Mineralogy, mineral chemistry, and genesis of precious metal-bearing volcanogenic massive sulfide deposits in the Newfoundland Appalachians, Canada: the Ming deposit as example

摘要

The Ming deposit, Newfoundland Appalachians, is a metamorphosedud(upper greenschist to lower amphibolite facies), Cambro-Ordovician, bimodalmaficudvolcanogenic massive sulfide (VMS) deposit that consists of several,udspatially-associated, elongated orebodies composed of stratabound semimassiveudto massive sulfides and/or discordant sulfide stringers in a rhyodaciticudfootwall. Copper is the main commodity; however, the deposit contains preciousudmetal-bearing zones with elevated Au grades.udIn this study, field observations, microscopy, and micro-analytical toolsudincluding electron microprobe, laser ablation inductively coupled plasma massudspectrometry, and secondary ion mass spectrometry were used to constrain theudrelative timing of precious metal emplacement, the physico-chemical conditionsudof hydrothermal fluid precipitation, and the sources of sulfur, precious metals,udsemi-metals and metals.udThe ore mineral assemblage is complex and indicates an intermediateudsulfidation state. Pyrite and chalcopyrite are the dominant ore minerals with minorudsphalerite and pyrrhotite, and trace galena, arsenopyrite and cubanite. Additionaludtrace phases include tellurides, NiSb phases, sulfosalts, electrum, AgHg±Auudalloys, and oxides. Silver phases and precious metals occur predominantly inudsemi-massive and massive sulfides as free grains, and as grains spatiallyudassociated with arsenopyrite and/or sulfosalts. Precious metal phases occurringudbetween recrystallized pyrite and within cataclastic pyrite are rare. Hence, the complex ore assemblage and textures strongly suggest syngenetic preciousudmetal emplacement, whereas metamorphism and deformation only internally andudlocally remobilized precious metal phases.udThe ore assemblage formed from reduced, acidic hydrothermal fluids overuda range of temperatures (≈350 to below 260ºC). The abundance of telluride andudAg-bearing tetrahedrite, however, varies strongly between the different orebodiesudindicating variable ƒTe₂, ƒSe₂, mBi, and mSb within the hydrothermal fluids. Theudvariations in the concentrations of semi-metals and metals (As, Bi, Hg, Sb, Se,udTe), as well as Au and Ag, were due to variations in temperature but also to audlikely contribution of magmatic fluids into the VMS hydrothermal system fromudpresumably different geothermal reservoirs.udSulfur isotope studies indicate at least two sulfur sources: sulfur fromudthermochemically-reduced seawater sulfate and igneous sulfur. The source ofudigneous sulfur is the igneous footwall, direct magmatic fluid/volatiles, or both.udUpper greenschist to lower amphibolite metamorphic conditions and deformationudhad no significant effect on the sulfur isotope composition of the sulfides at theudMing deposit.