Gaby Main, Papa Grande
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The Gaby Main and Papa Grande porphyry gold deposits are ~2 km apart (to the NW and SE respectively), and are located 350 km SW of Quito, 50 km NE of Machala and ~130 km south of Guayaquil in southwestern Ecuador (#Location: 3° 3' 31"N, 79° 42' 35"W).
Gaby Main and Papa Grande are two of a number of Miocene porphyry and epithermal copper-gold deposits that occur in two main clusters, which define two mineral districts in western Ecuador, including Chaucha, Gaby-Papa Grande, Quimsacocha and Portovelo-Zaruma (Azuay-El Oro District), and Junín and Alpala (Imbaoeste District). These districts are ~440 km apart and located within 100 km of the Pacific coast, 200 km SSE of Guayaquil and 75 km north of Quito, in southwestern and northwestern Ecuador respectively. Other deposits are distributed along the Miocene porphyry belt between these two districts, and to the north and south into Colombia and Peru.
Tectonic and Regional Setting
For detail of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador and the broader North Andes and Panama copper-gold province.
Gaby Main and Papa Grande
These two deposits are located near the western part of the Cordillera Occidental which is controlled by large NNE-trending transcurrent faults that truncate NW oriented tensional fracture zones. Mineralisation is associated with multiple early Miocene porphyry and seriate intrusive stocks and dykes hosted by oceanic plateau basalts of the Pallatanga Unit of the Oligocene to early Miocene Saraguro Group (PRODEMINCA 2000; Srivastava et al., 2008).
In the Gaby Main-Papa Grande area, Cretaceous basement rocks comprise basaltic lava flows, tuffs and volcanic breccias of the Piño de la Sierra Formation of the Pallatanga Unit. The Pallatanga Unit is largely composed of dark green and very hard basalts with very few large crystals. Apart from the tuffs and volcanic breccias, they are massive and generally featureless, except for local pillow structures, and are rich in ferromagnesian minerals, low in potassium and have very reactive lithologies. Away from the deposit area, the Cretaceous is overlain by Paleocene rocks of the Macuchi Formation, comprising up to 2000 m of basaltic/andesitic flows, tuffs and volcaniclastic rocks.
Within the deposit area, the Cretaceous sequence is intruded by porphyritic igneous rocks that range in composition from tonalite to granodiorite that contain conspicuous phenocrysts within a finer grained groundmass. Two distinct intrusive units have been identified in the Gaby area; an older hornblende porphyry and a younger plagioclase feldspar porphyry (Srivastava et al., 2008).
There are also numerous breccias with different origins, namely: i). A monomictic "jigsaw" crackle breccia with angular clasts; ii). A polymictic breccia with a mixture of more rounded clasts of several different types, including both felsic porphyries and basalts; and iii). Pebble dykes.
The bulk of the Gaby Main deposit is dominated by a 1400 x 750 m, north-south elongated, 20.26±0.07 Ma (U-Pb zircon; Schütte et al., 2010) hornblende (-plagioclase) porphyry stock with numerous smaller (50 to 100 m diameter) breccia units. Immediately to the south a 500 m diameter tonalite shares a common contact, while a number of elongated 700 x 200 m NNE to north-south oriented masses of plagioclase (-hornblende) porphyry overlap both intrusives to the south and continue north to the east of the main stock, to form an overall "U" shaped complex. The southwestern limb of the "U" at Gaby Main has little breccia material, and consists mainly of plagioclase porphyries that have intruded the basalts of the Pallatanga volcanics as NW-striking dyke swarms, that dip to the NE (Srivastava et al., 2008).
In contrast Papa Grande is dominated by breccias, with lesser volumes of the hornblende-plagioclase porphyry, and almost no feldspar porphyry. The main breccia mass has a central core and a number of irregular arms directed east-west and NNE-SSW with subsidiary NW-SE elements, covering an area of ~1750 x 750 m, with several surrounding smaller breccias. The main breccia is mainly within the Pallatanga volcanics, at the contact with a NE-SW elongated >1 x 350 m 19.89±0.07 Ma (U-Pb zircon; Schütte et al., 2010) hornblende-plagioclase porphyry stock. Another parallel dyke like, 100 to 200 m wide hornblende-plagioclase porphyry is located a further 200 m to the NW. The contacts between the breccias and porphyries at Papa Grande are thought to be steeply dipping to the NE.
The main types of alteration that seem to bear most strongly on Au-Cu mineralisation (Srivastava et al., 2008; Schütte et al., 2012):
Potassic, which includes a pervasive and weak assemblage that affects most of the porphyries, as well as a more intense and localised quartz-biotite alteration in many of the breccias. Little gold or copper was introduced to the deposits during these phases, although high metal grades often are associated with potassic alteration since many of the fluid conduits that controlled this hydrothermal event would also have been shared by later alteration episodes.
