Summary:
Deposit Type
The Tambomayo deposit is a low sulfidation epithermal-type deposit with gold and silver mineralization in the upper levels and occurrence of lead and zinc and base metals at depth. Structures in the deposit are phyllonian and rosary type with fracture filling.
Based on observations in underground workings and other characteristics, it is inferred that the mineralization was emplaced at very moderate to low temperatures (Mesothermal to Epithermal).
It is hydrothermal because the hot aqueous solutions originated as the last phase of a magmatic differentiation when exiting through the pre-existing fractures. They gave rise to alterations in the wall rock such as: kaolinization, pyritization, chloritization, etc., and finally the deposition of ore solutions took place.
It is mesothermal and epithermal due to the presence of medium temperature minerals such as galena, sphalerite, argentite, etc., although the pyrite, chalcopyrite and quartz minerals are of a different temperature.
Mineralization
The vein-type mineralization is emplaced in the Tambomayo volcanics, and related to intrusives, subvolcanic bodies and dikes in Miocene tuffs (Swanson, 1988).
The mineralization is mainly Au and Ag, with minor amounts of Pb, Zn and some Cu; this primarily presents as primary sulfide ore, with some secondary species generally occurring in outcrops.
The mineralogical filling of veins, according to their proportion, are constituted by abundant quartz, sphalerite, pyrite, galena, limonite, by scarce calcite, chalcopyrite, tetrahedrite, hematite, and rare proustite, pyrargyrite, marmatite, native Ag, electrum, covellite.
Five veins have been found in the area; from west to east, we have Mirtha vein, Susy vein, Erika vein, Paola vein, and Paola Norte vein. The most important are Mirtha and Paola.
The Mirtha and Paola system structures are epithermal structures recognized as a quartz-adularia system enriched in Au, Ag and base metals. Open spaces were filled by symmetrical banding textures, crustiform structures, replacement, breccias with ore and gangue minerals.
Mirtha Vein
Mirtha is an EW trending epithermal vein with an ore shoot of approximately 800 m, with the presence of minerals such as quartz-sericite-adularia-sulfides. The host rock is mainly andesitic tuff with breccia tuff sections, and small aphanitic andesitic lava horizons, with weak propylitic to moderate argillic alteration mainly in the north wall rock and silicification at the footwall of the vein. The crustiform, colloform, diamond-shaped and hydrothermal breccia textures in the Mirtha vein are associated with Ag-Au mineralization.
Paola Vein
Paola vein behaves as a strand of Mirtha vein that breaks off at the east end of the ore shoot and trends N 70°' E; unlike the Mirtha vein (N 80° W), it behaves as a new mineralized system and the occurrence of mineralization in this trend obeys to a structural context, where the sinistral movement of E-W faults has generated weakness zones of N 70° E strike that later were filled by hydrothermal solutions forming the Paola Vein with Ag-Au-Pb-Zn mineralization.
Paragenetic sequence
Precious metals are contained in galena, argentiferous galena, sphalerite, chalcopyrite, several sulfosalts such as: freibergite, tetrahedrite, polybasite, argentite, pyrargyrite, also, native gold and electrum. Gold occurs in disseminated form associated with sphalerite; gold-electrum occurs as anhedral grains, somewhat rounded and with sizes smaller than 0.03 mm associated to galena. Gold-electrum is considered because this mineral has yellowish shades, and in some cases it is mainly electrum that is sparsely disseminated in the gangue (BISA, 2011).
The gangue is mainly quartz (chalcedonic, gray, white and jasper varieties, calcite).
Types of Mineralization
Mineralization in veins (Fracture Filling) Fractures caused by tectonism and fault movement have been filled by irregularly shaped rosarytype hydrothermal solutions, with mineralogical filling of sulfides and silicates.
Mirtha vein is the main structure in Tambomayo. Surface outcrops are unusual, these are mostly covered (85%) and the outcropping structures occur as veins, veinlets and silicified bodies. Widths are variable and rosary-type, they narrow to centimeters and in other cases they widen to more than 1.0 m. The structures are oriented and controlled by three (3) main structural systems:
E-W system: belonging to this system are Karina, Luisa, Lucas, Ramal Norte Camila (Los Diques Zone), Venado 5, Venado 6 (Venado Zone), Venturosa, Maryori, Mirtha Este (Venturosa Zone), Asunta (Asunta Zone), Mirtha Oeste (Fase 3 Zone) veins;
NE System (anti-Andean system): this system comprises Camila, Rosa (Los Diques Zone), Oso 1, Oso 2, Oso 3, Oso 4 (Venado Zone), Soledad, Ramales Venturosa (Venturosa Zone) veins;
NW system (Andean system): this system includes Noelia, Shuleyca (Los Diques Zone), Venado 1, Venado 2, Venado 3 (Venado Zone), Clara (Clara Zone), Rocío, Fiorella (Venturosa Zone) and Katy ore body.
Mineralization in veinlets
Some main structures show veinlets parallel to the main structure; others show strands bifurcating from the main structure to the hanging wall and the footwall. Additionally, there is evidence of tension veinlets between the main strands in the cimoids and between two main veins that tend to join; therefore, there are multidirectional stockwork-type veinlets with iron oxide, white and gray quartz fillings.
Mineralization in bodies
The mapped area shows evidence of argillized bodies with brecciated texture, formed by rounded or angular fragments of the same host rock (monomictic) or foreign rocks, cemented with iron oxides with remnants of leached pyrite, quartz veinlets and presence of iron oxides (hematite).
The main structure is the Mirtha vein with a NW-SE strike; the Paola and Paola Norte veins are at its SE end; the Camila vein is at its lower level; and the main strike of the vein is N45°E and dip of 70ºSE. The length of the ore shoots is 70 m horizontal and 160 m vertical. To the north, it borders with the Carmen hanging wall and to the south it is dextrally displaced by the Carmen footwall and bordered by the north fault. The vein thickness ranges from a minimum of 1 m to a maximum thickness of 5 m.