The chlorite proximitor: A new tool for detecting porphyry ore deposits

The major, minor and trace element chemistry of chlorite were evaluated as a tool for mineral exploration in the propylitic environment of porphyry ore deposits. Chlorite from eighty propylitically altered samples, located up to 5 km from the Batu Hijau Cu-Au porphyry deposit in Indonesia, was analyzed using electron microprobe and laser ablation inductively-coupled plasma mass spectrometry. The results show that a variety of elements, including K, Li, Mg, Ca, Sr, Ba, Ti, V, Mn, Co, Ni, Zn and Pb, are probably incorporated in the chlorite lattice and display systematic spatial variations relative to the porphyry center. Ti, V and Mg decrease exponentially in concentration with increasing distance, whereas the others increase. Ratioing the former to the latter provides a variety of ratios that vary up to four orders of magnitude, providing sensitive vectoring parameters. Chlorite geothermometry suggests that Ti is substituted into chlorite as a function of crystallization temperature and thus maps out the thermal anomaly associated with the mineralized center. By contrast, Mn and Zn display a maximum in chlorite at a distance of ~ 1.3 km that mirrors the whole rock anomaly for these metals, reflecting their lateral advection into the wall rocks by magmatic-hydrothermal fluids. The recognizable footprint defined by chlorite compositions extends to at least 4.5 km, significantly beyond the whole rock anomalism (≤ 1.5 km) and thus represents a powerful new exploration tool for detecting porphyry systems. Variations in chlorite chemistry are very systematic in the inner propylitic zone (to distances of ~ 2.5 km), thereby providing a precise vectoring tool in a domain where other tools are typically ineffective. In this zone, equations of the form:x=lnRabcan be formulated, where the distance to center, x, is predicted based on a variety of element ratios in chlorite R, and where a and b are exponential fit parameters. Importantly, distal chlorite compositions in porphyry-related propylitic alteration systems are also shown to be distinct from metamorphic chlorite, allowing the external fringes of porphyry-related hydrothermal systems to be distinguished from “background” regional metamorphism or geothermal alteration.