Exploration & Mining Geology, Vol. 6, No. 2, 1997
The Campo Morado precious-metal-bearing, volcanogenic massive sulfide
deposits occur in a lower Cretaceous, bimodal, calc-alkaline volcanic sequence
in a major northerly trending belt in the Guerrero Terrane in northeastern
Guerrero, Mexico. During upper Cretaceous to Early Tertiary greenschist-facies
regional metamorphism, the rocks were deformed strongly into a northeast verging
fold-and-thrust belt. Three later stages of weak deformation were dominated,
respectively, by kink folds, broad warps, and extensional faults. Most deposits
occur in the upper part of a sequence of felsic flows and heterolithic
volcanoclastic rocks or at its contact with overlying chert and
argillite-sandstone. The Reforma and El Rey massive sulfide deposits are on the
overturned limb of a major, thrusted anticline, and the Naranjo and El Largo
massive sulfide deposits are to the south on the upright limb of the same major
fold. The La Lucha and San Rafael massive sulfide occurrences are in an upper
plate to the southwest which was thrusted over the plate containing the Naranjo
and El Largo deposits. In several of the deposits, Au, Ag, Zn, and Pb are
concentrated near the stratigraphic top, and Cu is concentrated near the
stratigraphic base. Major minerals are pyrite, quartz, ankerite, sphalerite,
chalcopyrite, and galena. Minor minerals are tennantite-freibergite,
arsenopyrite, and pyrrhotite. Gold and Ag occur in argentian gold, and Ag also
occurs in tennantite freibergite. The cumulative inferred resource of massive
sulfide for the Reforma, Naranjo, El Rey, and El Largo deposits exceeds 30 Mt,
with the latter two deposits incompletely delineated. Underlying pyrite-quartz
stockwork zones contain chalcopyrite, chlorite, and sphalerite. Hydrothermal
alteration minerals in the stratigraphic footwall are pyrite, quartz, chlorite,
ferroan dolomite, and ankerite. In the stratigraphic hangingwall, hydrothermal
alteration minerals are sericite, calcite-dolomite, and lesser clay minerals and
quartz. The deposits belong to a low-sulfidation, volcanogenic massive sulfide
system formed in a subaqueous environment, and are of the bimodal, siliciclastic
type.
