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Why are sulfur and selenium often found together in metal ores?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — oxygen group element Modern version of the periodic table of the elements (printable).

Sulfur and selenium are often found together in metal ores because they share significant chemical and physical similarities as members of the oxygen group (Group 16) of the periodic table.2

Their co-occurrence is primarily due to their shared chalcophile (sulfur-loving) affinity. In nature, both elements readily form compounds with heavy metals such as copper, lead, mercury, and silver. Because of these similar bonding behaviors, selenium frequently occurs as a minor component or impurity within sulfide minerals.12

During the industrial processing of these ores, selenium often becomes concentrated in the residual materials, such as the anode slimes produced during the electrolytic refining of copper. This geochemical relationship is so consistent that the primary commercial source of selenium is as a by-product of refining these metal ores.2