The Portovelo mining district: relation of mineralisation to

porphyry and epithermal environments

Richard Spencer, July Montenegro, Alina Gaibor, Patricio Perez,

Glenda Mantilla, Franklin Viera, Camille Spencer & Jorge Barreno

History

Mercadillo: 1549SADCO: 1905-1950 3.6Moz Au

12Moz AgCIMA 1950-1965 0.38Moz AuBIRA & others: To present 1MozTOTAL 5Moz Au

VeinsVein Width (m) Grade Tonnage Produced

Portovelo Mine

Abundancia 1.75 15.2 1,573,360

769,706

Jorupe 1.42 10.44 929,810 312,138

Portovelo 1.98 18.8 385,926 232,880

Cantabria 1.56 11.6 397,099 148,077

Tamayo 1.07 11.4 364,396 133,084

Soroche 1.53 13.1 212,264 89,598

Agua Dulce 1.36 17.4 148,993 83,451

Guillermo 1.62 24.0 97,216 75,115

Tablon 1.16 10.2 222,348 72,706

Nudo West 2.14 11.8 168,645 64,232

Tres Reyes 1.09 13.4 134,230 57,637

Veinteseis 1.70 24.9 61,304 49,062

Fortuna 1.33 13.8 79,510 35,238

Barbasco 1.50 11.83 88,547 33,689

Portovelo W 1.54 8.0 77,187 19,925

Quebrada 0.77 28.7 8827 8,145

Matalanga 0.73 19.9 11,792 7,546

Second phase production, Portovelo Mine 2,192,232

R-Nivel Mine

Cristina 0.91 18.2 422,093 246,768

Jane Sur 1.55 7.5 252,193 60,862

Jane Norte 1.36 4.9 71,914 11,387

Sub-total R-Nivel 319,018

Total second phase mining 2,511,250

Format

1. Tectonic setting

2. Alteration• Zoning• Tilting

3. Mineralisation

4. Future exploration• Hard rock• Footwall of thrusts• Ore shoot orientation

Format

1. Tectonic setting

2. Alteration• Zoning• Tilting

3. Mineralisation

4. Future exploration• Hard rock• Footwall of thrusts• Ore shoot orientation

Portovelo: location

Portovelo: structural setting – Oligocene extension

Oligocenegrowth sequences

Lopolithic intrusivesat onset of Miocene inversion?

Piñas normal fault

Whole rock data:intrusives

Alkali index:AI = (Na2O + K2O) (SiO2-43) x 0.17

0,00

0,50

1,00

1,50

2,00

2,50

3,00

0 5 10 15 20

Al2O3

Tholeiitic

Calc-alkaline

Al2O3

AI

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

SiO2

K2

O+

Na

O

Whole rock data:intrusives

Basaltic andesite

Andesite-diorite

SiO2

K2O

+N

aO Gabbro-basalt

“Melanocratic diorite”

Oligocene

Saraguro Group

Piñas fault

Jubones F

Miocene inversion of Piñas basin

Piñas fault

Late Oligocene – Miocene inversion

Jubones F

Piñas F

Portovelo: location

Portovelo vein system & intrusives

Intrusives:• RL sigmoids• Change in stress field? - dextral - compressional• “Tholeiitic” composition

Format

1. Tectonic setting

2. Alteration• Zoning• Tilting

3. Mineralisation

4. Future exploration• Hard rock• Footwall of thrusts• Ore shoot orientation

District-scalealterationzoning

Coo

ling

WESTWARD TILT

Cooling

Curipamba, vein: Sadco data, Au gradeContact of illite – illite-smectite inclined to SQuartz content increases with depth

90% calcite 50% calcite50% quartz

• 40% calcite• 60% quartz• Illite

SOUTHWARD TILT

Portovelo vein system & intrusives

Post-mineraltilt to the SW

Format

1. Tectonic setting

2. Alteration• Zoning• Tilting

3. Mineralisation

4. Future exploration• Hard rock• Footwall of thrusts• Ore shoot orientation

Base metal distribution

Coo

ling

Au distribution

Long section of the Sesmo vein Au & base metal distribution

Abundancia vein (Sadco data)

Grade Thickness

Tamayo vein: host-rock control

Intermediate sulphidation epithermal

• High base metal content• High NaCl in fluid inclusions (<18 wt %)• Near-neutral pH hydrothermal fluids

• Carbonate-bearing veins• Minor adularia• Fluorite• Au over large vertical interval (800m)

• Association with porphyry• Association with tholeiitic – calc-alkaline

transitional rocks (basaltic andesite)

1. Tectonic setting

2. Alteration• Zoning• Tilting

3. Mineralisation

4. Future exploration• Hard rock• Footwall of

thrusts• Ore shoots• New veins

Systematics of ore shoots: Abundancia vein

Brittle host rock

En echelon ore shoots hanging under thrusts

Extensions to vein system

New veins

West 1 Vein

West 2 Vein

Conclusions

• Continuum from porphyry to epithermal environments

• Intermediate sulphidation epithermal

• Controls on mineralisation:• Brittle host rock• Proximity to footwall of thrusts• Highest gold content in:

• illite-stable part of alteration system

• In sphalerite-dominated part of system

Exploration

• Focus on SW part of vein system

• Exploring existing veins: Go South beneath illite-smectite zone

• Exploring for new veins:Go west – extension to left-stepping en echelon system

• Controls on mineralisation – detail• Thrusts• Hard rock• Alteration

Explorationfocus

Area for explorationfocus

Chemical zoning in Viscaya vein