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PACRIM 2015 Congress, Hong Kong, China, 18-21 March 2015
High Grade Porphyry Copper-Gold Mineralisation in North-west
Ecuador - The Alpala Cu-Au Porphyry Discovery
B.Rohrlach1, O.Poma2, B.Rosero3, J.Silva4, J.Ward5
1.
MAusIMM, General Manager Exploration, SolGold plc, Level 27, 111 Eagle St, Brisbane Qld 4000, Australia,
[email protected]
2.
Senior Geologist, Cornerstone Ecuador S.A., Av 12 de Octubre N24-528, Edificio World Trade Centre, Tower B, Oficina
206, Quito, Ecuador. [email protected]
3.
Senior Geologist, Cornerstone Ecuador S.A., Av 12 de Octubre N24-528, Edificio World Trade Centre, Tower B, Oficina
206, Quito, Ecuador. [email protected]
4.
Senior Geologist, Cornerstone Ecuador S.A., Av 12 de Octubre N24-528, Edificio World Trade Centre, Tower B, Oficina
206, Quito, Ecuador. [email protected]
5.
MAusIMM (CP), President, Exploraciones Novomining S.A., Av. 12 de Octubre N26-97 y Abraham Lincoln Edificio, Torre
1492, Piso 11, Oficina 1101, Quito, Ecuador. [email protected]
1
ABSTRACT
The Alpala copper-gold (Cu-Au) deposit is a recent, high-grade, porphyry Cu-Au discovery located in the
Imbabura Province of northwestern (NW) Ecuador. The deposit lies along the crustal-scale Chimbo-Toachi
Fault Zone within the Dagua-Piñon terrane, an oceanic plateau accreted to the South American convergent
margin in the Late Cretaceous. The deposit is hosted by quartz diorite stocks that intrude andesitic volcanic
breccias of the Oligocene to mid-Miocene San Juan de Lachas Formation (SJDL). Miocene age calc-alkaline
plutonism is associated with porphyry Cu, Cu-Mo and Cu-Au deposits in southern and northern Ecuador.
The Alpala deposit lies along a sector of the Miocene arc that has recently been under-plated by the
Carnegie Ridge seamount chain that induced shoaling of the subducted Nazca plate beneath northern
Ecuador.
Surface expressions of the deposit were recognised in May 2012 during follow-up of stream sediment
anomalies, when outcropping sheeted and stockwork porphyry-related veins were discovered in Quebrada
Alpala. The roots of a NW-trending, structurally-controlled lithocap were recognised over an area of 4.5 by
TM
0.9 km. The remnant lithocap exhibits well-defined mineral zonation mapped by TerraSpec analyses on
grid soil samples. Soil sampling, heliborne magnetic surveying and channel sampling defined initial drill
targets.
Seven diamond drill-holes were completed in the year from September 2013. Drill hole CSD-13-005
intersected 672m grading 0.93% Cu, 0.91 g/t Au in high-temperature inner propylitic and locally potassicaltered quartz diorite intrusions. Overlying hydrothermal clay alteration assemblages define the base of the
lithocap. A NW-SE trending footwall structure defines the southwest margin of the deposit. Drill hole 5
intersected high-grade sheeted ‘B-type’ and ‘C-type’ veins (Gustafson and Hunt, 1975) that lie along the
southwest margin of an 1100m by 500m magnetic anomaly that is the focus of ongoing exploration.
INTRODUCTION
The Alpala porphyry copper-gold deposit is a recent discovery made by SolGold Plc and Cornerstone Capital
Resources Inc. in NW Ecuador. The deposit lies within the Cascabel exploration concession, 48 km northnorth-west (NNW) of Ibarra in Imbabura Province.
The first company to explore the region was Santa Barbara Copper and Gold S.A. in 2008, when stream silt
surveys and prospecting identified widespread geochemical anomalism. Cornerstone Ecuador S.A.
conducted prospecting, regional mapping and a stream-sediment survey in June and July 2011 and
identified several multi-element (Cu-Au-Mo-Ag-Pb-Zn) anomalous regions. A central 4 by 5 km area was
highlighted around surface exposures of porphyry Cu-Au mineralisation at Quebrada Moran. On 24 July
2012 SolGold entered into an option agreement with Cornerstone to explore the Cascabel property.
Miocene age calc-alkaline plutonism is associated with porphyry Cu (+/- Mo/Au) deposits in southern and
northern Ecuador. In northern Ecuador, porphyry and epithermal deposits of predominantly Miocene age
occur within the Imbaoeste Mineral District, and include the 982 Mt @ 0.89% Cu Junin porphyry Cu-Mo
deposit (Gribble, 2004). The Alpala porphyry Cu-Au discovery extends the northern margin of this mineral
district.
