Evidence for a Feeder Pipe and Associated Alteration at the Brunswick No. 12 Massive-Sulfide Deposit
Brunswick No. 12 is a vent-proximal massive sulfide deposit that formed above a feeder pipe with associated hydrothermal alteration. Because of the replacement of bedded sulfides deposited above the hydrothermal vent, individual sulfide lenses show the following vertical and lateral zoning outward from the vent complex: massive pyrite-pyrrhotite-chalcopyrite core interlayered pyrite-sphalerite-galena (Pb-Zn ore zone) — bedded pyrite. The feeder zone which underlies the Cu-rich core consists of an anastomosing network of pyrite-pyrrhotite-chalcopyrite-quartz veins with variable and generally minor contents of ferroan carbonate, Fe-rich chlorite, sphalerite, galena and arsenopyrite. Hydrothermal alteration of felsic igneous rocks hosting and adjacent to the feeder pipe is represented by an inner zone (zone I) characterized by sulfide stringers, Fe-rich chlorite, muscovite, quartz, and ferroan carbonate that formed at the expense of feldspars and what was probably glass; outer zones (II and HI) are represented by Mg-rich chlorite and muscovite formation, and the albitization and silicification of K-feldspar. Accompanying these alteration trends is a marked decrease of Na + Ca, and an increase in Fe + Mg + CO3 in zone and increases in Mg + Na in the outer zones. The classic zoning of metals within the sulfide lenses, the formation of a feeder pipe below the vent-proximal sulfide core, and the hydrothermal alteration of felsic volcanic rocks cut by the feeder pipe all support a vent-proximal origin for the Brunswick No. 12 deposit. Furthermore, restriction of sulfide stringer mineralization to the footwall sequence, the folding of the stringer zone by an F[ fold, and the difficulty of long distance mass transfer of metals from the sulfides into the host rocks all argue against a tectonic origin for the stringer zone. All of the evidence indicates that the feeder pipe formed during the ore forming hydrothermal event, but has been distorted and in some areas detached from the overlying massive sulfides during subsequent deformation.