Summary:
The deposits within the Penasquito Operations are examples of breccia pipe deposits developed as a result of intrusion-related hydrothermal activity.
Such deposits are hosted in a tectonic setting of continental magmatism, well-inboard of inferred or recognized convergent plate boundaries, and which commonly contains coeval intrusions of alkalic, metaluminous calc-alkalic, and peraluminous compositions. Preferred host strata include reducing basinal sedimentary or metasedimentary rocks. Deposit locations are often controlled by graben faults and ring complexes related to cauldron development.
Deposits typically consist of mineralized, funnel-shaped, pipe-like, discordant breccia bodies and sheeted fracture zones. Mineralization is hosted by a variety of breccia types, including magmatic hydrothermal, phreatomagmatic, hydraulic and collapse varieties. Breccia cement consists dominantly of quartz and carbonate (calcite, ankerite, siderite), with specularite and tourmaline at some deposits.
Mineralization characteristically has a low sulfide content (<5 volume %), and contains pyrite, chalcopyrite, sphalerite, galena, and pyrrhotite, with minor molybdenite, bismuthinite, tellurobismuthite and tetrahedrite, which occur either in the matrix or in rock fragments. It is typically silver-rich (goldsilver ratios of 1:10), with associated lead, zinc, copper, ± molybdenum, manganese, bismuth, tellurium, and tungsten), and a lateral (concentric) metal zoning is present at some deposits.
A sericite-quartz-carbonate-pyrite alteration assemblage and variably developed silicification is coincident with mineralized zones, grading outward into propylitic alteration. An early-stage potassium-silicate alteration locally occurs in some deposit areas.
Mineralization
The diatreme and sediments contain, and are surrounded by, disseminated, veinlet and veinhosted sulfides and sulfosalts containing base metals, silver, and gold. Mineralization is breccia or dike hosted, forms mantos, or is associated with skarns.
Mineralization consists of disseminations, veinlets and veins of various combinations of medium to coarse-grained pyrite, sphalerite, galena, and argentite (Ag2S). Sulfosalts of various compositions are also abundant in places, including bournonite (PbCuSbSa), jamesonite (PbSbzS.}), tetrahedrite, polybasite ((Ag.CuJicjSb.As Sn), and pyrargyrite (AgaSbSj). Stibnite (SbjSj), 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 veinlet-controlled mineralization in clasts. All breccia types host mineralization, but the favored host is the intrusion-clast breccia. Much of the mineralization within the Penasco 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 Penasco 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.
The Chile Colorado deposit is the largest known sediment-hosted mineralized zone, although others also occur adjacent to Penasco (e.g., El Sotol), and between the diatremes (e.g., La Palma). El Sotol, located to the west of Penasco, consists of small horizons mineralized with sulfides and sulfosalts, which are consistent with the stratification of the Caracol Formation.
Reforma is a northwest-southeast oriented vein system consisting of rhodochrosite, sulfides, and sulfosalts that occurs within the Chile Colorado deposit and to the south-southwest of the Penasco breccia.
There is a spatial association between strong QSP alteration and the highest degree of sulfide and sulfosalt mineralization. A halo of generally lower-grade disseminated zinc-lead-gold-silver mineralization lies within the QSPC assemblage surrounding the two breccia pipes.
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 characterized 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 Penasco 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.