Melange- and sediment-hosted gold-bearing quartz veins, Hodgkinson gold field, Queensland, Australia

Abstract

Gold-bearing quartz veins in the Hodgkinson gold field are hosted in subgreenschist facies Siluro-Devonian sandstone, conglomerate, carbonaceous shale, clast-in-matrix rock, chert, and spilite. Mineralization occurs in brittle to brittle-ductile shear zones which crosscut two previous deformations and melange. Vein systems are isolated in distinct structural and lithologic domains by postmineralization faulting. Scheelite-quartz veins may locally overprint the gold quartz mineralization, but stibnite-gold-quartz mineralization is restricted to separate domains. The strongest mineralized areas define elongate, elliptical ore shoots which plunge steeply down or across the dip of the host shear zones. Melange-hosted lodes are parallel in strike but have opposite dips to the melange cleavage. Sandstone-hosted shear zones lie within reactivated F 2 axial planes or are refracted through bedding planes. Splaying, fissure intersections, changes in attitude, and lithologic contacts are the main ore shoot controls.The gold quartz vein textural types are ribbon, buck, breccia, assimilation textured, and comb quartz. Multiple injection of quartz and multiple shear zone movement have produced complex zones of quartz separated by seams of gouge. Arsenopyrite, galena, sphalerite, pyrite, pyrrhotite, and gold (600-840 fine) constitute 5 percent of the ore shoots and occur as millimeter-scale clusters in quartz. Fluid inclusions in quartz are liquid and liquid-vapor types and give salinity ranges of 3 to 11 equiv wt percent NaCl, with a slight salinity decrease through time. CO 2 and other gases are not abundant. Homogenization temperatures of 170 degrees to 305 degrees C have been pressure corrected to 270 degrees to 355 degrees C, based on estimated formation pressure of about 1 kbar.Measured and calculated stable isotope compositions of the fluid depositing gold quartz are delta 18 O = 10 + or - 2 per mil, delta D = -100 + or - 20 per mil. Although meteoric water contributions cannot be discounted, geologic characteristics, isotopic data, quartz types, sulfide mineralogy, and ore controls favor gold deposition from an upwardly migrating, homogeneous, metamorphic, or distal magmatic fluid. Depletion of D is attributed to exchange reactions involving organic matter in the metasediments and reduced gas species rather than introduction of meteoric water. Gold mineralization accompanied brittle tectonism which was contemporaneous with regional plutonism and took place late in the evolution of the melange.