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Illustrated abstract of Markowski, A., (2003),
The Fortuna gold skarn, Nambija District (Cordillera Real, Ecuador).
MSc. Thesis, University of Geneva, 184 p.
(Supervisors Prof. L. Fontboté and Dr. M. Chiaradia)

More recent publications of A. Markowski on the Namibija skarn

The Fortuna gold skarn is located in the northern part of the Nambija gold district, in southern Ecuador, at elevations ranging from 1600 to 1700 meters (Figure 1, Regional geological map of Nambija District). Like the other skarn deposits in the Nambija district (Meinert, 1998 and 2000) it is an oxidized-calcic gold skarn. The skarn has developed nearby undated felsic intrusions on volcanic rocks belonging to the Triassic Piunza Unit (Figure 2, geological map of Fortuna mine). The felsic intrusions consist of porphyries with dioritic compositions, and of an equigranular intrusion (possibly belonging to the Jurassic Zamora batholith) with granodiorite to monzodiorite composition (Figure 3: cross-section through the Fortuna mine).

According to the geochemical classification of Pearce et al. (1984) these felsic intrusions plot in the volcanic arc granite field. In the Fortuna gold concession several prospects and mined sites are distinguished from north to south over a distance of 1 km. They are "Cuerpo 3", Mine 1, Mine 2 (the only presently mined site) and the "southern sector", located nearby the Fortuna porphyry intrusion. Gold grades are in the range of 5-10 g/t in Fortuna. Main gold concentrations occur near major N10°-60°E faults. Concentrations of other metals are low (typical values of selected mineralized samples range from up to 150 ppm for Cu and up to 300 ppm for Zn).

Two main skarn phases can be recognized based on their different mineralogy. The prograde phase consists of garnet, pyroxene, and subordinate epidote. The retrograde phase is weakly developed and does not strongly alter prograde minerals. In a first stage of the retrograde phase, quartz, epidote, and feldspars are the main minerals, in a second stage calcite, gold, hematite, pyrite, sericite, and chlorite are dominant (Figure 4: Paragenetic sequence).

Two types of garnet skarn were recognized in the field, displaying banded and irregular interfingering morphology at the outcrop scale: (1) a brown garnet skarn, which is generally massive and composed essentially by garnet grains (2) a blue-green garnet skarn, which consists of garnet, pyroxene, and epidote within a quartz matrix. The bluish color of this skarn is given by intergrown quartz (up to 50% vol) whereas the greenish colours are caused by epidote or pyroxene (up to 50% vol). Some large honey-reddish garnets form aggregates of up to a few centimeters in size within the brown garnet skarn, in places, also growing towards contiguous green garnet skarn.

Isotropic garnet forms most of the brown garnet skarn (Ad99-40). The green garnet skarn is mainly formed by colorless to slightly brown, strongly anisotropic grossular-rich garnet. The average composition is in the range Ad25-52 but in zoned garnets the compositional spectrum may be larger (in general Ad15-80). Isotropic, yellow and almost pure andraditic (Ad97-99.5) garnet constitutes the dark honey-reddish clusters, and, in general, constitutes the last generation of garnet which in part crystallizes into vugs (Figure 5: Garnet compositions).

Pyroxene is present as a minor phase except in the northern part of the concession and around the presently mined site (Mine 2) where pyroxene and pyroxene-garnet skarns are recognized. Pyroxene displays a wide range of compositions (Hd40.1-17.9Di69.7-46.8Jo19.1-7.05). Increases in Mn (2.5-6 wt%) and, less pronounced, in Fe (7-11 wt%) contents of the pyroxene occur from Mine 2 to 1 km to the north. Pyroxene occurs mainly as small subidiomorphic grains, which in part appear to replace garnet (mainly the green variety). (Figure 5: Pyroxene compositions).

In the northern part of the studied area, epidote (Epi9.8-17.7) occurs as the main phase, and is considered partly as a retrograde product of garnet. The abundance of epidote and clinopyroxene in the Cuerpo 3 and Mine 1 is interpreted as reflecting the nature of the protolith composition whereas garnet skarns of Mine 2 and of the southern sector have probably developed on a carbonate-rich protolith as indicated by some bioclast relicts.

