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Vallance, J., Markowski, A., Fontboté, L., & Chiaradia, M. (2003). Mineralogical and fluid inclusion constraints on the genesis of gold-skarn deposits in the Nambija district (Ecuador). in D. Eliopoulos et al. eds., Mineral Exploration and Sustainable Development, Millipress, in press.

Mineralogical and fluid inclusion constraints on the genesis of gold-skarn deposits in the Nambija district (Ecuador) *

Jean Vallance, Agnès Markowski, Lluís Fontboté
Section des Sciences de la Terre, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
Massimo Chiaradia
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.

* This version on line includes Figs A-E not contained in the original printed version.

Keywords: Ecuador, skarn, gold, fluid inclusions

ABSTRACT: The Nambija gold deposits are oxidized gold skarns developed on volcano-sedimentary rocks of the Triassic Piuntza Unit (southeastern Ecuador). A continuous process of skarn formation with a garnet-rich and pyroxene-poor prograde stage and gold deposition during a weak and sulfide poor retrograde stage has been recognized. Part of the late prograde and retrograde minerals (including native gold) are found in structurally-controlled irregular open spaces and veins (mainly along N10°E to N60°E directions). Gold precipitation is related to cooling of moderately saline fluids (2-11 wt% eq NaCl).

1. INTRODUCTION

The Nambija gold district is located in southeastern Ecuador (Zamora province) in the Cordillera del Condor at elevations ranging from 1600 to 2300 meters (Fig A, Fig. 1). It includes, from north to south, the Fortuna, Cambana, Campanillas, Nambija, Guaysimi and Sultana del Condor mines. The gold deposits occur within and close to skarn bodies developed on volcano-sedimentary rocks of the Triassic Piuntza Unit. Gold grades are typically high (up to 150 g/t) whereas the contents in Cu, Zn, Pb and other metals are in most mines very low.
Different genetic models have been evoked for the Nambija mineralization. According to Litherland et al. (1994) and Prodeminca (2000), the Piuntza Unit has been contact-metamorphosed and transformed into a skarn by the Zamora intrusion and gold is the result of a later epithermal event. Alternatively, a genetic link between (Tertiary?) porphyry felsic intrusions and skarn and gold mineralization is proposed by Hammarstrom, (1992). Nambija is described by Meinert (1998) as an "oxidized gold skarn" in his worldwide compilation of Au-bearing skarns.
Based on ongoing work on the gold deposits of Fortuna, Campanillas, Cambana, Nambija, and Guaysimi we provide here preliminary geological, mineralogical and fluid inclusion results.

Fig. A: View of the historical Nambija mining site with sluice gold recovery. (February 2002)

Fig. 1: Regional geological map of the Nambija region. In the inset, geotectonic map of Ecuador with the localization 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 (slightly modified from Litherland et al., 1994).

2. GEOLOGICAL SETTING

Ecuador consists of oceanic and continental terranes accreted to the Amazon craton between the Early Cretaceous and Early Tertiary (Fig. 1).
The Nambija district is situated on the western margin of the Amazon craton. The skarn bodies are hosted by the Triassic Piuntza Unit, which occurs in this area, as a NS trending lens (about 20 km long and 1-2 km wide) within the Jurassic batholith of Zamora. The Piuntza Unit is limited to the East and to the West by NS reverse faults (Prodeminca, 2000). The Zamora batholith consists of I-type tonalites and granodiorites and is Rb-Sr dated at 190-140 Ma (Litherland et al., 1994). Other magmatic rocks reported in the district are monzodiorite, monzonites, rhyodacites, syenites, quartz-feldspar porphyry dikes and stocks (Hammarstrom, 1992; Paladines & Rosero, 1996). The Piuntza Unit attains its maximum thickness (at least 300 m) at Nambija and consists of sandstone, siltstone, limestone, volcaniclastic rocks and andesitic flows (Paladines & Rosero, 1996).
At Fortuna, Cambana, Campanillas and Guaysimi, bedding strikes N90±20°E with 10 to 40° dips to the south. At Nambija, the Piuntza Unit forms a EW trending syncline. Main structures are NS reverse faults and NE-SW and NW-SE normal faults (Fig B).

Fig. B: Simplified geological map of the Nambija district
with main fault systems (modified from PRODEMINCA, 2000).

