Work Completed in this Study

1.6.1 Reviewed Literature

As the Central Andes is the end member example of Phanerozoic continental convergent margins and is also prolifically mineralised there is a rich and diverse body of previous research. The literature reviewed and cited in the study was largely sourced from the University of Tasmania Science and Technology library in Hobart, and from the collections of the author and of several members of staff at the University of Tasmania. The author also visited the Chilean Geological Survey (SERNAGEOMIN) library in Santiago for two weeks to access Latin American literature that is largely unavailable in Australia or online. The extended abstracts volumes from the triennial Congreso Geológico Chileno proved a particularly rich source of relevant literature not published elsewhere. Numerous unpublished resources were also acquired from the in-house libraries of the companies working in the project area. All literature sources are cited at appropriate locations in the text.

1.6.2 Field Work

Field work was carried out by the author over three visits to the Collahuasi district for a total of eight months based at Collahuasi, three weeks at Quebrada Blanca, and two weeks in the Quebrada Guatacondo-Copaquire area. Total field activity included detailed (1:200-500) mapping of ~5 km of excavated faces along roads and in open

pits (Fig. 1.6), 1:5,000 scale district mapping of approximately 150 km2 _{(a 30 x 5 km}

belt) and graphic logging of more than 15 kms of diamond drill core.

Figur

e 1.6.

An example of methods of raw data collection for this thesis, showing a vertical face sketch map of a road cut section along the main access road between the

Collahuasi camp and the Rosario Mine. In this complex section, aspects of the volcanology of the upper La Grande formation are evident, as well as Permo-T

riassic faults and

Early

Triassic dykes that were emplaced in the terminal magmatic event of the Choiyoi

core, and structural mapping in the Rosario and Ujina open pits and of new road and conveyor cuts in the eastern part of the district. The roads between Coposa, Ujina and Rosario mines and a new 12 km-long cutting for the Rosario – Ujina conveyor provided exposures that added significantly to the stratigraphic and structural information available. Regional (sterilisation) drill holes were re-logged in detail to augment locally sparse outcrop evidence of the relationships and correlations between units.

Work conducted between October and December 2004 included traverse mapping of the Domeyko FS in the Quebrada Guatacondo, regional mapping at Quebrada Yabricoyita, Cerro Chigliuno, on-going detailed structural mapping in the Rosario, Ujina and Quebrada Blanca open pit mines, and logging of oriented core at Cerro La Grande. The Rosario Pit was an extremely dangerous site throughout the study period and access was limited for safety reasons. The supergene resource at Ujina had been extracted by this time and the Ujina Pit was actively being backfilled, which also restricted access in some cases.

1.6.3 Map Compilation and Cartography

Map data compilation was done by the author between 2005-06. The mapping

incorporates all the existing map data available in 2004-05, as well as the lithological data collected from drill holes logged in this study, and the mineralogical data from the entire CMDIC exploration database. The stratigraphic and structural geometry presented is the simplest that satisfies the field data.

Cartography was done by the author in 2007-09 using ESRI ArcGIS 9.2 software with technical assistance from Willy Lynch at ESRI and Aysegul Domac at Newmont Altin Madencilik. A reduced-scale version of the new district geology map sheet is presented in Chapter 3, and a poster-size version with greater detail is included as an insert at the back of the thesis.

1.6.4 Petrography

Hand specimen and thin section petrography underpins the stratigraphic correlations presented in Chapter 3, and a table of petrographic descriptions is contained in the appendices. Petrography was conducted by the author at the University of Tasmania,

using a standard petrological microscope. Approximately 250 thin sections were investigated in this study, a small subset of which derived from Masterman’s PhD study (Masterman, 2003). The results of the microscopic investigation were used routinely to clarify or constrain more general hand specimen descriptions made by the author while mapping in the field.

1.6.5 U-Pb Geochronology

Zircon U-Pb geochronology provides a temporal framework for the mapped stratigraphy presented in Chapter 3, and constrains the interpretation of previous geochronological studies that utilised other methods. Heavy mineral separation and laser ablation ICP-MS geochronology was performed by the author in July 2004 and June 2006, with a small number of samples prepared and analysed by Dr. Sebastien Meffre at CODES in May 2005.

