BATHOLITHIC AND EARLY HALO TYPE COPPER-MOLYBDENUM DEPOSITS

At the 1972 and 1975 GSA annual meetings Eric Cheney and John Trammell proposed a new style of batholithic Cu-Mo deposit which diverged in many respects from the Lowell-Guilbert or “classic” porphyry Cu model. Their type examples, Butte, MT, Quartz Creek, WA, and Brenda, BC are hosted mostly in equigranular plutons and exhibit “inside-out” alteration patterns with sericitic zones internal to, cutting and extending above potassic assemblages. They also lack peripheral propylitic zones. The batholithic model did not gain wide acceptance at the time, and was not published until 1996. However, Cheney and Trammell anticipated and formalized several advances in the understanding of porphyry Cu systems such as ubiquitous late quartz-sericite-pyrite (Gustafson and Hunt, 1971, 1975; Carson and Jambor, 1977), and the inside-out Chilean porphyry model (Seedorff et al., 2005). Subsequent work has shown that most batholithic Cu-Mo deposits are examples of early-halo type (“EHT”) porphyry systems. Higher Cu grades in these deposits are localized in vein or fracture halos of coarse muscovite ± biotite ± K-feldspar with abundant Cu sulfides (Cheney and Trammell's “biotitic envelopes”). These halos contrast with the A-type sugary quartz-sulfide veins controlling initial Cu deposition in the more widely recognized A-vein type (“AVT”) or classic systems such as El Salvador. EHT deposits are interpreted to form above deeper magma chambers than AVT deposits. Magmatic-hydrothermal fluids in EHT deposits exsolve at pressures greater than the critical point of saline water, leading to a different path of fluid and mineralogic evolution. Other EHT systems include Chuquicamata, RT, MM, Lomos Bayas, and Los Pelambres, Chile; Haquira, Peru; Copper Creek, Arizona; Ann-Mason Pass and Bear (Yerington district), Nevada; and the Valley deposit (Highland Valley district), BC. Most of these are end-member or transitional batholithic deposits in the sense of Cheney and Trammell, 1975.