3. MINE GEOLOGY AND ORE MINERALOGY

At the mine scale the skarn bodies are largely controlled by bedding, form up to 30 m thick massive layers and mainly replace fine-grained andesitic volcaniclastic rocks and, locally, limestones as at Guaysimi and Fortuna. In the latter deposit bioclast relicts have been observed (Markowski, 2003). At the outcrop scale, the transition from skarn to volcaniclastic rocks is sharp, crosscuts bedding, and is in places marked by a 1 to 2 cm thick rim of K-feldspar (Fig C).

Fig. C Typical aspect of skarn outcrops. A: The transition from skarn to not skarnified rocks (here quartzite) is sharp and concordant to bedding to the right and discordant to the left (Guaysimi mine); B: In places, a zonation from skarn to volcaniclastic rock is observed. The inner part of the skarn body consists of brown garnet (grt), a first rim is made of epidote and pyroxene, and a second one of K-feldspar (kfs), which forms the transition to volcaniclastic rocks. Cambana mine.

Skarn consists mainly of massive brown garnet > pyroxene assemblages (Fig D). In places, preferently within volcaniclastic rocks, epidote occurs as cm-sized clusters. Small amounts of K-feldspar are present at the boundary between skarn and unskarnified rocks. Small amounts of K-feldspar are present at the boundary between skarn and unskarnified rocks. This brown garnet skarn is locally replaced by a green skarn assemblage consisting mainly of garnet and quartz. Garnet in the brown skarn ranges Ad99-40, whereas in the green skarn it is generally more aluminous, typically Ad80-20.

Fig. D: Brown garnet skarn (grt 1) grading into blue-green garnet skarn (grt 2 + qtz).
Crosscuting red-brown garnet veins (grt 2 veins) are observed. Guaysimi mine.

Andraditic garnet occurs mainly at the border of vugs and forming yellow-honey clusters (a few mm in size) within brown skarn. Strong zoning with anisotropic granditic cores and isotropic andraditic rims is often observed and suggests multiple pulse precipitation from a hydrothermal fluid. Pyroxene is mainly diopsidic (Di92-47) with Mn contents which may be significant (Jo0-19). Epidote is locally abundant and at Fortuna has Ep10-18 compositions (Markowski, 2003).
The skarn often shows irregular “open spaces” filled with, in order of abundance, quartz, K-feldspar, garnet, calcite, chlorite, epidote ± plagioclase ± muscovite and subordinate pyrite, hematite, sphalerite and chalcopyrite. The open spaces are aligned in several mines preferentially along N10° to N60°E directions and grade into irregularly-shaped, 2 cm to 30 cm thick, generally steep dipping “Type I” veins. Type I veins crosscut the skarn and the volcaniclastics rocks (Fig. 2), the vein character being better developed in the latter. In places they are roughly concordant to the bedding. Garnet is found on the walls of some Type I veins and does not show corrosion against quartz and other infilling minerals. Native gold preferentially occurs in these veins, building grains up to several mm in size. The spacing between Type I veins ranges from a few to tens of meters. No evidence of vertical movements along the veins has been observed.
Type II veins are thin (< 1 mm) filled with quartz, calcite, chlorite, K-feldspar, and small amounts of hematite, pyrite, chalcopyrite, and native gold. They crosscut but roughly follow the same orientations (N10° to 60°E) as Type I veins.
The irregular open spaces, and Type I and Type II veins are crosscut by 1 to 2 mm wide, steep dipping, N70°E to N100°E trending Type III veins. They are filled by quartz, pyrite, and, subordinate, K-feldspar, muscovite, epidote, chlorite, calcite, and actinolite. Native gold has not been observed in these veins.
Several porphyritic quartz – feldspar - amphibole intrusions crosscut the Zamora granite or the Piuntza unit in the Nambija district. They are present in all studied mines close to the skarn bodies, suggesting a genetic connection. They are highly weathered into clays and iron oxides and are crosscut by a network of 1 to 5 mm thick pyrite ± K-feldspar ± quartz veins.
In the El Tierrero mine at Nambija, the garnet skarn is cut by 0.5 to 5 cm wide veins filled by quartz - K-feldspar – epidote – pyrite – chalcopyrite - molybdenite ± sphalerite. These veins are tentatively correlated with Type I veins.
Calcite ± quartz fill late normal N10°E to N70°E faults.

