Mexico's Rich Geologic Past Holds the Key to Prolific Gold Deposits

- October 3rd, 2015

The majority of gold mined in Mexico is as a co-product, first, of silver, and secondly of copper. The bedrock beneath Mexico is laden with an intricate, and valuable network of vein deposits of minerals.

This article was first published on Gold Investing News on August 26, 2010.
By Leia Michele Toovey- Exclusive to Gold Investing News
The majority of gold mined in Mexico is as a co-product first of silver, and secondly of copper. The bedrock beneath Mexico is laden with an intricate, and valuable network of vein deposits of minerals.
The bedrock deposits of the great silver-gold vein system of the Veta Madre at Guanajuato were discovered in the year 1550 and unearthed almost immediately. Numbered amongst the leading gold districts that have since been exhausted, El Oro was discovered in 1521 and mined for nearly 400 years, generating an output in excess of 5 million ounces of gold.
A history of intense volcanism and pyroclastic activity is the reason behind Mexico’s rich mineral deposits. Most mineral producing regions of the world have some areas where magnetism is the predominant force behind the geologic development; however, in the case of Mexico, virtually all of the crust has been intruded by magma.
These intrusions occurred over a span of many years—giving rise to the mineral deposits that are still being discovered to date.  The magma that traveled through the crust either stayed below the surface, creating heat and pressure necessary for metamorphism and the deposition of mineral assemblages, or extruded onto the surface. Both extrusive and intrusive igneous rocks are therefore common to the area.
Porphyry skarn replacement and vein deposits host variable amounts of minerals such as gold, molybdenum, copper, silver, tungsten, tin, lead, iron. These intrusive deposits are primarily Mesozoic to mid tertiary in age, with three prominent classifications. They are a result of volcanism toward the pacific margin of the continental crust in northwestern Mexico.
The magma that reached the earth surface, (extrusive) formed massive batholith sized domes and shields. Much of this material exited through zones of weakness in the Earth’s crust flowing out through vents, fissure, and blown out through volcanism, however, large amounts of magma also cooled and solidified below the surface (intrusive).
Secondary occurrences of less intense volcanism likely contributed to molten material surging in the fault zones created by the prior period of magmatic activity  that occurred as a result of ongoing plate tectonics.  This secondary magma intrusion/extrusion cooled and solidified into igneous rocks. These rocks are thought to be the origin of what is now the rugged topography and deep cliff-sided canyons of the Sierra Madre. After this framework was set, a secondary enrichment followed as less extensive intrusive material permeated the framework.  This intrusive material’s heat and pressure created a hydrothermal system, where minerals precipitated into already formed cracks and faults in the crust. Therefore, mineral assemblages in the area are often referred to as hydrothermal, and structurally controlled.
These geological forces are the reason behind why gold is mined as a co-product to silver in the area. Hydrothermal veins are the most important source of silver in Mexico, and they form under nearly identical conditions to gold.  But silver, more reactive than gold, can be leached from surface rocks and carried downward in solution. This process can concentrate silver into exceedingly rich deposits deep underground.
The mineral deposits in Mexico are so wide-spread that the region lacks thorough testing.  As a result, exploration in order to define the extent of mineralization is ongoing in the region.  Mining stalled in Mexico during the last century, but now is rapidly returning in concert with the rapid ascent of the metal’s price.  Adding further impetus, of course, is the congruent price increase of gold’s two main co-products, silver and copper.
Mexico’s rich geologic history has resulted in a variety of mineral deposits, occurring in many environments.  Exploration along areas of faulting, thrusting, fractures, and fissures is key to this area; to reference back to the beginning of the article, many of the gold deposits are structurally controlled, therefore, minerals can be found in the crack and breaks created by crustal pressures.
NAFTA is one reason why Mexico is a politically stable region for mineral exploration and mining.  Rich mineral deposits and fair mining regulations in Mexico have also been contributing factors to an increased mining community, and mines that were shut down in the first part of the twentieth century are being restarted, turning profits once again.
Active exploration and mining companies in the area include Vancouver-based Goldcorp (TSX:G; NYSE:GG), Oro Gold (TSXV: OGR), Canasil Resources (TSXV: CLZ), and Piedmont Gold Corp (TSXV: PZE).  Goldcorp has interest in the following properties: Peñasquito, Los Filos, El Sauzal, and San Dimas gold/silver mines in Mexico. Oro’s wholly-owned gold and silver Trinidad property is located near Mazatlan, Mexico. Canasil also owns a portfolio of exploration projects in Mexico. Piedmont  is an exploration company focused on the Mojave-Sonora Megashear, a fold-thrust belt extending into Mexico.

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