1.6.6 Geochronology 40_Ar/39_Ar

40_Ar/39_{Ar geochronology was used in this study to date magmatic emplacement of}

rock units that lack abundant zircon and to date hydrothermal events. Hornblende and illite separations were performed by Amelia Rainbow at Queens University, Kingston, Ontario. Geochronolgical analyses were performed by the author in September, 2005, under the supervision of Dr. Doug Archibald at Queens University.

1.6.7 Electron Microprobe

Electron microprobe analyses were undertaken during this study to determine the

potassium content of samples used in 40_Ar/39_{Ar geochronology and to investigate}

the chemistry of minor accessory phases in some magmatic rocks. Analyses were performed by the author under the supervision of Dr. Karsten Goemann at the Central Science Laboratories, University of Tasmania, in late 2006.

1.6.8 Structural Analysis

The results of structural mapping are presented in Chapter 4. Structural analysis was done by the author during field mapping and intermittently thereafter at the University of Tasmania. There are few published examples of the analysis of stockwork vein geometries such as that attempted in this study, and interpretation

has relied heavily on the results of several fundamental works on brittle structural geology and transpressive fault systems (Etchecopar et al., 1981; Sanderson and Marchini, 1984; Hancock, 1985; Dunne and Hancock, 1994; Tikoff and Teyssier, 1994; Jones and Tanner, 1995; Jones et al., 1997; Casas et al., 2001; Jones et al., 2004).

1.6.9 U-Th/He Thermochronology

U-Th/He thermogeochronology is used in this study to place constraints on the magnitude and timing of relative vertical displacements across principal faults. Grain picking and analyses were performed by members of Dr. Peter Reiners’ team, then at Yale University, Connecticut, USA, during late 2005 and early 2006. Interpretation of the geological significance of the data was done by the author.

1.6.10 Whole Rock Geochemistry

Whole rock geochemistry of least-altered magmatic rocks is used in this study to constrain the tectonic environment of basement volcanism and sedimentation and to make inferences about the tectonic environment of fertile magma generation (Chapter 5). X-ray fluroescence (XRF) and laser ablation ICP-MS analyses were performed by Dr. Phil Robinson at CODES in August 2004, May 2005, and July 2006. Further LA ICP-MS and XRF geochemistry was done at the Ontario Geoscience Laboratories, Thunder Bay, Ontario (OGS), in collaboration with Dr. Peter Hollings at Lakehead University.

1.6.11 Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) mineral mapping

ASTER spectral filter mineral maps were made during this study as a comparator to alteration mapped in drilling and at surface (Chapter 4). ASTER data was generously donated by Bob Agar, a consultant to Newmont Asia Pacific, in Perth, Australia. The data was processed by the author in August 2007, but the raw imagery is younger than the mining operations, so spectral anomalies in the vicinity of the major deposits (notably over cover sequences east of Ujina) are likely to be anthropogenic. Little of geological value could therefore be drawn from this work and the results are not presented.

1.6.12 Concurrent and Subsequent Studies

This study ran in parallel with the broader goals of AMIRA International project P765. It is therefore associated with-, and provides important background to investigations of wholerock geochemistry of altered rocks and the trace element chemistry of alunite, enargite and epidote that were important aspects of that broader research project.

Shortly before this study commenced, Anglo American had sponsored the PhD project of Esteban Urqueta, that dealt primarily with surface lithogeochemical mapping across the eastern Collahuasi district. Publications related to this study include the conference abstract of Oates et al. (2007), and Urqueta et al.’s (2009) paper concerning the major results of their lithogeochemical study.

During field work for this thesis, Professors Victor Maksaev and Fransisco

Munizaga at the University of Chile in Santiago were working on a regional Re-Os geochronology and Os isotope study of Chilean porphyry belts, although the results are yet to be published. Subsequent to reporting of mapping and geochronology results contained in this thesis, Munizaga et al. (2008) published 13 SHRIMP zircon U-Pb ages for the Peine Group. However, these authors failed to constrain the stratigraphic position of their samples and appear to have repeatedly dated the lowermost part of the package. This is perhaps not surprising since it comprises erosionally resistant felsic ingmibrites that are widespread and distinctive. During 2008, CMDIC also commissioned a study of the magmatic chemistry of the mineralised intrusive rocks at Collahuasi. This work largely duplicates results reported to AMIRA P765 sponsors in December 2006, but together the datasets provide a more statistically robust data population on which petrogenetic interpretations may be made (e.g., Selles et al., 2009).