Download Evidences for a Link betweenLa Caridad

Evidence for a Link between La Caridad Porphyry Copper Deposit and
La Caridad Antigua High Sulfidation Epithermal Deposit.
Victor A. Valencia1, Joaquin Ruiz1, Mihai Ducea1, Fernando Barra 1, Lucas Ochoa2
1
2
Department of Geosciences, University of Arizona, Tucson, Az. 85721, USA
Departamento de Geologia, Universidad de Sonora, Hermosillo, Son., 83000, Mexico
Spatial and temporal links between epithermal and porphyry copper deposits have
been documented in many areas in the Circum-Pacific Ring (Lepanto, Philippines,
Arribas et. al., 1995; Hedenquist et. al., 1998; Nevados de Famatina and La Mejicana,
Argentina, Losada-Calderon and McPhail, 1996; Maricunga Belt, Chile, Muntean and
Eunaudi, 2001). In region of Southwestern North America with more than 50 porphyry
copper deposits, the transition between porphyry and epithermal ore deposits is poorly
documented. Here we present field observations, major and trace element data along with
Rb-Sr and Re-Os measurements, which suggest that La Caridad, a porphyry coppermolybdenum deposit and La Caridad Antigua, a high-sulfidation epithermal system, are
related.
The La Caridad porphyry is a world-class copper-molybdenum deposit located in
northeastern Sonora, Mexico, and is the most productive Cu deposit in Mexico (~150,000
ton Cu/y). Reserves as of 2001 were ~four million tons of Cu and 250,000 tons of Mo.
The deposit occurs within igneous extrusive rocks, which range from andesites to dacites.
These host rocks were intruded by diorite, granodiorite, quartz monzonite porphyries and
irregular pegmatites bodies of Laramide age. Mineralization at La Caridad deposit is
associated to the quartz monzonite porphyry and took place over a short period of time,
as is shown by preliminary Re-Os ages in molybdenite associated with hydrothermal
alteration. Molybdenite associated with potassic alteration yields an age of 53.8 ± 0.3 Ma
(2) and molybdenite associated with phyllic alteration yields an age of 53.6 ±0.3 Ma
(2).
La Caridad deposit is cut by La Caridad postmineral fault in the eastern part of the
deposit, this is a NW-SE striking normal fault with a 45o NE dip.
La Caridad Antigua is located at three km east of La Caridad ore body, in the
hanging wall of La Caridad Fault, and was described by Wandke (1925). This deposit
was mined in the beginning of the past century (1907-1917) and is hosted in latites and
rhyolites (K-Ar in biotite 51.3 ± 2.0 Ma (2), Livingston, 1973) that are intruded by a
small quartz feldespatic porphyry. This deposit contains a acid-sulfate epithermal mineral
association, which includes pyrophillite, kaolinite, sericite, alunite, quartz and barite, as
alteration minerals along with a variety of sulfides including chalcopyrite, pyrite,
enargite, bornite and tennantite. This event generated massive a silicic alteration halo that
extends for more than 100 m laterally away from the deposit.
We analyzed Rb-Sr for 8 samples from mineralized and pre-mineralized rocks, in
both deposits. Four samples of mineralized whole rocks from the La Caridad pit and
from La Caridad Antigua yield an isochron with a 87Sr/86Sr initial ratio of 0.7075 and an
age of 50.6 ± 4.8 Ma. Although the error on the age is somewhat large it overlaps with
the molybdenite ages. This correlation along with the similar initial isotopic ratios
suggests a co-magmatic origin for both deposits. Pre-mineralization host rocks have
87
Sr/86Sr initial values ranging from 0.7065-0.7068 (calculated at 53.6 Ma) with a gradual
increase which correlates with increase of silica content.
La Caridad Antigua is believed to represent the high level portion of a porphyry
copper system with a elevation between both deposit of ~2-3 km. Although the difference
between the K-Ar ages of the La Caridad Antigua deposit and the Re-Os ages of the La
Caridad deposit suggest that they are two separate events, the intimate spatial association,
the identical
87
Sr/86Sr initial values and the identical ages for the host rocks of the
porphyry and epithermal mineralization combined suggests a genetic relation between the
La Caridad and La Caridad Antigua ore bodies. We suggest that the K-Ar age is probably
disturbed and it is more likely that the two deposits were formed from a single magmatichydrothermal system that evolved from porphyry Cu-Mo deposit at depth to a high
sulphidation epithermal deposit at shallow levels.
Acknowledgements
We are most grateful to Grupo Mexico for allowing access to the mine operations and
logistics (Ing. Remigio Martinez, Ing. Narcizo Javier Olvera, Ing. Jose Contla, Ing.
Marco Figueroa), and to the Arizona Geological Society for generous financial support.
In particular, we would like to thank to Dr. John Chesley Dr Chris Eastoe, Sergio Castro
for their help and for interesting discussions.
References
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