Summary:
The Franke area exhibits a shallowly dipping anticline fold striking N20-40°W. It is slightly steeper on the southwest flank (up to 20°) than on the northeast flank (5-15°). Bedding along the axis of the anticline is an almost horizontal crest zone about 100-200 m wide, as observed in the underground workings dug into this area. However, previous studies have suggested that this anticline also plunges to the NNW and SSE, defining a gently dipping structural dome.
Flanking the sub-horizontal crest zone of the anticline, several steeply dipping N25-35°W/70- 80°E faults are apparently one of the main mineralization controls for the emplacement of high grade copper in the Franke deposit. The other control is given by the segments of the N60-70°W structural trend.
The geometry of the ore zones in the Franke deposit is closely related with this particular structural arrangement: while the mineralization at the flanks of the anticline is typically formed by one or more ore horizons of limited thicknesses (1 to 5 m), the mineralization within and near the structural corridors located near the anticline crest, tends to be thicker and higher in grade, forming 10 to 30 m thick mineralized blocks separated by relatively thin barren horizons.
There appears to be a crude outward zonation of alteration centered roughly on the top of the Franke deposit but it is distorted by the several vein-stockwork zones that similarly exhibit more intensive alteration and primary mineralization. The central and most intensive alteration includes moderate to strong alteration of plagioclase phenocrysts to sericite and clays, plus total conversion of magnetite to specularite; a strong propylitic to weak sericitic assemblage. Outward from this central area the alteration generally decreases in intensity until finally grading with the regional green schist assemblage.
The deposits of the Altamira district, as those of the La Negra Formation, are hydrothermal copper (± silver) andesite hosted stockwork sulphide deposits that have been partially oxidized. The injected hydrothermal materials were rich in copper but poor in iron and sulphur, consequently the resulting primary mineralization is largely chalcocite (minor covellite and bornite and traces of chalcopyrite). Due to the lack of pyrite in the primary assemblages, no sulphuric acid was generated during the oxidation process and therefore, the conversion of the primary sulphides into secondary assemblages (copper oxide minerals and minor secondary sulphides) took place largely “in situ”, with limited displacement of the mineralization from its original location.
Mineralization.
The mineralization of copper is found in the form of oxides species (Atacamite, Malachite, Chrysocolla, etc.) and to a lesser extent, sulphides (Chalcocite). The Franke - Altamira “manto-type” copper deposit presents a Cu-Ag association (Flores, 2006). The flow breccias are of sub-aerial origin and they are found in most of the deeper drilling under the Franke deposit.
The uppermost unit of the Aeropuerto Formation is dominantly equigranular andesite, in contrast with the porphyritic flows of the middle portion and has little exposure on the surface of the district.
The middle portion of the Aeropuerto Formation that hosts most of the copper mineralization in the Altamira district is about 100 – 200 m thick at the Franke deposit and is dominated by distinctively discontinuous lava flows of porphyritic phases interbedded, in the upper part, with sedimentary rocks that are highly lenticular, indicating a fluvial or lacustrine terrestrial origin.
Most of the copper mineralization is hosted in porphyritic amygdaloidal andesites and ocoitas, but an important amount is hosted in equigranular andesites and in a lesser amount in volcaniclastic and calcareous sediments. Basal andesitic flow breccias and upper chocolate – brown andesites are generally barren units.
The high grade portion of the Franke deposit is formed by a large number of stratabound pods of different sizes, each one connected to one or more faults striking NW, WNW or N. These faults are now sealed by hydrothermal material and were used by the hydrothermal fluids to pervade into the favourable horizons (i.e. vesicular flows, contacts, micro fractures and stockworks). The horizontal size of these pods varies from a few metres to tens of metres from the feeders and the fading of the mineralization pinches within a very short distance, even if the favourable horizon persists.
The oxidation profile at Franke is rather complex, lacking clear definitive surfaces defining the top of the sulphides and the bottom of the oxides. The upper 20 m of the stratabound mineralization is largely, but not completely oxidized. Between 20 and 60 m depth, the mixed ores with subordinated sulphides are the most common assemblages while at deeper levels the sulphides start to predominate. Inversions of this sequence are very common and oxidized horizons may reappear under sulphide ore zones.