Some thoughts on gold-rich porphyry copper deposits

Contributions to commodity geology research

“…At Dos Pobres (Langton and Williams, 1982), 0.005 to 0.015 percent molybdenum occurs in a quartz-sericite-pyrite alteration zone lying partly within the outer fringe of the> 0.5-percent-copper orebody and partly outside of the ore body. A similar relation is cited by Sillitoe ( 1979) at Saindak, Pakistan; Bajo de Alumbrera, Argentina; and Dizon, Philippines.…”

“…Deposit Tonnage Sillitoe (1979) noted no relation between deposit tonnage and gold grade in his set of 16 deposits having gold grade greater than or equal to 0.4 ppm. For our set of 55 deposits, which includes a wider range of gold grades, however, there is a low correlation of -0.42 between deposit tonnage and gold grade; this correlation is significant at the 1-percent level.…”

“…The richest deposits contain 0.9 g/t (Sillitoe, 1979). Median molybdenum grades are 0.003 percent within copper-gold orebodies, but peripheral zones reaching 0.01 percent molybdenum may occur outside the coppergold orebody.…”

“…Magnetite in veinlets makes up more than 1 percent by volume of most porphyry copper-gold orebodies (Saegart and Lewis, 1977;Sillitoe, 1979). Magnetite forms stable assemblages with any of the minerals that characterize potassic or feldspar-stable alteration: chalcopyrite, biotite, anhydrite, K-feldspar, chlorite, or actinolite.…”

“…Previous contributors in this area are Kesler (1973) who plotted gold, molybdenum, and copper content of porphyry ores on a triangular coordinate system and related gold and molybdenum content to tectonic environment; Sillitoe (1979Sillitoe ( , 1982 who noted the correlation between gold grade and magnetite content in the potassic alteration zones of porphyry copper deposits; and Sinclair and others (1982) who noted other factors, such as deposit morphology, that are correlated with gold content.…”