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The Alpala porphyry copper-gold deposit, part of a cluster of mineralised systems in the Cascabel district, is located ~110 km north of Quito, and 20 km south of the Colombian border in far northern Ecuador, ~60 km to the NE of the Junín deposit (#Location: 0° 45' 13"N, 78°: 19' 54"W).
Tectonic and Regional Setting
The bulk of Ecuador's porphyry Cu±Mo±Au±Ag and porphyry-related epithermal Au±Ag±Cu deposits are of Jurassic and Tertiary (mostly Miocene) age, that define two distinct and parallel, temporal metallogenic belts to the east and west respectively (PRODEMINCA 2000; Sillitoe and Perelló 2005; Chiaradia et al., 2009).
Alpala is one of a number of porphyry and epithermal deposits that form two main geographically separated mineral districts within the broader Tertiary metallogenic belt that follows the western Andean range or Cordillera Occidental.
The southern of these, the Azuay-El Oro District, is defined by a series of Miocene porphyry Cu-Mo, Cu-Au and high, low and intermediate sulphidation epithermal copper-gold deposits and prospects ~200 km SSE of Guayaquil in southern Ecuador.
The Imbaoeste District, ~440 km to the north, includes the Eocene Alpala porphyry Cu-Au and Miocene Junín porphyry Cu-Mo deposits. These deposits are respectively associated with the Oligocene to early Miocene Saraguro Group and overlying similar Late Miocene to Recent arc sequences which are both developed and overlap throughout the belt, with the older succession predominating to the west.
Other deposits and occurrences are distributed along the Tertiary porphyry belt between these two districts, and to the north into Colombia and SE into Peru.
For more 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.
The Alpala deposit lies along the NNW trending crustal scale Chimbo-Toachi Fault Zone and is hosted by quartz diorite stocks. It that intrudes a sequence that is underlain by basement tholeiitic MORB basalts of the Dagua-Piñón Terrane, which were progressively overlain by Palaeocene to Eocene submarine volcanosedimentary rocks along the Macuchi Volcanic Arc in northern Ecuador, followed sedimentary rocks of the Eocene Tortuga Formation and then by emergent andesitic volcanic breccias that formed the Oligocene to mid-Miocene San Juan de Lachas Formation.
The Tortuga Formation is the oldest exposed sequence in the deposit area and comprises sedimentary rocks of inferred Eocene age that outcrop SW of the Alpala deposit where they comprise interbedded shale and greywacke (Graves, 2012). Equivalent units are observed elsewhere in the district, and locally interdigitate with the lower parts of the San Juan de Lachas Formation.
In the deposit area, the San Juan de Lachas Formation comprises argillic to propylitic altered andesitic to locally basaltic andesitic volcaniclastic breccias, forming a thick, poorly differentiated sequence. The breccias vary from coarse lithic tuffs to finer lapilli and crystal tuffs that dip gently NE. The breccias are strongly fractured in proximity to intrusives.
Several intrusive phases are evident in the deposit area, the most prominent of which is a 5 x 2 km diorite porphyry stock that outcrops extensively north of the deposit, but forms discrete, high-level apophyses penetrating andesitic volcaniclastic rocks in the south, straddling Alpala. A dacite porphyry intrusion is mapped at central Alpala, possibly a phase of a tonalite porphyry encountered at depth (Coote, 2014), whilst quartz-diorite and microdiorite are the dominant intrusive types, all of which exhibit porphyry style alteration and stockwork veining. An ~5 x up to 400 m wide, late-mineral, NNW trending hornblende-rich diorite occurs to the north of the main Alpala porphyry system. Younger, probable Quaternary age perched conglomerates cover the host rocks north of Alpala.
Mineralisation and Alteration
Porphyry Cu-Au mineralisation at Alpala occurs along the SW margin of a deep (top at ~800 m) 1100 x 500 m magnetic core caused by primary magnetite in intrusions and a substantial development of hydrothermal magnetite as veins and disseminations in association with chalcopyrite and lesser bornite in sheeted and stockwork veins. Above this core, high-grade porphyry Cu-Au mineralisation occurs along the transition zone between an inner potassic assemblage and a marginal, epidote- and actinolite-bearing and magnetite-rich high temperature propylitic zone. Minor relict hydrothermal biotite is recognised in thin-section where it is pervasively overprinted by chlorite at deeper levels and by retrograde phyllic and intermediate argillic assemblages at shallower levels.
At surface there is a 2.5 x 1 km acid lithocap, composed of illite (phengite) alteration surrounding two main cores of kaolinitic argillic alteration, the northern of which encloses a zone of pyrophyllite, while the southern surrounds smaller zones of pyrophyllite and dickite. This lithocap is enclosed within a more extensive and diffuse zone of muscovite, with patchy internal and marginal Fe-Mg chlorite and an irregularly developed outer rim of Mg-chlorite. A structurally controlled, 4.5 x 9 km, NNW trending argillic zone surrounds and overprints the main illite-kaolinite trend and the more extensive and broader north-south trending muscovite and chlorite zones.
Within the ore deposit and its halo, hydrothermal and primary magnetite is variably to pervasively oxidised to hematite within shallower feldspar-destructive clay alteration assemblages, whilst chalcocite is encountered near surface within the lithocap. The absence of enargite is interpreted to reflect erosion of the lithocap.
Up to 11 morphological generations of veins have been identified, although most of the recognised Cu and Au mineralisation intersected to 2015 is associated with B-veins and C-veins, with chalcopyrite-rich C-veins commonly re-opening existing B-veins. Some Cu occurs in transitional C-D veins (intergrown chalcopyrite-pyrite with bleached selvedges). Minor bornite is found in high-grade intersections and appears to increase at depth and northeast of the NW-trending footwall structure.
In high-grade drill-core, the Cu (%) to Au (g/t) ratio is close to 1:1, whilst at lower grades there is a tendency for Cu to be dominate over Au.
No reserve estimated have been released. Better intersections include: 1306 m @ 0.62% Cu, 0.54 g/t Au from 24 m depth, including 672 m @ 0.93% Cu, 0.91 g/t Au from 658 to1330 m; and 50 m @ 1.80% Cu, 2.26 g/t Au from 1096 to 1146 m.
This preliminary summary is derived from Rohrlach et al., 2015.
The most recent source geological information used to prepare this summary was dated: 2015.
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:
Rohrlach, B., Poma, O., Rosero, B., Silva, J. and Ward, J., 2015 - High Grade Porphyry Copper-Gold Mineralisation in North-west Ecuador - The Alpala Cu-Au Porphyry Discovery: in Proceeding, PACRIM 2015 Congress, Hong Kong, China, 18-21 March 2015 The AusIMM, Melbourne, pp. 369-376|
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