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ENTRANCE TO THE MILPILLAS MINE

In the close-up photograph (ca. 2008), the entrance to the Milpillas Mine is clearly visible, a testament to its prominent presence. Even from approximately one kilometer away, as seen in the more distant image (ca. 2017), the mine’s entrance remains discernible. Looking at the lower photo, the impressive solar thermal plant at Milpillas comes into clear view. This facility, notably one of Mexico’s largest solar thermal plants, was constructed in 2016. It boasts a substantial collector area of 6,270 square metres and delivers an impressive output of 8,600 MWh/year. This innovative installation has played a crucial role in significantly reducing the mine’s reliance on diesel for its Copper electro-winning process.

The mine’s operational history was marked by a transition from easily accessible, high-grade oxide ores to deeper sulphide deposits. This shift, influenced by fluctuating Copper prices and geological realities, led to a temporary closure and subsequent reopening. This illustrates the inherent volatility and adaptive nature of the mining industry. Environmentally, the mine, like all extractive operations, faced challenges related to water use in an arid region and waste management. Socially, it contributed to local employment while navigating the broader complexities of large-scale industry within small local communities. Ultimately, Milpillas served as a reminder that mining, even when geologically exceptional, is a complex endeavor balancing economic gain with environmental responsibility and social considerations.

Copper mineralization at Milpillas was primarily hosted within altered intrusive rocks and, to a lesser extent, surrounding host rocks. The process began with the emplacement of a large, hydrous, and metal-rich magma at shallow to moderate crustal depths. As this magma cooled and crystallized, residual volatile phases, including water, sulfur, and metals like Copper, become progressively concentrated in the remaining melt. When the pressure of these exsolving fluids exceeded the lithostatic pressure, they were released, often explosively, through fractures and faults, forming a pervasive network of veins and disseminations.
These hot, acidic, and metal-laden hydrothermal fluids interacted with the surrounding rocks, leading to extensive hydrothermal alteration, such as potassic, phyllic, and argillic zones. As the fluids migrated and cooled, changes in temperature, pressure and pH, along with mixing with meteoric waters, caused the precipitation of Copper-bearing minerals.

Chalcocite underwent additional oxidation within the upper supergene zone. This process, driven by oxygenated waters, led to the dissolution of Chalcocite and the subsequent precipitation of various secondary Copper minerals. Notably, vibrant blue Azurite and green Malachite formed, often as stunning crystal specimens. Frequently, pseudomorphs were observed, where Malachite replaced Azurite while retaining its original crystal habit. Other secondary minerals, such as Cuprite and Brochantite, were also formed, reflecting the complex chemical transformations that occurred in this rich Copper deposit.

Original photos courtesy of Jaime Suberville (Author: silvia)