Ore-grade gold mineralization within the Upper Zone at
BarrickÕs Rodeo Deposit is controlled by a combination of stratigraphy
and structure. The mineralization is hosted in fine-grained, carbonaceous,
muddy limestone, located within the upper portion of the
Popovich Formation and lamprophyre. Mineralization is concentrated
directly below, and locally within, a selectively silicified, fossiliferous,
debris flow. Structurally, gold mineralization is confined
between 330¡ striking, east-dipping faults. The regionally dominant,
70-85¡ east-dipping Post Fault Zone forms the eastern extent of mineralization
drilled to date. Ernie and Bert, 50-65¡ east-dipping
reverse faults, form the western extent of ore-grade mineralization.
Between the faults, a 330-340¡ trending, 10-20¡N plunging syncline
occurs with west-dipping, gold-enriched lamprophyres concentrated
on the eastern limb.
Gold occurs in two distinct forms and is associated with two distinct
geochemical patterns. The first, ÒtypicalÓ of Carlin-style gold
mineralization, consists of gold occurring within arsenical pyrite-rich
rims on pyrite grains and exhibits elevated concentrations of As, Hg,
Sb, Ag, Tl, Te and W. The second form of gold mineralization occurs
as relatively coarse, native gold particles associated with a potentially
older gold event. Gold in the second form is associated with enrichment
of Zn, Pb, Ag, Cd, Ba, Hg, Sb, As and Cu.
Hydrothermal alteration consists of decalcification, producing
a residual carbon-rich mud, selective silicification, plus localized
quartz-sericite-pyrite alteration of the lamprophyre. The competency
contrast between the selectively silicified beds, primarily fossiliferous,
debris flows, and the adjacent muddy limestone may have been
sufficient to concentrate bedding-parallel faults leading to the establishment
of conduits for gold-bearing fluids. Fault-controlled, mineralized
lamprophyre are frequently bounded by fault gouge, indicative
of post-intrusive reactivation, and exhibit ductile deformation.
Reactivation and local reverse movement provided open channels
for gold mineralizing fluids.
Underground reserves within the Upper Zone at the end of
1998 are 1,500,884 tonnes (1,654,425 tons) containing 28.1 tonnes
(904,163 oz) gold averaging 18.75 g/tonne (0.547 oz/ton).