The small amount of amphibole present at Fortuna is attributed to the original scarcity of pyroxene of which amphibole is the typical retrograde product. Moreover, as mentioned above, the skarn is only weakly retrograded.

Chlorite may be an abundant phase of the skarn, mainly nearby faults, and reveals variable compositions with a Fe/Fe+Mg ratio ranging from 0.87 to 0.38. Three geothermometers have been applied to chlorite analyses and they reveal bimodal distributions of temperatures ranging from 300 to 340 °C for the first population and from 400 to 420°C for the second population. Calculated temperatures are lower for samples collected in Cuerpo 3, Mine 1, and in the area south of Mine 2 than in Mine 2 around the main N60°E fault. As pyroxene, chlorite shows an enrichment in Mn (up to 4 wt.%) to the north. Other retrograde minerals of the skarn are K-feldspar, plagioclase, quartz, calcite and sericite. These minerals, together with epidote and chlorite, occur mainly within centimeter-wide veins (N°40E-N°60E) and irregular open spaces. Later, thinner calcite veinlets do not show any preferential orientation. Observation of calcite in cathodoluminescence does not provide evidence for the existence of different fluid generations.

Native gold (Ag content: 5.9 to 14.6 wt%, traces of Cu and Hg) occurs together with calcite and quartz within garnet fractures or between mineral joints (Figure 6: Gold occurrences). It occurs always within skarn assemblages. Hematite is more abundant than pyrite when gold is present and could indicate a high oxygen fugacity during gold deposition.

Whole rock geochemistry has been carried out on skarn samples of the four different sectors and suggests an FeO enrichment in Cuerpo 3 and in the southern sector, and MgO and MnO enrichments toward the northern sector (Cuerpo 3). The whole skarn rock geochemistry reflects the mineralogy and the composition of the skarn minerals. Gold contents are higher around Mine 1 and 2 (up to 2000 ppb generally and 3 to 33 g/t), and Zn and Cu exhibit an opposite behavior with higher values in Cuerpo 3 and in the southern sector.

Quartz primary inclusions have homogenization temperatures between 350°C and 200°C, moderate salinities (1.6-9.7 wt% NaCl eq.), and variable liquid/vapor ratios. Other primary fluid inclusions containing halite (32-36 wt% NaCl eq.) have total homogenization temperatures by halite melting between 218°C and 268°C. They may be the result of boiling of the moderate saline fluid. Lead isotopes were analysed on four samples of igneous rocks from Fortuna. They show similar lead isotope compositions than ores analysed in the Nambija District.

Several arguments suggest that gold is associated to the skarn formation. These arguments include the occurrence of gold always in association with the retrograde phases of the skarn and the mineralogical and paragenetic similarity of the Fortuna skarn with other oxidized gold skarns such as McCoy, Nevada (Meinert, 1998).

In the studied area, more grossular-rich compositions of garnet and more diopside-rich compositions of pyroxene are observed close to the main N10°E to N60°E trending faults at Mine 2 and in the southern sector. This zonation, according to Meinert (1997), may be used to determine the proximal zone of the pluton or fluid source responsible for the metasomatic process. Also the chlorite geothermomethers indicate higher temperatures around the main N60°E fault of Mine 2. Thus, it is proposed that the main N10°E to N60°E faults at Mine 2 and in the southern sector have sourced the fluids responsible for the skarn development and gold mineralization. This is consistent with the higher gold grades of Mine 2. Gold transport was probably as a chloride complex in a fluid close of the pyrite/hematite buffer and close to neutral conditions (based on the presence of retrograde hematite in areas close to the inferred fluid source, and of pyrite to the north, and the absence of acid alteration). The deposition process is probably due to cooling. Very low metal contents are also consistent with the hypothesis that the fluid was poor in reduced sulfur.

Acknowledgment: This work has benefited from support by Society of Economic Geologists, Académie Suisse des Sciences Naturelles and the Swiss National Science Foundation n° 2000-062 000.00 and Fortuna Gold mining Corp.

