Very low-grade metamorphism in the Portage Lake Volcanics on the Keweenaw Peninsula, Michigan, USA

The Portage Lake Volcanics (PLV) on the Keweenaw Peninsula, Michigan, USA used to host
one of most famous and biggest native copper mining areas in the world and are now closed. The
rock sequence is also the locality of the first description of the low-grade metamorphic index
mineral pumpellyite (Palache and Vassar, 1925). Exploration and mining of the native copper
deposits in the approx. 5 km thick Keweenawan (Precambrian) Portage Lake Volcanics (PLV) on
the Keweenaw Peninsula have provided many drill cores. Due to a lucky coincidence, 195 km of
drill core material is still accessible. Part of this drill core is the basis for studying the history of
the low-grade metamorphism of the basaltic rocks of the Portage Lake Volcanics. Ten drill cores
were finally selected and studied, representing three vertical stratigraphic profiles through the
PLV. The three profiles are located northeast of the former main mining area along the SW-NE
striking PLV. They are correlated by interflow sediments. Furthermore, two samples from the
Calumet mine were studied to complement the fluid inclusion data. Amygdaloidal flow tops and veins with abundant secondary alteration minerals, less altered
transition zones and massive flow interiors were sampled. The macroscopical and microscopical
description combined with X-ray diffraction analysis lead to an overview of the mineral
distribution in the studied area. The two already described low-grade metamorphic facies, a
pumpellyite-prehnite and a zeolite facies, were confirmed (Jolly and Smith, 1972; Livnat, 1983).
In contrast to the model of Jolly and Smith (1972) the samples do not indicate a stratigraphic
zonation, which was demonstrated by Livnat (1983). His data support a low-grade metamorphic
alteration plane, which crosscuts the stratigraphy of the PLV dipping towards the north at a
shallower angle than the original bedding. In contrast, this study suggests an irregular and
discontinuous distribution of the pumpellyite-prehnite index minerals with a slight trend to more
pumpellyite-prehnite abundance in the SW towards the main mining area. The zeolite facies is a
later, superimposed low-grade metamorphic alteration event related to late crosscutting veins.
Based on the results of the microscopical studies, a three-stage alteration model of the PLV is
postulated. These stages are subdivided into different pulses on the basis of electron microprobe
analysis, fluid inclusion study and cathode luminescence data. The alteration minerals, such as
quartz, pumpellyite and calcite, show major zonations, from 3 to 7 zones. The investigation of alteration minerals in the different units of the PLV revealed that the growth
of the low-grade metamorphic minerals was strictly controlled by the fluid/rock interaction and
thus by the permeability of the surrounding rocks. In permeable rocks, secondary minerals like
epidote, pumpellyite and prehnite are abundant as initial secondary alteration minerals. In nonpermeable
rocks such as massive flow interiors and in zones with non-connected small (<2 mm)
amygdules, phyllosilicates dominate as minerals of initial alteration.
Phyllosilicates are the dominant minerals in the PLV. They can be considered as relative
indicators of metamorphic grade (e.g. Kristmansdottir, 1979; Schiffman and Fridleifsson, 1991).
In the Ahmeek profile, containing slightly more pumpellyite-prehnite index minerals, two distinct
populations of phyllosilicates developped (chlorites and mixed-layered phyllosilicates). In
contrast in the Copper Harbor profile, simultaneously with less abundant prehnite-pumpellyite
facies index minerals, a continuous population of phyllosilicates (chlorite-corrensite-mixedlayered)
is present.
The first and the third alteration stages are well distinguished on the basis of their d18O values,
where epidote, clinochlore and quartz of the first stage have a range of 5 Ð 15ä and calcite of the
third stage shows a range between 20 Ð 30 ä. The fluid inclusion studies in quartz and calcite close to the native copper precipitation revealed
two main fluid inclusion populations. In the Copper Harbor profile, these populations can be
divided into a high saline (> 30 wt%) and a low saline (£ 10 wt%) population. Towards the
southwest, the differences between the fluid populations vanish. In the studied mine samples, no
populations could be distinguished so far.
Based on fluid inclusion and stable isotope data fluid mixing as suggested by Livnat (1983) was
confirmed. The recently obtained data indicate that meteoric and metamorphic waters were
mixed. The postulated seawater input (Livnat, 1983), however, could not be observed.
The process of native copper precipitation was probably different in the drill core profiles, where
only little native copper occurs, and in the mine samples. In the mine samples corrosive fluids
dissolved and precipitated quartz and calcite up to three times before the native copper
precipitation occurred. The gangue minerals of the available mine samples are epidote and quartz.
In the studied drill cores copper occurs together with prehnite. Two main types of native copper
emplacements could be observed. The first type was likely precipitated as a result of a short,
powerful, hydrothermal process such as pressure release, which is indicated by native copper
droplets in vein quartz and in quartz cement of the conglomerates. The second type formed
slowly as native copper sheets parallel to calcite cleavage or along grain boundaries. In contrast
to the typical mine samples, no corrosion could be observed here.
The low-grade metamorphism in the PLV must have been related to a high heat flux, which was
probably caused by an underplated gabbro at ca. 40km depth. Later thrusting along the
Keweenawan fault did not change the metamorphic conditions considerably but generated new
pathways for fluids depositing the same minerals or similar assemblages of alteration stage II.
The deposition of the native copper ores is connected to this alteration stage. A late retrograde
zeolite facies overprints the early alteration stages.