13o Simpósio de Geologia da Amazônia Belém | 22 a 26 de setembro de 2013
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MINERALOGY AND STABLE ISOTOPES AT THE CANAHUIRE EPITHERMAL Au (Cu-Ag) DEPOSIT, SOUTHERN PERU: PRELIMINARY DATA
Elena Malásquez López, Instituto de Geociências – UNICAMP UNICAMP (
[email protected]) Roberto Perez Xavier, Instituto de Geociências – UNICAMP UNICAMP (
[email protected]) Regina Baumgartner, Gold Gold Fields Exploration (
[email protected]) INTRODUCTION
The Canahuire Au-(Cu-Ag) deposit is located in the southern Peruvian Andes, at 4300 m.a.s.l. and it is the first precious metal carbonate-hosted intermediate sulphidation epithermal deposit in the region. Similar systems in Peru consist dominantly of base metal replacement deposits, with limited gold content, such as Cerro de Pasco (Baumgartner et al . 2008) and Colquijirca (Bendezú et al . 2009), among others. The main outcropping rocks at Canahuire consist of a folded and faulted sedimentary sequence of the Yura Group (Late Jurassic to early Cretaceous) which includes, from bottom to top, shale, sandstone, and limestone. The limestones are part of the Gramadal Formation and are the main host of the mineralisation. A set of WNW-ENE-striking faults developed a dilational jog-type structure which hosts a breccia complex, with phreatomagmatic phreatomagmatic and hydrothermal breccias, and part of the mineralisation (Santos et al . 2011). The phreatomagmatic breccia (also called diatreme breccia) contains rounded clasts of all types of rocks present in the area floating in a fine-grained rock flour matrix. Hydrothermal breccias are monomictic or polymictic with sedimentary and rhyolite clasts and display a poorly developed matrix but a well-developed cement composed of siderite and sulphides, mostly pyrite. The Chucapaca volcanic complex, located about 1.5km to the east of Canahuire is composed mainly of rhyolite domes and is probably temporally temporally related to the breccia breccia complex at Canahuire. Canahuire. HYDROTHERMAL ALTERATION AND ORE PETROGRAPHY
Hydrothermal alteration is dominated by siderite which replaces mainly limestone of the Gramadal Formation, but also occurs filling cavities in the breccia complex. Argillic alteration has been mainly documented in the phreatomagmatic breccia and consists of kaolinite, smectite and illite. Most part of the ore occurs as replacement bodies in the Gramadal limestone and to a lesser extent in the hydrothermal breccias from the breccia complex. In both cases, the ore contains gold, chalcopyrite ± tennantite – tetrahedrite accompanied by abundant pyrite and siderite. Marcasite, arsenopyrite arsenopyrite and minor pyrrhotite are also present. Field relationships, drill core descriptions and petrographic studies define two mineralisation stages for Canahuire. The first (stage I) is marked by the assemblage pyrite I, pyrrhotite, arsenopyrite arsenopyrite and magnetite. The second second stage (stage II), in which most most of the gold has been introduced, is characterized by native gold – electrum - pyrite II – arsenopyrite – chalcopyrite – tennantite tennantite – tetrahedrite tetrahedrite - stibiobismuthinite accompanied by siderite – ankerite ankerite – quartz – chalcedony chalcedony – adularia adularia – kaolinite kaolinite – smectite smectite - illite. Two generations of pyrite have been described, based on their morphologies: pyrite I with a spongy-type texture and present in both mineralization stages; and polygonal pyrite II, generally intergrown with pyrite I but restricted to the second stage (Fig. 1). In some cases pyrite I displays well- developed “bird´s eye” texture, typical of the replacement of pyrrhotite by pyrite. Additionally, microprobe analyses (Jeol JXA8230) reveal that spongy pyrite I contains variable amounts of Mo, Se, Co, and Cu, whereas polygonal pyrite II is characterized by concentrations concentrations of As, Ag, Te, Bi, Se, Au, Cd, Co, and Cu, above the detection limit (Fig. 2). Furthermore, native gold occurs with native bismuth as inclusions in arsenopyrite.
13o Simpósio de Geologia da Amazônia Belém | 22 a 26 de setembro de 2013
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Figure 1 - Reflected light photomicrographs of typical ore assemblages at the Canahuire Au-(CuAg) deposit. A) Spongy pyrite (Py I) grain with chalcopyrite (Cp) rimmed with sphalerite (Sph) and cut by thin veinlets of tennantite-tetrahedrite (Tn-Th). B) Intergrowth of marcasite (Mrc) and polygonal pyrite (Py II) together with chalcopyrite (Cp) grains in the matrix. C) Arsenopyrite (Apy) with inclusions of native gold (Au). D) Galena (Gn) in contact with intergrowth of pyrite I (Py I) and pyrite II (Py II).
