Ecuador’s Gold Importer: The Volcano
There is one common goal that individuals such as sixteenth century explorer
Francisco Vasquez de Coronado and Discovery Channel star Todd Hoffman have in
common: the drive to find and extract gold.
Gold is one of the most sought after metals in today’s era primarily because of its popularity in jewellery. Because of this, many pursuers of this precious metal must learn different methods to locate it and, therefore, learn the environment that is responsible for producing gold.
The country of Ecuador, located on the western coast of South America, is
geographically located in a volcanically active area. Ecuador is split into four
regions: The coast (“Costa”), the jungle (“Oriente”), the mountains (“Sierra), and
the volcanically-formed island chain known as the Galapagos Islands.
The Ecuadorian Andes, also known as “Volcano Alley”, is a central valley of parallel
running mountains in which active volcanoes still erupt. The volcanoes of Cotopaxi,
Tungurahua and Sangay are particularly important as they are listed in the world’s
top ten most active volcanoes of this century. In fact, the Tungurahua volcano,
which translates to “throat of fire” has had ongoing eruptions between 1999 and
July 2013 and shows no signs of quieting down any time soon.
East of the Andes, the Oriente Basin forms a subandean retroarc basin system that stretches across Venezuela and Ecuador and ending in Southern Chile. Ecuador is made up of several important structures. Two mountainous regions of Ecuador, called “Cordilleras” after the Spanish word cordilla (“cord”), are known as the Cordillera Occidental and the Cordillera Real (aka Central). These chains of mountains contain more accessible peaks in Ecuador than anywhere else in the world. The Cordillera Occidental is a fully marine cordillera consisting of Pacific Ocean Floor crust connected with the Molleturo massif, an elevated accretion of continental and marine crusts in the southern coast of Ecuador. Important volcanoes in the Cordillera Occidental are the Quicocha, Pululahua, and Quilotoa caldera
volcanoes. The Cordillera Occidental was subsequently uplifted against the Cordillera Real along with the associated underwater Cordillera Macuchi Island Arch.
South of the Molleturo Massif, a continental block known as the Tahuin Cordillera
split and later reconnected with the Cordillera Real. In addition to the two Cordilleras, Ecuador has an elevated continental block located in the northern Oriente named the Napo Uplift. The Napo Uplift is unique because it houses the Reventador volcano, the eastern-most active andesitic volcano of the Andes, and the Sumaco volcano, a southern volcano known for its completely different alkaline lava composition.
Considering the geology of Ecuador, vulcanism is arguably the most influential process of creating gold in Ecuador. There are four probable causes of gold formation due to volcanoes in the area: volcanogenic massive sulphides, skarns, granodiorite intrusions, porphyry-related mineralization and epithermal mineralization.
Volcanogenic massive sulphides are ore deposits that are created by volcanic-associated hydrothermal events, such as sea floor smokers, and are rich in metal sulphides. Minerals accumulate at hydrothermal vents on or below the sea floor after magma is released from a volcano. The sulphide deposits are sustained by high heat flow from subvolcanic intrusions and/or upwelling plumes as well as circulating hot fluids that are continuously released by the volcano. Skarns are created by the reaction between igneous (granitic or rhyolitic) intrusions and sediment and are also contributors to gold formation. In this case, gold deposits are likely generated at shallow levels in geothermal systems, where it is scavenged
from host sedimentary sequences by meteoric (land) hydrothermal fluids and subsequently deposited on the peripheries which can be up to several kilometres from the initial intrusion. Magnetite and hematite compositions in black sand that is found in alluvial deposits of gold support this hypothesis as they are commonly associated with skarns. Similar to the environment that creates skarns, granodiorite intrusions are plutonic igneous rock intrusions that form due to the intrusion of silica-rich magma. The magma then cools in batholiths below the Earth’s surface and is only exposed at the surface of the Earth after uplift and erosion.
This is a common theme in Ecuador due to multiple faulting and uplift of oceanic and continental crust in the cordilleras. Intrusion-related gold deposits depend primarily on the existence of its mineral form in magma. Mineralization of gold can also occur in veins and assemblages parallel to the intrusion, such as quartz-exsolved felsic dykes, and usually continue below the exposed pluton. Once exposed at the surface, the intrusion will transfer gold into waterfalls, streams and rivers as they act as eroders and gold will be carried for great distances as the water moves it. Porphyry deposits can also form during intrusion of magma. This occurs when two stages of cooling occurs: one deep in the earth at the early stages of magma generation in the crust at a very slow pace to allow the growth of phenocrysts (larger crystals that are measured to be 2 mm or greater), and one at a much quicker pace when the magma reaches shallow depths of considerable cooler temperatures or as the magma erupts onto the surface. The secondary cooling process creates aphanitic (very fine grained) texture in the igneous rock surrounding the larger phenocrysts, presumably gold, as the rock cools. Epithermal mineralization is similar to skarns in that it involves the mineralization of gold due to a change in chemistry, but in this case the reaction is between hydrothermal systems, which do not have to originate from vulcanism, and the surrounding sediment layers.
Both low-sulfidation and high-sulfidation systems are common in this process of gold generation. Low-sulfidation systems involve fluids that rise along permeable zones in rock and deposits ore along near-neutral pH hot springs. Low-sulfidation systems usually consist of cavity-filling veins with sharp boundaries, where gold is present. High-sulfidation systems involve the ascent of magmatic volatiles where they are absorbed by meteoric water and form an acidic solution that leaches the surrounding rock. The leached country rock is then receptive to being filled with ore metals by other magmatic fluids that pass by at a later time.
This process is common in young volcanoes and can be directly related to vulcanism
in contrast with low-sulfidation systems. An alternative method of gold generation not due to vulcanism is also plausible and seen in shear zone-hosted mesothermal quartz. Shear zones are areas where strong rock deformation happens due to a high strain rate surrounded by lower strain rates. Ecuador is susceptible to this method of generation as it is a common process in mountain building, which can be seen in the existence of the Andes and further faulting in the area.
During mountain building events, pathways of deposition are created from deep mesothermal events as shearing happens between fault blocks. This creates mesothermal quartz, which is often associated with gold deposits.
The understanding of the geology of Ecuador is very important for successful gold exploration in the country. The multiple orogenies and continual vulcanism in the area, as seen primarily in the Ecuadorian Andes, is a building block for several methods of gold generation such as volcanogenic massive sulphides, skarns, granodioritic intrusions, porphyry deposits, and shear zone-hosted mesothermal quartz. By considering the environments that are responsible for supplying and moving deposits of gold, exploration of gold in Ecuador can reach a whole new level.