Relative age of Cordilleran base metal lode and replacement deposits, and high sulfidation Au-(Ag) epithermal mineralization in the Colquijirca mining district, central Peru

At Colquijirca, central Peru, a predominantly dacitic Miocene diatreme-dome complex of 12.4 to 12.7Ma (40Ar/39Ar biotite ages), is spatially related to two distinct mineralization types. Disseminated Au-(Ag) associated with advanced argillic alteration and local vuggy silica typical of high- sulfidation epithermal ores are hosted exclusively within the volcanic center at Marcapunta. A second economically more important mineralization type is characterized as "Cordilleran base metal lode and replacement deposits." These ores are hosted in Mesozoic and Cenozoic carbonate rocks surrounding the diatreme-dome complex and are zoned outward from pyrite-enargite-quartz-alunite to pyrite-chalcopyrite-dickite-kaolinite to pyrite-sphalerite-galena-kaolinite-siderite. Alunite samples related to the Au-(Ag) epithermal ores have been dated by the 40Ar/39Ar method at 11.3-11.6Ma and those from the Cordilleran base metal ores in the northern part of the district (Smelter and Colquijirca) at 10.6-10.8Ma. The significant time gap (~0.5 My) between the ages of the two mineralization types in the Colquijirca district indicates they were formed by different hydrothermal events within the same magmatic cycle. The estimated time interval between the younger mineralization event (base metal mineralization) at ~10.6Ma and the ages of ~12.5Ma obtained on biotites from unmineralized dacitic domes flanking the vicinity of the diatreme vent, suggest a minimum duration of the magmatic-hydrothermal cycle of around 2Ma. This study on the Colquijirca district offers for the first time precise absolute ages indicating that the Cordilleran base metal lode and replacement deposits were formed by a late hydrothermal event in an intrusive-related district, in this case post Au-(Ag) high-sulfidation epithermal mineralizatio

New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au-Ag and Cordilleran polymetallic ores

We present 40Ar/39Ar data acquired by infra-red (CO2) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au-(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO2-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9Ma and lasted until ∼10.6Ma. Four alunite samples associated with high-sulfidation epithermal Au-(Ag) ore gave 40Ar/39Ar plateau ages between ∼11.9 and ∼11.1Ma (compared to the previously documented ∼11.6 to ∼11.3Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au-(Ag) deposition period can be estimated to at least 0.4My. Three new 40Ar/39Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au-(Ag) and the base-metal Cordilleran ores may suggest that SO2-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO2-bearing vapor degassing longer than 11.9 to 10.6Ma may be recognized with further more detailed wor

Shallow polymetallic and precious metal mineralization associated with a Miocene diatreme-dome complex : the Colquijirca district in the Peruvian Andes

Ore characterization based mainly on microscopic and microanalytical methods allow to discriminate two type of epithermal mineralization in the Colquijirca district, central Peru: disseminated high sulfidation epithermal Au-(Ag) ores and sulfide-rich zoned cordilleran polymetallic deposits. High precision ⁴⁰Ar/³⁹Ar radiometric dating reveals that high sulfidation Au-(Ag) ores, formed from 11.9±0.07 to 11.10±0.06 Ma, predated cordilleran mineralization which formed from 10.83±0.06 to 10.59±0.06 Ma. Both systems entrained commonly strongly acidic fluids recorded by episodic precipitation of alunite derived from magmatic vapor plumes. Two end-member fluids are recognized in the Colquijirca system: a cold (<150°C) close-to-zero salinity fluid likely of meteoric origin and hot (250°C-300°C) 3-7 wt% NaCl equiv. dominantly magmatic fluid. It is postulated that the ore fluids resulted of a strong dilution of metal-bearing saline magmatic fluids with meteoric water that exchanges isotopically with host rocks

The carbonate-hosted Zn-Pb San Gregorio deposit (Colquijirca District, Central Peru) as part of a high sulfidation epithermal system

The recently discovered world class San Gregorio Zn-Pb deposit in Central Peru, has formed by high level carbonate replacement through very acid fluids coming from a high sulfidation system related to a Tertiary volcanic neck, which produces also Au-Ag, Cu-Au and Zn-Pb-Ag ores in other parts of the same district. To our knowledge San Gregorio is the first "carbonate-hosted epithermal Zn-Pb high sulfidation deposit" recognized as such

Cordilleran or Butte-type veins and replacement bodies as a deposit class in porphyry systems

Cordilleran or Butte type polymetallic mineralization in a porphyry-related setting is known in numerous districts. Most of it is largely epithermal, is found in the upper part of the systems, cuts earlier veins with potassic and phyllic alteration assemblages, and occurs as veins, massive replacement bodies, and sulfide-cemented breccia

New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au–Ag and Cordilleran polymetallic ores

We present 40Ar/39Ar data acquired by infra-red (CO2) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au–(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO2-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9 Ma and lasted until ∼10.6 Ma. Four alunite samples associated with high-sulfidation epithermal Au–(Ag) ore gave 40Ar/39Ar plateau ages between ∼11.9 and ∼11.1 Ma (compared to the previously documented ∼11.6 to ∼11.3 Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au–(Ag) deposition period can be estimated to at least 0.4 My. Three new 40Ar/39Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06 Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au–(Ag) and the base-metal Cordilleran ores may suggest that SO2-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO2-bearing vapor degassing longer than 11.9 to 10.6 Ma may be recognized with further more detailed work

Early mineralization at Cerro de Pasco (central Peru) revisited

The large Cerro de Pasco Cordilleran base metal deposit in central Peru is located on the eastern margin of a Middle Miocene diatreme-dome complex. A striking characteristic is the presence of a N-S trending massive funnel-shape pyrite-quartz replacement ore body that contains pyrrhotite pipes grading outwards to lead-zinc replacement bodies, along the eastern contact of the diatreme-dome complex. Earlier workers interpreted the pyrrhotite pipes as postdating the pyritequartz body. This study, that has been possible through access to new mining areas and drill cores, allows an alternative interpretation. The new data strongly suggest that the pyrrhotite pipes and their associated lead-zinc replacement bodies predated the formation of the pyritequartz bod