REGIONAL GEOLOGY
The Alpala porphyry deposit lies near the northern end of a Miocene intrusive belt that trends north-south
(NS) through Ecuador (Figure 1). The basement sequence comprises tholeiitic MORB basalts of the DaguaPiñon (DAP) terrane (Cedial et al., 2003), an oceanic plateau that accreted to the South American
convergent margin during the Late Cretaceous (Cruz, 2007). Palaeocene to Eocene submarine volcanism
along the Macuchi Volcanic Arc in northern Ecuador erupted onto the DAP, and was followed by emergent
andesitic volcanism that formed the Oligocene to mid-Miocene San Juan de Lachas Formation, which are
the host-rocks for the Alpala deposit.
Magmatism evolved from volcanic and associated plutonic activity prior to the Mid-Miocene to dominantly
plutonic activity during the Late-Miocene. The Alpala deposit lies above the subducted eastern extension of
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the Carnegie Ridge, a seamount chain that extends from the Galapagos Islands to the Ecuador Trench. This
ridge of thickened oceanic crust began to subduct beneath northern Ecuador in the Late-Miocene at around
8 Ma (Gutscher et al., 1999). Mid- to Late-Miocene age plutons of hornblende diorite, quartz diorite and
tonalite intrude andesitic breccias of the San Juan de Lachas Formation (SJDL) and older Cretaceous and
Eocene units that lie along the east flank of the Macuchi Arc.
The Chimbo-Toachi Fault Zone is a north-north-east (NNE) trending crustal-scale structure that focussed
magmas into the upper crust during the Miocene, and transects the southern half of the Cascabel property.
LOCAL GEOLOGY
The geology of the property is dominated by argillic to propylitic altered andesitic volcaniclastic breccias of
the SJDL which are intruded by hornblende diorite and quartz diorite stocks. The intrusives outcrop
extensively north of the Alpala deposit, and form discrete, high-level apophyses at the southern end of the
property at Alpala (Figure 2). Younger, probable Quaternary age perched conglomerates cover the SJDL
along the south bank of Rio Mira north of Alpala.
FIG 1 – Location of the Alpala porphyry deposit within the Miocene belt in north-western Ecuador.
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Stratigraphy
Tortuga Unit
The oldest sequence on the property comprises sedimentary rocks of inferred Eocene age (Tortuga Unit).
These rocks outcrop south-west (SW) of the Alpala deposit where they comprise interbedded shale and
greywacke (Graves, 2012). Equivalent units are observed elsewhere on the property and are locally
interdigitated with the lower parts of the SJDL.
San Juan de Lachas Formation
The dominant host rocks to the Alpala porphyry system are andesitic to locally basaltic andesitic
volcaniclastic breccias that form a thick, poorly differentiated sequence. The breccias vary from coarse lithic
tuffs to finer lapilli and crystal tuff that dip gently northeast. The breccias are strongly fractured in proximity to
intrusives and are propylitic-altered and commonly epidote-bearing, with traces of malachite on fracture
surfaces.
Intrusions
Several phases of intrusions are identified on the property. The most prominent intrusive unit is a diorite
porphyry stock (5 by 2 km) that outcrops north of Alpala (Figure 2). This intrusion plunges southward
beneath andesitic volcaniclastic cover. Smaller apophyses of this diorite extend to surface east and west of
the Alpala porphyry. A dacite porphyry intrusion is mapped at central Alpala, and is tentatively correlated with
a tonalite porphyry identified in drill core petrographic samples (Coote, 2014). Quartz diorite and microdiorite
are the dominant intrusive type recognised in drill core at Alpala. All three intrusions exhibit porphyry style
alteration and stockwork veining. A late-mineral hornblende-rich diorite that trends NNW lies between the
Alpala porphyry system and the Tandayama-America porphyry stockworks, located towards the north.
FIG 2 – (A) Lithology and alteration of the Cascabel concession. (B) Alteration zonation within the Alpala
TM
lithocap defined by TerraSpec mapping.
