| dc.description.abstract | The Santa Cecilia gold deposit is located in the south-central portion of the Alta
Floresta Gold Province, southwest portion of Amazon Craton in the north of Mato
Grosso. The deposit is formed by three gold mineralized quartz veins known as Otta,
Tatá and Rato. The first two are hosted in an isotropic granodiorite while the latter in a
locally foliated metadacite. Hydrothermal alteration is similar for the veins and
developed in six stages respectively: sericitic, potassic, muscovite+pyrite, calcic,
silicification 1 and silicification 2, with gold deposited in the last silicification stage.
The quartz veins occur slightly to intensely recrystallized. Sulfides occur within the
quartz veins and consists of pyrite, pyrrhotite, arsenopyrite, chalcopyrite and sphalerite.
Fluid inclusion petrography in the veins revealed the presence of five Fluid Inclusion
Assemblage (FIA). FIA 1 and 2 in the Rato vein, FIA 3 e 4 in the Tatá vein, and FIA 5
in Otta vein. FIA 1 is primary, aqueous-carbonic, two-phase and present chlatrate
melting temperature ranging from -5,1 to +1,1 °C, with the calculated salinity ranging
from 12 to 20 wt% NaCl, total homogenization temperature – Th (total) – from 220 to
450°C with a mode at 360°C. FIA 2 is secondary, aqueous, two-phase, with Te in the
range of -42° to -25,1°C, suggesting a H2O-NaCl system with variations to KCl, MgCl2
and FeCl2. The ice melting temperature – Tm (ice) – range from -2,6 to -20,8°C
reflecting salinities between 4,34 to 22,91 wt% NaCl equiv. and Th (total) from 155 to
300°C with a mode around 240°C. FIA 3 and 4 are aqueous, two-phase, and
respectively of primary and pseudo-secundary origin. FIA 3 shows Te ranging from -
21,2 to -39 °C suggesting a H2O-NaCl system with variations to KCl, MgCl2 and
FeCl2. Tm (ice) ranges from -6,5 to -20,7 °C, calculated salinity between 9,86 to 22,85
wt% NaCl equiv. and Th (total) from 166 to 252°C, with a mode around 200°C. FIA 4
shows Te between -24,6 to -44,4°C, suggesting a H2O-NaCl-KCl system with variation
to MgCl2 and FeCl2, Tm (ice) ranges from -6,3 to -20,9°C, calculated salinity between
9,6 and 22,98 wt% NaCl equiv. and Th (total) from 198 to 267°C. FIA 5 is represented
by primary, two-phase aqueous inclusions with Te ranging from -22 to -24,7°C and Tm
(ice) ranging from -0,8 to -7°C. The Te suggests a H2O-NaCl system with variation to
KCl. Th (total) range from 185 to 290°C, with a mode around 200°C and the calculated
salinity ranges from 1,4 to 10,49 wt% NaCl equiv. Oxygen isotopic analyzes in quartz
returned values of δ
18O of 13,27‰, 12,46‰, e 13,70‰ respectively for Rato, Tatá and
Otta veins. The isotopic compositions of the quartz equilibrium fluids were calculated
based on the end-member’s values of Th (total) variation for each vein and respectively
returned values of δ
18O between +3,07‰ e +10,07‰, -1,64‰ e +3,56‰ e +1‰ e
7,8‰; the obtained variation suggests the existence of magmatic and meteoric sources.
Fluid inclusion data suggests a sequential evolution/release of a CO2-rich saline fluid
possibly magmatic to a CO2-poor/free aqueous saline fluid, which during evolution
mixed with a low-salinity aqueous fluid, likely meteoric in origin, and underwent a
boiling process; this set of processes possibly led to the gold deposition. The depth
boiling depth calculated at 300 meters for FIA 2, 150 meters for FIA 5 and 100 meters
for FIA 3 and boiling pressure at 30, 14 and 13 bars, respectively. Characteristics such
as hydrothermal alteration zones forming narrow halos around the veins, the small
volume of the reduced sulfide assembly, inclusions of moderate to high salinity likely of
magmatic origin, as shown by the oxygen isotope data, suggest that the Santa Cecília
deposit resembles mainly the intrusion-related gold systems (IRGS) model. LA-ICP-MS
zircon U-Pb ages of 1816 Ma for hosting granodiorite suggests a correlation with the
gold-bearing Paranaíta Suite while the age 1895 Ma for the metadacite suggests a
southwest extension of the Tapajós-Parima province into the northern Mato Grosso
state. | |
