Abstract: Wall-rock alteration along veins in the Central City-Idaho Springs region, Gilpin and Clear Creek Counties, Colorado, superficially resembles alteration around similar base-metal sulfide deposits, but differs in some details. Mesothermal-type veins of early Tertiary age cut deformed Precambrian metasedimentary, igneous, and metamorphosed igneous rocks; the veins contain pyrite, galena, sphalerite, chalcopyrite, and tennantite as principal constituents and minor gold, silver, and uranium minerals. A spatial zonation of ore minerals relates to distinct stages of ore deposition.
Detailed studies show that three of the wall rocks in the area—granodiorite, biotite-muscovite granite, and biotite-quartz-plagioclase gneiss—are altered to varying degrees and in varying amounts. Clay-mineral formation was a major feature of the process. The alteration sheath around fractures and veins is divided into four gradational but regular zones: (1) Least altered (fresh) rock; (2) weakly altered, hard rock; (3) moderately altered, soft clay-mineral rock—(a) montmorillonite-rich, (b) kaolinite-rich, and (c) illite-sericite-rich; and (4) strongly altered, hard (silicified) rock. The sequence of silicate alteration is believed to relate first to the relative stability of primary mineral structures in the alteration environment, but ultimately to the chemistry of the environment.
The host rocks are significant factors in localization of altered mineral assemblages as they supply the requisite elements for argillic-type alteration. Structures of the host rocks and veins play a prominent role in localization of altered rocks and the zoned ore deposits. Coincidence of alteration and mineralization processes may have more spatial than temporal significance.
A working hypothesis explaining the geological evidence proposes that most alteration was accomplished by meteoric and/or hypogene solutions before ore minerals were deposited. Later solutions of the pyrite stage may have intensified the early zone 4 alteration assemblage close to the source channels, but base-metal sulfide stage solutions were not competent to impress additional alteration effects. Most supergene alteration occurs close to the surface. Therefore, in these areas alteration zonation does not afford a reliable guide to ore.