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Geology

Detailed geological data from our current drill program can be found HERE.

 

A map of Keweenaw Copper Co. projects can be found HERE.

 

 

Copper in the Keweenaw

 

The copper-bearing rock sequence in the Keweenaw consists of several hundred lava flows with interbedded conglomerate and sandstone layers. The entire sequence is called the Portage Lake Lava Series (PLLS). Structurally, the series is the southeastern  limb of a major syncline  that dips down below Lake Superior. The northwest limb of the syncline is exposed on Isle Royale near the northwestern  shore of Lake Superior. The rocks are Keweenawan  in age (Upper Precambrian) and represent a sequence of flood basalts that can be traced in the subsurface several hundred miles to the southwest by means of their magnetic signature.

 

The  lava  flows  are  basaltic  in  composition,  and  the  entire  sequence  is  several  thousand  feet  thick. Individual flows range in thickness from a few feet to at least 1,600 feet and in strike length from just a few hundred feet to at least 40 miles.

 

The base of Keweenawan  basaltic flows is truncated by a major low- to high-angle reverse fault known as the Keweenaw  Fault. The fault is generally parallel to the strike of the PLLS and thrusts these rocks over a younger red bed sequence named the Jacobsville sandstone.

 

Historically,  three  types  of deposits  were  mined.  Most  spectacular  were  the fissure  veins that hosted individual masses of native copper, often weighing hundreds of tons. More productive, however, were the native   copper   lodes   in   brecciated   and   scoriaceous   amygdaloid   flow   tops  and   lode   deposits   in conglomerate  horizons.

 

At the properties  of interest, copper  is hosted in at least three stratigraphic  horizons. At the Centennial Mine,  copper  is contained  within  the Calumet  & Hecla conglomerate,  the most productive  host in the district. At the Kingston Mine, copper is hosted in the Kingston conglomerate, a minor host in the district.

 

At the 543S and other copper sulfide deposits, copper is hosted mainly in amygdaloid flow tops and rarely in conglomerate in basal units of the PLLS.

 

Native copper and associated native silver were generally associated with calcite, quartz, epidote, and chlorite; frequently with prehnite, datolite, analcite, and laumontite; and uncommonly with apophyllite. Rock alteration in the dense trap rock is not extensive; the most common alteration minerals are reddish adularia feldspar, chlorite, epidote, pumpellyite, and laumontite. In the northern area sulfide deposits, the primary copper mineral is chalcocite, with minor amounts of native copper and silver. Alteration is similar to alteration at the native copper mines.