La Herradura Gold Mine
La Herradura Gold Mine . |
Location: Sonora, Puerto Penasco, Mexico (MX).
Products: Gold.
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.
great and useful information
ReplyDelete