Cerro del Mercado Mine

 This is a summary from my article in the German mineral magazine “Mineralien Welt” with the original title: “Fluorapatit und seine Begleitmineralien vom Cerro del Mercado in Durango, Mexiko”; from July-August 2001

 INTRODUCTION

Cerro del Mercado is a hill about 1 km long and nearly 200 m high that rises above the tableland of Durango. It is located at the northern edge of the city of Durango, the capital of the state of Durango, in Mexico. Lying at an altitude of about 1880 meters, the city has approximately 450,000 habitants. Cerro del Mercado is a medium sized iron deposit with a long history of production. It was discovered in 1552 by a Spaniard, Capitán Ginés Vazquez de Mercado.
Systematic exploitation commenced in the year 1828. Between 1975 and 1992 the mine was idle, shut down because it was only marginally profitable. Since 1993 the mine has been operated by the company “Cerro de Mercado S.A. de C.V.”, a subsidiary of  “Grupo Acero del Norte”. Currently the deposit is mined as an open pit, and the ore is concentrated by flotation and magnetic/gravimetric separation. Daily production in 2001 has been about 2000 metric tons of iron ore.

The Cerro del Mercado hill, view to the northwest, image from April 2000

 GEOLOGY

The Cerro del Mercado iron deposit has been interpreted as a resurgent dome situated within the Chupaderos caldera. The iron ore mineralization is related in space and time with an about 350 m long and up to 180 m wide dome of quartz latite, occupying the central portion of the deposit. The quartz latite, but also older rhyolitic/dacitic rocks exhibit intense metasomatic alteration. The high-grade iron ore consists of massive martite (hematite that is pseudomorphous after magnetite) that occurs not only as irregular, steeply dipping bodies and dikes, but also as mantos. These high-grade iron ore bodies are found especially at the western, southern and northern margin of the quartz latite dome.  In addition, there are large, low-grade martite ore bodies in brecciated quartz latite. Another type of low-grade ore consists of stratiform, unconsolidated “sandy magnetite” blankets, intercalated with tuffs. Radiometric dating of the volcanic rocks and direct fission track analysis of apatite has indicated that the iron deposit formed approximately 30 million years ago.

View from the Cerro del Mercado to the southwest, showing the western portion of the main pit.
In the background the city of Durango, image taken in April 2000

 GENESIS OF THE DEPOSIT

The genesis of the Cerro del Mercado deposit has been debated for a long time: one theory postulates the intrusion/extrusion of an iron oxide magma, another theory is based on a hydrothermal-metasomatic origin, and a combination of both processes is also possible.

  MINERALOGY   

 The massive iron ore is composed of martite, a pseudomorph of hematite after magnetite where the shape of the magnetite crystals is perfectly preserved, yielding beautiful octahedrons up to 10 cm across.  The sandy iron ore bodies are composed of fine grained magnetite and some martite.

The apatite is present as an accessory mineral throughout the iron deposit, but locally it occurs in abundance within pockets or veins at the margin of the massive iron ore bodies and in fractures or voids in the quartz latite breccias. Associated minerals include the syngenetic magnetite (now martite) and pyroxene (diopside-hedenbergite); other associated minerals appear to have formed later and at lower temperatures.  These include: calcite, chalcedony, quartz, titanite, opal, hematite, limonite, sepiolite (a soft, white paper-like mineral, it is a hydrous magnesium silicate), nontronite (replaces pyroxene together with mitridalite and maghemite C),  and rarely gypsum.
The apatite from Cerro del Mercado is a nearly pure fluorapatite, having the formula Ca₅(PO₄)₃F with a maximum of 0.45 wt% Cl.  The hardness (after Mohs) of apatite is 5, its density is 3.2, and the index of refraction is 1.63-1.64.
The crystal form of the apatite from Cerro del Mercado consists of well-shaped elongated hexagonal prisms with 6 or 12 faces and 6 faces on the low pyramidal termination.  Doubly terminated apatite crystals have not been found at Cerro del Mercado.  Smaller crystals are especially lustrous and transparent, with a color that is generally a deep honey-yellow. Yellow-green crystals also are found, and very seldom green-yellow crystals.
Larger crystals tend to be heavily included and fractured and therefore are only translucent. Most of the apatite crystals from this location are smaller than 2 cm. Large apatite crystals are really rare and, when found, may approach a length of 8 to 12 cm with a diameter of 4 to 7 cm. The inclusions observed in the apatite are martite, magnetite, hematite, pyroxene and fluid/gas inclusions.

A nice honey-yellow apatite crystal on matrix, the crystal is 18 mm high.

 CONCLUSIONS

The Cerro del Mercado iron mine in Durango is one of the most important and famous localities for fine apatite specimens in the world. Almost every collector has seen and can easily recognize apatite from this well known mine.   The fluorapatite at Cerro del Mercado was most abundant in the western ore body; sadly, this ore body is almost exhausted and mining has been inactive there for over 25 years.  The apatite from Durango is not only of interest for mineral collectors, but has other uses too. Clear apatite that is chemically very pure has been used in small amounts for many years by the pharmaceutical industry for the production of certain medications. In addition, the Durango apatite has research applications.  Some scientists use it as a standard for the thermochronologic analysis of rocks (apatite fission track analysis). These measurements represent an alternative method for the dating of rocks and in some cases, they make it possible to reconstruct the thermal history of rocks in the low temperature environment.

REFERENCES

 Lyons, J. I., 1988, Volcanogenetic Iron Oxide Deposits, Cerro de Mercado and vicinity, Durango, Mexico: Economic Geology, Vol. 83, p. 1886-1906.

Megaw, P., 1999, The Geology and Minerals of Cerro de Mercado, Durango, Mexico: Rocks & Minerals, Vol. 74:1, Mineral of Mexico, p. 20-29.

Jurgeit, M., 2001, Fluorapatit und seine Begleitmineralien vom Cerro del Mercado in Durango, Mexiko: Mineralienwelt, Vol. 4, p. 56-61.