EPITHERMAL SILVER-GOLD-BASE METAL DEPOSITS RELATED TO CALDERA DEVELOPMENT IN A NEWLY-IDENTIFIED GRABEN IN OAXACA, SOUTHEASTERN MEXICO

A new silver-gold-base metal producing region is rapidly emerging in the state of Oaxaca in southeastern Mexico. Production mainly comes from high-grade epithermal silver-gold-base metal deposits hosted by Tertiary-age volcanic rocks and underlying Mesozoic sedimentary rocks. These deposits are among the youngest epithermal systems in Mexico. The Tertiary volcanism and ore deposits are superimposed on composite thrust-bound basement terranes associated with the evolution of the north-west trending oblique right-lateral convergent Cocos-North American plate margin. This continental margin is still active with more than 8,000 earthquakes with magnitudes greater than 4.0 recorded during the past 40 years.

Extension along the plate margin formed a newly-identified west-northwest–trending graben critical in controlling the large, diffuse Tertiary volcanic field and related epithermal deposits in the Oaxaca region. Large-scale calderas with medium- to high-level intrusions, including andesitic to dacitic flow domes, overlain by thick ignimbrite units are associated with the epithermal deposits. Mineralization is intimately related to regional strike-slip shearing and caldera magmatism. There is also a particularly close association between dome margins, dikes, and epithermal vein mineralization.

Precious and base metals were introduced in two separate events, one related to an earlier skarn event at depth, followed by the main epithermal event of precious-base metal deposition. Veins are zoned from sulfide-dominant near the surface to increasing amounts of calc-silicate minerals at depth. Fluid inclusion temperatures and salinities are both higher than typical of epithermal systems, supporting a skarn connection for this district. Characteristics of a skarn environment are also indicated by geochemical sampling studies demonstrating a Au+Ag+As+Sb+Hg +Cu+Pb+Zn+Mo+Bi+W association, whereas characteristics of the epithermal silver-gold-base metal veins are demonstrated by a more limited element suite consisting of Au+Ag+As+Sb+Hg.

Modern exploration initiated during the late 1990’s resulted in several significant new discoveries in Oaxaca. Since 2010, production from two new underground mines totals nearly 280,000 ounces of gold and 28 million ounces of silver.