Hidden gold mines and stone forests of Northern Peru
Intro: While Chile may be well-endowed in giant copper mines, Peru has a more golden-flavour to its country-side. After my travels in Chile we flew to the northern Peruvian region of Cajamarca. The main town is also called Cajamarca (which is the place where they killed the last Inca). From here it was off to the golden hills where hidden over 3,000 metres above sea level are some of the richest gold deposits in the world, plus some neat anthropogenic geological formations (i.e. canals carved into stone from the pre-Inca Cajamarca culture)
Science Spiel: Gold and Stone (Geology, Ore Deposit Geology)
Peru is divided into roughly three geographic regions; these are known quite well to local Peruvians as the Highlands, Coast and Jungle. While all three regions host their own treasures, the golden gems are hidden more-or-less in the Highlands. Like with Chile, the Peru Highlands are part of the long Andean Cordillera. This is a parallel range of mountains at the western edge of South America, related to the great compressive forces along the margin due to the subduction of the oceanic Nazca plate under the continental South American plate. Areas of flat slab subduction and crustal thickening has uplifted regions to over 4,000 m above sea level (Benavides-Caceres, 1996).
Within the Cajamarca region of northern Peru lies the countries most mineralized belt. There are at least 14 known porphyry copper-gold deposits and 19 epithermal gold-silver deposits, all within ~60 km north of Cajamarca (Gustafson et al., 2004)! This is known as the Yanacocha Epithermal Gold Belt (named after the largest gold deposit in Peru, Yanacocha). Gold deposits in this region as largely classified as epithermal; a term used to call certain types of gold deposits that formed in settings that were < 1 km deep and were subjected to < 250 °C hot fluids (Buchanan, 1981; Heald, 1987; White and Hedenquist, 1990; Corbett and Leach, 1998).
The gold mines I visited in Peru were La Zanja, Yanacocha and Tantahuatay, and these are all classified as a sub-set of epithermal deposits known as “high-sulfidation” epithermal deposits. Sub-classfications to the epithermal deposits are based on specific paramenters of the hot fluids (e.g. acidic, oxidation state, etc.) which precipitate certain mineral assemblages (e.g. enargite and alunite; White and Hedenquist, 1990). The Cajamarca region is of particularly interest because not only does it host these large epithermal deposit, but they are spatial and temporally associated and overprinting porphyry copper-gold deposits (e.g. The Andes, copper mines and volcanoes of Chile), sometimes resulting in odd wormy dissolution textures known as “gusano” texture. It is not entirely understood why the gold deposits are here and how they formed, but a common feature is their occurrence on a northeast-striking 30 – 40 km wide, 200 km long belt known as the Chicama-Yanacocha structural corridor (Quiroz, 1997). The intersection with the northeast structures and the Andean parallel and trans-Andean structures (i.e. faults sub-parallel and perpendicular to the Andes) are thought to have localized these giant gold deposits (Vidal et al., 1997; Longo, 2000).
And finally, when my field course on gold deposits of Peru was over, I spent some extra time checking out a place nearby Cajamarca, known locally as the “Stone Forest” or “Cumbemayo”. This is a site of volcanic breccias (i.e. ignimbrites) that have been cooled and weathered in such a way (i.e. column jointing) that they now resemble beautiful pillars and trees. What’s also unique about these rocks are the numerous and undulating carved canals. Canals were made by the 2,000 year old pre-Inca culture of the Cajamarca people in order to funnel and divert water to the nearby towns.
Final Thoughts: The long (~ 3 hour) drive to and from the gold mines really put into perspective not only the beauty of the Peruvian highlights, but also how well it can hide these large gold deposits. As I mentioned initially, most of these sites are over 4,000 m above sea level, and unless you know where to look and what to look for they can easily be missed. Who know how many more large gold deposits underlie the rugged highlands of Cajamarca? I guess that’s what us exploration geologists are for!
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