Economic Geology Vol. 70, 1975, pp. 388-404

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CHRONOLOGY OF LATE TERTIARY VOLCANISM AND MIN- ERALIZATION, HUACHOCOLPA DISTRICT, CENTRAL PERU

Introduction and Geologic Setting The Huachocolpa District, part of the base and

precious metal province of central Peru (Petersen, 1965), is located near the crest of the Andes in Huancavelica Department.

Rocks of the District consist of a basement of moderately to strongly folded strata of Paleozoic, Mesozoic, and early and middle Tertiary age uncon- formably overlain by late Tertiary volcanic and vol- canogenic sedimentary rocks (Salazar, 1970; Noble, 1971; Noble and others, 1972, 1974). The late Tertiary rocks are the surficial part of a major vol- canic and plutonic belt and consist of great volumes of andesite, dacite, and rhyodacite that form an inter- penetrating complex of domes, plug domes, dikes, flows, and composite volcanoes. The intrusive rocks of the belt are represented by such bodies as the batholith of the Cordillera Blanca in northern Peru (Noble and others, 1972). In many places in the Huachocolpa District the middle and late Miocene and Pliocene volcanic rocks are moderately to strongly propylitized and are cut by a widely dis- tributed system of generally steeply dipping east- west-, west-northwest-, or east-northeast-trending Pb-Zn-Ag veins.

The domes, flows, and breccias also are cut by a distinctive suite of discontinuous and somewhat ir- regular north-south-trending dacite-rhyodacite dikes and plug domes. In contrast to the rocks that they intrude these dikes and domes are virtually unaltered. The dikes cut major vein systems in the Blenda Rubia and Tangaria mines, and dikes and a plug

dome intrude highly altered and mineralized rock at and near the Consuelo mine in the vicinity of the town of Huachocolpa. That the intrusive bodies are younger than mineralization is confirmed by detailed mineragraphic studies of ore minerals adjacent to the dike in the Blenda Rubia mine (Ulrich Petersen, unpub. data).

Radiometric Dating Potassium-argon age determinations (Table 1)

were made in the laboratories of the U.S. Geological Survey in Menlo Park, California, using standard isotope-dilution techniques for Ar analysis (Dal- rymple and Lanphere, 1969) and flame photometry with a lithium internal standard for 1{20 analysis. The plus-minus value represents estimated analytical uncertainty of Ar and K analysis at one standard deviation. The specimens dated were carefully col- lected from unaltered outcrops. Their unaltered condition was confirmed by examination in thin sec- tion. In all samples the biotite grains appeared fresh, with the exception of a small amount of deu- teric(?) chlorite in the biotite of specimen 3. The internal consistency of the age determinations and the high K20 contents of the biotite separates (a measure of alteration) combined with the petro- graphic data argue against the possibility of K-Ar systems reset by later thermal or alteration events.

Results

Samples 1 and 2, for which ages of 10.4 m.y. and 10.1 m.y. (Table 1) were obtained, are blocks of

TABLE 1. K-Ar Analytical Data for Samples from the Huachocolpa District, Central Peru (Constants: K4X, = 0.584 X 10-X/yr. Xa = 4.70 X 10-X/yr. Atomic abundance K 4 = 1.19 X 10 -4.

Argon analyses by E. H. McKee; Potassium analyses by L. B. Schlocker)

Radiogenic Ar 4 Radiogenic Sample Location KO (moles/gm Ar Age

no. Field no. Lat S. Long W. (wt %) X 10 -x) (percent) (m.y.) 1 B-25 1259 ' 75002 ' 8.29 1.274 67.9 10.4 4- 0.3 2 B-60 1304 ' 75001 ' 8.09 1.208 67.6 10.1 4- 0.3 3 B-69 13006.3 ' 74055.2 ' 8.09 1.167 50.7 9.7 4- 0.3 4 B-23B 12058 ' 75001 ' 9.29 1.251 46.7 9.1 4- 0.3 5 B-29 13003.3 ' 74054.6 ' 8.63 1.043 43./ 8.2 4- 0.3 6 B-8 13006.6 ' 74057.5 ' 8.87 0.5995 44.8 4.6 4- 0.2 7 B-19 1308.4 ' 7458.7 ' 8.36 0.4654 41.3 3.8 4- 0.2 8 B-36 13002.8 ' 74.57.4' 8.76 0.4728 25.6 3.7 4- 0.2

388

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10 o 770 780 750 ! r r

Cede De Pasco Colquiji rca

La Oroya

O 3, Moro(:oc ha %LIMA

CD " Yauricocha

I I 7 70 760

Huancayo 0

Huacavelica Juh:ani

13 '

