Fissure veins
Fissure veins
Commercial production began in 1844 at the Phoenix mine. Most early miners began with little knowledge or planning, and few mines ever saw production, much less profit. The first successful copper mine, the Cliff mine, began operations in 1845, and many others quickly followed. These first mines worked copper-filled fissure veins that cut across stratigraphic layers.
Although the copper-mining region stretched about 100 miles from northeast to southwest, the most productive early mines, working fissure veins, were those at the north end in Keweenaw County (such as the Central, Cliff, and Phoenix mines), or at the south end in Ontonagon County (such as the Minesota Mine).
In Keweenaw County, the fissure lodes were nearly vertical mineralized zones with strike nearly perpendicular to that of the enclosing basalts and conglomerates. In Ontanogan county, by contrast, the fissures had strikes nearly parallel to, and dips slightly steeper than, the surrounding beds.
The miners sometimes found masses of native copper up to hundreds of tons. To extract a single mass of copper, miners could spend months chiseling it into pieces small enough to hoist out of the mine. Although they were pure copper, removing the masses took a great deal of effort, and was sometimes not even profitable. The majority of the copper recovered was "barrel copper" (pieces broken from the rock and hand sorted in the "rock house," and shipped to the smelter in barrels), and finer copper broken loose from the rock in stamp mills and separated by gravity in "buddles" or "jigs."
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