The sphalerite in the Broken Hill orebody at Aggeneys, Northwest Cape Province, South Africa, comprises a primary suite which equilibrated at approximately 650 degrees C and 5.9 X 10 8 Pa (5.9 kb), and a secondary suite which formed during a process of supergene alteration. The primary sphalerite that equilibrated with pyrrhotite contains 14 to 22 mole percent FeS, that with pyrrhotite and pyrite contains 13.7 mole percent FeS, and that with chalcopyrite less than 4 mole percent FeS. All the primary varieties are low in copper (less than c.a. 1.6 mole % CuS). The secondary varieties include a chemically-altered population, here referred to as copper-rich sphalerite, and a neoformed population (mole % FeS less than 4.5, and mole % CuS between 1.6 and 2.4). The copper-rich sphalerite is characterized by minute intergrowths of a chalcopyrite-like sulfide (similar to chalcopyrite disease of Barton, 1978), is most frequently developed close to the water table, and is interpreted as the result of copper enrichment during the process of supergene enrichment. The reaction envisaged is: yCu (super +n) + Zn x Fe (sub 1-x) S <--> Zn x Fe (sub 1-x-2y) S (sub 1-2y) + yCuFeS 2 + yFe (super +2) + (2-n)ye (n = 1 or 2). The net effect of this reaction is that the primary sphalerite is gradually depleted in iron as the process of supergene alteration proceeds, and this effect may have serious implications for the sphalerite geobarometer. The wide variation in the iron content of the sphalerite should also be taken into account in flotation operations.