The San Jose-Carachugo-Chaquicoacha Gold Trend, Yanacocha District, Northern Perú

YanacochaÕs San Jose-Carachugo-Chaquicocha corridor (SCC)
is presently the most productive segment of the Yanacocha district in
northern Perœ. The trend is delineated by a cluster of seven high sulfidation
gold deposits (San Jose Sur and Norte, Carachugo Norte,
Sur and Este, and Chaquicocha Norte and Sur) that range in size
from 400,000 to over 3 million ounces of gold. Resource/reserve ore
is based on oxide gold ore only, and little is known about the sulfide
ore potential. Present resource/reserve figures within the SCC gold
trend are 8.45 M oz gold for the Carachugo-Chaquicocha Complex
(Carachugo Norte, Sur and Este, and Chaquicocha Norte, Sur and
Alta) and 1.10 M oz gold for the San Jose Complex (San Jose Sur
and Norte). Total gold production to date exceeds 3 million ounces.
All rock units within the SCC are part of a sequence of volcanic
rocks classified as the Miocene (11.8 to 12.5 Ma) Yanacocha
Volcanic Complex. New stratigraphic relationships have been recognized
that indicate pyroclastic rocks are far more abundant, and
flow-dome complexes are less prolific, than previously interpreted.
Advancements in the understanding of the volcanic stratigraphy
have led to the present interpretation that defines the evolution of the
SCC through processes of eruptive cycles. Four eruptive cycles have
been interpreted from the volcanic stratigraphy; from oldest to
youngest they include: 1) An early eruptive cycle produced a
sequence of hornblende andesite tuffs and flow-dome complexes.
These andesites represent the oldest volcanics within the SCC. Local
accumulations of water-lain tuffs and colloidal silica overlaid the
early andesite volcanics. The laminated units are interpreted to represent
deposition in a subaqueous environment, or crater lakes, during
brief periods of volcanic quiescence. They are spatially associated
with the gold deposits, and may have accumulated during the
early stages of acidic alteration. 2) An early explosive cycle produced
ash-flow tuffs and undifferentiated pyroclastic rock that characteristically
contain accidental fragments of previously acidicaltered
andesite. These tuffs overlaid early andesite volcanic rocks
of eruptive cycle 1, and the laminated sediments and chemical precipitates
related to the period of quiescence. 3) Rare dacite domes
and their associated quartz-bearing pyroclastic rocks intruded and
overlay the volcanic rocks from eruptive cycles 1 and 2, and represent
the continuation of explosive volcanism. 4) Late phreatic eruptions
formed breccias that crosscut all lithologies and alteration
types, and entrained fragments of all rock types from eruptive cycles
1 through 3. These late explosive eruptions served as precursors to
a late resurgent phase represented by coarse feldspar porphyry
andesite/latite plutons and diatremes.
The SCC is a complex structural corridor that has an overall
N50¼E trend, and includes zones of major structural intersections
with N50¼W and E-W trending faults. These faults were important
controls for gold mineralization, and ore bodies formed at the intersections.
Styles of acidic alteration within the corridor are, in general,
typical of acid-leached and advanced argillic assemblages, but
the deposits are unique in other aspects. Tremendous volumes of
quartz are present with lesser amounts of advanced argillic and
argillic alteration. Massive quartz and granular quartz alteration
assemblages are abundant, and although genetically quite different,
they both overprint vuggy quartz. Essentially, rocks within a surface
area of 10 square kilometers consist almost entirely of quartz.
Domes localized within the structural corridor are overprinted by
acidic alteration, crosscut by gold-bearing hydrothermal breccias,
and are considered part of the pre-gold ore host stratigraphy.
Surface and down-hole trace element data are most complete
for the Chaquicocha and the Carachugo deposit complexes. Surface
geochemistry shows a strong relationship with anomalous mercury
and arsenic directly above concealed gold deposits that lack a surface
gold anomaly. Tin and barium have proven useful as surface
indicators over deposits, but these elements are erratic in behavior.
Within SCC gold ore bodies, mercury displays the strongest relationship
with gold ore in the oxide zones. Barium is secondary in
importance. Copper is depleted in the oxide zones, and shows only
a weak positive correlation with ore; however, copper displays the
strongest positive correlation with gold in the sulfide ore zones.
Arsenic and barium also show strong positive correlations in the sulfide
ore zones; however, mercury is erratic and displays a poor correlation
with sulfide ore. Elements that form halos to gold ore
include lead, antimony, and occasionally zinc and silver.
Gold deposits within the SCC occur as perched and rootless
tabular to sub-vertical bodies within structural corridors. Much evidence
exists to suggest gold deposition within SCC deposits
occurred later than the pervasive acidic alteration, and may represent
the latest phases of mineralization within the Yanacocha district.
Gold-bearing fluids favored the more permeable alteration assemblages
such as vuggy and granular residual quartz, or hydraulic-fractured
massive quartz. High-grade (>2.5 g/tn) and bonanza-grade
(>100 g/tn) gold occur as discrete zones associated with moderate to
high-angle fault zones and hydrothermal breccias surrounded by
tabular bodies of low-grade gold. Root zones with high grade gold
have not been found within the SCC deposits.
Four stages of mineralization have been interpreted from the
present observations of SCC deposits. Early pyrite is found encapsulated
within the dense silicification of massive quartz alteration.
Later pyrite-enargite occurs as veinlets that crosscut the pyritic massive
quartz. Late tennatite overprints pyrite in the veinlets, and a latest
barite-coarse gold event crosscuts the later three. Late stage crystalline
barite is found as stockwork veinlets and euhedral crystals
along fractures and vugs in zones of high-grade gold. Coarse gold
has been observed intergrown with barite and late hydrothermal
quartz in vugs and along fractures within zones of vuggy and massive
quartz alteration.