Fig. 2: Sketches of gold-bearing outcrops in the Nambija District. A: steep dipping auriferous (Au) Type I vein in the Campanillas mine filled by garnet (grt), quartz (qtz), K-feldspar (kfs), and calcite (cal) crosscuting volcaniclastic rocks with epidote clusters (ep). Type II veins filled by calcite (cal) and hematite (hm) crosscut the whole. B: Native gold in an irregular “opening” zone filled by garnet, K-feldspar, quartz, plagioclase and calcite in the Guaysimi mine. Native gold grains occur preferentially at the contacts between garnet and quartz.

In summary (Fig. 3), the prograde stage corresponds to the formation of a brown garnet – pyroxene (±K-feldspar) assemblage. Formation of irregular open spaces and Type I veins took place at the end of the prograde stage. The retrograde stage consists of epidote, quartz, K-feldspar, calcite, chlorite ± plagioclase ± muscovite, pyrite and subordinate chalcopyrite, hematite. and sphalerite. These minerals occur both within the massive skarn (mainly chlorite, calcite and hematite which replace garnet and pyroxene) as well as in the irregular open spaces and in Type I, II and III veins. Pyrite occurs mainly in open spaces and in Type I veins and shows at least at Cambana and Campanillas a decreasing abundance from porphyritic intrusion to the outer zone.

Fig. 3: Paragenetic mineral sequence in the Nambija district.

Gold precipitates during the retrograde stage in fractures within the garnet or at mineral joints within the irregular “open spaces” and in Type I and II veins (Fig. 4 and E). Gold rich areas in several mines seem to be associated with hematite rather than with pyrite. This could indicate high oxygen fugacity and/or low sulfur fugacity during gold deposition.
Sealing of late normal faults and veins by calcite and quartz characterize the post ore stage.

Fig. 4: Gold (Au) at mineral joint between quartz (qtz) and anisotropic garnet (grt) in irregular “open spaces” at Guaysimi. Gold and calcite (cal) seem to be coeval. Garnet suffered retrograde alteration into calcite and chlorite (cal + chl). Sample DTR 305. Crossed polarizers.

Fig. E: Native gold in a Type I vein (quartz, subordinate K-feldspar and chlorite) within green pyroxene-epidote skarn. Thin type II calcite veins are also visible. Campanillas mine.

4. FLUID INCLUSIONS

Fluid inclusions have been observed in garnet, pyroxene, quartz, and calcite from the Guaysimi, Campanillas, and Fortuna mines. High temperature fluids are recorded in garnet and pyroxene (preliminary fluid inclusion data: 450-420°C, 16-7 wt% eq NaCl in garnet; 450-400°C, 53-12 wt% eq NaCl in pyroxene).
Microthermometric analysis of primary and secondary fluid inclusions from quartz in Type I veins from the three mines are plotted in Fig. 5. Primary fluid inclusions show V/L ratios from 0.3 to 1. Eutectic temperatures below –30°C indicate the presence of cations like Ca2+, Mg2+ or Fe2+/3+. Salinity ranges from 2 to 11 wt% eq NaCl and minimal trapping temperatures (Th) to the liquid or to the vapor phase range between 350 and 430°C. Inclusions are named L2 and V2 when homogenizing to the liquid or to the vapor phase respectively.
Secondary L3 fluid inclusions were trapped as fluid inclusion planes. V/L ratios are between 0.1 and 0.2. Salinities are in the range 2-10 wt% eq NaCl with Th to the liquid phase ranging from 140°C to 250°C. Eutectic temperatures indicate the presence of cations like Ca2+, Mg2+ or Fe2+/3+.
Three types of primary fluid inclusions were recognized in quartz that exhibits pyramidal form at Campanillas. V4 fluid inclusions are 100% vapor. L4 are two-phase fluid inclusions (vapor/liquid ratios from 0.1 to 0.2). Lh4 type inclusions have similar V/L ratios but contain halite crystals of variable size and were also observed in quartz from type I vein at Fortuna (Markowski, 2003).
L4 fluid inclusion salinity is in the range 0.1-19 wt% eq NaCl with Th between 140 and 170°C. Lh4 fluid inclusions show total homogenization by melting of halite at temperature ranging from 180°C to more than 300°C (30 to >38 wt% eq NaCl) and temperature of vapor disappearance around 150°C at Campanillas and in the range of 170-220°C at Fortuna. Eutectic temperatures indicate the presence of cations like Ca2+, Mg2+ or Fe2+/3+ in L4 and Lh4 fluid inclusions.

