DESCRIPTIVE MODEL OF PORPHYRY Cu, SKARN-RELATED DEPOSITS
By Dennis P. Cox
DESCRIPTION Chalcopyrite in stockwork veinlets in hydrothermally altered intrusives and in skarn with extensive retrograde alteration (see fig. 50).
GENERAL REFERENCE Einaudi and others (1981), p. 341-354.
Rock Types Tonalite to monzogranite intruding carbonate rocks or calcareous clastic rocks.
Textures Porphyry has microaplitic groundmass.
Age Range Mainly Mesozoic and Tertiary, but may be any age.
Depositional Environment Epizonal intrusion of granitic stocks into carbonate rocks. Intense fracturing.
Tectonic Setting(s) Andean-type volcanism and intrusion superimposed on older continental shelf carbonate terrane.
Associated Deposit Types Skarn copper, replacement Pb-Zn-Ag.
Mineralogy Chalcopyrite + pyrite + magnetite in inner garnet pyroxene zone; bornite + chalcopyrite + sphalerite + tennantite in outer wollastonite zone. Scheelite and traces of molybdenite and galena may be present. Hematite or pyrrhotite may be predominant.
Texture/Structure Fine granular calc-silicates and quartz sulfide veinlets.
Alteration Potassic alteration in pluton is associated with andradite and diopside in calcareous rocks. Farther from contact are zones of wollastonite or tremolite with minor garnet, idocrase, and clinopyroxene. These grade outward to marble. Phyllic alteration in pluton is associated with retrograde actinolite, chlorite, and clay in skarn.
Ore Controls Intense stockwork veining in igneous and skarn rocks contains most of the copper minerals. Cu commonly accompanies retrograde alteration.
Weathering Cu carbonates, silicates, Fe-rich gossan.
Geochemical Signature Cu, Mo, Pb, Zn, Au, Ag, W, Bi, Sn, As, Sb.
Ruth (Ely), USNV (Westra, 1982a_)
Gaspe', CNQU (Allcock, 1982_)
Christmas, USAZ (Koski and Cook, 1982)
Silver Bell, USAZ (Graybeal, 1982_)