La Herradura Gold Mine
|La Herradura Gold Mine .|
Location: Sonora, Puerto Penasco, Mexico (MX).
Owner: Fresnillo plc.
Average ore grade in reserves: 0.80 g/t Gold
Total Reserves: 1.5 Moz Gold
Mine Life: 4.1 years
MINING AND EXPLORATION HISTORY
The La Herradura mine contains 5.4 million ounces of contained gold in production plus reserves. The deposit is owned by Minera Penmont, a Joint Venture between Peñoles and Newmont. As a result of an aggressive grassroot exploration program in northwestern Mexico that started in 1987, the first economic drill intersection in La Herradura came in 1991 (100m @ 0.85 g/t Au). Subsequent and continuous drilling campaigns resulted in the definition of an orebody containing 1.7 M oz by May 1998, when mine operations started. To date, 2 M oz of gold have been produced. Present reserves are 3.4 M oz of gold in ore with an average grade of 1 g/t, using a cut-off of 0.35 g/t Au. The mine produces 210,000 ounces of gold per year ( Jose de la Torre, pers. commun., 2008).
Regional Geologic and Tectonic Setting
La Herradura mine is located in northwestern Sonora, Mexico. This deposit occurs within a northwest trending belt that consists of metamorphic rocks of greenschist and amphibolite facies and granitoids of Proterozoic age (Nourse et al., 2005). These rocks are intruded by a series of Triassic and Middle Jurassic granitoids and are overlain by younger sedimentary and volcanic rocks of Middle to Late Jurassic age (Figure 2.1). All these units are intruded by Late Cretaceous to early Tertiary granitoids related to the Laramide orogeny and are overlain by Miocene rhyolites, andesites, and basalts and Quaternary basalts. Basin and Range tectonics affect this area, as they do much of Sonora and adjacent Arizona. Basin and Range faulting occurred in the mid to late Tertiary. Faulting resulted in the formation of NW-trending linear ranges of crystalline rock, separated by deep basins filled with sand and gravel derived from the ranges. Correlation is difficult between ranges.
The Geologic Setting of La Herradura
La Herradura mine occurs within a northwest trending belt of Proterozoic rocks consisting of greenschist and amphibolite grade metamorphic rocks and granitoids. The deposit is hosted in biotite-quartz-feldspar and quartz-feldspathic gneisses that are bordered to the east by Jurassic clastic rocks and subvolcanic intrusions and to the west by upper Paleozoic limestone. Isolated outcrops of fresh andesite, trachyte, and basalt occur locally northeast of the mine.
The Structural Setting of La Herradura
Based on structural mapping in the La Herradura mine area, it is possible to identify at least five tectonics events superimposed on all stratigraphic units outcropping in this area (de la Torre, 2004; Romero 2005, Table 2.1). These observations indicate that gold mineralization is associated with the third tectonic event, and they also tend to constrain the age of this mineralization to between 80 and 45 Ma.
Alteration of La Herradura
Reported alterations assemblages of this deposit (de la Torre, 2004; Romero, 2005) are quartz-sericite-albite in the core of the deposit and selectively follow the quartz-feldspar gneiss bands in the outer zones of the deposit. Iron-carbonates (ankerite-siderite) are widespread within the deposit, mainly restricted to haloes adjacent to quartz-sulfide veins within the core of the orebody. Iron carbonates also are found in the outer alteration aureoles of the deposits. Propylitic alteration islocated in the outermost portions of the deposit, and it occurs mainly in the biotite-bearing gneiss and in Jurassic rhyolitic and andesitic volcanic rocks.