|
Rodolfo Corona-Esquivel, Instituto de Geologia, UNAM (Mexico)
Fernando Henriquez, Universidad de Santiago de Chile (Chile)
Jordi Tritlla, CGEO-UNAM (Mexico)
Enrique Martinez-Hernandez, Instituto de Geologia, UNAM (Mexico)
|
|
The iron deposit of La Perla, located in the center-east part of the Chihuahua State, Mexico is one of the most important iron ore suppliers for the steel industry in Mexico. Like other iron ore deposits abroad, its origin (hydrothermal vs. magmatic) is still a matter of debate.
This deposit is located in the southwest portion of the Chihuahuense Geological province, formed by Mesozoic to Cenozoic rocks. The mine is located in a dome-shaped structure emplaced at the main fracture in a felsic volcanic field of Oligocene age. Originally, the deposit was composed by several lens-shaped, elongated bodies up to 400 m long and with 50 m thick at its thickest part.
The ore consists mainly in dominant hematite, subordinate martitized magnetite and minor limonite, all mixed up with different quantities of pyroxene, quartz, apatite and calcite. One of the more striking features of La Perla is the presence of very important deposits of powder-like powdery magnetite-hematite. The powder-like ore is presently exposed over a nearly continuous belt at the lower perimeter of the open pit, forming a blanket whose thickness varies from 3 to more than 15 m. Within this powdery deposit, variations in the size of the crystals, texture and composition exist.
In the western wall of the pit, the powdery ore is constituted by a pile of several layers with different textures, ranging from those formed by sub-milimetric crystal aggregates to accumulation of platy-sized hematite crystals of 2 to 3 mm in diameter arranged in an isotropic texture. The powder-like ore present a grain sorting pseudo-stratification, with a fining-upward sequence and cross bedding, and includes fragments of volcanic rocks (riodacite). It is also common to find spherical aggregates, between 8 and 12 cm in diameter, formed by a lattice of tabular hematite crystals.
The presence of clinopyroxene crystals as well as euhedral apatites within the powdery hematite ore, strongly suggest a magmatic involvement for the origin of this deposit. Moreover, the textural evidence suggests a near-surface, pneumatolytic-magmatic scenario, closely related to pyroclastic volcanic explosions, as found in similar iron ore deposits (Cerro de Mercado, Durango, Mexico).
|