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ALPALA PORPHYRY DEPOSIT
Discovery Tools
The discovery outcrop identified in May 2012 in the south-draining Alpala Creek during reconnaissance
mapping comprised intensely sheeted B-veins and surrounding quartz stockwork with high Cu and Au
grades (Table 1). It was one of three outcrop areas on the property where porphyry stockworks were
identified.
2
Three regional datasets were collected in the second half of 2012. A 20 km grid soil survey yielded
widespread geochemical anomalies on the property, identifying an extensive geochemical ‘footprint’ and a
coincident Cu-Au-Mo soil anomaly in Alpala creek, within a regional-scale Mo anomaly (>1.4 ppm over 1.5 x
2.2 km). Heli-magnetic surveying over the property by GPR International Inc. (October 2012) identified a 3.0
x 2.2 km, structurally-dissected magnetic high/low complex coincident with the largest of three regional soil
TM
2
Mo anomalies. TerraSpec analysis of grid soil samples over 20 km was conducted to map alteration
mineralogy within the lithocap. This work defined a neutral-to-acid alteration assemblage zonation over an
area of 1.0 x 2.5 km, centred over the discovery outcrops. Highly acidic parts of the lithocap lie NW and SE
of the stockworks exposed in the Alpala stream (Figure 2).
TABLE 1
Significant exploration drill intersections at Alpala.
Hole ID
Depth From (m)
Depth To (m)
Interval (m)
Cu_%
Au_g/t
Discovery Outcrop
Channel 46
Channel 56A
Channel 56B
Trench 56
CSD-13-001
Includes
CSD-13-002
0
0
0
0
0
16
226
6
126
184
4
160
24
24
436
658
778
1052
1096
580
808
1136
654
1056
1160
1200
396
862
904
0
0
0
0
0
318
284
24
418
226
751.3
318.31
1330
420
658
1330
1330
1310
1146
1401.5
1006
1182
1612
1294
1294
1294
1310.45
1310.45
1186
4.00
45.64
56.93
34.35
45.50
302m
58m
18m
292m
42m
747.33m
158.31m
1306m
396m
222m
672m
552 m
258 m
50m
821.5m
198m
46m
958
238
134
94
914.45
448.45
282
0.99
0.59
0.34
0.46
0.25
0.39
0.96
0.33
0.37
0.50
0.11
0.11
0.62
0.32
0.26
0.93
1.03
1.27
1.80
0.14
0.20
0.27
0.40
0.65
0.75
0.84
0.41
0.56
0.60
3.30
0.81
1.16
0.19
0.46
0.48
1.67
0.41
0.30
0.68
0.05
0.05
0.54
0.17
0.11
0.91
1.05
1.40
2.26
0.10
0.15
0.35
0.17
0.35
0.50
0.62
0.44
0.64
0.76
Includes
CSD-13-003
CSD-13-004
CSD-13-005
Includes
Includes
Includes
Includes
Includes
Includes
CSD-14-006
Includes
Includes
CSD-14-007
Includes
Includes
Includes
CSD-14-008
Includes
Includes
5
In the first half of 2013 extensive channel and trench sampling was conducted over an area 430m NS by
200m EW to map surface grade distribution, and identify the extent of stockwork veins at Alpala (Table 1).
Channel sampling was also conducted over quartz stockworks at the Tandayama-America and Quebrada
Moran prospects.
At higher elevations north of Alpala, a “shell” of oxidised pyrite stockworks (D-veins) occurs within illite- and
kaolinite-altered volcaniclastic rocks, and these pass downward into erosionally exposed windows of
mineralised B-vein quartz stockworks in Alpala creek.
Diamond drilling commenced on 1 September 2013, with the first hole CSD-13-001 intersecting 302m
grading 0.39% Cu and 0.48 g/t Au from 16m depth below mineralised Channel 46 (Table 1). Hole CSD-13003 intersected extensive runs of typical halo grade (Table 1) before hole CD-13-005 encountered highgrade Cu and Au mineralisation. High grade Cu-Au mineralisation in hole CSD-13-005 occurs in a contact
zone between potassic and high-temperature inner propyltic alteration, and is overprinted at shallower levels
by intense hydrothermal clay alteration.
