5th Swiss Geoscience Meeting, Geneva 2007
Multiple intrusion-centered Miocene Morococha
polymetallic district, central Peru
Bendezú Aldo *, Catchpole Honza *, Kouzmanov Kalin *, Fontboté Lluís * &
Astorga Carlos**
* Départment de Minéralogie, Rue de Maraîchaire 13, CH-1205 Genève
aldo.bendezujuarez@terre.unige.ch
**Pan American Silver Corp, Lima, Peru
The historical mining district of Morococha that has been known since the PreColonial times is part of the Miocene Polymetallic Belt of central Peru. It is
located 150 km east of Lima and covers an area of about 70 km2 in the northwestern part of the Domo de Yauli complex. The main regional structure in the
area is the Morococha anticlinorium which axis trends roughly NW-SE. The
geology of the district consists of continental volcanic rocks and red beds of the
Mitu Group (Permian), sedimentary carbonate, volcanic rocks and basalts of the
Pucará Group (Triassic-Jurassic), Goyllarisquizga Group, and Chulec,
Pariatambo, Jumasha and Celendin Formations (Late Cretaceous), cut by
Miocene intrusions with different ages. The oldest intrusion (14.1 Ma) covering
large area in the western part of the district is known as Anticona diorite. The
Anticona diorite doesn’t show any direct relationship with the Miocene
mineralization. The later is related to the emplacement of numerous younger
intrusions with Late Miocene age (7-9 Ma) – Potosí, San Francisco, Gertrudis,
San Nicolas, Yantac, San Miguel and Ticlio stocks. The whole district is cut by
dacitic dykes trending N100-120 which are probably the last magmatic event
that occurred in the studied area and that predated the polymetallic
mineralization.
Several mineralization styles have been recognized in the Morococha district:
porphyry mineralization; endo- and exsoskarns; two types of replacement
bodies: (a) pyrite-quartz bodies formed at the contact between the Pucará
limestones and certain intrusive stocks and/or the volcanic rocks of the Mitu
Group, and (b) polymetallic bodies (“mantos”) hosted by the Pucará limestones
as replacement of breccia zones and/or specific horizons with strong lithological
control; and polymetallic veins.
Porphyry style alteration and mineralization has been recognized in three areas:
the Toromocho porphyry Cu (-Mo) deposit in the central part of the district; the
Potosí stock in the north-eastern part, and the recently discovered Ticlio
porphyritic stock in the westernmost part. The porphyry mineralization at
Toromocho is related mainly to quartz-chalcopyrite-pyrite and quartzmolybdenite-pyrite stockwork veinlets hosted by quartz-porphyry and feldsparporphyry intrusions affected by K-alteration and superimposed phyllic alteration
assemblages. In the Potosí stock four different intrusions - microdiorite,
porphyritic microdiorite, quartz-feldspar porphyry and amphibole-biotite
porphyry, have been distinguished, all of them cut by later aplitic dykes. The
intrusions display strong Na-Ca and moderate K-alteration, related to weak
porphyry-style veining consisting of quartz-pyrite-chalcopyrite±pyrrhotite and
quartz-molybdenite. In the Ticlio area quartz-monzonite intrusion with strong Kalteration hosts high-density porphyry stockwork consisting of several
generations of quartz, quartz-feldspar and quartz-magnetite veinlets. Sulfides
5th Swiss Geoscience Meeting, Geneva 2007
are almost not present in these veins. The porphyry mineralization in the three
areas is overprinted by later polymetallic veins and related phyllic to argillic
alteration halos and strong silicification.
Magnetite and serpentine-dominated exoskarns and garnet-diopside-epidotedominated endoskarns usually form along the contacts of the Late-Miocene
intrusions with the Pucará Group and Upper Cretaceous limestones of the
Jumasha Formation. In places the skarn bodies host polymetallic mineralization.
Replacement pyrite-quartz bodies have been recognized only in the northern
sector of the district – the Manuelita, Codiciada and Tashiman areas. There is
no strong evidence of structural control on the formation of these bodies. In
parts they can reach a size of more than 850 m and a thickness of 100 m. We
consider the formation of the replacement pyrite-quartz bodies as a transitional
event between the porphyry mineralization and the emplacement of the
polymetallic manto bodies and veins.
The polymetallic manto bodies occur mainly as a replacement of: (a) tectonic
and dissolution breccias overlying the Montero Basalt which is a marker bed
into the Pucará Group; (b) overthrust-related breccias; (c) particular horizons of
the Pucará Group situated few meters above the contact between the Mitu
volcanic rocks and the Pucará limestones; (d) previously skarnified beds of
limestones within the lower units of the Pucará Group. The skarn-replacement
mantos contain pyrrhotite, chalcopyrite, Fe-rich sphalerite and galena in
different proportions, while the breccia-replacement mantos consist of Fe-poor
sphalerite, galena, Cu-sulfosalts, rhodochrosite and quartz. Polymetallic veins
with the same composition played like feeders for the formation of the manto
bodies.
The polymetallic veins are the economically most important ore type in the
Morococha district. They are fault-controlled and belong to two main systems:
(a) normal dextral or sinistral faults striking N60-80, and (b) normal dextral faults
striking N20-30. The relative temporal relationship between the two vein
systems indicates that the N60-80 system predate the N20-30 system. Field
observations in particular places suggest that the N60-80 vein system consists
mainly of quartz and pyrite, and the second vein system is enriched in basemetal minerals. The later polymetallic vein system cut all previously described
ore body types and temporally corresponds to the latest mineralizing event in
the district.
In the Morococha area, Late Miocene magmatic activity has produced one of
the economically most important polymetallic districts in the Miocene
Polymetallic Belt of central Peru. Porphyry mineralization and alteration is
overprinted by epithermal style of polymetallic mineralization, thus indicating ore
formation in two different environments, separated in time or forming a
continuum. Structural data suggest also change in the orientation of the stress
regime between the emplacement of the porphyries and the ore formation
related to the polymetallic veins. Polymetallic mineralization dominated by Ag,
Zn and Pb produced the most of the economic ore in the peripheral part of the
district, while the large Toromocho porphyry Cu (-Mo) system dominates its
central part.