GEOCHEMICAL AND SPECTRAL TRENDS IN THE LEPANTO LITHOCAP, PHILIPPINES, AND THEIR GENETIC IMPLICATIONS

In the Mankayan district, Philippines, a 4-km long NE-trending lithocap dominated by quartz-alunite hosts the Lepanto high sulfidation epithermal deposit. Both the lithocap and the epithermal deposit are linked genetically to the underlying FSE porphyry Cu deposit and related quartz diorite intrusions. The ~1480 nm peak of alunite in Short Wavelength Infra-Red (SWIR) spectrum shifts to higher wavelength position in rocks closer to the FSE porphyry. The peak position increases with the alunite Na/(Na+K) ratio, with higher ratio in turn indicating higher formation temperature, according to existing experimental results. Alunite also contains more Ca closer to FSE, with the Ca end-member, huangite, only occurring above FSE. Alunite trace element contents vary systematically with respect to their location in the lithocap. The Sr (up to ~6500 ppm), La, Sr/Pb and La/Pb values for alunite increase towards the intrusive center, whereas Pb (up to ~8000 ppm) and Ag/Au increase in distal locations. The lithocap rocks with advanced argillic (AA) alteration, including quartz-alunite, typically contain <50 ppb Au, and Cu, Mo, As and other metals are also low. Plots of all samples, including AA-altered lithocap samples and mineralized samples, show no trends. However, whole rock data of filtered samples containing alunite, and < 0.1 wt% Cu and < 0.1 ppm Au, demonstrate trends similar to the alunite mineral compositions noted above.

The presence of geochemical trends in filtered quartz-alunite lithocap samples and the lack of any trends in the unfiltered dataset are consistent with a two-stage formation process; the lithocap forms early due to acidic condensates of magmatic vapors, and Au and Cu ore minerals form later, due to the ascent of a muscovite-stable liquid. The unfiltered dataset contains signals from both stages, which explains the lack of trends. The low abundance of Au, Cu and other metals in AA-altered lithocap samples indicate that the early magmatic vapors contained low contents of these metals in the near-surface, low pressure environment. The Pb, Sr and other elements in alunite are inferred to have been derived from preexisting wall rocks through acid leaching. Lead preferably substitutes for K whereas Sr is related to Ca, explaining why Pb is distal and Sr is proximal in alunite and thus quartz-alunite altered rocks.