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Visit of porphyry and epithermal environments at the Portovelo-Zaruma Mining Camp, Southwest Ecuador

R. Spencer, IAMGOLD, Quito, Ecuador

The Portovelo-Zaruma mining camp

The Portovelo-Zaruma mining camp is an unusual mining district in Ecuador, with historic gold production exceeding 4.5 Moz. The production has mostly come from an extensive set of base metal-rich veins that lie adjacent to a cluster of porphyry intrusions, and around which alteration and vein mineralogy are zoned. The source fluids were near neutral pH, moderately saline (11-18 wt% NaCl equiv) and moderately oxidized, as they precipitated pyrite and magnetite. Veins in the north and east formed at high temperatures, characteristic of cordilleran veins, and grade into veins that are more typical of intermediate sulfidation epithermal systems in the western and southern part of the system. Auriferous shoots are located in dilational structural sites in the footwall of oblique thrust ramps in competent host rocks. Boiling in such sites played an important role in gold deposition in the shallower portion of the tilted vein field, as indicated by abundant adularia and bladed calcite. A second phase of gold mineralization was associated with base metal-poor epithermal veins, and is apparently restricted to the extreme south and west of the vein trend. (From Spencer et al., 2003)

Visit Program

Wednesday 10 June: Meet at view-site from which the whole of the Portovelo-Zaruma belt can be seen, from lithocap remnants overlying porphyries in the north to the coolest parts of the vein system in the south. Field stops at the highest parts of the epithermal system - granular silica above vein mineralisation at El Calvario and relict lithocap with pyrophyllite and alunite at Zaruma Urcu. Thursday overnight in Hotel Roland, Zaruma.

Thursday afternoon and Friday morning: review of core organised from north (most proximal to porphyries), to the south where quartz gives way to calcite as the main gangue mineral in the veins. Friday afternoon, depart for Machala.

Reference

Spencer, R.M., Montenegro, J.L., Gaibor, A., Perez, E.P., Mantilla, G., Viera, F., and Spencer, C.E., 2002, The Portovelo-Zaruma Mining Camp, SW Ecuador: Porphyry and Epithermal Environments: SEG Newsletter, v. 49, p. 1, 8-14.

[Quito 2003 field trip guide] [On line publications of the ore deposits group]

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