Sociedad Geológica de Chile: July 2011

Zhaoshan Chang, Noel White, Jeffrey Hedenquist, David Cooke, Ana Liza Cuison, Joey Garcia, Jr., Cari Deyell

FSE porphyry NW end of lithocap

Lithocap

!  Horizontal to sub-horizontal body

!  Residual silicic core (± ore), halo of advance argillic (AA) alteration

"  Structure-controlled feeder zone

"  Lithology-controlled lithocap horizon

Mineralization

Structure- controlled

Lithocap

FSE porphyry NW end of lithocap

http://www.ngdc.noaa.go

v/mgg/image/2minrelief.html SE end of lithocap

Cretaceous-Paleogene Lepanto metavolcanics

Late Oligocene to mid-Miocene Apaoan volcaniclastics

12-13 Ma Bagon intrusive complex

Slightly younger Balili volcaniclastics

1 km Qtz diorite porphyry

Pliocene volcanism

3.3 - 1.8 Ma, multiple eruptions

1 km

Imbanguila dacite porphyry (3.3-1.8 Ma)

Imbanguila pyroclastics (3.3-1.8 Ma)

Cretaceous-Paleogene Lepanto metavolcanics

Late Oligocene to mid-Miocene Apaoan volcaniclastics

12-13 Ma Bagon intrusive complex

Slightly younger Balili volcaniclastics

Qtz diorite porphyry

Young cover:

<1.2 Ma

Lapangan Tuff (0.19 Ma)

1 km

Imbanguila dacite porphyry (3.3-1.8 Ma)

Imbanguila pyroclastics (3.3-1.8 Ma)

Cretaceous-Paleogene Lepanto metavolcanics

Late Oligocene to mid-Miocene Apaoan volcaniclastics

12-13 Ma Bagon intrusive complex

Slightly younger Balili volcaniclastics

Qtz diorite porphyry

Bato dacite porphyry (1.2 Ma)

Bato pyroclastics (1.2 Ma)

Spanish workings: quartz-luzonite-Au

1 km

Artisanal workings,

Nayak qtz-Au veins

NW end of lithocap: qtz-alunite cliffs at unconformity, with kaolinite halo

1 km 1 km

X Small workings

Breccias < 50 ppb Au

Lepanto HS:> 0.9 Mt Cu & 102 t Au

FSE porphyry: 650 Mt @ 0.65% Cu & 1.2 g/t Au

Guinaoang porphyry, 500 Mt @ 0.4% Cu & 0.4 g/t Au

Victoria veins, 11 Mt @ 7.3 g/t Au +

Ag-Cu-Pb-Zn

Mohong Hill porphyry + HS

1 km

Nayak veins

Teresa veins, 0.8 Mt @ 5.74 g/t Au

Buaki porphyry

Lithocap geometry

and base of the

Imbanguila units

Contour of the Imbanguila base from Garcia (1991)

Mankayan district, Philippines

Mohong Hill quartz-alunite

lithocap

Lepanto fault

Looking NNW

Dickite ± kaolinite

Dickite ± kaolinite

Quartz-alunite

Hedenquist et al., 1998; Chang et al., 2011

Most ore (~70%) in root zone of lithocap, in Lepanto fault or its splay branches

Lepanto fault

Hedenquist et al., 1998; Chang et al., 2011

1.42 Ma

1.41 Ma

1.35 Ma

Hedenquist et al., 1998

FSE – Lepanto alteration: Coupled potassic and quartz-alunite (lithocap), later phyllic overprint

  Early alteration: AA: Quartz – alunite – pyrite ±

pyrophyllite ± diaspore ± dickite ± kaolinite lithocap

Locally silicic: Vuggy quartz – pyrite

  Later mineralization + silicification ! Pyrite ! Pyrite, enargite, luzonite ! Tennantite, chalcopyrite, sphalerite,

galena, tellurides, selenides, native Au

Gonzalez, 1959; Tejada, 1989; Claveria, 1997, 1998, 2001

Early convecting magma, rapid crystallization =

High rate of fluid advection, fluid rises rapidly with little cooling, and intersects its solvus

High-temperature, ductile conditions at shallow depth

Brine forms deep potassic zone, vapor separates from brine and discharges through ductile zone: part of vapor condenses near the surface to acidic liquid, creating lithocap

Shinohara & Hedenquist, 1997

Early convecting magma (30 - 50 % crystals), flux ~ equal to White Island quiescent eruption

Late stagnant magma (>50% crystals, advective flux sharply decreases ~ 10x

Effect on fluid exsolution, advection, and nature of fluid ascent?

