Physical Volcanology and Hydrothermal Alteration of the Archean Volcanic Rocks at the Eagles Nest Volcanogenic Massive Sulphide Prospect, Northern Minnesota

Abstract

The Eagles Nest Volcanogenic Massive Sulphide Prospect occurs within the Lower Ely Member of the 2.7 Ga Ely Greenstone Formation located within the Vermilion District of the Wawa Subprovince of the Superior Province of the Canadian Shield in Northeastern Minnesota. The object of this study is to characterize the physical volcanology and hydrothermal alteration of the rocks of the footwall to the massive sulphide mineralization. This was done by a combination of geological mapping, diamond drill core logging, thin section petrology, and major and trace element geochemistry. The entire sequence has been metamorphosed to greenschist facies. The mineralization is not exposed at the surface and was discovered by airborne Electromagnetic (EM) and Magnetism surveys, followed up by ground EM Loop and Magnetism surveys. This defined a target that was intersected by three diamond drill holes drilled by Newmont Exploration. The footwall to the mineralization is composed of approximately 1300 meters of pillowed and massive andesite lava flows, with one lobe-hyaloclastite lava flow. The base of the footwall at Eagles Nest is intruded by the dioritic to tonalitic Purvis Lake Pluton. This contact is locally an intrusive breccia. The mineralized horizon consists of approximately 100-150 meters of oxide facies banded iron formation with thin lenses of massive pyrite +/- chalcopyrite +/- sphalerite, and is interlayered with thin, strongly altered andesite lava flows, and sedimentary rocks. The hanging wall is composed of pillowed and massive andesite, with one chemically distinctive basalt lava flow. Hydrothermal alteration mineral assemblages throughout the stratigraphic package have been defined by thin section analysis. In the footwall, these assemblages are epidote +/- quartz, quartz +/- epidote, actinolite +/- epidote, and chlorite +/- quartz +/- sericite. The epidote +/- quartz and quartz +/- epidote assemblages are visible in outcrop as meter scale, locally intense, discontinuous, pale green alteration patches of pervasive alteration, and/or as fillings of amygdules and veinlets. Footwall alteration is semi-conformable, and widespread for many kilometers east and west along strike from the Eagles Nest map area (Peterson, in prep). Mineralized horizon alteration mineral assemblages are chlorite + quartz, magnesium chlorite +/- sericite, and tremolite. Tremolite and magnesium chlorite alteration is interpreted to have occurred on, or immediately below the seafloor, and is syn- to post-mineralization in timing. Most rocks in the mineralized horizon have been intensely, to completely altered, and protolith is often impossible to distinguish. All alteration in the mineralized horizon is stratiform and no evidence of crosscutting alteration has been observed. Alteration mineral assemblages in the hanging wall consist of epidote +/-quartz, quartz+/- epidote, actinolite + epidote +/-quartz, chlorite +/- epidote +/-quartz. This alteration is younger than, but very similar in character to, that of the footwall. Grant's isocon method (1986) was used to define changes in rock mass and the concentration of individual components as a result of hydrothermal alteration. The Eagles Nest Prospect is interpreted to have been located on the flank of a subaqueous volcanic center during the time hydrothermal activity was precipitating the oxide and sulphide minerals. Water depth at Eagles Nest is estimated to have been 500 meters or less, based on observations of amygdule size and percentage within the andesite lava flows. The Purvis Lake Pluton was likely the synvolcanic intrusion driving volcanism at Eagles Nest and the surrounding area, but has intruded upwards in the stratigraphic sequence since the mineralizing event. Composition of oxide and sulphide minerals, alteration mineral assemblages, results from isocon analysis, and experimental work done by Seyfried and Janecky (1985) on hydrothermal fluids beneath mid-ocean ridges, all suggest that hydrothermal fluids in the Eagles Nest footwall never reached temperatures high enough to mobilize and concentrate significant amounts of Cu and Zn. At Eagles Nest the mineralized horizon does not contain significant Cu - Zn sulphides. Epidote +/- quartz alteration in the footwall is not a good exploration guide for VMS mineralization in the Lower Ely Greenstone.