Mineral hotspots in the Pan Amazon

By Timothy J. Killeen

The mineral resources of the Pan Amazon are not randomly distributed across the region but are located on landscapes with specific geological histories. There are three conspicuous macro regions:

A. The Amazon Craton is the term used to describe the complex of ancient rocks that form the heart of the continent, which was created via the serial accretion of continental plates between one to three billion years ago. Each such orogenic event was accompanied by magmatic intrusions that are imbued with the mineral resources.

B. The Andean Cordillera was formed after the Amazon Craton collided with the Nazca plate starting about 500 million years ago; this massive orogenic process occurred in phases with subregional variations characterized by volcanisms and magmatic intrusions that are impregnated with mineral deposits of global importance.

C. The Sedimentary Plain of the Western Amazon can be subdivided into basins that are the repository of significant hydrocarbon reserves, which were formed in what was once a shallow sea located between the Amazon Craton and the Andean Cordillera. Older sedimentary rocks that overlie parts of the Craton hold globally important reserves of bauxite and potash, while the upper horizons of multiple landscapes are rich in alluvial gold.

The distribution of corporate mines among the Tectonic Provinces of the superficial exposure of the Amazon Craton: (I) Carajás – Imataca, (II) Transamazon – Guiana Coast, (III) Central Amazon, (IV) Tapajós – Parima, (V) Rondônia – Juruena, (VI) Rio Negro, and (VII) Sunsas / Macarena, as well as Proterozoic formations of the Atlantic Shield (VIII). The stippled rectangles show the major deflection and shear zones that have influenced the tectonic history of the Amazon Craton and the Andean Cordillera. Source: Santos et al. (2001) and Porter Geo (2022).

There are two superficial manifestations of the Amazon Craton: the Brazilian Shield and Guiana Shield, which were separated about 500 million years ago (Ma) by a deformation zone in the Earth’s crust. The deformation zone is known as the Huancabamba Deflection / Amazon Mega-shear and extends from the floor of the Pacific through the Andes, across the Amazon Craton and into the Atlantic Ocean. A fundamental structural feature of the South American continent, it is responsible for creating a biologically important low spot in the Andean Cordillera (Huancabamba Gap) and the creation of Amazon Rift Valley. Sediments eroding off the two shield formations were deposited into the Proto-Amazon River and are the source of the bauxite and potash resources of the Central Amazon.

Geophysicists are still debating the details of the geological history of the Amazon Craton, but they all agree that it was created and has been modified repeatedly by tectonic forces over billions of years. A recent interpretation recognized six provinces linked to chronological age, tectonic origin and the types of rock formations found within them.

The Andean orogeny started in the Jurassic soon after the breakup of Gondwona (~140 Ma) with key structural stages occurring in the Cretaceous (70–100 Ma) and the Miocene (5–23 Ma) when most of the vertical uplift ensued. The Andes are organized into parallel mountain ranges, referred to as cordilleras; the western cordilleras were formed by volcanic and magmatic phenomena linked to the subduction zone of the colliding continental plates, while the eastern cordilleras are composed of uplifted blocks of sedimentary and metamorphic rocks that originated on the western margin of the Amazon Craton. Magma intrusions invaded the eastern slope of the Cordillera Occidental and the western slope of the Cordillera Oriental to create the polymetallic porphyries, which are the foundation of the Peruvian mining industry.

The distribution of economically important industrial metals, as well as gold, is a function of this geological history and the future of the mining industry is well-known among professional geologists and the investors that finance their industry

The rapid uplift of the Andes during the Miocene contributed to a continental-scale change in climate, which created orographic rainfall that dramatically increased erosion off the Andes and filled the hydrocarbon-rich sedimentary basins located in the subsidence zones immediately adjacent to the western Cordillera. A shallow sea between the Andes and the Amazon Craton eventually filled with sediment and forced a reversal in the direction of the Proto-Amazon River, which began to flow from west to east only about ten million years ago. In the Pleistocene (< 2 Ma) the cyclical advance and retreat of glaciers pulverized the highly mineralized peaks of the western Cordillera adjacent to the Altiplano. Glaciation released thousands of tonnes of gold into the fast-flowing sediment-laden rivers of the Andean foothills and created the alluvial gold fields on the piedmont and adjacent alluvial plain.

The Sedimentary Basins of the Pan Amazon were created by the ongoing phenomena of orogeny and erosion. Several are located within the Andean foredeep where the western reaches of the Amazon Craton are being subducted underneath the Andean Cordillera. Individual basins are separated by ‘arches,’ underground ridges that roughly correspond to modern hydrological basins. To the North and East, sedimentary basins are arranged along the outer margins of the Guiana Shield where they have accumulated sediments and fossil fuels over hundreds of millions of years.

The sedimentary basins in the Pan Amazon. Data sources: Schenk et al. (1999) and Codato et al. (2019).

There are two sedimentary basins in the middle of the continent, both formed when the Proto Amazon flowed from East to West within the Amazon Rift Valley. All these basins, to different degrees of importance, are the source of the region’s oil and gas reserves, which are the fossilized remains of photosynthetic microorganisms that once dominated the shallow water ecosystems above the continental shelves of the Amazon Craton in the Mesozoic (100–250 Ma) and Paleozoic (250–450 Ma).

“A Perfect Storm in the Amazon” is a book by Timothy Killeen and contains the author’s viewpoints and analysis. The second edition was published by The White Horse in 2021, under the terms of a Creative Commons license (CC BY 4.0 license).

To read earlier chapters of the book, find Chapter One here, Chapter Two here, Chapter Three here and Chapter Four here.

Chapter 5. Mineral commodities: a small footprint, a large impact and a great deal of money

This article was originally published on Mongabay