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Illustrated abstract of Jobin Y., (2004),
High sulfidation enargite-pyrite veins at Cerro de Pasco, Peru. A mineralogical study of ore and alteration minerals and an infra-red fluid inclusion study on enargite.
MSc. Thesis, University of Geneva, 139 p.
(Supervisor Prof. L. Fontboté)

The well-known Zn-Pb-Ag-Bi-(Cu) Cerro de Pasco deposit is located in central Peru, 170 km N-NE of Lima at a mean elevation of 4334 m within and mainly to the east of a felsic Miocene diatreme-dome complex. A first mineralizing stage is constituted by a large pyrite-quartz body replacing volcanic rocks (Rumiallana Agglomerate and Lourdes fragmental units) and mainly Upper Triassic - Lower Jurassic carbonate rocks (Pucará Group). Pipes and tabular bodies of pyrrhotite probably formed during the same stage are found within the pyrite-quartz body and are zoned outward to Zn-Pb bodies. The sphalerite associated with this stage is Fe-rich (typically 15 to 30 % mole FeS ).

A later stage is constituted by E-W trending enargite-pyrite veins hosted by the pyrite-quartz body and also emplaced in the diatreme and basement rocks (Excelsior Group). In the carbonate rocks, Zn-Pb veins and replacement bodies are well developed and are characterized by Fe-poor sphalerite (typical range 0.4 to 4 mole % FeS ).

This work presents results of a field, mineralogical, geochemical and microthermometric study carried out at six mineralized sites of the western part of the Raúl Rojas open pit (three sites with enargite-pyrite veins, two with carbonate-hosted Fe-poor sphalerite and galena veins, and one site with both ore types), as well as at the Santa Rosa Au-prospect (Figure 1: Position of the different sampled sites, plotted in a modified map of Baumgartner et al., 2003).

The studied enargite-pyrite veins display a main mineralization stage characterized by an enargite/luzonite-pyrite assemblage which is typical of high sulfidation and oxidation states. Assemblages bearing tennantite-tetrahedrite (Sb content in tetrahedrite average 17.8, up to 19.87 wt.%, and average 3.7 wt.% in tennantite), chalcopyrite, bismuthinite, stibnite, Fe-poor sphalerite (FeS %mole in the range of 0.5-1.0, up to 2.62 FeS %mole), and galena, replace the main stage (Figure 2: Paragenetic sequence for site 1, 2, and 3) . This widespread replacement records the transition from high to intermediate sulfidation states. Ag contents in enargite-pyrite vein samples range 80 to 200 ppm (up to 216 ppm) with tetrahedrite, in part intergrown with bismuthinite, as predominant silver host (average 0.17, up to 0.27 wt.% Ag). Relatively high contents of Au (up to 16 ppm) seem to correlate with the main stage as microprobe analyses indicate that significant Au-amounts are contained in pyrite (up to 0.12 wt.%) and in enargite (up to 0.15 wt.%). No native gold or electrum has been found (Figure 3: Microphotographs of selected samples). Hydrothermal alteration in the volcanic wall rocks shows a distinct zoning from an inner part (3 to 5 m) showing advanced arigillic alteration assemblages, with vuggy silica, alunite, APS-minerals (svanbergite and natroalunite), dickite and/or kaolinite, and a more external part (tens of meter of wide) characterized by argillic alteration assemblages, where kaolinite may be the predominant mineral (Figure 4: Scheme of enargite-pyrite vein related alteration within volcanic rocks, inspired from site 2) . Fluid inclusion data obtained by infra-red microthermometry on enargite indicate that the enargite-pyrite veins deposited at epithermal conditions (200-270°C), from moderately saline fluids (around 5-7 wt.% NaCl eq. ). Figure 5: Picture under reflected light of an enargite grain showing cleavage and position of a fluid inclusion (green cross), used for infra-red light microthermometry (CPR-439-9).

