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VISIT OF THE FORTUNA MINE, NAMBIJA DISTRICT, ECUADOR
(UNESCO-SEG METALLOGENY COURSE, JUNE 2003)Agnès Markowski (1), Lluís Fontboté (1), Massimo Chiaradia (2), Jean Vallance (2)
(1) Section des Sciences de la Terre, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland;
(2) School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.INTRODUCTION:
Fortuna is located in the Nambija District (Zamora Province) in the Cordillera del Condor, which is in the Sub-Andean zone (southern Ecuador, Figure 1 of Markowski 2003b) at ca. 4°S and 78°W, and at an altitude ranging between 1500 and 1650 m. The Fortuna mine belongs to the Fortuna Gold mining Corp., which is located in Quito, Av. De los Shyris 760 y Rep. Del Salvador. An overview of the geological, mineralogical, and geochemical characteristics is contained in Markowski (2003b).
The purpose of this half-day tour is to examine the main types of skarn outcropping in Fortuna. The Fortuna mine is smaller and less Au-rich compared to Campanillas, Nambija and the other mines more to the south. The outcropping conditions forced us to focus on a detailed petrographical work in Fortuna.
The Fortuna mine was divided into four different sectors represented on the schematic geological map of Fortuna (Figure 2 of Markowski, 2003b). A schematic cross-section is proposed in Figure 3 (Markowski, 2003b). We will examine with more attention Mine 1 and especially Mine 2. We will also stop to observe the Fortuna porphyry, which is an undated felsic intrusion of diorite compositions.
An overview of the geological, mineralogical, and geochemical characteristics of the Fortuna gold skarn deposit is contained in Markowski (2003b) and of the Nambija District in Vallance et al. (2003).
MINE 1:
This site is currently inactive. Grades were about 5 to 8 g/t gold. The mineralogy is made up mainly (90%) of epidote, subordinate pink feldspar, and anhedral pyroxene (Figure 1). Feldspar and quartz crystallize generally in veins, and are in some places spatially associated to calcite. Some samples present brown to honey garnet (Ad40-99.5) veins crosscutting epidote rich rocks. Millimeter to centimeter sized calcite veins crosscut these lithologies.
Figure 1: A: Epidote-pyroxene rich rock with areas richer in epidote (in light green). Brown spots are relicts of garnets, which crystallize also in veins. B: Epidote rich rock, with K-feldspar - plagioclase – quartz veins. Dark spots are chlorite (Markowski, 2003a) MINE 2:
The presently mined site is Mine 2 (Figure 2). Grades range from 5 to 10 g/t gold. We will focus on this part of the Fortuna mine where gold can be observed macroscopically. In this mine we will distinguish the different garnet skarn types.
Figure 2: Mine 2 is the presently mined site of Fortuna mine.
We will first examine the northern side of one of the N60°E fault of Mine 2 (Figure 3). From field evidence, two types of garnet skarns were recognized with two types of morphologies (Figure 4): a brown garnet skarn, which is massive and composed generally of large garnet grains and a green/blue one which consists of small garnet, pyroxene, epidote, calcite grains cemented by quartz.
The relationship between these two types of garnet skarns is not always clear and rocks yielding intermediate colour and textures are often present. The petrographical study and microprobe analyses on garnets have permitted to define three garnet types: Isotropic garnet to zoned garnet (Ad99-40) forms most of the brown garnet skarn; a colourless to slightly brown, strongly anisotropic (Ng-Np: 0.010), grossular-rich garnet with average composition of Ad25-52 constitutes the green garnet skarn, but if zoning exists the compositional spectrum is larger (Ad15-80). The third garnet type constitutes dark honey-reddish clusters and is completely isotropic, yellow in natural light and has almost pure andraditic compositions (Ad99.5-97).
These two skarns were mainly mapped as an undifferentiated garnet skarn, illustrated in brownish colour on Figure 2 (Markowski, 2003b) and 3. Gold grade is estimated at 10 g/t on this side of the fault. Ore seems to be associated to subparallel faults with a spacing of 30 to 40 centimeters and parallel to the big N60°E trending faults.
Figure 3: Zoom of the main studied area from Fortuna mine (modified from Fig. 2 in Markowski 2003b).
On the northern side of the fault two types of garnet skarn and morphologies can be observed.
On the southern side the garnet skarn is lighter (see text).The southern outcrop (Figure 5) is partly composed by skarn and partly by silicified and recrystallized tuffs. These two lithologies are clearly stratified in the upper part of the outcrop. The garnet skarn is greenish and display lighter colours more to the south. The ratio of garnet:pyroxene is about 100:1 on the northernmost edge of the outcrop and is almost 1:1 on the southern most edge of the outcrop. Figure 6 presents a representative sample of light green skarn outcropping in this area. Gold grade is < 2 g/t (considered as barren) close to the fault and grows to 7-8 g/t to the south. Some discontinuous bands up to 40 centimeters in length and containing recrystallized material and relicts of bioclasts can be observed in this area.
Figure 5: Southern outcrop showing three main fracture types and white discontinuous bands
containing relicts of bioclasts (slightly modified from Markowski, 2003a). See Figure 3 for location.The southern outcrop displays also fractures/veins filled with garnet, epidote with feldspars walls (Figure 7). These fractures crosscut silicified and sometimes recrystallized tuffs. These epidote-garnet veins are oriented N20°E to N60°E.
References:
Markowski, A. (2003a). The Fortuna gold skarn, Nambija District (Cordillera Real, Ecuador). MSc Thesis, University of Geneva, 184 p.
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