SULFIDE MINERALOGY OF LOW-T / HIGH-P ROCKS FROM SERPENTINITE MÉLANGES OF THE MOTAGUA FAULT ZONE, GUATEMALA

A study of the sulfide minerals in eclogite, jadeitite, serpentinite and other high-pressure/low-temperature assemblages within two serpentinite-matrix mélange belts in Guatemala is part of a larger study of the petrogenesis and geochemistry of these rocks. The belts straddle the Motagua fault zone (MFZ) and represent subduction zone products emplaced ~130 Ma that were dissected into northern and southern belts by lateral faulting along the MFZ and conjugate faults. Sulfides are minor to common in these rocks and in adjacent antigorite serpentinite and manifest the chalcophile element and sulfur content in these rocks, whether they are inherited or have been moved by fluids. Sulfide mineralogy interpretation is the first step in evaluating their origin.

Texturally sulfide in eclogite, jadeitites, blueschists and garnet amphibolites varies from fine interstitial dustings of individual sulfide as rounded to euhedral grains (to ~100 μm) to vein decorating swaths of polygranular sulfide to porphyroblasts (to ~3 mm). Larger clusters typically exhibit multiple sulfide assemblages. Certain jadeitites and omphacitites exhibit pyrite phenoblasts (cubes and pyritohedra), particularly from Río Comaja, north of the MFZ. Sulfides are common in metabasite/eclogite and rare in jadeitite, and rocks from each belt share similar assemblages. North of the MFZ, pyrite (Py) and Py + chalcopyrite (Ccp) is common; some rocks contain Py + Ccp + pyrrhotite (Po) and Py + Ccp + pentlandite (Pn). South of the MFZ, Ccp+ intermixed covellite-chalcocite (Cv-Cc) is common; in some samples Py is the only sulfide and rare. Sulfides in serpentinite vary from fine-grained disseminations to veinlets. Serpentinite from both sides of the MFZ typically contains Pn + Py or Pn + Po while individual samples contain Py + mackinawite and Pn + millerite + heazlewoodite. Co ranges from ~0.2 to > 10 wt% in the Ni-rich phases, it is generally much lower in Fe-rich phases but can reach 5 wt% on Py rims. All S-phases are in apparent low-T equilibrium.

We interpret (1) sulfide cations originated from basaltic rocks (and sediment) for both eclogite/metabasite and jadeitite, and from peridotite for the serpentinite, largely based on the systematic differences in Cu and Ni+Co and (2) S was largely immobile. Sulfur isotopic analysis is underway to evaluate S sourcing.