MINERALS OF AU, AG AND U IN VOLCANIC-ROCK-ASSOCIATED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN APENNINE OPHIOLITE, ITALY

Abstract

Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in the Northern Apennine belt of Italy were found to contain gold, acanthite or argentite (Ag2S), and freibergite (Ag,Cu,Fe)12(Sb,As)4S13. These precious-metal minerals are part of a complex assemblage of microscopic to submicroscopic (^lt;10 mm) accessory phases, including uraninite, barite, galena, smithsonite, siderite, a La–Ce carbonate and a Sn hydroxide, attributable to the rare mineral hydroromarchite Sn3O2(OH)2. The textures of the sulfide host are indicative of metal deposition in a range of conditions, involving initial precipitation from hot, metal-charged solutions (syngenesis), followed by postdepositional replacement of biological material and resedimentation of clasts, at low temperature (epigenesis). Syngenetic gold is Ag-rich and associated with uraninite in Fe–Cu-rich ore. The fineness of native gold increases by loss of Ag into secondary Ag2S during seafloor weathering of the sulfides. Freibergite forms during recrystallization of sphalerite in massive Zn–Fe ore. Hydroromarchite occurs in massive Zn–Fe ore, closely related with late deposition of siderite and secondary pyrite, possibly due to the action of neutral to basic and reducing waters flushing through the ore in the initial stages of seafloor diagenesis. The widespread evidence for seafloor resedimentation and for biogenic- activity-driven reworking of the sulfides suggests a transition from proximal VMS to a more distal type of deposit formed by resedimentation processes in a dynamic tectonic environment. This scenario seems to be consistent with the interpretation of the Jurassic Ligurian ocean as a narrow basin in the early stage of its opening, having its modern analogue in the Red Sea.