Tenth in a series of articles on the mines and minerals of Michigan's Copper Country

Part 1 of this two-part article (Rosemeyer 2003a) discussed historical information, collecting history, and color and zoning in datolite nodules, along with datolite localities in northern Keweenaw County and Isle Royale National Park. Part 2 discusses collecting history and datolite localities associated with amygdaloidal copper lodes (see A Word to the Wise, this issue) that occurred in the Portage Lake Volcanics (PLV) in southern Keweenaw, Houghton, and Ontonagon counties.

GEOLOGY AND MINERALIZATION IN THE PLV

The PLV are 9,000-15,000 feet thick and are made up of more than two hundred individual basaltic lava flows that range from 3 feet to over 1,200 feet in thickness. The flood basalts were extruded on a relatively flat land surface and flowed away from feeder vents located along the axis of the Middle Proterozoic Midcontinent rift zone.

Interbedded with the basalt flows are twenty-two major conglomerate and sandstone units. The sedimentary rocks were deposited on the weathered and brecciated vesicular tops of the basaltic lava flows during periods of volcanic inactivity. Water-transported sediments formed large alluvial fans on the relatively flat-lying lava flows, forming lenticular beds that covered wide areas and ranged from a few inches to 120 feet in thickness.

The basalts and sedimentary beds collectively comprise the PLV of the Keweenawan Series (now known as the Keweenawan Supergroup) and crop out in a narrow belt for more than 160 miles, forming the "spine" of the Keweenaw Peninsula.

Regional contraction of the rift resulted in tilting and fracturing, uplifted the rocks on the edge of the rift, and formed the Lake Superior Basin. The rocks of the Keweenawan Supergroup of northern Michigan and Wisconsin lie on the south flank and dip north to northwest toward the center of the basin. The beds strike parallel with the Keweenaw Peninsula and dip 20[degrees]-75[degrees] toward the northwest.

The main part of the Lake Superior copper district lies within the band of PLV, which forms the south limb of the Lake Superior syncline. Native copper and a host of gangue minerals were deposited in fissure veins, brecciated and fractured vesicular lava flow tops, and conglomerate beds during the same period of metal mineralization. A number of theories are postulated for the formation of the copper deposits, but all generally agree that the deposits are epigenetic and that the ore-bearing solutions were hydrothermal. Orebodies in the fissure veins and both rock types were formed by hot watery solutions moving upward and laterally along fractures and fault zones, brecciated vesicular flow tops, and permeable conglomerate beds. Ore minerals were probably deposited because of a combination of fluid-wallrock reactions, fluid mixing, and cooling of the solutions (Bornhorst and Rose 1994). Copper mineralization occurred sporadically along the full length of the PLV but was mainly concentrated in a zone 28 miles long that extends from Painesdale, Houghton County, to just past Mohawk, Keweenaw County. This zone is referred to as the central zone in this article. This 28-mile stretch along the PLV hosted the most intensely developed conglomerate and amygdaloidal lodes and produced 10,266,299,321 pounds of copper, approximately 70 percent of the total production for the entire district (which includes the sediment-hosted, stratiform chalcocite deposit of the White Pine mine). Total production for the district from 1844 to 1995 was 14,595,357,516 pounds of copper.

LOCALITIES IN THE CENTRAL ZONE OF THE COPPER COUNTRY

Porcelaneous and crystallized datolite has been found sparingly in all of the major amygdaloidal copper lodes mined in the central zone but does not occur in the conglomerate lodes. In their monumental work on the copper deposits of Michigan, Butler and Burbank (1929) described the occurrence of datolite in amygdaloidal lodes and fissure veins. They noted that in the sequence of mineral deposition in the copper deposits, datolite was formed in the intermediate to later stage of ore deposition.

Porcelaneous datolite occurred in the amygdaloidal lodes as veins, vesicle filling, and nodules. Vein fillings in fractures range from a fraction of an inch to more than 2 inches in width and can be continuous for hundreds of feet. The datolite is typically fine grained and translucent to opaque; colors range from white to butterscotch and from peach to orange to red. The most notable localities for vein datolite were the Pewabic, Isle Royale, Arcadian, and Baltic lodes. Porcelaneous masses to 2 inches across that completely fill irregular-shaped vesicles occur in the upper portion of some lava flows. This datolite conforms to the outline of the vesicles, and the rim or rinds of the masses are semismooth and usually coated with a dark green to black chloritic mineral. The colors of these datolites are some of the most beautiful seen in the Copper Country and include yellow, salmon-pink, orange, and dark red. The best examples of these vesicle-filled datolites are recovered from the Pewabic lode. However, the most common type of porcelaneous datolite forms nodules that exhibit a cauliflower-like outer surface and occur in altered zones or mud pockets in the amygdaloidal lodes. The mud pockets were called "nests" by the miners and could contain many nodules. Individual nodules range from a fraction of an inch to more than 14 inches across. The most common color is an opaque off-white but can include pinks and reds. Many of the nodules contain copper inclusions that color the datolite pale to dark gray, black, brown, or various shades of green and blue. The cauliflower-like nodules occurred in all of the amygdaloidal lodes mined and were especially noteworthy in the Pewabic, Kearsarge, Isle Royale, and Arcadian lodes; they were noticeably lacking in the Osceola lode.