cerro Verde. Dacite-Monzonite Porphyry cut by hematite veins. All
mafic minerals and feldspars have been altered to white phyllosilicates. See
also Plate 1, A, C (7310027)
B CV 0+25: Cerro Verde. Dacite-Monzonite Porphyry cut by hematite veins.
All mafic minerals and feldspars have been altered to white phyllosilicates.
What criteria can we see in the actual rocks that indicate whether the
precursor mineral to hematite was chalcopyrite or chalcocite? See also Plate
1, B, D (7310026)
C CV 2633B: Cerro Verde. Dacite-Monzonite Porphyry cut by jarosite veins.
All mafic minerals and feldspars have been altered to white phyllosilicates.
All disseminated sulfides have been converted to jarosite. This suggests that
the pyrite was disseminated in the rock and dominated the vein filling. See
also Plate 2, A, B, C (7310025)
D SR Alun: Santa Rosa. Massive, tan-colored alunite vein about 6 cm thick. See
also Plate 2, D (7310023)
Rock Plate B
A SR 208: Santa Rosa. Tourmaline breccia. See also Plate 3, A, B (7310033)
B SR SW B: Santa Rosa. Silicified and pyritized dacite-monzonite porphyry.
See also Plate 3, D (7310032)
C SR 2558: Santa Rosa. "D" vein cuts dacite-monzonite porphyry. Note ~1.5
cm wide silicified halo on pyrite vein. See also Plate 3, C; Plate 4, A, B
(7310034)
D CVS: Cerro Verde. Cerro Verde Schist. Massive phlogopite; folliated. See
also Plate 4, C, D (7310030)
9. Rock Plate C
A BRECCIA: Santa Rosa. Polylithic breccia. Subrounded to angular fragments
range in size from less than 1 cm to over 5 cm. Breccias Silíseas. (8090003)
B BON: Santa Rosa. Chalcopyrite breccia from high-grade 'bonanza' zone.
Breccia fragments contain disseminated chalcopyrite. Chalcopyrite cements
angular breccia fragments. (8090005)
Photomicrographs Plate 1
A CV 0+0: Cerro Verde. Dacite-monzonite porphyry showing leached pyrite
(hematite-lined voids). Minor tourmaline is present throughout the rock. X-
ray diffraction shows that the sample contains considerable kaolinite. PL,
50X, FOV = 2mm (7310027)
C CV 0+0: Cerro Verde. Same as A. Photomicrograph shows that all
groundmass, feldspar and mafic minerals have been altered to white
phyllosilicate. XP, 50X, FOV = 2mm (7310027)
B CV 0+25: Cerro Verde. Dacite-monzonite porphyry showing leached pyrite
(hematite lined voids). Late torumaline occurs along fractures and is also
disseminated throughout the rock. PL, 50X, FOV = 2mm (7310026)
D CV 0+25: Cerro Verde. Same as B. Hematite boxworks after pyrite. PL,
50X, FOV = 2mm (7310026)
Photomicrographs Plate 2
A CV 2633B: Cerro Verde. Dacite-monzonite porphyry showing that all
disseminated precursor sulfides (pyrite) are altered to pale yellow jarosite.
PL, 50X, FOV = 2mm (7310025)
B CV 2633B: Cerro Verde. Same as B. Photomicrograph shows that all
feldspars and mafic mineral have been converted to white phyllosilicate. XP,
50X, FOV = 2mm (7310025)
C CV 2633B: Cerro Verde. 1200-micron wide 'vein' cuts from lower right to
upper left. "Vein" contains fragments of wall rock (microbreccia) in a matrix
of jarosite. Elsewhere, younger jarosite-only veins are developed within this
vein. PL, 50X, FOV = 2mm (7310025)
10. D SR Alun: Santa Rosa. Contact between vein fill and wall rock. Vein fill is
fine-grained (10 x 40 micron) alunite crystals (top right). XP, 50X, FOV =
2mm (7310023)
Photomicrographs Plate 3
A SR 208: Santa Rosa. Three habits of tourmaline are present. A fibrous, light
colored variety; a very coarse, pale-dark green variety that commonly forms
rosettes; and clusters of isolated crystals. Crystals of the latter variety can
range from 80 to 1000 microns but are commonly less than 100 microns long.
The fibrous variety commonly overgrows the coarse variety. The
"groundmass” is coarse mosaic quartz. PL, 50X, FOV = 2mm (7310033)
B SR 208: Santa Rosa. Same as A. XP, 50X, FOV = 2mm (7310033)
C SR 2558: Santa Rosa. Fine-grained quartz, white phyllosilicate and pyrite
(opaque). All the mafic minerals and feldspar have been converted to white
phyllosilicate. XRD shows that quartz, phyllosilicate, kaolinite and pyrite are
present. XP, 50X, FOV = 2mm (7310034)
D SR SW B: Santa Rosa. X-ray confirms that this sample consists of quartz,
white phyllosilicate and pyrite. White phyllosilicate pseudomorphs of
euhedral feldspar can be recognized. This sample is much coarser grained
than SR 2558 and contains no Cu-sulfides as disseminations or 'eggs'. XP,
50X, FOV = 2mm (7310032)
Photomicrographs Plate 4
A SR 2558: Santa Rosa. Vein pyrite showing "eggs" of chalcopyrite, bornite
and bornite-chalcopyrite. RL, 200X, FOV = 0.5mm (7310034)
B SR 2558: Santa Rosa. Disseminated sulfides in sample. Pyrite abundance is
greater than Cu-sulfide abundance. Dark blue is covellite; lighter blue is
chalcocite. Elsewhere covellite replaces chalcopyrite. Replacement by
covellite indicates low copper activity solutions. RL, 200X, FOV = 0.5mm
(7310034)
C CVS: PL, Cerro Verde. This sample has strong folliation that is imparted by
the alignment of pale green trioctahedral phyllosilicate ('phlogopite'). Knots
of chlorite and white phyllosilicate (relic feldspar?) are common. 100X, FOV
= 1mm (7310030)
D CVS: XP, Cerro Verde. Same as C. 100X, FOV = 1mm (7310030)
28. Useful References
Titley, S.R., Ed., 1982, Advances in Geology of the Porphyry Copper Deposits,
Southwestern North America. University of Arizona Press, Tucson, AZ, 560 p.
Titley, S.R. and Hicks, C.L., Eds., 1966, Geology of the Porphyry Copper Deposits,
Southwestern North America, University of Arizona Press, Tucson, AZ, 287 p.
Compton, R.R., 1985, Geology in the Field. John Wiley and Sons, New York, 398 p.
Billings, M.P., 1972, Structural Geology, third edition, Prentice Hall, New York, 606 p.
Davis, G.H., 1984, Structural Geology of Rocks and Regions. John Wiley and Sons, New
York, 492 p.
Pierce, F.W and Bolm, J.G., Eds., 1995, Porphyry Copper Deposits of the American
Cordillera. Arizona Geological Society Digest 20, 656 p.
Chávez. W.X., Jr., 2000, Supergene Oxidation of Copper Deposits: Zoning and
Distribution of Copper Oxide Minerals. SEG Newsletter No. 41, April 2000.
Phelps Dodge, 2000, Geologia de los Porfidos de Cobre Cerro Verde Y Santa Rosa,
Arequipa, Peru. Departamento de Geologia, 21 p.