References:

Einaudi, M.T., Meinert, L.D., Newberry, R.J., (1981), Skarn deposits, Economic Geology, 75th Anniversary Volume, p.317-391

Litherland, M., Aspden, J.A., and Jemielta, R.A., (1994), The metamorphic belts of Ecuador: Overseas Memoir of the British Geological Survey 11, p.147 .

Markowski, A. (2003a). The Fortuna gold skarn, Nambija District (Cordillera Real, Ecuador). Ms Thesis, University of Geneva, 184 p. (Supervisors: Pr. L.Fontboté and Dr. M. Chiaradia)

Markowski, A. (2003b). Version on line of abstract of The Fortuna gold skarn, Nambija District (Cordillera Real, Ecuador). Ms Thesis, University of Geneva, 184 p. with additional figures.

Meinert L.D., (1989), Gold skarn deposits-geology and exploration criteria: Economic Geology Monograph 6, p.537-552

Figures

Markowski Fortuna

Figure 1: Regional geological map of the Nambija region. In the inset, geotectonic map of Ecuador with the location of the Nambija area. Legend: 1, Intrusives; 2, Napo Formation; 3, Hollín Formation; 4, Alao-Paute Unit; 5, Misahualli Unit; 6, Zamora batholith; 7, Metamorphosed Piuntza Unit; 8, skarn (schematic); 9, Piuntza Unit; 10, metagranite (Tres Lagunas); 11, Sabanilla Unit; 12, Chiguinda Unit; 13, Isimanchi Unit; 14, migmatic gneiss; white circles: skarn-related Au ore; black squares=epithermal Au-Ag-Cu-Pb-Zn ore; gray circle: Cu-Mo porphyre ore (slightly modified from Litherland et al., 1994)

Markowski Fortuna

Figure 2: Geological map of Fortuna mine (modified from Markowski, 2003) [click to enlarge]

Markowski, Fortuna

Figure 3: Schematic cross-section through the Fortuna mine. (modified from Markowski, 2003)

Markowski Fortuna

Figure 4: Paragenetic sequence of the skarn from Fortuna mine (modified from Markowski, 2003)

1:Massive grt skarn, different pulses explain zonation, first rich in iron; last garnet generation poor in iron (very anisotropic) and fine grained.
2: cpx begins to grow after garnet; relationship of cpx and ep is not clear: grt => ep+cpx, or grt = ep?, or grt=> cpx+cal?
3: Epidote intimately associated to clinopyroxene, relation not well defined.
4: fds+ep as retrograde alteration of grt
5: amph+ep+py veins in volcanic andesite overprinted by strong potassic alteration;
6: Early quartz grows in apparent equilibrium with late garnet in up to cm-sized irregular openings, in part with brecciated borders. Later quartz occurs frequently in calcite veins.
7: Calcite apparently resulting from grt corrosion by cpx
8: Calcite tend fist to fill open spaces, then to infiltrate quartz joints and grt fractures, and later occurs preferentially as thin veins
9: Late mm size veins crosscutting the skarn.
10: Gold almost always filling open spaces within calcite and quartz, in garnet-quartz joints, and in garnet fractures calcite. Gold is in most cases spatially linked to the close occurrence of calcite.
11: Hematite present with calcite, in the same position as gold
12: Pyrolusite crust

Markowski Fortuna

Figure 5: Comparison of garnet and pyroxene compositions with general skarns

Markowski Fortuna

Figure 6: Microphotographs of gold occurrences in garnet skarn (Markowski, 2003) [click to enlarge]

A, plane polarized light, B, crossed nicols; C, reflected light of sample DTR 54b_6907: Gold crystallizes between garnet and quartz, and infiltrates the garnet. The other opaque mineral present on this photo is hematite and it is also associated to calcite what suggest high oxygen fugacity conditions when gold precipitates. White bars: 200 mm

D, plain polarized light, E, crossed nicols, F, reflected light of sample DTR 54a_6906: Microphotographs of gold associated spatially with calcite in quartz open spaces. Quartz filling the open spaces in the garnet skarn. White bars: 200 mm

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mlevy

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