Near infrared and XRD quantification of porphyry copper alteration at Cerro Colorado and Spence, Chile.

摘要

The need for quantitative mineralogical analyses is widespread throughout nearly all disciplines of the geological sciences. If reliable quantitative determinations are available, limitless possibilities exist regarding the subsequent analysis of data. This research involves (1) the development of a chemometric model to quantify mineralogy in natural samples using visible and near infrared reflectance spectroscopy (VIS-NIR), (2) the utilization of VIS-NIR to determine the composition of alunite and chlorite group minerals, and (3) the application of these methods to study the alteration mineralogy of the Cerro Colorado and Spence porphyry copper deposits, located in northern Chile.; A Partial Least Squares (PLS) chemometric model to predict mineral abundance was constructed by performing a regression between VIS-NIR spectra and quantitative X-ray diffraction analyses of natural samples. Separate models were constructed for sawed rock slabs and powders of samples from the Cerro Colorado and Spence porphyry copper deposits. Excellent results were obtained for the minerals kaolinite and alunite, and satisfactory results were obtained for illite+muscovite. These models were used to quantify alteration intensity in both deposits, and results were modeled in a 3-D visualization program. This approach is rapid, accurate, and reproducible method to quantify mineralogy and improve the quality of core logging programs.; New Re-Os age determinations of molybdenite indicate that mineralization occurred at Cerro Colorado at 55 +/- 0.3 Ma, nearly 3 million years earlier than indicated by 40Ar/39Ar ages of igneous biotite and hydrothermal sericite. These results suggest that the Re-Os technique records the actual sulfide mineralization age, whereas the 40Ar/ 39Ar technique records cooling of the hydrothermal system to below the closure temperature of argon diffusion in biotite and muscovite (300--400°C).; Supergene advanced argillic alteration at Cerro Colorado is most intense above areas with original high sulfide content associated with quartz-sericite-pyrite mineralization. Alunite group composition is bimodal, with natroalunite centered on the pyrite-rich Cerro Colorado intrusive breccia. Supergene mineral zoning and composition reflect the precursor hypogene mineralization and host rock composition as well as local groundwater chemistry and flow direction. Hypogene copper mineralization is associated with widespread sericite-chlorite-pyrite alteration present below the supergene profile at Cerro Colorado. Chlorite composition is zoned inward from Fe-rich compositions along the fringes of the deposit to distinctly Mg-rich compositions in mineralized areas. This zoning is attributed to a higher sulfidation state and oxygen fugacity in pyritic assemblages that favor Mg-chlorite compositions.; Quantitative mineralogical analyses of the Spence porphyry copper deposit indicate that the nature of supergene advanced argillic alteration is distinct from that present at Cerro Colorado. Differences are attributed to precursor hypogene mineralogy, host rock composition, and district geomorphology.