Figure 2 - A) Backscattered electron image showing the intergrowth of pyrite I (Py I) and pyrite II (Py II). The yellow circles are the points analysed on the electron microprobe. Points 1 to 4 are within pyrite spongy I (Py I) and points 5 to 8 are within pyrite II (Py II). Polygonal pyrite (Py II) contains minor amounts of As – Au – Ag.
13o Simpósio de Geologia da Amazônia Belém | 22 a 26 de setembro de 2013
_____________________________________________________________________________________ CARBON AND OXYGEN ISOTOPE DATA δ13C and δ 18O data were obtained from the
analyses of thirteen carbonates samples from the Canahuire deposit. Carbonate samples included: (1) calcite and calcite - siderite veinlets in the Gramadal Formation limestones, respectively, at distal and intermediate distances from the ore zones and; (2) siderite replacement zones associated with the mineralization in the Canahuire diatreme complex (Table 1). Distal limestone-hosted calcite and intermediate calcite – siderite veinlets have relatively close δ13C values that vary, respectively, from 1.31‰ to 1.32‰ and from 18 0.09‰ to 0.31‰. On the other hand, these veinlets differ considerably in δ O values, which in calcite veinlets range from 16.56‰ to 18.91‰ and in siderite – calcite veinlets from 6.21‰ to 13 8.59‰. Compared to veinlets, ore-related siderite is more depleted in δ C (- 0.03‰ to -6.21‰). In addition, siderite δ18O values are much lower (4.35‰ to 10.93‰) than those of distal calcite veinlets, but are not distinguished from δ18O values of siderite – calcite veinlets from intermediate zones, except for the wider variation.
Figure 3 - δ13C and δ18O values of carbonates (calcite and siderite) from the Canahuire Au-(CuAg) deposit. Table 1 - δ13C and δ18O values of carbonates from the Canahuire Au-(Cu-Ag) deposit.
13o Simpósio de Geologia da Amazônia Belém | 22 a 26 de setembro de 2013
_____________________________________________________________________________________ PRELIMINARY CONCLUSIONS
The change in ore paragenesis from stage I, with pyrite I, pyrrhotite, arsenopyrite and magnetite, to stage II with arsenopyrite – chalcopyrite – tennantite – tetrahedrite stibiobismuthinite marks a hydrothermal system evolving from low to intermediate sulphidation state fluids (see Fig. 1 in Einaudi et al . 2003). Collectively, δ13C and δ18O values from calcite veinlets are comparable with marine limestone values and may reflect fluids in equilibrium with the Gramadal Formation limestone, under low fluid/rock ratios, in distal zones from the mineralization. The marked shift to slightly more depleted δ13C values and much lower δ 18O values, as observed in siderite replacement zones towards the mineralization, suggests that the Canahuire hydrothermal system evolved through varying degrees of interaction of the Gramadal limestone with magmatic and meteoric fluids. This interaction may have probably promoted the change from low to intermediate sulphidation state of the fluids and gold deposition. ACKNOWLEDGEMENTS
This research project is part of a M.Sc. dissertation under progress at the Institute of Geosciences of the University of Campinas and with support from Gold Fields Exploration. Special acknowledgements are due to Dr. Erika Tonetto (SEM – UNICAMP), Profs. Alcides Sial and Valderez Ferreira (NEG LABISE – UFPE) and Nilson Botelho (electron microprobe – UNB). REFERENCES
BAUMGARTNER R. & FONTBOTÉ L. Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru. In: ECONOMIC GEOLOGY, 2008. v.103, p.493 – 537 BENEDEZÚ R., FONTOBTÉ L. Cordilleran epithermal Cu-Zn-Pb-(Au-Ag) mineralization in the Colquijirca district, central Peru: Deposit-scale mineralogical patterns. In: ECONOMIC GEOLOGY, 2009. v.104, p.905-944. EINAUDI M., HEDENQUIST J., INAN E. Sulfidation state of fluids in active and extinct hydrothermal systems: transition from porphyry to epithermal environments. In: ECONOMIC GEOLOGY SPECIAL PUBLICATION, 2003. v.10, p.285-313 SANTOS A., BAUMGARTNER R., GAIBOR A., DUSCI M., AZEVEDO F., GRADIM R., DUNKLEY P., DENBOER D., VALER R. 2011. Geology and mineralisation of the AuCu-Ag Canahuire epithermal deposit, Chucapaca Project, Southern Peru. In: SGA, The Biennial SGA Conference, 11, Proceedings, Antofagasta (Chile) available on: https://www.esga.org/index.php?id=228&tx_commerce_pi1%5BshowUid%5D=1903&tx_commerce_pi 1%5BcatUid%5D=2&cHash=03f281e3a8’