Later stagnant magma, slow crystallization =

Low rate of fluid advection, fluid rises slowly and loses heat, such that it does not intersect solvus

Lower temperature, brittle conditions at shallow depth

Creation of the phyllic (muscovite, “sericite”) stage

Shinohara & Hedenquist, 1997

Early magmatic fluid = Hot, plastic, lithostatic P (potassic, A veins) Rapid ascent, intersection of solvus, forms brine plus vapor

Later magmatic fluid = Cooler, brittle, hydrostatic P (muscovite, D veins) Critical fluid never intersects solvus due to slow ascent and cooling

Normal progression of exsolving magma chambers, magma convection early (fast crystallization), to later stagnant xstallization (conductive heat loss)

Shinohara and Hedenquist, 1997; Hedenquist et al., 1998

Chang et al., 2011

Chang et al., 2011

Higher Na/(Na+K) ratio indicates higher formation temperature (Stoffregen and Cygan, 1990)

- Related to Na content Chang et al., 2011

Milagros: Garcia, 2009

Whole rock Cu<0.1%, Au<0.1ppm samples with alunite

La/Pb increases; Hg, Ag, Ag/Au, Te, As/Zn decrease

Chang et al., 2011

At Far Southeast, magnetic low over porphyry: phyllic destruction of magnetite-bearing potassic alteration

Decrease: Alunite Pb, Ag/Au Whole-rock (alunite-bearing only): Pb, Ag, Ag/Au, Hg

Lepanto FSE porphyry

Victoria veins Lepanto

FSE 500 m

Hedenquist et al., 2001

Surface projections of Victoria-Teresa IS vein & Lepanto HS enargite, over Far Southeast porphyry

Teresa veins

X Biot 1.18 Ma X Illite

1.4-1.15 Ma

X Horn 1.45 Ma

Hydro Biot 1.40-1.45 Ma

Alunite 1.40-1.45 Ma

Illite ~1.35 Ma

Bt 2.2-1.8 Ma

Enargite-Au Porphyry Cu

IS Au-Ag veins

Weak smectite + pyrite, illite-smectite + pyrite at lower elevation

DNK: dickite, nacrite, kaolinite, or any combinations

DNK: dickite, nacrite, kaolinite, or any combinations iX: illite crystallinity

Hedenquist et al., 1998; Hedenquist and Taran, 2011

Upward flare of leaching due to cooling; hydraulic gradient caused offset of lithocap (and ore) from causative intrusion

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Ag $!Au $!

LS (alkalic)

Low sulfidation Intermediate sulfidation

Bill

ion

$ (U

S)

Au $700 US/ozAg $10 US/oz

N=6

N=12

~5 Moz Au eq.

N=4

Modified from values compiled by B. Gemmell, in prep.

M

M

M

M

M: Mexico

M D

D

D: diatreme related

D

D

D

Pb-Zn or Cu

100 Ag

100 Au

IS IS

LS

10-20%

IS 3-7% <5%

Au-Ag (LS) Au-Te (alkalic) Ag-(Au)-Pb-Zn (IS) Au-Cu (HS)

Geologic setting of intermediate- (and low-)sulfidation vein deposits (Americas, SW Pacific)

  LS: Rhyolite-basalt association in extensional settings:   Intra, near, and backarc; postcollision rifts (non porphyry)

migration of volcanism

S Kyushu, looking south

  Tectonics determine type of epithermal Au deposits (early HS, IS, later LS)

Kushikino: Mt Kamuridake to east capped by silicic zones, adv. argillic halos

Kushikino IS veins 2 km west

Kushikino IS veins: qtz-calcite-Au

OR, perched, barren lithocap, marginal intermediate sulf’n veins

Muscovite halo, Ag-Au w/ spl, ccp, tn

barren

2 km

2M mica

Hudson (2003)

1 km

Dietrich et al., 2007

14.7-14.1 Ma

13.7 Ma

13.5 Ma

COLQUIJIRCA MINE

Modified from Petersen & Vidal (1996)

14.5-14.4 Ma

15.4-14.5 Ma

Colquijirca mine, Cerro de Pasco district, Merced pit (to north)

Cerro Marcapunta

Fontbote & Bendezu (1999)

Enargite-py, cc-cv-dg

S N

Dacite flow dome

Bendezu et al., 2008

Marcapunta Norte

Bendezu et al., 2008

Colquijirca

Dome - diatreme

N

Marcapunta Norte

Marcapunta Oeste

Cerro Marcapunta: dacite

Smelter: pyrite + enargite

Ref: Colquijirca mine staff (Fontbote & Bendezu, 2000) Bendezu et al., 2008

ARUNTANI

Andean arc, Peru: 75 Moz in HS deposits

10 S

64 N

Alto Chicama, Peru

2001, discovery 2 km from road in mining district

2003, 10.5 Moz resource

Dante Loayza and Jorge Barreda (Aruntani SAC), Alvaro Crósta, Wolfgang Morche, and Jeffrey Hedenquist

Canahuire resource model & pit shell: Chucapaca JV

1.3 km

350 m Mineralisation

Open 700W

300W

100E

W E

Looking North g/t Au

Erosion level?

Muscovite up to pyrophyllite