THE PETROLOGY AND LOW-TEMPERATURE GEOCHEMISTRY OF THE SULFIDE-BEARING MAFIC-ULTRAMAFIC UNITS OF THE ACOJE MASSIF, ZAMBALES OPHIOLITE, PHILIPPINES: CHARACTERIZATION OF MINERAL AND FLUID EQUILIBRIA (GEOLOGY, TECTONICS, MIXED-VOLATILE, SULFUR ISOTOPE, SPECT

The Acoje massif is a fragment of a late Eocene oceanic crust that is part of a mafic-ultramafic complex collectively known as the Zambales ophiolite. This study documents the occurrence and examines the petrology and low-temperature geochemistry of the sulfide-, native metal-, and alloy-bearing lenses and layers near the base of this cumulate sequence (BCS) as they are exposed in the surface and subsurface workings of the Acoje Mining Company, Chromite Mine East, of the municipality of Sta. Cruz, Zambales.; The BCS originally consists of interlayered dunite, peridotite, pyroxenite, gabbro, gabbronorite, and minor anorthosite. However, serpentine and associated phases, brucite, talc, anthophyllite, tremolite, chlorite, epidote, magnesite, magnetite, and ferrit-chromite comprise more than 50 vol. % of the presently observed mineralogy of the sequence.; The serpentinization paragenesis is dominated by the assemblage lizardite + chrysotile + magnetite +/- brucite. An upper limit to the temperature of hydration of the BCS thus may be defined by the reaction: 2Fo + 3H(,2)O = Chry + Br. This reaction limits the hydration temperature of the BCS below 350(DEGREES)C depending on the forsterite activity and partial pressure of H(,2)O. This hydration paragenesis also suggests the potentially dominant control of the oxygen fugacity by the highly reducing (>5 log units below QFM) buffer reaction: 3Fa + Fo + 4Br + O(,2) = 2Mt + 2Chry.; The attainment of highly reducing conditions during hydration of ultramafic rocks, as exemplified in the BCS, is favored by (1) stable or metastable involvement of olivine in the buffering reactions, (2) high Fe:Mg ratios in olivine and/or pyroxene, and (3) low silica activity during hydration.; Two distinct sulfide associations are distinguished in the BCS, a troilite (+/- pyrrhotite)-dominated group and a pentlandite-dominated group. The sulfide assemblages in the mafic zone are mostly troilite (+/- pyrrhotite)-dominated whereas those in the ultramafic zone are pentlandite-dominated. The textural occurrence (e.g., interstitial or amoeboid surrounding, and as inclusions in, silicates and chromite) and sulfur isotopic signature ((delta)('34)S = 0.75 - 4.93) of the sulfides, as well as the bulk composition and mineralogy of the least altered sulfides (i.e., troilite- or pyrrhotite-dominated) strongly support an initial magmatic origin, by liquid immiscibility, of the BCS sulfides. (Abstract shortened with permission of author.)...