Fig. 5: Th versus salinity diagram (Campanillas, Guaysimi and Fortuna mine).
Possible fluid evolution trends are shown. Symbol explanation in the text.

Transition from L3 to V4/L4/Lh4 fluids could be explained by boiling around 220°C at Fortuna and 160°C at Campanillas. This and the transitions from the high temperature fluids recorded in garnet and pyroxene and in quartz to the low temperature fluids should be studied by additional work. Fluid inclusions in post ore calcite (L5) display intermediate salinities (24-21 wt% eq NaCl) and low temperatures (100-80°C).

5. CONCLUSIONS

The Nambija mineral assemblage is typical of oxidized calcic gold skarns (Meinert, 1998). Petrographic evidence indicates a continuous process of skarn formation with a garnet-rich and pyroxene-poor prograde stage and gold deposition during a weak and sulfide-poor retrograde stage characterized by the precipitation of epidote, quartz, K-feldspar, calcite, chlorite ± plagioclase ± muscovite, pyrite and subordinate chalcopyrite, hematite. and sphalerite. In several mines, part of the late prograde and retrograde minerals (including native gold) are found in structurally-controlled irregular open spaces and veins (mainly along N10°E to N60°E directions). This feature, which has not been described in most other skarn deposits, may be important for exploration.
Gold deposition is probably the result of dilution and cooling of high saline (magmatic?), oxidized (poor in reduced sulfur, deposition of hematite) fluids transporting gold as chlorine complexes. Deposition was at temperatures lower than 350°C, probably prior boiling (~220-160°C). This is consistent with negligible chloride complex efficiency below 250°C (e.g. Gammons & William Jones, 1997).
Dating is necessary to determine whether the mineralization is related to a phase of the Jurassic Zamora batholith intrusion or to later events.

ACKNOWLEDGEMENT

We would like to thank Comcumay S.A., Cominzasa, Compañía Minera del Ecuador – Andos, Compañía Minera sol de Oriente, Cooperativa Once de Julio, and Fortuna Gold Mining Corp. for access to their mines and properties and for their assistance in the field. This paper has benefited from critical suggestions by L. D. Meinert and F. Tornos. This investigation was supported by the Swiss National Science Foundation, project n° 2000-062 000.00.

REFERENCES

Gammons, C.H., & Williams-Jones, A.E., 1997. Chemical mobility of gold in the porphyry-epithermal environment. Economic Geology, 92, p. 45-59.
Hammarstrom, J.M. 1992. Mineralogy and chemistry of gold-associated skarn from Nambija, Zamora Province, Ecuador: A reconnaissance study: Advances related to U.S. and International Mineral Resources, USGS, Chapter K, p. 107-118.
Litherland M., Aspden J.A. & Jemielita R.A. 1994. The metamorphic belts of Ecuador. Overseas Memoir 11. BGS, Keyworth, U.K. 147 p.
Markowski, A. 2003. The gold skarn of Fortuna, (Nambija District, Cordillera del Condor, Ecuador). Ms Thesis, University of Geneva, 184 p. Also accessible on line under:
http://www.unige.ch//sciences/terre/research/Groups/mineral_resources/archive/pub_archive/markowski_fortuna_2003/markowski_fortuna_2003.html.
Meinert, L.D. 1998. A review of skarns that contain gold, in Lentz, D. R., ed., Mineralized porphyry/skarn systems, Min. Assoc. Can. Short Course Series, v. 26, p. 359-414.
Paladines, A. & Rosero, G. 1996. Zonificación mineralogénica del Ecuador, Ed. Laser, Quito 146 p.
Prodeminca. (2000). Depositos porfidicos y epi-mesotermales relacionados con intrusiones de la Cordillera del Condor. Evaluacion de distritos mineros del Ecuador. Vol 5. UCP Prodeminca Proyecto MEM BIRF 36-55 EC. 223 p.

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