New Mexico Mineral Symposium — Abstracts

The Flambeau mine was located about 1.25 mi southwest of the town of Ladysmith, Rusk County, northwestern Wisconsin. The mine site is at an elevation of 1,130 ft above sea level and only 140 ft from the Flambeau River, which the mine takes its name from. The general topography of this glaciated region is low rolling countryside with numerous lakes and rivers. The picturesque scenery of the area is agricultural, with dairy farms interspaced with timbered lands.

The open pit Flambeau mine was a supergene enriched deposit that contained a high-grade copper orebody that was mined from May 1993 to March 1997. During the short life span of the mine, 1.9 million tons of ore was extracted that averaged 10% copper and 0.18 ounce of gold per ton. The deposit was located in a belt of Precambrian sedimentary and volcanic rocks known as the Wisconsin magmatic terranes, which are located on the southern margin of the Canadian Shield. The orebody was a volcanogenic massive sulfide deposit similar to other deposits that occur over the length of the volcanic belt in northern Wisconsin. The primary ore consisted of poorly layered, fine-grained chalcopyrite, sphalerite, pyrrhotite, pyrite, and minor galena.

Extensive folding and faulting during the period of Penokean Mountain building about 1.85 b.y. ago tilted the volcanic rocks to a vertical position. A long period of erosion followed that reduced the mountains to low rolling hills. An Upper Cambrian sea covered the area about 520 m.y. ago and deposited a layer of sandstone that covered the deposit. During relatively recent times part of this sandstone was removed by glaciation. The original deposit was extensively altered and enriched by chemical weathering that formed a supergene high-grade chalcocite orebody. An iron-rich cap (gossan) formed in the sandstone over the orebody that also contained appreciable fine-grained native gold, which was mined and paid for most of the development costs.

Chalcocite was the dominant supergene ore mineral and occurred as granular and massive material. It also occurred as fine crystals that rival those found at Cornwall, England, and Bristol, Connecticut. The crystals also made the mine world famous in mineralogical circles for the size, crystallization, and number of specimens produced. Other minerals that occurred in the supergene zone and were of interest to collectors were native copper along with gold and silver. Many stunning specimens of microcrystals were also recovered, which included complex crystals of chalcocite along with colorful groups of cuprite, malachite, and azurite crystals.

The Flambeau mine will be remembered as a source of fine chacocite specimens mainly due to the efforts of Casey Jones who collected on site the majority of specimens produced from the short-lived mining operation. Today, reclamation has been completed and wetlands have been created from the back-filled open pit.