Chemostratigraphy, alteration and metamorphism of Paleoproterozoic volcanic rocks at the Kristineberg VHMS deposit, northern Sweden

摘要

The Kristineberg massive sulfide deposit is hosted by volcanic and subvolcanic rocks of the Paleoproterozoic Skellefte group. It consists of two main massive sulfide horizons, the A- and B-ores, separated by 100-150 m, and the Einarsson Zone, a complex interval of Cu-Au-rich 'stockwork' sulfides in deformed rocks near the 1000 m level. In many areas, precursor rock types are almost impossible to identify due to abundant secondary phases such as chlorite, muscovite, cordierite, andalusite, phlogopite and quartz, combined with schistose fabrics. Application of immobile-element methods to 249 new whole-rock lithogeo-chemical analyses has, however, allowed several rock types to be chemically identified and correlated. Massive rhyolite A lies immediately south of the B-ore horizon and Einarsson Zone, and is interpreted to form much of the stratigraphic footwall. A series of andesitic to dacitic to rhyolitic rocks makes up the interval between the A- and B-ore horizons. Rhyolite X lies immediately north of the A-ore horizon. Mass changes calculated for all rock types outline asymmetrical alteration effects flanking the massive sulfide lenses, ranging from major mass gains (Si, Fe and Mg) to major mass losses (Si and alkalis). Immobile-element ratios reveal that very different secondary assemblages, e.g. andalusite-quartz-muscovite and cordierite-chlorite-talc, can be produced from the same parental volcanic unit. The common presence of cordierite + andalusite but without anthophyllite is proposed to have formed from the original low-pH alteration phase kaolinite, which reacted with chlorite during metamorphism to form cordierite, and, where present in excess, to also produce andalusite. Rhyolite A is interpreted as a shallow footwall intrusion that promoted hydrothermal activity.