Porphyritic Sulfide Deposits

A porphyritic sulfide deposit is a massive copper sulfide deposit that usually includes other heavy metals such as zinc, lead, silver, and gold

The original sulfide deposits form when hot magma intrudes into country rock.  As the magma cools, it differentiates, with the more stable components crystallizing out first.  The remaining magma is rich in water and heavy metals.  It percolates beyond the boundaries of the original body of magma and deposits heavy metal sulfides in a system of veins.

Later, rainwater may dissolve the upper layers of the porphyritic deposit and redeposit an even more concentrated layer of ore known as “copper hat.”  This is the richest copper ore commonly mined.  Most U.S. mines have exhausted this ore and are now working the underlying porphyritic bodies.

One serious difficulty with working porphyritic sulfide deposits is that the sulfide readily oxidizes in air to form sulphuric acid.  Acid mine discharge can be a serious environmental hazard, as at the Ashio mine in the late 1890s, where discharge contaminated a significant area of the Kanto Plain.

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