Mineralisation and Alteration
Porphyry Cu-Au mineralisation encountered by drilling at Alpala occurs along the SW margin of an 1100m by
500m magnetic complex that has an upper surface at around 800m depth and which extends to below
1800m depth (Figure 3). Drilling has revealed that the magnetic signature is caused by primary magnetite in
intrusions and by substantial hydrothermal magnetite as veins and disseminations in association with
chalcopyrite and lesser bornite in sheeted and stockwork veins. Drilling has intersected high-grade porphyry
Cu-Au mineralisation along the transition zone between an inner potassic assemblage and a marginal,
epidote- and actinolite-bearing and magnetite-rich high temperature propylitic zone. Minor relic hydrothermal
biotite is identifiable in thin-section where it is pervasively overprinted by chlorite at deeper levels and by
retrograde phyllic and intermediate argillic assemblages at shallower levels. Hydrothermal and primary
magnetite is variably to pervasively oxidised to hematite within the overlying feldspar-destructive clay
alteration assemblages. Chalcocite is present near surface within the lithocap. The absence of enargite likely
reflects the deep erosional level within the lithocap.
FIG 3 – Cu and Au intersections at the Alpala porphyry system.
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Whilst 11 morphological generations of veins have been identified, most of the Cu and Au mineralisation
intersected to date is associated with B-veins and C-veins (nomenclature of Gustafson and Hunt, 1975), with
chalcopyrite-rich C-veins commonly re-opening existing B-veins. Some Cu is introduced in transitional C-D
veins (intergrown chalcopyrite-pyrite with bleached selvedges). Minor bornite is present in the high-grade
intersections in drill holes 5 and 7, however bornite is anticipated 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 dominant over Au. The mineralisation at Alpala represents a near gold-rich end-member to the
porphyry Cu-Au systems in South America.
CONCLUSIONS
Alpala is a recent high grade porphyry Cu-Au discovery in a poorly explored section of the Miocene belt in
northwestern Ecuador. Porphyry Cu prospects and intrusions of Miocene age have long been recognised in
southern Colombia (e.g. central sub-belt of Sillitoe et al., 1982) and to the south at Junin, however the region
is poorly explored. The country risk in Ecuador and Colombia has improved markedly in recent years, giving
foreign companies the confidence to invest exploration funds.
Alpala is a Au-rich porphyry Cu-Au system of probable Late Miocene age. Intrusive bodies are controlled by
the second-order NNW, NW and WNW fault architecture in the vicinity of the Toachi Fault Zone. Although
drill exploration is at an early stage, Alpala may have geometric similarities to the Wafi-Golpu porphyry
system in Papua New Guinea and the La Colosa Au-porphyry system in Colombia (S Garwin; August 2014,
personal communication).
The principal factors leading to the discovery of extensive high-grade porphyry Cu-Au mineralization in drill
hole CSD-13-005 at Alpala are:






Comprehensive early-stage prospecting, rock chip sampling and reconnaissance mapping.
Judicious ground selection - understanding the tectonic environment prior to commercial entry.
Definition - and size recognition - of the geochemical footprint early in the exploration program.
Acquisition of multiple, spatially comprehensive datasets (soils, magnetics and spectral data).
Application of spectral mapping to define alteration mineral vectors within the remnant lithocap.
Early focus of exploration on the key Alpala region, highlighted by multiple datasets that fit the geologic
model for a world-class porphyry system: Mapped vein stockworks, magnetic signature, spectral
signature and geochemical signature and scale.
ACKNOWLEDGEMENTS
SolGold Plc and Cornerstone Capital Resources Inc. are gratefully acknowledged for permission to publish
this paper. The ideas summarised in this paper are the culmination of several years of exploration activity
and contributions by Cornerstone and SolGold geologists. Special mention is given to many past and
present technical contributors to the project: Y.Crepeau, M.Norris, A.Martin, N.Mather, C.Moore, D.Lulofs,
S.Mantilla, F.Ordonez, D.Hinojosa, A.Roberston, L.Munoz, L.Burns, G.Graves, R.Harrison, A.Coote, S.Vaca,
B.Whistler and S.Garwin.
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Circum-Gulf of Mexico and the Caribbean: Hydrocarbon habitats, basin formation, and plate tectonics
(ed: C Bartolini, R T Buffler and J Blickwede), pp 815-848 (AAPG Memoir 79).
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Porphyry Copper-Gold Deposit, Cascabel Exploration Area Ecuador, Applied Petrologic Services &
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