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At Colquijirca, central Peru, a predominantly dacitic Miocene diatreme-dome complex of 12.4 to 12.7 Ma ( 40 Ar/ 39 Ar 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 pyritechalcopyrite-dickite-kaolinite to pyrite-sphalerite-galena-kaolinite-siderite. Alunite samples related to the Au-(Ag) epithermal ores have been dated by the 40 Ar/ 39 Ar method at 11.3-11.6 Ma and those from the Cordilleran base metal ores in the northern part of the district (Smelter and Colquijirca) at 10.6-10.8 Ma. 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 $10.6 Ma and the ages of $12.5 Ma 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 2 Ma. 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 mineralization.

Recently identified occurrences of porphyry-style mineralization evidence the link of the world's second largest known epithermal base metal Cerro de Pasco deposit (Peru) to a porphyry system emplaced at depth. They consist of (i) quartz-monzonite dykes and (ii) the south-western part of the large diatreme-dome complex adjacent to the main ore bodies of Cerro de Pasco, and (iii) stockwork of banded quartz-magnetite-chalcopyrite-(pyrite) porphyry-type veinlets crosscutting trachyte porphyritic intrusion cropping out at surface in the central part of the diatreme-dome complex. The latter porphyry-type mineralization observed at the same erosion level as the main epithermal base metal carbonate-replacement ore bodies is the subject of this work. Geological constraints indicate a shallow emplacement level (depth < 1 km, P < 270 bar), implying rather unusual low-pressure and high-temperature environment for the formation of porphyry-style mineralization. The banded porphyry-type veinlets record a multiphase history of formation with two successive high-temperature (N600 °C) stages, followed by a lower-temperature (b350 °C) stage.More than 90% of the quartz in veinlets precipitates during the first two high-temperature stages. Stage 1 is characterized by the entrapment in hydrothermal quartz of inclusions containing variable proportions of both silicate melt and metal-rich hypersaline (N90 wt% NaCl eq.) liquid, hereafter referred to as heterogeneous silicate melt inclusions (HSMIs). The latter are rarely described in porphyry-type mineralization. We suggest that during stage 1, the inclusions result from heterogeneous entrapment of an evolved hydrous rhyolitic melt mixed with a hypersaline fluid phase at low pressure (270 bar) and high temperature (N600 °C). The stage 2 is marked by the entrapment of metaland sulfur-rich hypersaline liquid inclusions, with salinity around 70 wt% NaCl eq., originated from the adiabatic ascent of magmatic hypersaline fluids transferred fromdeeper parts of the system. The lower-temperature stage 3 is characterized by an important temperature drop from N600 °C to b350 °C as revealed bymicrothermometry of aqueous two-phase liquid-vapor (L-V) inclusions. Quartz textures revealed by SEM-CL imaging allowascribing the sulfide precipitation to the low-temperaturemineralization stage 3. In-situ SIMS 18O/16O isotope analyses of quartz across the veinlets are indicative of a magmatic signature of the fluids during the first two stages; while quartz from stage 3 has oxygen isotopic compositions suggestive of minor contribution of meteoric waters to a predominantly magmatic aqueous fluid (~10 vol.% of meteoric input), which probably triggered Cu-Fe sulfide precipitation in the stockwork. High metal and sulfur contents of HSMIs and hypersaline liquid inclusions determined by LA-ICP-MS are interpreted to represent the fluid composition prior to the main sulfide precipitation event. The similar Pb-Zn ratio of the bulk ore extracted from the epithermal ore bodies at Cerro de Pasco and the HSMIs and hyper...

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

Abstract0361-0128/09/3845/905-40 905 Introduction PERU is a major producer of silver, copper, zinc, lead, bismuth, tin, and gold. Of these metals, zinc, lead, silver, tin, and bismuth are produced dominantly from deposits of several styles but all sharing the following major features: (1) polymetallic (Cu-Zn-Pb-Ag-Au) suite; (2) zoned from inner Cu-bearing to outer Zn-Pb−bearing ores; (3) sulfide-rich character, commonly massive; and (4) primary occurrence as open-space fillings (veins, breccia bodies) in silicate host rocks and as replacements in carbonate rocks. Sawkins (1972) classified ores displaying these features as Cordilleran vein-type deposits. Later, Einaudi (1982) described these deposits as Cordilleran vein or lode deposits within the context of porphyry-related systems. Additional general discussions on Cordilleran vein deposits are given by Guilbert and Park (1986), Bartos (1989), andBendezú (2009). Recently, this type of mineralization has been termed "zoned base metal veins" by Einaudi et al. (2003); however, a clear zonation is not always recognized, and the ores in many districts are dominantly mantos and not veins, and they commonly contain Au and Ag in addition to base metals. We therefore prefer the more general term "Cordilleran polymetallic deposits" Fontboté and Bendezú, 2009 AbstractAt Colquijirca, central Peru, a Miocene diatreme-dome complex is associated in space and time with several large epithermal polymetallic (Cu-Zn-Pb-Au-Ag) deposits of the Cordilleran class. Of these deposits, Smelter and Colquijirca, located in the northern sector of the district, are part of a continuously mineralized northsouth corridor that extends for nearly 4 km outward from the diatreme-dome complex and to depths of 1 km below surface. This corridor is zoned from Cu-(Au) ores in its inner parts (Smelter deposit) to peripheral ZnPb-(Ag) ores (Colquijirca deposit). The Smelter-Colquijirca corridor has undergone minor erosion, providing a good example of a nearly intact paleo-epithermal system of the Cordilleran class.This description of the hypogene mineralogical patterns of the Smelter-Colquijirca corridor leads to the proposal that they are the result of superimposition in time and space of three main stages. During an early quartzpyrite stage, in which basically no economic ore deposition occurred, carbonate rocks surrounding the Marcapunta diatreme-dome complex were replaced by quartz and pyrite. This was followed by the main ore stage, which was largely superimposed on the quartz-pyrite replacements and produced zonation of ore minerals and metals along much of the Smelter-Colquijirca corridor. The zoning from Cu ores to Zn-Pb ores is complex and comprises a number of distinct and well-defined zones that display abrupt or gradual interfaces between zones. From internal to external parts, these zones consist mainly of the following mineral associations and assemblages: (1) enargite ± (luzonite, pyrite, colusite, tennantite, goldfieldite, ferberite, gold-silver tellurides, bismuthinite, gold, alunite,...