Sodic-calcic, which is widespread and accompanied the introduction of most of the gold and copper that is now present in the two deposits, including the vast majority of the visible gold. The alteration assemblage includes veinlets, disseminations, replacement and hydrothal breccias with albite, actinolite, chlorite, epidote, titanite, magnetite and tourmaline. Intersections with good gold grades show a stockwork of sodic-calcic veinlets that are thought to be contemporaneous with the matrix of the breccias.
Phyllic, which post-dated brecciation and is not as widespread as the earlier potassic and sodic-calcic assemblages. Though locally restricted, this sericitic-muscovite phase is thought to have accompanied the introduction of most of the arsenic within the deposits, and also some of the late-stage gold and copper.
The highest gold and copper grades are generally found within the breccias, although the porphyries can also host moderate to high grade mineralisation, but the volcanics rarely do. However, although the breccias and porphyries are the most favourable host rocks for strong gold and copper mineralisation, they are not consistently well mineralised, with some being barren or only very weakly mineralised. Furthermore, strong mineralisation in one lithology often extends across boundaries into neighbouring units of a different rock type (Srivastava et al., 2008).
Both deposits have a well developed weathering profile over the upper few metres and with decreasing intensity as depth increases. The near-surface weathering has created a saprolite layer in which the texture of the original bedrock has largely been obliterated and where original bedrock minerals have been oxidised.
Hypogene mineralisation occurs as: i). disseminated sulphides in porphyry intrusives and in the matrix of breccias; ii). quartz veining; iii). gold occurring as an enrichment in the saprolite that overlies the intrusives and the breccia zones; and, iv). minor eluvial and placer gold.
Sulphides constitute up to 2 to 3% of the ore, consisting mostly of pyrite, pyrrhotite and chalcopyrite that is disseminated or in small veinlets. Minor amounts of magnetite and ilmenite are also noted in this main ore type.
At the Gaby Main deposit, gold-bearing mineralization and pervasive hydrothermal alteration are contained within the porphyry-style intrusive complex and flanking Cretaceous basic volcanic rocks as a "U" shaped zone measuring ~1800 by up to 300 m. over a vertical interval of >500 m (International Minerals website, 2012). At Papa Grande, both gold-bearing veins and disseminated gold occur. While the veins are formed along NNW trending faults, there is no obvious control on the distribution of disseminated gold (International Minerals website, 2012). Srivastava et al., (2008) describe the main gold mineralisation, which is associated with sodic-calcic alteration, as being particularly well developed in previously fractured porphyry intrusions and in hydrothermal breccias which penetrate the porphyry intrusions, accompanied by low-grade (<0.1%) Cu mineralisation as chalcopyrite.
Copper occurs only as chalcopyrite, while arsenic is only found as arsenopyrite, which is seen only in trace amounts in the main ore type, but is more abundant in the late-stage quartz veins that host the second style of mineralisation. Gold particles mostly occur as liberated, solid nuggets, as flakes and, less commonly, as elongated particles. Most of the gold grains observed are not readily visible in hand specimen, ranging from 40 to 200 µm in size, with an average of ~75 µm. Minor amounts of fine refractory gold, less than 5 µm are also evident, typically as inclusions in pyrite (Srivastava et al., 2008, after Hazen, 1995?).
Mineralisation associated with the different alteration events include: i). Potassic - chalcopyrite, mainly disseminated within the breccia matrix; ii). Sodic-calcic - veinlets, disseminations of silicate-pyrrhotite-pyrite-chalcopyrite±molybdenite, with associated free gold; iii). Phyllic - veinlets of quartz-pyrite±chalcopyrite±pyrrhotite±arsenopyrite±galena±sphalerite (Schütte et al., 2012).
Molybdenum mineralisation from the breccia matrix at Gaby Main has been dated at 20.54±0.08 Ma (RE-Os molybdenite; Schütte et al., 2010), and associated alteration at 20.17±0.16 Ma (U-Pb hydrothermal titanite; Schütte et al., 2010).
Published Canadian NI 43-101 compliant resource figures at July 2012 (International Minerals, 2012) at a 0.4 g/t Au cut-off, are as follows:
Measured + indicated resources - 356.4 Mt @ 0.6 g/t Au, 0.09% Cu; plus
Inferred resources - 143.2 Mt @ 0.6 g/t Au, 0.09% Cu.
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Schutte P, Chiaradia M, Barra F, Villagomez D and Beate B, 2012 - Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology: in Mineralium Deposita v.47 pp. 383-410|
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