Gr. Olivo et al., The Au-Pd mineralization at the Conceicao iron mine, Itabira district, southern Sao Francisco craton, Brazil: An example of a Jacutinga-type deposit, ECON GEOL B, 96(1), 2001, pp. 61-74
Citations number
59
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
The Cunceicao mine, which is located in the Itabira district, southern Sao
Francisco craton, is a rare example of gold and palladium mineralization ho
sted by jacutinga, an altered Lake Superior-type iron-formation. The mine s
equence consists of three metamorphosed lithostratigraphic units, which are
from the oldest to the youngest: (1) an Arehean volcano-sedimentary sequen
ce, (2) a Paleoproternzoic iron-formation, and (3) a Paleoprotcrozoic quart
zite. These units are cut by mafic intrusive rocks and, with the exception
of intrusive contacts, all contacts between the various units are structura
l and interpreted as thrust faults. The rocks were subjected to three phase
s of folding: (1) D-1, characterized by tight asymmetric and isoclinal fold
s, which develop locally into sheath folds (the D-1 fabrics are the S-1 fol
iation and elongation lineation); (2) D-2, represented by tight, large-scal
e folds and associated parasitic folds; and (3) D-3, characterized by open
folds with an associated crenulation cleavage. The D-1 and D-2 deformation
are interpreted to be products of simple shear with a. transport direction
consistently east over west. These rocks were metamorphosed to typical mine
ralogy of amphibolite facies conditions during the D-1 phase of folding.
The Conceicao orebody consists of a single quartz vein surrounded by jacuti
nga. The vein is parallel to the S-1 foliation, which is locally oriented w
est-southwest and dips 55 degrees to 75 degrees NW, and was folded by De de
formation . It extends 100 m downdip in the direction of the L-e elongation
lineation and 400 m along strike. The jacutinga consists of centimeter-sca
le layers of muscovite, specular hematite (hematite I), and octahedral hema
tite grains (hematite II) with remnants of magnetite, and a finely laminate
d envelope around the quartz vein, which is composed mainly of Fe hydroxide
s, hematite (hematite III), kaolinite; muscovite, and tourmaline. The quart
z vein is dismembered and quartz grains are partially corroded and filled b
y Fe hydroxides and kaolinite. The ore minerals, interpreted to have formed
during the main hydrothermal event, are palladian gold (up to 3.2 wt % Pd)
and an unknown Pd-Sb-As mineral (tentatively identified as mertierite II),
which are typically included in hematite II octahedra. A few grains of pal
ladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Nat
ive gold is commonly associated with Fe hydroxide in the finely laminated a
lteration envelope around the quartz vein and is interpreted to have precip
itated during weathering.
The similarities between the Au-Pd orebodies at die Conceicao and Caue depo
sits (referred to as "jacutinga-type" deposits) suggest that they were form
ed by the same processes and their characteristics favor an epigenetic mode
l for gold and palladium concentration in the iron-formation. Since the Cre
taceous, these Archean and Proterozoic locks have been weathered, and the h
ydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn)
hydroxides; gold and palladium have been remobilized and reprecipitated As
pure metals or Pd oxides.