Summary:
The mineralization at Peñasquito contains gold, silver, lead and zinc. Deposits currently mined within the Peñasquito operations are considered to be examples of breccia pipes developed as a result of intrusion-related hydrothermal activity.
The breccia pipes are believed to be related to quartz–feldspar porphyry stocks beneath the Peñasquito area. The current bedrock surface is estimated to be a minimum of 50 m (and possibly several hundred meters) below the original paleo-surface when the diatremes were formed.
Alluvium thickness averages 30–50 m at Peñasquito, and this cover obscured the diatremes. There is one small outcrop of breccia near the center of the Peñasco diatreme, rising about 5 m above the valley surface. The single outcrop near the center of the Peñasco pipe contained weak sulfide mineralization along the south and west side of the outcrop, representing the uppermost expression of much larger mineralized zones at depth.
Peñasco and Brecha Azul are funnel-shaped breccia pipes, which flare upward, and are filled with brecciated sedimentary and intrusive rocks, cut by intrusive dikes.
The larger diatreme, Peñasco, has a diameter of 900 m by 800 m immediately beneath surface alluvial cover, and diatreme breccias extend to at least 1,000 m below surface. The Brecha Azul diatreme, which lies to the southeast of Peñasco, is about 500 m in diameter immediately below alluvium, and diatreme breccias also extend to at least 1,000 m below surface.
Chile Colorado is a mineralized stockwork located southwest of Brecha Azul, hosted in sediments of the Caracol Formation. It has dimensions of approximately 600 m by 400 m immediately beneath surface alluvial cover, and extends to at least 500 m below the current land surface.
Polymetallic mineralization is hosted by the diatreme breccias, intrusive dikes, and surrounding siltstone and sandstone units of the Caracol Formation. The diatreme breccias are broadly classified into three units, in order of occurrence from top to bottom within the breccia column, which are determined by clast composition:
• Sediment-clast breccia;
• Mixed-clast breccia (sedimentary and igneous clasts);
• Intrusive-clast breccia.
Sedimentary rock clasts consist of Caracol Formation siltstone and sandstone. Intrusive rock clasts are dominated by quartz–feldspar porphyry. For the purposes of the geological block model, the sediment-clast breccia (BXS), the sediment-crackle breccia (CkBx), mixed-clast breccia (BXM) and intrusion-clast breccia (BXI) are modeled as separate lithological solids.
A variety of dikes cut the breccia pipes and the immediately adjacent clastic wall-rocks. These dikes display a range of textures from porphyry breccia, to quartz–feldspar and quartz-eye porphyries, to aphanitic micro breccias. For block modelling purposes, the units are simplified into three intrusive lithologies; brecciated intrusive rocks (IBX), felsites and felsic breccias (FI/FBX), and quartz–feldspar porphyry (QFP).
Both of the breccia pipes lie within a hydrothermal alteration shell consisting of a proximal sericite–pyrite–quartz (phyllic) alteration (QSP) assemblage, distal sericite–pyrite–quartz– calcite (QSPC) assemblage, and peripheral pyrite–calcite (PC) alteration halo.
Mineralization
The diatreme and sediments contain, and are surrounded by, disseminated, veinlet and vein-hosted sulfides and sulfosalts containing base metals, silver, and gold. Mineralization is breccia or dike hosted, mantos, or associated with skarns.
Mineralization consists of disseminations, veinlets and veins of various combinations of medium to coarse-grained pyrite, sphalerite, galena, and argentite (Ag S). Sulfosalts of various compositions are also abundant in places, including bournonite (PbCuSbS), jamesonite (PbSb S), tetrahedrite, polybasite ((Ag,Cu) (Sb,As) S), and pyrargyrite (Ag SbS). Stibnite (Sb S), rare hessite (AgTe), chalcopyrite, and molybdenite have also been identified. Telluride minerals are the main gold-bearing phase, with electrum and native gold also identified.
Gangue mineralogy includes calcite, sericite, and quartz, with rhodochrosite, fluorite, magnetite, hematite, garnets (grossularite– andradite) and chlorite–epidote. Carbonate is more abundant than quartz as a gangue mineral in veins and veinlets, particularly in the “crackle breccia” that occurs commonly at the diatreme margins.
Breccia- and Dike-Hosted Mineralization
Breccia-hosted mineralization is dominated by sulfide disseminations within the matrix with lesser disseminated and veinletcontrolled mineralization in clasts. All breccia types host mineralization, but the favored host is the intrusion-clast breccia. Much of the mineralization within the Peñasco and Brecha Azul pipes lie within the intrusion-clast breccia.
All of the dike varieties are locally mineralized, and they are almost always strongly altered. Mineralization of dikes occurs as breccia matrix fillings, disseminations and minor veinlet stockworks at intrusion margins, and veinlets or veins cutting the more massive dikes. Mineralized dikes form an important ore host in the Peñasco diatreme but are not as abundant in Brecha Azul.
Mineralization of the Caracol Formation clastic sedimentary units where the units are cut by the diatremes is dominated by sulfide replacement of calcite matrix in sandstone beds and lenses and disseminated sulfides and sulfide clusters in sandstone and siltstones. Cross-cutting vein and veinlet mineralization consists of sulfide and sulfide-calcite fillings.
Mantos-Style Mineralization
Mantos-style sulfide replacements of carbonate strata have been identified within and beneath the Caracol Formation adjacent to the diatreme pipes, beneath the clastic-hosted disseminated sulfide zones. They consist of semi-massive to massive sulfide replacements of sub-horizontal limestone beds, as well as structurally-controlled cross-cutting chimney-style, steeply dipping, fracture and breccia zones filled with high sulfide concentrations. The sulfides are generally dominated by sphalerite and galena, but also contain significant pyrite. Gangue minerals (commonly carbonates) are subordinate in these strata-replacement mantos and cross-cutting chimneys. Stratiform and chimney mantos are haracterized by their very high zinc, lead, and silver contents, with variable copper and gold contributions.
Skarn Mineralization
Garnet skarn-hosted copper–gold–silver–zinc–lead mineralization (carbonate replacement deposits or CRDs) within dissolution breccias was identified at depth between the Peñasco and Brecha Azul diatremes. The mineralized skarns trend northwest–southeast, and have been divided into the following zones:
• CRD Upper zone: a garnet skarn hosted within the Indidura and Cuesta del Cura Formations; x, y, z dimensions of 1,500 x 600 x 450 m;
• CRD Deeps zone: a garnet skarn hosted within the Taraises and La Caja Formations; x, y, z dimensions of 1,300 x 550 x 250 m.
Polymetallic mineralization is hosted by garnet skarn and associated breccias, mainly as chalcopyrite and sphalerite with some gold and silver. Gangue minerals consist of pyrite, calcite, garnet, and magnetite. The garnet skarns are often surrounded by halos of hornfels, especially in siliciclastic units, and/or marble and recrystallized limestone in carbonate units. Deep exploration programs identified quartz feldspar porphyry with strong QSPC and potassic alteration that contains occasional veinlets of quartz with molybdenite, and veins with secondary biotite and magnetite disseminated in the wall rocks.