The trace-element compositions of amphibole, magnetite and ilmenite as potential exploration guides to metamorphosed Proterozoic Cu–Zn±Pb±Au±Ag volcanogenic massive sulfide deposits in Colorado, USA

Abstract

Abstract Orthoamphibole, clinoamphibole and magnetite are common minerals in altered rocks associated spatially with Palaeoproterozoic volcanogenic massive sulfide (VMS) deposits in Colorado, USA and metamorphosed to the amphibolite facies. These altered rocks are dominated by the assemblage orthoamphibole (anthophyllite/gedrite)–cordierite–magnetite±gahnite±sulfides. Magnetite also occurs in granitoids, banded iron formations, quartz garnetite, and in metallic mineralisation consisting of semi-massive pyrite, pyrrhotite, chalcopyrite, and sphalerite with subordinate galena, gahnite and magnetite; amphibole also occurs in amphibolite. The precursor to the anthophyllite/gedrite–cordierite assemblages was probably the assemblage quartz–chlorite formed from hydrothermal ore-bearing fluids (~250° to 400°C) associated with the formation of metallic minerals in the massive sulfide deposits. Element–element variation diagrams for amphibole, magnetite and ilmenite based on LA-ICP-MS data and Principal Component Analysis (PCA) for orthoamphiboles and magnetite show a broad range of compositions which are primarily dependent upon the nature of the host rock associated spatially with the deposits. Although discrimination plots of Al/(Zn+Ca) vs Cu/(Si+Ca) and Sn/Ga vs Al/Co for magnetite do not indicate a VMS origin, the concentration of Al+Mn together with Ti+V and Sn vs Ti support a hydrothermal rather than a magmatic origin for magnetite. Principal Component Analyses also show that magnetite and orthoamphibole in metamorphosed altered rocks and sulfide zones have distinctive eigenvalues that allow them to be used as prospective pathfinders for VMS deposits in Colorado. This, in conjunction with the contents of Zn and Al in magnetite, Zn and Pb in amphibole, ilmenite and magnetite, the Cu content of orthoamphibole and ilmenite, and possibly the Ga and Sn concentrations of magnetite constitute effective exploration vectors.