Summary:
Mineralization on the Los Filos Mine property is typical of intrusion-related gold–silver iron-oxide skarn deposits.
Los Filos Deposit Area Mineralization
In the Los Filos area, mineralization is associated with two early Tertiary granodiorite stocks that were emplaced in carbonate rocks and resulted in development of high-temperature calc-silicate and oxide metasomatic alteration (skarn) assemblages that were followed by distinct meso to epithermal alteration. Hematite and magnetite are typical skarn minerals, but diopside, which is usually recognized in skarn assemblages, is not present.
The Nukay and Sur deposits formed along the north and southern margins of the West Stock . The Peninsular and Zona 70 deposits formed on the northern and southwest margins of the East Stock, respectively.
The Los Filos Open Pit mineralization is hosted primarily within or on the lower contact of a diorite sill that dips from 20° to 50° to the east, away from the East Stock. The diorite was emplaced into a large, moderately dipping active structure that parallels bedding in the marble. The sill has a sigmoidal shape that starts out roughly flat at the stock, extends east at a moderate dip for approximately 200 m, turns south, flattens out, and extends to the Bermejal Intrusion, located approximately 2 km to the south.
Mineralization—Eastern Los Filos Open Pit
The diorite sill hosts a significant proportion of the mineralization on the eastern side of the Los Filos Open Pit. Mineralization is structurally controlled by breccias and quartz–hematite–gold (± calcite) veins that occur relatively late in the paragenetic sequence, and probably represent the last stage of hydrothermal activity in the deposit. The veins dip at moderate to steep angles (50° to 80°), while the breccias dip more moderately (30° to 40°). Both veins and breccias are developed preferentially within the intrusive rocks and their contacts with the carbonate rocks. The veins typically occur in clusters, with spacings of 5 to 50 cm. The breccias tend to occur as isolated or bifurcating structures.
Mineralization—Central and Western Los Filos Open Pit
The central portion of the Los Filos Open Pit occurs above the underground areas Peninsular and Sur, as well as the previously mined El Grande, Agüita, Zona 70, and Crestón Rojo zones. The western portion of the Los Filos Open Pit occurs above the underground areas of Nukay and Independencia.
Gold mineralization is dominantly hosted within Cretaceous-aged medium-bedded to massive fossiliferous limestone of the Morelos Formation, as well as locally within the Los Filos granodiorite intrusions and associated diorite sill. The carbonate rocks were intruded by granodioritic plutons, with contact metamorphism resulting in the formation of marble within the calcareous rocks and local development of calc–silicate endoskarn in the intrusive rocks. Pods of calc–silicate and iron-rich exoskarn in the marble formed along contacts. The iron-rich exoskarn deposits occur along the contacts of the eastern stock.
Bermejal–Guadalupe Deposit Area Mineralization
The Bermejal–Guadalupe Open Pit (BOP-GOP) and Bermejal Underground (BUG) area of mineralization along the contact of the Bermejal–Guadalupe stock with the carbonate rocks of the Morelos Formation. The Bermejal–Guadalupe Open Pit mineralization occurs along the top and flanks of the intrusion. Mineralization that extends below the open pit and along the northern end of the steeply dipping intrusion flanks is referred to as the Bermejal Underground deposit. The Guadalupe deposit is on the eastern extension of the Bermejal–Guadalupe stock and comprises the southeastern portion of the Bermejal–Guadalupe pit.
Mineralization—Bermejal
The major mineralized bodies at Bermejal consist of iron-oxide gold skarn with minor amounts of copper and silver at the intrusion–limestone contact. Disseminated endoskarn mineralization also occurs within the hydrothermally altered intrusive rocks.
Surface drilling defined several mineralized areas around the Bermejal–Guadalupe stock with dips ranging from 30° to vertical. Quartz, iron-oxides, high-grade gold-mineralized veins, stockwork, and disseminated mineralization are locally abundant. Both limestone and intrusive rocks host the quartz– iron-oxide and high-grade gold mineralization. Stockworks and disseminated mineralization are restricted to the intrusion.
The Bermejal–Guadalupe stock has at least a 900 m vertical extent, and the contact is sub-vertical below the open pit. The Bermejal Underground mineralization extends below the open pit, runs dominantly northwest–southeast, parallel to and under the diorite sill, with additional mineralization running from southwest to northeast along the northern margin of the intrusion.
The mineralization at Bermejal underground is divided into five zones, including:
1. Upper intrusion—along the intrusion contact and above the junction of the diorite sill with the Bermejal intrusion
2. Upper sill contact—along the upper side of the diorite sill
3. Sill—mineralization within the diorite sill, including interior oxide zones sub-parallel to the sill margins
4. Lower sill contact—along the lower side of the diorite sill
5. Lower intrusion—along the intrusion contact and below the junction of the diorite sill and the Bermejal Intrusion.
The deposit is open laterally along the sill and down-dip along the granodiorite intrusion. Bermejal mineralization is predominantly iron-oxide skarn. At depths of more than 250 m, oxidation is pervasive and continuous, while sulphides can occur locally and increase within the intrusion (endoskarn). Although most gold is associated with iron-oxide bodies at the intrusive–limestone contact, there is also gold contained within mineralized structures, quartz veins, and pyroxene-rich zones of both endoskarn and exoskarn.
The volume of limestone replacement (exoskarn) is minor compared to the volume of endoskarn. Thickness perpendicular to the intrusion contact of the combined endoskarn and exoskarn zones varies from 10 to 150 m, with an average thickness of 80 m. Mineralization extends continuously around the intrusion, with the deposit forming a dome-shaped shell around it. Important structural controls strike north–south and east–west, resulting in local bends and widening of the mineralized zones.
Mineralization—Guadalupe
The Guadalupe deposit is on the eastern extension of the Bermejal–Guadalupe stock, on the southeastern end of the Bermejal–Guadalupe resource pit. A portion of the mineralization in this area was mined on several levels as a small-scale underground mine between 1939 and 1956. Mineralization comprises iron-oxide gold skarn, with minor amounts of copper and silver developed along the intrusion–limestone contact. Mineralization also occurs within exoskarn and can occur as stockworks or disseminations within the hydrothermally altered intrusive rocks. Both limestones and intrusive rocks host the quartz–iron-oxide and high-grade gold veins.
Oxidation is pervasive and continuous along the granodiorite–carbonate contact down to elevations of 1,200 masl (up to 350 m from surface), while minor sulphides occur locally. Although most gold is associated with massive iron-oxide bodies at the granodiorite–carbonate contact, there is also gold within structures, quartz veins, and the pyroxene skarn zone. Potential exists for additional Mineral Resources at depth.