Aurania has discovered copper-silver in sedimentary layering in its Lost Cities – Cututcu Project in Ecuador. This type of copper deposit is also known as “red-bed” copper, amongst others. Sedimentary-hosted deposits have produced about 20% of the world’s copper; the best-known are:
- The Central African Copper Belt that extends from of Zambia into the Congo over 900km – and contained resources of 410Blbs of copper since its discovery in 1895. Discoveries are still being made in the Copper Belt, attested to by Ivanhoe’s 38Blb copper deposit at Kamoa-Kakula in the Congo; and
- The Kupferschiefer of Germany and Poland that extends over 600km and has been mined from at least 1199 AD. Today, four mines are in production in Poland by KGMH, operating at depths of 800m – 1,200m. The Kupferschiefer has produced 130Blbs of copper and over one billion ounces of silver.
Sediment-hosted copper deposits are typically sheet-like, lying within layering of the enclosing sedimentary rock pile.
Copper-silver has been found in carbon-bearing layers within a red bed sedimentary rock sequence in an area measuring 23km north-south by 7km east-west in Aurania’s properties in Ecuador. Similar copper-silver mineralization has been described by Hannan Metals some 300km south of Aurania’s Ecuador property in northern Peru and by Max Resources from the border area of Colombia and Venezuela in the north – some 2,000km.
The association of copper with silver, without notable enrichment in other metals through this belt, is unusual – the exception being the Kupferschiefer and parts of the Central African Copper Belt. The copper-silver mineralization is present as malachite, chrysocolla, tenorite, chalcocite, cuprite and native copper, and is hosted by bleached, reduced sandstone and carbonaceous shale. The strongest mineralization is found in strata with fossilized plant fragments (Figure 1).
Copper-silver mineralization in sedimentary layer with carbonized plant stems.
Sample C175636: 5.6% copper, 146g/t silver
These giant deposits occur in a unique geological setting; at the top of red bed sequences, which are porous and permeable oxidized sandstones (as you see today in the Grand Canyon) where they are capped by organic carbon-rich strata – typically carbon-bearing shales, limestones or volcanic ash layers. These deposits formed in inland seas like today’s Baltic or Caspian, which periodically evaporated completely, to be flooded again over geological time.
These deposits typically form sheets of high-grade copper that extend over large areas. At Kamoa-Kakula, the copper is concentrated in specific parts of elongate zones within the sedimentary layer like popsicle sticks laid out on a sheet of paper. To give an idea of potential scale, the Kakula “popsicle stick” alone contains 38Blbs of copper. Copper in the Kupferschiefer occurs with silver – making it the biggest historic producer of silver in the world. The Central African Copper Belt, in turn, has the largest by-product cobalt resource in the world.
Dominant features of the Kuferschiefer and Central Africa Copperbelt provide a useful exploration model. Classic models for the Kupferschiefer and Central African Copperbelt are based on the following concepts:
- Sand, made up of rock fragments and quartz, was washed into lakes that formed in slight depressions that formed against faults. Over time, and as the depression (basin) against each fault continued to sag, the sand consolidated into rock (sandstone). During unusually high rainfall, mud would wash into the basin and would form layers over the sand. In contrast, excessive dryness led to evaporation, which concentrated salts in the shrinking lakes to the extent that salt layers developed. As the basin evolved and aged, the sand layers would become sandstone, which would be red because of the oxidation (rusting) of iron-bearing minerals, and the mud layers would harden into mudstone or shale;
- The rock fragments in the sandstone contain a little copper, and perhaps a few other metals such as silver. As the basin subsides, the salt from the salt (or “evaporite”) layers, corrodes the metals and these metal ions are dissolved in the solution as salts. Copper chloride (CuCl), which is very similar in chemistry to table salt (NaCl) remains in solution in the saline water that lies below surface in these basins – and it can stay in solution for millions of years – tens of millions of years;
- These metal-bearing saline solutions are warmed gradually as the sand layers subside deeper into the basin, and as the fluids warm, so they can hold more metal as chloride complexes, like copper chloride and silver chloride;
- The layering of the sandstone sedimentary rock, and especially the layers of impermeable mudstone, would minimize the extent to which these metal bearing fluids could move around the basin – the layering would restrict most of the very slow flow to be confined within cells of sandstone layers capped by shales. If the basin were squeezed by tectonic forces, the fluids would start to move from the high-pressure areas to areas of lower pressure, and they would be forced up against the faults that originally gave rise to the basin – and they would flow up and along the faults with fluids from each cell mixing as they flowed in the fault. Where these fluids encounter a permeable layer with carbon, the carbon reduces the copper and silver in the chloride complexes and the metals concentrate in the layers with carbon, or at the base of carbon-bearing mudstone or shale. The carbon is from fossilized plant matter that was originally washed into the basin with the sand or may have accumulated in the muddy layers that formed when the sand was inundated by high-water.
An additional potential source of copper in the red-beds of the Chapiza Formation is from the subvolcanic and plutonic rocks that intruded the area in the Mid- to Late-Jurassic (Figure 47). The heliborne geophysical survey undertaken over the Project by Aurania shows a large number of magnetic features that are interpreted to be Mid- to Late-Jurassic intrusions, and if a proportion of these are mineralized porphyries or IOCGs, very significant quantities of copper may have been introduced into the red-bed system from this additional source. Since IOCG systems tend to develop in extensional tectonic settings, they may have developed contemporaneously with the basin in which the red-beds accumulated. The porphyries are considered to have formed in a late Jurassic island arc that was superimposed on the mid-Jurassic rift basin in which the red-beds accumulated.
The genetic model developed for the sedimentary-hosted mineralization, with copper and silver leached from sediments and introduced from hydrothermal fluids related to mineralized porphyry or IOCG systems, was maintained in solution as copper-chloride; the chloride derived from salt layers and domes. The copper- and silver- rich brine would have maintained these metals in solution until such time as they were driven laterally along more permeable units within the red bed sequence in response to tectonic compression that would also have inverted the extensional basin. Fault zones would have provided the cross-stratal permeability that allowed the warm metal-rich fluid to ascend until it came in contact with reduced strata that induced precipitation of copper and silver (see Figure 1).