Summary:
The Uchucchacua mineral structures are hosted by Mesozoic limestone of the Jumasha Formation and are classified as a mesothermal polymetallic deposit of silver-lead-zinc with important contents of manganese. The main mineralized structures are veins and ore bodies with high-grade silver content.
The geology consists of sedimentary rocks from the Upper Cretaceous carbonate sequence. At the base, limestones from the Jumasha and CelendÃn formations exist; these sedimentary rocks have been strongly folded and faulted. On top of these units and in erosional unconformity, the red layers of Casapalca formation were deposited, and were eventually covered by Calipuy group volcanic rocks and Tertiary intrusives.
Uchucchacua is a polymetallic deposit associated with replacement bodies and veins. Its mineralization (Ag, Zn, Pb, Fe and Mn) is located in a sequence of carbonate rocks from the Upper Cretaceous Jumasha Formation.
Mineralized structures are mostly between 1 and 4 meters thick, with occurrences in some sectors with thicknesses in the order of 15 meters.
The mineralization processes at Uchucchacua have been complex and multiple; therefore, its mineralogy is unusually varied. Among the main mineral groups are: Oxides, Silicates, Carbonates, Sulfides and Sulfosalts. Among the main ore minerals, we have: Galena, Proustite, Argentite, Pyrargyrite, Native Silver, Sphalerite, Marmatite, Jamesonite, Polybasite, Boulangerite, Chalcopyrite, Covellite, Jalpaite, Stromeyerite, Golfieldite. Gangue minerals include Pyrite, Alabandite, Rhodochrosite, Calcite, Pyrrhotite, Fluorite, Psilomelane, Pyrolusite, Johansonite, Bustamite, Arsenopyrite, Marcasite, Magnetite, Stibnite, Quartz, Orpiment, Realgar, Benavidesite, Tephroite and Gypsum.
It is important to mention that the silver mineralization with base metals is mainly embedded in rocks of the Jumasha Formation middle member, and occurs in different styles:
- Socorro Zone: mineralization mainly in the form of veins.
- Carmen Zone: veins and bodies in the form of replacement chimneys and mantles.
- Huantajalla Zone: veins and replacement chimneys.
- Plomopampa zone: veins.
- Lucrecia Zone: replacement bodies and veins.
Uchucchacua is a polymetallic epithermal deposit of veins (fracture filling) and metasomatic replacement, emplaced in carbonate rocks of the Jumasha Formation. Mineralization is complex, occurring in multiple stages or pulses, controlled by well-defined vein structures, replacement bodies or shoots and skarn.
Carbonate replacement deposits (CRD) related to intrusives are an important global resource for base metal production; these deposits present a variety of manifestations ranging from Pb-Zn-Cu skarns, to polymetallic replacement bodies in carbonate rocks with Pb-Zn-Ag, to distal skarns with Pb-Zn-Ag-Mn.
In Peru, these deposits are generally associated with Miocene calc-alkaline intrusions resulting from the subduction of the oceanic plate under the continental plate. They show a zoning pattern characterized by Cu±Au±Ag in the higher temperature core grading towards Pb-Zn-Ag and Mn zones in the distal low-temperature epithermal parts of the hydrothermal system. Uchucchacua is an excellent example of the latter manifestation.
Although the deposits at Uchucchacua have many features in common with other skarn-associated Zn-Pb deposits, they possess a combination of important distinguishing features (Bussell, y otros, 1990):
1. Minerals have unusually high Ag values.
2. The mineral assemblages are enriched in Mn, which can be considered to indicate Mn enrichment in the late stage of the Pb-Zn skarn series. The main mineralization took place at lower temperatures compared to other similar deposits and developed at low temperatures towards the end of a skarn hydrothermal system.
3. The fluid was polygenetic with a significant contribution of brines mixed with hot meteoric and (probably) magmatic waters.
4. It is uncommon to find a closed systematic association of the mineralization in contact with the intrusive. Mineralization develops by fissure filling and limestone replacement along fractured rock zones.