Arsenic in Porphyry Copper Deposits: Economic Geology of a Polluting Element

Abstract

Air pollution caused by the smelting of ore concentrates from porphyry copper deposits represents a serious environmental problem. Porphyry copper deposits can be grouped into an arsenic-poor class, without arsenic minerals, and an arsenic-rich class, of equal size, with arsenic minerals. Tennantite-group minerals are the most frequently observed arsenic minerals; enargite/luzonite and arsenopyrite are less common. The presence or absence of arsenic minerals does not correlate with variations in geotectonic setting of porphyry copper deposits, but the occurrence of arsenic minerals is closely related to hydrogen metasomatism (sericitic, intermediate argillic, and advanced argillic alteration). Fluid evolution paths leading to low K+, Na+, and Cu+ versus H+ ion activity ratios promote the deposition of arsenic sulfosalts. Tennantite is a stable phase at low temperatures and low Cu+/H+ activity ratios of the hydrothermal fluid. The As concentrations of hydrothermal fluids generally are sufficiently high to allow precipitation of tennantite at appropriate configurations of other variables (fluid composition and temperature). The presence of arsenic-rich country rock is not envisaged as a necessary condition for the formation of arsenic-rich porphyry copper deposits. Chances for the development of arsenic-poor zones are best where alkali metasomatism is dominant in porphyry copper deposits.