Two carbonate hosted Zn-Pb E-W trending veins located in the northwestern part of the open pit, close to the contact to the Rumiallana volcanic rocks and the pyrite-quartz body, have been studied. Galena and Fe-poor sphalerite occurs mainly as fine grained bearing "sulfide rock" replacement bodies. The sphalerite is slightly Fe-richer (1.94 and 5.91 %mole) than that of the enargite-pyrite veins, but still far below the composition of the Fe-rich sphalerite related to the pyrrhotite pipes. The veins show a core with APS-minerals (svanbergite and hindsalite), which grades into quartz and kaolinite dominated and dickite-bearing assemblages, and external zones with replacive magnetite, hematite and Fe-Mn-Zn carbonates (Figure 6: Scheme of carbonate hosted Zn-Pb vein related alteration, inspired from site 5) . The mineralization and alteration patterns are similar to those of the carbonate-hosted Fe-poor sphalerite and galena Cordilleran veins and replacement bodies being presently mined at the eastern part of the open-pit (Diamante and Matagente) which are interpreted by Baumgartner et al. (2003) to be the result of epithermal fluids under high sulfur and oxygen fugacities and acidic conditions

In the sixth site, in the northwestern part of the open-pit, a N120°E trending vein has been identified in which, along strike and within 15 m, apparently a transition between enargite-pyrite and sphalerite-galena-pyrite assemblages has been observed. Additional detailed mapping of this area could offer strong arguments supporting the hypothesis of a lateral continuity and common genesis of enargite-pyrite veins and carbonate hosted Fe-poor sphalerite-galena veins and related replacement bodies.

A preliminary study of samples from the Santa Rosa Au-prospect (southern part of the open-pit) has been carried out showing similarities in paragenesis and alteration assemblages with the enargite-pyrite veins. However, acidic alteration is more intense, with larger development of vuggy silica and advanced argillic alteration assemblages. Temperatures during hydrothermal alteration are probably also higher as indicated by the presence of diaspore, pyrophyllite, and zunyite.

Acknowledgment:
This work has benefited from support by the Swiss National Science Foundation n° 2000-067836.02 and Volcan Cia Minera S.A.

References:
Baumgartner, R., Fontboté, L., and Bendezú, R., (2003), Low temperature, late Zn-Pb-(Bi-Ag-Cu) mineralization and related acid alteration replacing carbonate rocks at Cerro de Pasco, Central Peru. Mineral and sustainable development, Eliopoulos et al. (p. 441-444), Millpress, Rotterdam.

Figure 1. Position of the different sampled sites, plotted in a modified map of Baumgartner et al. (2003).

F igure 2. Paragenetic sequence for sites 1, 2, and 3.

Figure 3. Microphotographs of selected samples:

3A: Enargite-pyrite vein with enargite (enr) replacing porous band of zoned pyrite (py), luzonite (luz) in vugs,
and tetrahedrite (td) replacing an earlier sulfide (CPR-135, reflected light).

3B: Enargite-pyrite vein showing selective enargite replacement within zoned pyrite (CPR-109, reflected light).

3C: Enargite-pyrite vein with luzonite replacing enargite and tennantite (tn) replacing luzonite (CPR-139, reflected light).

3D: Same as C, with crossed nicols (CPR-139).

3E: Enargite-pyrite vein with pyrite, tennantite, chalcopyrite (cp), and galena (gn) replacing enargite (CPR-109, reflected light).

3F: Same as E, with crossed nicols (CPR-109).

Figure 4. Scheme of enargite-pyrite vein related alteration within volcanic rocks, inspired from site 2,
alteration assemblages: sericitic (ser), argilic (A), and advanced argilic (AA),
minerals: quartz (qtz), sericite (ser), pyrite (py), kaolinite (kao), dickite (dick),
alunite (alu), and minerals from the alunite group (APS).

Figure 5. Picture under reflected light of an enargite grain showing cleavage
and position of a fluid inclusion (green cross), used for infra-red light microthermometry (CPR-439-9).

Figure 6. Scheme of carbonate hosted Zn-Pb vein related alteration, inspired from site 5,
with hematite (hm), magnetite (mt), sphalerite (sl), galena (gn) and other abbreviations as in Fig.4.

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