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- Baumgartner, Regina, et al.
(författare)
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Bracketing the Age of Magmatic-Hydrothermal Activity at the Cerro de Pasco Epithermal Polymetallic Deposit, Central Peru: A U-Pb and Ar-40/Ar-39 Study
- 2009
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Ingår i: Economic Geology and the Bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128. ; 104:4, s. 479-504
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Tidskriftsartikel (refereegranskat)abstract
- The Cerro de Pasco epithermal Cordilleran base metal deposit in central Peru consists of two mineralization stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. In order to constrain the duration of the magmatic-hydrothermal system at Cerro de Pasco, U-Ph age determinations and Hf isotope determinations have been conducted on single zircon grains. Biotite from intrusive rocks, and sericite and alunite related to the first and second mineralization stages, respectively, have been used for stepwise IR-CO2 laser Ar-40/Ar-39 age determinations. This study has been complemented by Rb-Sr analyses of sericite and associated pyrite. Single crystal zircon U-Pb analyses yield ages of 15.36 +/- 0.03 Ma for an accretionary lapilli tuff from a collapsed block within the diatreme, 15.40 +/- 0.07 Ma for it dacite porphyry dome in the diatreme, 15.35 +/- 0.05 and 15.16 +/- 0.04 Ma for two quartz monzonite porphyry dikes emplaced into the diatreme-dome complex that are within a time span of 350,000 yr. There is no field evidence for any magmatic activity predating the formation of the diatreme-dome complex. Stepwise IR-CO2 laser Ar-40/Ar-39 ages have been determined on sericite and alunite. The latter formed during the second mineralization stage in advanced argillically altered domes and from halos of oxidized enargite-pyrite veins in the Venecocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively, Ar-40/Ar-39 alunite ages cluster between 14.54 +/- 0.08 and 14.41 +/- 0.07 Ma. This age consistency suggests that advanced argillic alteration related to the second mineralization stage at Cerro de Pasco formed during a period of 100,000 yr. Three significantly younger alunite ages of 1.2.39 +/- 0.06, 12.13 +/- 0.07 Ma, and 10.94 +/- 0.10 M, probably reflect partial resetting by late circulating fluids. Therefore, it appears that magmatic-hydrothermal activity at Cerro de Pasco Pasco lasted about 1 m.y., from 15.4 to.14.4 Ma, similar to the duration in the nearby mid-Miocene Colquijirca district. An attempt to more precisely date the first mineralization stage, bracketed by the quartz monzonite. dikes (15.1 Ma) and the second mineralization stage (14.5 Ma) failed: the Ar-40/Ar-39 ages of sericite in the alteration halo of the pyrite-quartz body are not reliable, Probably due to inherited argon derived from Precursor micas in Paleozoic clasts of the diatreme breccia. This is indicated by Rb-Sr data on sericite which indicate binary mixing between Miocene magmatic mid Paleozoic basement sources and do not carry any age, information.
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| 2. |
- Slama, Jiri, et al.
(författare)
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Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis
- 2008
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 249:02-jan, s. 1-35
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Tidskriftsartikel (refereegranskat)abstract
- Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in inicrobeam U-Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plesovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U-Pb age with a weighted mean Pb-206/U-238 date of 337.13 +/- 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U-Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to similar to 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plesovice zircon (>0.9 wt.% Hf) is homogenous within and between the grains with a mean Hf-176/Hf-177 value of 0.282492 +/- 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plesovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.
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