inequigranular one: centimetrical megacrystals and 
megaporphyroclasts embedded in a medium grained 
matrix. Associated basic dikes of dioritic composition 
intrude the metagranites and show features of 
mechanic mixing of magmas. 
The IGS is composed by biotite monzogranites and 
amphibole monzogranites to  sienogranites. These 
lithotypes have two main petrographic facies: a) coarse 
grained facies and b) medium to fine grained 
equigranular facies. All of these rocks are crosscut by 
aplitic and diabase dikes. There are also acidic volcanic 
rocks, mainly riolites. The IGS rocks host many gold- 
quartz veins of different types and thicknesses. The 
granitic rocks of the IGS are mainly isotropic, but 
locally may show some evidence of magmatic foliation 
and ductile and brittle-ductile deformational structures. 
The MCF is composed by polimitic conglomerates and 
arcose sandstones, with intercalation of intermediate 
volcanic rocks. The conglomerates have a framework 
constituted by pebbles of quartz, quartzites, granites 
and basic rocks in a matrix composed by sandy shale. 
The PBSS is mainly composed by sandstones, 
siltstones and mudstones. The sandstones are fine to 
medium grained, while the conglomerates are coarse 
grained. The rocks are regarded to the Pimenteiras and 
Serra Grande formations. 
The cenozoic deposits of the area are constituted by 
lateritic covers and by alluvial sediments associated 
with the main drainage lines. 
2.2. Structural geology 
The structural framework of the Porto Nacional region 
is characterized by four different deformational 
surfaces, denominated: S,, Sn+1, Sn+2/Sm+2 and Sn+3 
(Gottardo, 1996). 
2.2.1. S, and S,+1 deformational surfaces 
The S, is registered by a regular gneissic banding, 
characterized by the alternation of mafic bands and 
felsic bands. The gneissic banding is variably 
transposed by folds (Fn+1) and boudins; the axial plane 
foliation (Sn+1) is an anastomosed structure present in 
the PNC, CCGgS and NMS. In the NMS, S, is a 
transposed schistosity, still preserved in micro and 
meso lithons. The spatial and geometric relations 
between S, and Sn in the PNC, CCGgS and NMS 
show that both surfaces were paralleled during Dn+1 
deformational event. 
2.2.2. Sn+2/Sm+2 deformational surface 
The transcurrent to oblique shear zones crosscutting 
the Porto Nacional region are the main feature regarded 
to the Sm+2 deformational surface at large scale. These 
shear zones are ductile to brittle-ductile and trend 
N30°E. They extend more than 100 km and show 
thicknesses from 10 to more than 1000 m. The 
nucleation and the movement in these shear zones 
changed the spatial disposal of the previous S, and Sn+1 
in the PNC, CCGgS and NMS; the structures resulting 
174 
from this superposition are complexes and may be 
characterized as follows: 
a) the main mesoscopic feature is a mylonitic foliation 
(Sm+2) trending NE, which is strongly anastomosed. 
There are also isoclinal small scale folds, rootless folds 
made upon the gneissic banding; 
b) the Sm+2 Mylonitic foliation is very heterogeneous: 
ortomylonites are present in the high strain zones, 
disposing a highly anastomosed pattern; there are also 
phyllonites in zones of high fluid pressure. Sometimes, 
ortomylonite and  phyllonite textured zones are 
alternated to compound a coarse banding. There are 
central-type gold-quartz veins in this S,,,2 shear zones; 
c) in the CCGgS, there are brittle subsidiary shear 
zones of variable orientation and thicknesses; their 
movement direction were recovered by slickensides and 
associated steps and grooves. Most of these shear 
zones are dilational structures, which control the 
emplacement of gold-quartz veins; 
d) the shear zones gave origin to en echelon or large 
open folds developed upon the PNC, CCGgS and NMS; 
these folds have axial plane cleavage or foliation (Sn) 
and show saddle reefs quartz veins in the hinge zone; 
e) in the MMgS, the shear structure is a composite 
foliation (Sc+2) made up by magmatic flow features and 
solid-state strain features. The deformational features 
are derived from dynamic  recrystallization and 
reorientation of megacrystals, that transformed them in 
porphyroclasts with asymmetric recrystallization tails. 
The deformational features include high strain shear 
zones (Sm+2) Of variable thicknesses, to where Sc+2 
foliation merge in a S/C relation (s.s.); 
f) in the IGS, the Sm+2 foliation is observed as discrete, 
brittle-ductile shear zones or as cleavage zones. The 
brittle-ductile shear zones are variable in thicknesses 
and display S/C relation (s./.) to the previous Sp//Sn+1. 
These shear zones form extensional strike-slip duplexes 
and pinnate synthetic ramifications. The relative 
movement direction is sinistral and was characterized 
mainly by slickensides and their grooves and steps; 
All of these structures (S,//Sn+1 and Sn+2) are truncated 
by low angle NE cleavage and minor faults. They show 
regular planes, spaced from 1 to 10 m, which were 
regarded as Sn+3 structure. 
3. METODOLOGY 
3.1. Revision of the concepts of lineaments 
The name lineament was introduced by Hobbs (1904) 
to characterize any linear feature in the ground. From 
the advent of aerial photograph and, later, satellite 
images in geology, there was an increasingly interest in 
defining and analysing linear features of the Earth. 
Then, some names were introduced as synonymous to 
lineament. O'Leary et al (1976) concluded that 
lineament is the most suitable term to describe these 
linear features. The lineaments may be simple or 
composite, and rectilinear or slightly curved. They must 
differ from the nearby linear patterns, and are the 
consequence of underground structures. Sabins (1986) 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996