The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are
hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean
zone of southeastern Ecuador. The skarns consist dominantly of granditic
garnet (Ad(20-98)) with subordinate pyroxene
(Di(46-92)Hd(17-42)Jo(0-19)) and epidote and are spatially associated
with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is
developed at the intrusion margins and grades inwards into a potassic
alteration zone. Exoskarn has an outer K- and Na-enriched zone in the
volcanosedimentary unit. Gold mineralization is associated with the
weakly developed retrograde alteration of the exoskarn and occurs mainly
in sulfide-poor vugs and milky quartz veins and veinlets in association
with hematite. Fluid inclusion data for the main part of the prograde
stage indicate the coexistence of high-temperature (500A degrees C to >
600A degrees C), high-salinity (up to 65 wt.% eq. NaCl), and moderate-
to low-salinity aqueous-carbonic fluids interpreted to have been trapped
at pressures around 100-120 MPa, corresponding to about 4-km depth.
Lower-temperature (510-300A degrees C) and moderate- to low-salinity
(23-2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote
of the end of the prograde stage. The microthermometric data (Th from
513A degrees C to 318A degrees C and salinity from 1.0 to 23 wt.% eq.
NaCl) and delta(18)O values between 6.2aEuro degrees and 11.5aEuro
degrees for gold-bearing milky quartz from the retrograde stage suggest
that the ore-forming fluid was dominantly magmatic. Pressures during the
early retrograde stage were in the range of 50-100 MPa, in line with the
evidence for CO(2) effervescence and probable local boiling. The
dominance of magmatic low-saline to moderately saline oxidizing fluids
during the retrograde stage is consistent with the depth of the skarn
system, which could have delayed the ingression of external fluids until
relatively low temperatures were reached. The resulting low
water-to-rock ratios explain the weak retrograde alteration and the
compositional variability of chlorite, essentially controlled by host
rock compositions. Gold was precipitated at this stage as a result of
cooling and pH increase related to CO(2) effervescence, which both
result in destabilization of gold-bearing chloride complexes.
Significant ingression of external fluids took place after gold
deposition only, as recorded by delta(18)O values of 0.4aEuro degrees to
6.2aEuro degrees for fluids depositing quartz (below 350A degrees C) in
sulfide-rich barren veins. Low-temperature (< 300A degrees C) meteoric
fluids (delta(18)O(water) between -10.0aEuro degrees and -2.0aEuro
degrees) are responsible for the precipitation of late comb quartz and
calcite in cavities and veins and indicate mixing with cooler fluids of
higher salinities (about 100A degrees C and 25 wt.% eq. NaCl). The
latter are similar to low-temperature fluids (202-74.5A degrees C) with
delta(18)O values of -0.5aEuro degrees to 3.1aEuro degrees and
salinities in the range of 21.1 to 17.3 wt.% eq. CaCl(2), trapped in
calcite of late veins and interpreted as basinal brines. Nambija
represents a deep equivalent of the oxidized gold skarn class, the
presence of CO(2) in the fluids being partly a consequence of the
relatively deep setting at about 4-km depth. As in other Au-bearing
skarn deposits, not only the prograde stage but also the
gold-precipitating retrograde stage is dominated by fluids of magmatic
origin.