Summary:
Mineralogical, structural and geochemical features of the Mallay Mine and surrounding exploration targets, such as Tres Cerros, fit with the ‘Cordilleran polymetallic deposit’ type as described by Sawkins (1972), Einaudi (1982) and Bendezú et al (2008).
Mallay Mine Geology
Structure
The structural grain through the Property follows the Andean trend as shown by the northwesterly trend of fold axes and general strike direction of bedding. Sediments are tightly folded into nearly isoclinal folds and show plunges to the north. Reverse faults were developed along bedding planes that strike northerly, e.g., Isguiz, Pierina and San Sebastian veins. Tension faults host mineralized veins that are at high angle to bedding faults, trending 270 deg - 290 deg and cutting through the sedimentary section in the Isguiz anticline. Lithology contrast controls vein width-veins are wider in Farrat sandstone and narrower with splits in the Carhuaz siltstone and carbonate sediments in core of the anticline. Ore shoots (clavos) along the Isguiz bedding-plane fault formed at intersections with the WNW-trending conjugate tension structures forming vertical bodies of massive sulfide. Mineralization is also found in bedding plane faults at the contact between the Farrat sandstone and Pariahuanca limestone, mostly as replacement-style mineralization.
The principal ore shoots measure up to 8 meters in width and 30 meters in length (horizontal dimension), averaging 0.94m in width above level 4090 m and 2.05 m below level 4090 m. Lengths of the principal ore shoots range from 15 to 30 meters. Mine workings along the WNW veins not in contact with Veta Isguiz produce from ore shoots ranging between 0.5 and 1.0 meters in width, and 15-25 meters in length.
Alteration and Mineralization
The principal ore shoots along the Isguiz structure have been filled with massive sulfide: pyrrhotite, sphalerite (marmatite), galena, chalcopyrite, pyrite, and marcasite with minor arsenopyrite and tetrahedrite. The sulfides are accompanied by orthoclase, diopside, siderite and minor actinolite; late, very fine veinlets of carbonate and siderite cut these minerals. Petrographic studies suggest a paragenetic sequence of pyrrhotite-marcasite-sphalerite-chalcopyritel-chalcopyrite II-galena. Alteration of wall rock in Isguiz is subtle in clastic wall rocks and better developed along contact with the carbonate sediments featuring calc-silicate alteration minerals, commonly diopside and actinolite.
In the Rosa Vein, parallel to Isguiz on the opposite limb of the anticline, lead-antimony sulfosalts (boulangerite-geocronite) are found with the same mineral assemblage as Isguiz. The E-W system of veins, e.g., Maria Vein, features a suite of sulfides similar to that of Isguiz: pyrite arsenopyrite sphalerite-galena-chalcopyrite-tetrahedrite.
The Pierina Vein, hosted in the Jumasha limestone, shows a different suite of minerals that includes electrum and tellurides of Au-Ag. Ore and gangue minerals are pyrite, pyrargyrite, galena, arsenopyrite, sphalerite (low and high Fe), pyrrhotite, marcasite, native silver, tetrahedrite, and bournonite. Tellurides of Au-Ag were found as inclusions measuring 6 µm to 200 µm in galena, calcite and pyrite. Fluid inclusion studies indicate two mineralizing events: a high-temperature pulse at 300° 350°C followed by a cooler pulse at 200° to 300 °C. Observed fluid salinity was <10% equivalent weight NaCl.
Alteration in the Pierina zone is characteristic of skarn with veins containing gangue minerals of diopside, actinolite, epidote, chlorite. The massive limestone wall rock is commonly marbleized. Further east of the Pierina zone other E-W orientated veins (Nicole, Elizabeth, Margarita, etc.), also hosted in the Jumasha limestone, exhibit a similar style of mineralization including notably high gold grades (up to 5 g/t Au). These veins are located within a brecciated skarn that has been mapped at surface and warrants further examination.
Tres Cerros Geology
Structure
The mineralized area in Tres Cerros is located along a fault contact between tightly folded quartz arenite (mapped by BVN as Farrat Fm, however, is more characteristic of Chimu Fm.) locally overlain by flat-lying Calipuy volcanic rocks. Isolated blocks of sediment have been mapped on the volcanic side of this fault in a complex structural zone.
The structural pattern through the sediments in Tres Cerros is remarkably similar to that of the Mallay Mine vein pattern where N-S primary faults (as bedding-plane slip) intersect with conjugate WNW-trending tension faults.
The fault contact between sediments and volcanic rocks is suggested as running parallel to the rim of a large caldera. Volcanic rocks west of the fault contact mapped as breccia may be outflow of volcanic flow breccias from the caldera, although the structural pattern shown in the mapped area is not as expected along a rim zone of a caldera.
Alteration and mineralization Surface sampling and geologic mapping in the Tres Cerros zone shows that gold and silver mineralization is hosted almost exclusively in the quartz arenite (sandstone) along the primary, northerly trending contact fault (“Contact Fault”) between sedimentary and volcanic rocks. Volcanic rocks are rarely mineralized even in contact with fragmented, mineralized sandstone blocks found on the volcanic side of the contact.
Mineralization is associated with epithermal, high-sulfidation alteration with reported assemblages of advanced argillic alteration minerals (alunite, dickite, quartz) and the presence of vuggy silica texture presumably found only in volcanic rocks. Quartz arenite is relatively non-reactive to alteration as it is composed mostly of quartz grains, so any alteration in sandstone is difficult to identify in the field. Buenaventura reported minor alunite in stockwork veining in the sandstone.
An IP geophysical survey outlined coinciding, strong chargeability and resistivity anomalies at the southern end of the target area that may be interpreted as indicating a zone of disseminated sulfides in silicified host rock.
A zone of strong Zn values is located to the west of the fault/contact zone closer to the center of the hypothesized caldera. Since zinc is found outboard from high-temperature Au-Ag-Cu zones proximal to the heat source of hydrothermal fluids, the proximity of this zinc occurrence to the center of the caldera suggests that the zinc mineralization came much later than the caldera formation and is related to a hydrothermal system located well outside of the caldera.
The form of the volcanic breccia in cross section and its association with a sub-volcanic dome suggests that this volcanic breccia represents a diatreme.