Characteristics of the Tops of Porphyry Deposits: Relevance to Exploration

摘要

Porphyry deposits typically form in several stages, with characteristic alteration assemblages and zoning associated with each stage. In the early high-temperature potassic stage, during ductile formation of quartz veins, vapor that separates from a hypersaline liquid rises toward the surface. A portion of the vapor may discharge as volcanic fumaroles, but some of the vapor will condense into groundwater, forming acidic liquid from the magmatic HC1 and SO_2. This creates advanced argillic alteration of alunite and kaolinite, as well as pyrophyllite, dickite, diaspore, etc. Extreme alteration and leaching will create a core of residual silica, and where this is controlled by a lithologic unit, a lithocap will form, with a halo of advanced argillic alteration minerals. The subsequent, lower temperature porphyry stage forms straight quartz veins with center lines, due to the brittle conditions, and is commonly associated with muscovite. As this liquid ascends and cools, it may become stable with respect to pyrophyllite. If a lithocap is present, such pyrophyllite, along with muscovite, will occur in its roots, overlapping the mineral assemblage of the lithocap halo, albeit formed in by a distinctly different process. Distinguishing between the origins of these two advanced argillic assemblages, which form beneath and adjacent to the lithocap, respectively, is essential to assess the erosional level of a prospect, i.e., the depth to a potential porphyry system.