David P. Braxton
Thesis: Thermo-chemical Profiling WithinThe Alteration Envelope: Chlorite Geothermometry and Mass Transfer Around the Au-Ag Calera Vein, Orcopampa, Peru
Support: University of Utah, Cia de Minas Buenaventura, S.A.
The Calera vein system of Orcopampa Peru is a fault-controlled epithermal Au-Ag deposit, flanked by an extensive, zoned alteration envelope. Silicic-argillic alteration (quartz + illite + pyrite + kaolinite) and phyllic alteration (quartz + illite + pyrite ± calcite) dominate within 20 meters of the vein, while propylitic alteration (chlorite + illite + calcite + pyrite + albite ± smectite ± epidote) persists outward for hundreds of meters from mineralized structures. Electron microprobe analyses of chlorite within the propylitic alteration zone reveal a systematic increase in tetrahedral Al occupancy with increasing proximity to the Calera vein. Empirically linked to formation temperature, this compositional variation defines vein-centered thermal gradients cooling outward from 370oC to 280oC over 100 meters.
Maximum homogenization temperatures from secondary fluid inclusions in quartz phenocrysts correlate well with chlorite formation temperature maxima, supporting the assertion that chlorite composition is temperature sensitive. The illite crystallinity index, known to vary inversely with formation temperature, also validates the use of chlorite as a geothermometer. Ilite crystalinity indices are high (0.76-0.84o2theta) in samples with low average chlorite formation temperature (270oC-290oC), and low (0.42-0.52o2theta) in samples with high average chlorite formation temperature (322oC-338oC).
Enabling estimates of mass transfer in the altered wall rocks adjacent to the Calera vein, whole rock geochemistry documents the progressive conversion of propylitic to phyllic alteration. Rocks with strong phyllic alteration are enriched in SiO2 by up to 40% and are largely devoid of original Na2O, CaO, MnO, and MgO. Rocks 'within the propylitic zone show considerably less variation in enrichment Idepletion factors, except within 30 meters of the phyllic-propylitic transition, where systematic depletion in Na2O and destruction of smectite indicate incipient conversion to phyllic alteration. Given the lack of any clear relationship between Al in chlonte and whole rock Al enrichment, chlorite composition is probably not significantly influenced by differential alteration within the propylitic zone, further supporting chlorite as a geothermometer.
Cooling trends around the Calera vein persist for at least 100 meters outward into the propylitic zone. These thermal gradients provide large-scale vectors toward mineralized centers reaching far beyond the proximal zones of silicic-argillic and phyllic alteration. Chlorite geothermometry may well prove to be a useful exploration tool in similar volcanic-hosted epithermal districts.
Dave commutes to field area.
Outcrop of Calera Vein, Orcopampa, Peru.
Bonanaza stage mineralization, Calera Vein, Orcopampa, Peru.
Location of Orcopampa District, Peru.
Presumed thermal zoning pattern.
Correlation of illite crystalinity with average chlorite temperatures.
Chlorite geothermometric results.
Dave leads discussion on tourmaline breccias. (SEG Short course on mapping in porphyry copper systems, Cerro Verde, Arequipa, Peru, 2001) .
Dave reviews stereonet diagram of course participant. (SEG Short course on mapping in porphyry copper systems, Cerro Verde, Arequipa, Peru, 2001).
Braxton, D.P., and Petersen, E.U., 1999, Thermal aureoles of fossil epithermal systems: A case study from the Calera Vein, Orcopampa, Peru: Anniversary Volume No. 5 "75 th Anniversary of the Geological Society of Peru", 81 - 102, (invited paper). pdf
Braxton, D.P., 1997, Thermo-cemical profiling within the alteration envelope: chlorite geothermometry and mass transfer around the Au-Ag Calera vein, Orcopampa, Peru. MS. Thesis, University of Utah, 90 p.
Braxton, D.P. and Petersen, E.U., 1966, Profiling thermo-chemical zonation distal to epithermal Cu-Au-Ag Systems, Orcopampa, Peru. Geological Society of America Abstracts, 28, 7, A-334. Theme session on Transition between porphyry copper and epithermal systems.