Huachocolpa Ayacucho

Alunsulla

, 750 \

7foo'

Chosecc demi' /

complex 5 Min

. TaREnrta , ..... Trnilut ,0,:,,,: /... center

Consuel, [ Nunscar

o t 2 4 . gfna Maeckayils Viii,meters glenda / YelClelC

lukii cemplex

Fro. 1. Location in south-central Peru of Huachocolpa 'and an enlargement of that district.

rhyodacite from tuff breccia of the Tinqui com- posite volcano. Rocks of this volcanic center, which overlie tightly folded volcanic and fresh-water sedi- -mentary rocks of early Tertiary age, consist of thick .outwardly dipping sequences of interbedded tuff 'breccia, tuff, lava, and volcanic sandstone intruded by irregular radial dikes, an equant central(?) plug, and other irregular intrusive bodies (Noble, 1971).

Specimens 3 and 4 are from major dome com- plexes. Field evidence indicates that the Chosecc dome (specimen 3; 9.7 m.y.) postdates the Tinqui stratovolcano, whereas the time relations of the Manchaylla Complex (specimen 4; 9.1 m.y.) to the Tinqui center have not been unequivocally deter- mined in the field. The relatively small Huascar in- trusion (specimen 5; 8.2 m.y.) postdates the Man- chaylla Complex and from field relations appears to be one of the younger members of the late Miocene suite of central volcanoes and domes.

Closely spaced ages of 4.6, 3.8, and 3.7 m.y. (speci- mens 6, 7, and 8) were obtained on three representa-

tive specimens of the younger rhyodacite dikes and domes.

Conclusions

The unconformity between virtually undeformed volcanic rocks of the Huachocolpa and nearby Julcani Districts and underlying moderately to strongly de- formed early and middle Tertiary rocks provides a younger limit of about 10.5 m.y. on a major pulse of late Tertiary deformation in this part of the Peruvian Andes (Noble and others, 1972, 1974). The age of the Pb-Zn-Ag veins of the Huachocolpa District is bracketed by the 10.4- to 8.2-m.y.-old volcanic host rocks that they cut and the 4.0-m.y.- old dikes that cut the veins; the later stages of min- eralization may overlay the time of dike emplacement and thus be about 4.0 m.y. old.

Acknowledgments This study would have been impossible without

the generous assistance of a number of individuals of

390 SCIENTIFIC COMMUNICATIONS

Cia. de Minas Buenaventura, S. A., particularly Elmer Vidal D. and Socrates Nufiez L. A part of the work was supported by National Science Founda- tion Grants GA-1546 and GA-35756 to Noble.

EDWIN H. MCKEE DONALD C. N0,BLE ULRICH PETERSEN :MARIO ARENAS F. ALBERTO BE:AVIDES Q.

E. H. M. U.S. GEOLOGICAL SURVEY

MENLO PARK, CALIFORNIA 94025

D.C. N. AND M. A. F. CIA. DE MINAS BUENAVENTURA, S.A.

LIA, PERu

U.P. DEPARTMENT OF GEOLOGICAL SCIENCES

HARVARD UNIVERSITY CAMBRIDGE, MASSACHUSETTS 02138

A. B. Q. PONTIFICIA UNIVERSIDAD CATOLICA DEL PERU

LIMA, PERU May 31, September 11, 1974

REFERENCES

Dairytopic, G. B., and Lanphere, M. A., 1969, Potassium- argon dating: San Francisco, W. H. Freeman, 258 p.

Noble, D.C., 1971, Preliminary report on the Cenozoic geology of the Huachocolpa District, Huancavelica Depart- ment, Peru: Private report, Cia. de Minas Buenaventura,_ S.A.

Petersen, Ulrich, McKee, E. H., Arenas F., M., and_ Benavides Q., A., 1972, Cenozoic volcano-tectonic evolu- tion of the Julcani-Huachocolpa-Castrovirreyna area, cen- tral Peru: Geol. Soc. America Abstracts with Program,. v. 4, p. 613.

Noble, D.C., McKee, E. H., Farrar, Edward, and Petersen,. Ulrich, 1974, Episodic Cenozoic volcanism and tectonism in the Andes of Peru: Earth Planet. Sci. Letters, v. 21,_ p. 213-220.

Petersen, Ulrich, 1965, Regional geology and major ore de- posits of central Peru: EcoN. GF. oL, v. 60, p. 407--476.

Salazar, D., H., 1970, Mapa geologico del Dpto. de Huan- cavelica: Senrico Nacional de Geologica y Mineria [Peru],_ escala 1: 250,000.