DISTINCTIVE CHLORINE AND TRACE ELEMENT CONTENTS IN SILICATES AND SULFIDES FROM METASOMATIC IRON-OXIDE SYSTEMS

As part of a broad study comparing mineral compositions from diverse hydrothermal systems, we analyzed a suite of silicates and sulfides from two dozen worldwide Fe-oxide-(Cu-Au-REE) deposits and related systems. We observed a distinctive association of Cl-rich biopyriboles accompanied by Co(±As)-bearing sulfides, allanite and/or monazite, magnetite, REE-enriched apatite and titanite, marialitic scapolite, alkali feldspar, and ferropyrosmalite. Cl-bearing biotite (to 5.6 wt% Cl; ~40 mol% Cl in the OH site) and/or Cl-bearing amphibole (to 2.9 wt% Cl) were found at many of the localities examined. Associated are Co-bearing sulfides: typically pyrite (to 4 wt% Co) and less commonly pyrrhotite (0.5 wt% Co), cobaltite, or löllingite (8 wt% Co). Most of the Cl-enriched amphiboles classify as chlorian hastingsite to chlorian-potassian hastingsite. Amphiboles and pyroxenes from these systems commonly show weak peralkaline tendencies regardless of the alkalinity of associated igneous rocks.

These and other associated silicates contain notable scandium and vanadium. Amphibole and epidote contain up to 550 ppm Sc, although one exceptional amphibole has 1.5 wt% Sc. We found vanadium-enriched epidote and diopside (to 0.57 wt% V; others report goldmanite garnet and mukhinite from similar deposits). REE enrichment is pronounced in many of these occurrences. Allanite occurs as a dominant phase in several samples. Monazite is ubiquitous, typically as common inclusions in apatite.

Overall, this suite of minor elements, Cl-bearing silicates, and Co(±As)-enriched sulfides characterize a rather diverse spectrum of metasomatic Fe-oxide occurrences. They contrast with ore-forming environments from many magmatic-hydrothermal systems (e.g. porphyry-type occurrences) and with those dominated by relatively dilute external fluids (e.g. epithermal and most VMS systems). These differences can be attributed in part to variations in the alkalinity and salinity of the ore-forming fluid, and are most compatible with non-magmatic hypersaline brines.