| Description |
Igneous apatite is an early magmatic phase that accepts a wide variety of elemental substitutions. These substitutions include porphyry Cu hypogene mineralization elements S and Cu, as well as other potentially mineralization related elements. These characteristics make apatite useful in understanding pre-exsolution concentrations of these elements in melts and, subsequently, useful as porphyry system/intrusive unit fertility indicators. Apatite element substitution concentrations in pre-, syn- and late-mineralization emplaced units of the San Enrique Monolito deposit in the Los Bronces-Río Blanco porphyry district, Central Chile were assessed to evaluate the use of apatite substitutions as porphyry system/intrusive unit fertility indicators. Major, trace and rare earth element (REE) concentrations in apatite grains were determined, sulfur substitution mechanisms and controls were assessed, melt S was calculated using apatite/melt S partition coefficients and apatite S, melt S, and other elemental substitution correlations to mineralization were determined. The ratio of S to Na substitution indicates S in analyzed apatite grains is incorporated by the coupled exchange S<super>6+</super> + Na<super>+</super> <> P<super>5+</super> + Ca<super>2+</super> and systematic fluctuations in Cl vs. Mn, Fe and light rare earth elements (LREE) suggest a change in apatite structure associated with concomitant increases in these elements. Neither S exchange nor crystal structure, however, display a preferential control on S uptake. This is interpreted to indicate S concentrations are wholly a function of S availability in melt. Apatite S and melt S are low in premineralization emplaced units, elevated in syn and late-mineralization units and highest in units emplaced contemporaneously with the height of mineralization. Apatite Cu correlates similarly. Apatite Mg was detected only in grains from synmineralization emplaced units, possibly substantiating mafic magma input and underplating as Cu source models for porphyry mineralization. Concentrations of apatite S and melt S above 750 ppm and 200 ppm, respectively, are proposed as fertility indicators. However, because of the cumulative uncertainty associated with melt S calculations, it is proposed that apatite S is a more robust discriminant than melt S. Apatite Cu above 700 ppm and apatite Mg above 300 ppm are also proposed as mineralization potential indicators. |
