New Mexico Mineral Symposium — Abstracts

For the past 60 yrs (1952–2012) copper-banded agates of exceptional beauty and rarity have been recovered from various mine dumps along the Kearsarge copper-bearing amygdaloidal lode in Houghton County, Michigan. The unusual agates, which range in size from 1 to 4 cm, occur in a basaltic lava flow that is about 1.1 Ga (billion years old).

The Keweenaw Peninsula, which juts out into Lake Superior at the western end of Upper Michigan, is located along the arch-shaped Middle Proterozoic midcontinent failed rift system. Over a period of geologic time 1,109 to 1,060 Ma (million years ago) the rift valley was filled with a thick sequence of subaerial tholeiitic lava flows and intercalated sedimentary beds now collectively called the Portage Lake Volcanics (PLV).

The agate-bearing Kearsarge lava flow lies within the PLV and can be traced for a strike distance of 35 mi. The flow pinches out at both ends, but the central portion may be up to 200 ft thick and can be divided into four zones from top to bottom. The uppermost zone is a brecciated flow top consisting of a porous and permeable basalt that ranges up to 30 ft thick. This was the zone that was commercially mined from 1882 to 1967 and produced 2,271,997,325 lbs of native copper. Below the amygdaloidal flow top, the flow changes into a semi-permeable zone that is about 10 ft thick and contains the copper-banded agates. Below this zone is a widespread layer of plagioclase phenocrysts embedded in a dense basalt that may be up to 15 ft thick. The lowest zone is an impermeable aphanitic basalt up to 145 ft thick that rests on the Wolverine Sandstone.

Initial filling of some of the vesicles with silica in the agate-bearing zone probably took place within a few million years after the deposition of the lava flow and formed agates in about 25% of the vesicles. The copper mineralization event that formed the commercial orebodies to place about 32–45 m.y. after the deposition of the PLV and lasted for a duration of 13 m.y. Copper-bearing hydrothermal solutions permeated the uppermost zones of the Kearsarge flow and deposited copper in vesicles, fractures, and replaced certain bands of chalcedony in the earlier formed agates.

Copper manifests itself in a variety of forms in the partially replaced agates. The simplest is an outer band of copper along the contact of the host rock and initial band of chalcedony. More intense replacement took place where the pervasive copper solutions were more concentrated and replaced the more porous band of chalcedony within the agate structure. In some cases almost total replacement has taken place leaving only shards of chalcedony remaining.

Collecting of the rare agates has continued at the Wolverine #2 and C & H 21 mines—the two most productive localities for the agates. With luck, the elusive can still be collected with hard work and digging.