5th Swiss Geoscience Meeting, Geneva 2007
Geology and mineral occurrences in the Toldojirca area
(Eastern part of the Morococha district, central Peru)
Pérez José, Kouzmanov Kalin & Fontboté Lluís
Département de Minéralogie, Rue des Maraîchers 13, CH-1205 Genève
perezja6@etu.unige.ch
The polymetallic Miocene Morococha mining district is located in the central
Peruvian Andes, 150 km east of Lima and consists of different types of ore
bodies such as porphyry mineralization, skarn, replacement bodies (locally
known as mantos) and polymetallic epithermal veins hosted by sedimentary and
magmatic rocks of various ages. The main mineralized areas in the district
occupy its central, northern and eastern zones.
The present work studies Au anomalies of so far unknown origin located in the
Toldojirca area, in the extreme eastern part of the district. The main topographic
marker in the studied area is the Toldojirca hill, with a N-NE to S-SW orientation
and maximum elevation of 4759.4 m. Two mineralization styles are present:
several veins occurring as mineralized structures with sinuous and
discontinuous shapes; and a replacement body (manto). Both types are hosted
by carbonate and volcanic rocks (Terrones, 1949) of the Pucará Group
(Mégard, 1978; Rosas, 1994). Carbonate rocks with siliciclastic components of
the Aramachay Formation and mainly dolomitic packstones and grainstones of
the Condorsinga Formation crop out in the Toldojirca area. They correspond to
the middle and the upper formations of the Pucará Group, Hettangian to Upper
Toarcian in age. Several beds of the Aramachay Formation present variable
intensities of dolomitization or silicification. Tuff intercalations with typical fluidal
textures occur in the middle and upper part of the Aramachay carbonate
sequence. In the lower part of the Condorsinga Formation a group of mafic
volcanic rocks known as the Montero Basalt crops out. The later belongs
geochemicaly to the within-plate basalts. Several layers of carbonate clastsupported breccias occur in the upper part or the Aramachay Formation and all
along the Condorsinga sequence. The breccia bodies are roughly bed-parallel,
and are interpreted to have formed by dissolution of tectonically brecciated
carbonate rocks along several overthrust planes. In the southern part of the
Toldojirca hill, the stratigraphic beds show constant strike and dip (N135°E and
29°NE in average), in the central part the dips are gradually steeper (up to
70°NE). Beds in the northern part of the Toldojirca hill show less regular strike
(N143°E ± 15°) whereas the dips are constant (close to 33°NE).
Vein-type bodies are present in the central and southern part or the Toldojirca
area and form a structurally controlled discontinuous set of mineralized bodies
which are roughly oriented N40°-50°E and dip 80°NW. Their size is in average,
about 270 m long and 60 cm thick with unknown depth. The vein-type bodies
are strongly weathered and only supergene minerals (mainly Fe- and Mnoxides/hydroxides, as well as native silver) and quartz forming replacement
textures and filling open spaces as veinlets and geodes can be found. Locally,
traces of pyrite in association with quartz, sphalerite, galena, marcasite,
sulfosalts and rutile are preserved. Analysis of these strongly weathered vein
yields Au and Ag values as high as 8.7 ppm and 92.0 ppm, respectively.
Arsenic (up to 1%), Sb (up to 494 ppm), Pb (up to 806 ppm) and Zn (up to
5th Swiss Geoscience Meeting, Geneva 2007
2.25%) are also present. Detailed mineralogical, S-isotopic and fluid inclusion
studies are in progress.
A replacement body 80 cm thick and with an approximate surface of 0.25 km2
located in the northeastern and northwestern upper part of the Toldojirca hill
occurs as a Si-Mn rich carbonate rock strongly weathered to Mn and Fe
hydroxides. The replacement affects part of the stratigraphically highest
packstone beds of the Condorsinga Formation, and in particular packstone
levels within grainstone beds, suggesting that fluid flow and replacement are
lithologically controlled. The mineralogy of the manto is dominated by Mnoxides in a strongly silicified carbonate rock. In zones with neoformed crystalline
quartz and/or chalcedony, disseminated fine-grained pyrite was found. Several
quartz veinlets crosscut the manto and partly also silicify it following the
lithological control described above. Main primary minerals in veinlets are
sulfosalts intergrown with idiomorphic quartz. Pyrite occurs fine-grained
disseminated in the veinlets and in the silicified host rock. Relictic rhodochrosite
is also found. Supergene Fe- and Mn-oxides are present in both veinlets and
host rock in fractures and/or forming boxwork textures. The presence of native
silver filling open spaces in the northwestern part of the hill is due to weathering
and secondary enrichment. The replacement body is interpreted to be feed by a
N50°E trending vein which intersects it.
Immediately below the Montero Basalt, in the southern part of the Toldojirca
Hill, a bed parallel 4.5 m thick and 100 m long along strike of quartz is found. It
shows the same texture and mineral composition than a N50°E trending vein
which appears to feed it.
The present work suggests that the Au-anomalies in the Toldojirca area are the
product of weathering of polymetallic veins. Due to the similarities in
mineralization style and structural control of the ore bodies in the Toldojirca
area and of veins in the southern part of the Manuelita zone (the San Andres
vein for instance), situated in the central part of the district, we consider the
Toldojirca zone as the easternmost continuation of the well developed ENEWSW trending set of polymetallic veins presenting the main type of ore in the
central part of the Morococha district.
REFERENCES
Mégard, F. (1978) Etude Géologique des Andes du Pérou Central. Mém.
ORSTROM, Paris, 86, 310 p.
Rosas, S. (1994) Facies, diagenetic evolution and sequence analysis along a
SW-NE profile in the Southern Pucará Basin (Upper Triassic-Lower Jurassic),
central Peru. Heidelberger Geowiss. Abh., 80, 295 p.
Terrones, A. J. (1949) La Estratigrafía del distrito minero de Morococha.
Sociedad Geológica del Perú, V. Jubilar, XXV Aniversario, Parte II, Fasc. 8, 115.