Geological Survey Bulletin 1309 The Geologic Story of Isle Royale National Park

Many interesting minerals occur on Isle Royale in addition to the basic rock-forming minerals that make up the bulk of the lava flows and other rocks on the island. Among these are the native copper that played such an important part in the early explorations of the island and chlorastrolite, the official state gem of Michigan. These minerals as a group are known as secondary minerals, ones introduced into the rocks after the rocks themselves were formed. They chiefly fill holes in the volcanic rocks and the conglomerates or form veins filling fractures. Some of these minerals, especially those occurring as amygdules in the volcanic rocks, are quite attractive when polished and long have been sought by collectors, although collection has been restricted since Isle Royale became a national park. Fortunately for the collector, these minerals, except for chlorastrolite, are equally or more abundant on the Keweenaw Peninsula or elsewhere in the Lake Superior region.

Considerable evidence obtained during mining activities on the Keweenaw Peninsula indicate that the copper and most other secondary minerals were deposited from solutions that percolated upward through the rocks. We can only speculate upon the ultimate source of the copper and other elements in the mineralizing solutions, but one of the more generally accepted theories is that those elements were "sweated" out of the lower part of the pile of volcanic rocks after those rocks were warped downward in the Lake Superior syncline, into a region of higher temperature and pressure. The elements then migrated upward and were deposited as minerals in open spaces higher in the rock sequence. If the source of the mineralizing solutions was indeed the deeper part of the volcanic pile, then the source also would have been closer to the Keweenaw Peninsula, for the axis, or deepest part of the Lake Superior syncline, is closer to the Keweenaw Peninsula than to Isle Royale. This asymmetry of the syncline may have been one of the more important factors in producing economically valuable deposits of copper on the peninsula but not on Isle Royale.

Among the many secondary minerals on Isle Royale, those occurring most often are barite, calcite, chlorite, copper, datolite, epidote, laumontite, natrolite, prehnite, chlorastrolite (pumpellyite), quartz (including agate), and thomsonite. Only a few of special interest on Isle Royale are described further in this report. Information on the others can be obtained from many sources, including the booklet "Rocks and Minerals of Michigan" (Poindexter and others, 1965).

NATIVE COPPER—WIDESPREAD BUT NOT ABUNDANT

Copper, like most other metals, most commonly occurs in nature bound up with other elements into minerals such as chalcocite (Cu₂S), chalcopyrite (CuFeS₂), and cuprite (Cu₂O), and such minerals form the bulk of copper ores. Of all the metals that do occur in the native or metallic state, copper is by far the most common; however, major concentrations of native copper are rare, and so it is noteworthy that the most important deposits known in the world are those of the Keweenaw Peninsula.

Economically valuable concentrations or deposits of native copper on the peninsula can be divided into two broad groups—lode deposits and fissure deposits. The lode deposits comprise mineralized conglomerate beds and the vesicular tops of lava flows; in each case the primary porosity of the rock or presence and continuity of open spaces was a factor in determining where the ore would be deposited. The fissure deposits are along fracture zones that generally cut across the beds. Some of the fissure deposits contained great masses of metallic copper as much as hundreds of tons in weight. Extensive Indian mining pits on the fissures later led prospectors to most of the known deposits of this type. However, such deposits, rich as they were, have been much less important than the lower grade but vastly larger lode deposits, which have produced about 98 percent of the total copper mined in the Native Copper district, about 5,400,000 tons.

Native copper is widely distributed on Isle Royale, but mineralization was apparently too weak to develop large lode deposits. Except for the Island mine, which was on a conglomerate lode, most of the prospects and short-lived mines on the island were opened on small fissure deposits. In the fissure occurrences, such as at the Minong, Siskiwit, and most other mines and prospects, native copper occurs in nodules and irregular masses in highly altered rock in fracture zones a few inches to many feet wide. Several large masses more than a ton in weight were found at the Minong mine (fig. 69); these were rare and most pieces mined were probably similar to that in figure 70, or smaller. Although approximately 250 tons of copper were produced from the Minong mine, the copper was too sparse and widely distributed to be mined profitably.

COPPER NUGGET weighing 5,720 pounds found at a depth of 16-1/2 feet in a pit dug by prehistoric Indians at the site of the Minong mine. Note the uneven surface resulting from attempts to remove sections for implements (Burton collection, Detroit Public Library). (Fig. 69)

NATIVE COPPER MASS from the Minong mine. This is perhaps typical of specimens from the fissure deposits on Isle Royale, but larger than average. (Fig. 70)

At the Island mine, near the west end of Siskiwit Bay, native copper occurs in the matrix of a conglomerate, the only known occurrence of this type on Isle Royale. The specimen illustrated (fig. 71), however, is much richer in copper than average for the mine, and this mine too was a financial failure.

CONGLOMERATE WITH NATIVE COPPER from the Island mine. The specimen has been sawed and slightly polished so that the copper reflects light and shows up as light colored irregular-shaped patches. Specimen is 8 centimetres wide. (Fig. 71)

In other places in the park, most notably on the chain of islands south of Rock Harbor, veins of quartz, prehnite, and calcite contain scattered grains of copper. Native copper also occurs in amygdules in lava flows, especially in prehnite amygdules.

Rakestraw (1966) has described the historic mining ventures on Isle Royale and what the visitor can see at some of the abandoned mine sites.

CHLORASTROLITE—MICHIGAN'S STATE GEM

Isle Royale has long been famous as the home of chlorastrolite, known informally in rock-collecting and lapidary circles as "Isle Royale greenstone." This usage of "greenstone" should not be confused with the use of the same term for a volcanic rock with a greenish hue, such as makes up Greenstone Ridge on the island. In 1972 the governor of Michigan signed a bill designating chlorastrolite as the official state gem.

Chlorastrolite, meaning "green star stone," occurs as amygdules or cavity fillings in certain of the lava flows on Isle Royale. When weathered out of the lava flows, it can be found on some of the island beaches as pea-sized pebbles, generally greenish in color. When polished, either by wave action on the beaches or artificially, the "greenstones" generally exhibit a distinctive and attractive mosaic or segmented pattern, sometimes referred to as "turtleback" (fig. 72). The polished stones also commonly are chatoyant—the property of having a luster resembling the changing luster of the eye of a cat. Chatoyancy is probably best known in the gemstone called tiger eye and is a property of translucent material that contains fibrous structures capable of scattering light. The grouping together of bundles of such fibers produces the mosaic pattern of the "greenstones."

CHLORASTROLITE AMYGDULES showing characteristic segmented pattern. (Fig. 72)

Chlorastrolite was first discovered on Isle Royale and named and described by C. T Jackson and J. D. Whitney in 1847. Long afterward it was found to be the same material as another mineral, pumpellyite, first described from the Keweenaw Peninsula in 1925 and named for Raphael Pumpelly, a pioneer student of the minerals of the Keweenawan copper deposits. The material from the peninsula was described in much greater detail than that from the island, and the name "pumpellyite" became deeply entrenched in the world mineralogical literature long before it was realized that the material from both areas was mineralogically the same. Consequently, pumpellyite has been adopted as the only valid name for the mineral species, although chlorastrolite is still useful as a term to designate the variety with the peculiar crystal habit of the Isle Royale "greenstone." Pumpellyite is common in many parts of the world, but the chlorastrolite variety is apparently rare outside of Isle Royale.