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Roots of Porphyry Copper Systems - Possible Signs?

by

JUN ANGELES June 15, 2012

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Parts of the Porphyry Copper System

• Tops – advanced argillic lithocap

• Middle part – porphyritic intrusive(s) – quartz-sulphide stockworks – potassic or phyllic

• Fringes – propylitic

• Root parts – what is that?

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Why Understand Roots?

• Lots of funds expended on wrong concepts • Geologists always think of tops, fringes and middle parts of poprhyry systems • Could not deal with root parts - none in the literature, or - it kills the goose that lays the golden egg

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Definition

• Porphyry Cu deposit is a copper orebody Associated with a porphyritic intrusive

Mineralization is contained in Cu sulphides in quartz stockworks, fracture fills and disseminations

Commonly contains either Mo or Au as by products and in some cases, both metals

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Tonnage & Grade

• Porphyry Cu systems – contribute ¾ of world’s Cu, half of Mo, 1/5 of Au & most of Rhenium

• Porphyry Cu orebodies contain <10 million to 10 billion tonnes @ 0.3 to 1.5% Cu @ nil to 1.5 g/t Au @ <1 – 2 g/t Ag @ nil to 0.04% Mo

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Distribution of Major Porphyry Cu Deposits

Sillitoe (2010)

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Philippine Porphyry Cu

Deposits/ Prospects

Modified after

Sillitoe & Gappe (1984)

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District Scale Setting

• Closely associated to underlying composite parental pluton at paleodepths of 5-15 kms Supply chambers for the magmas & hydrothermal fluids

Sillitoe (2010)

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Deposit Scale Characteristics

• Centered on I-type porphyry intrusions of calc-alkaline affinity, sometimes shoshonitic to rarely alkaline vertical pluglike stocks, circular to elongate in plan dike arrays to small irregular bodies Stocks and dikes have diameters/lengths of <1km up to 14km, eg. Chiquicamata

• Vertical extents up to >2 kms

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Deposit Scale Characteristics

• Generally in multiple phases, eg. 9 in Grasberg • Ore proportion in intrusive host rocks is ~ >50%

• Breccias are common

Typical Porphyry Alteration Types

PHASE ALTERATION TYPE ALTN SILICATES SULPHIDES % Sx Py: Cu Sx

Retograde

Advanced Argillic Quartz, Alunite, Pyrophyllite

Pyrite, Energite, Covellite 3 - ≥ 10 ≥5 : 1

Sericitic (Phyllic) Quartz, Sericite Pyrite, Chalcopyrite 3 -5 3 : 1

Sericite-Chlorite Sericite, Chlorite, Quartz

Pyrite, Chalcopyrite, ± Bornite 1 - 3 2 : 1

Prograde Potassic (K Silicate)

Biotite, Kfeldspar, Quartz

Pyrite, Chalcopyrite, Bornite 1 - 3 1 : 1

Propylitic Chlorite, Epidote, Carbonates

Pyrite, ± Sphalerite, ± Galena 1 - 3 N.A.

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Sulphide Occurrence in Copper Zone

Corbett & Leach (1998)

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Empirical Telescoped

Model

Sillitoe (2010)

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Empirical Telescoped Model – Alteration Zoning

Sillitoe (2010)

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Mankayan District

(after Hedenquist et al., 2001)

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Possible Signs of Roots of Porphyry

Copper Systems

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Geological Setting

• Prospect is well within a batholith - not near or in contact with the batholith - presence of aplites and/or pegmatites

• The causative intrusive is equigranular instead of porphyritic - the coarser the grain size, the nearer it is to the parental pluton

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Alteration Characteristics

• Sodic-calcic type – albite/oligoclase, actinolite & accessory diopside, epidote & garnet (Sillitoe, 2010) • Cu mineralization is in propylitic to chloritic rocks, usually intrusives but also in volcanics • Epidote becomes ubiquitous in the Cu-bearing potassic and sericite-chlorite alteration zones

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Copper Sulphide Occurrence

• Absence of quartz-sulphide stockworks - i.e. <2 veins/meter

• Sheeted quartz-sulphide veining is present but sporadic - <1-3m zones within generally disseminated mineralization

• Fracture fills and disseminations is the main mode of Cu sulphide occurrence

• If Potassic alteration is present, it has a high pyrite: Cu sulphide ratio, i.e. ≥3 : 1

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Soil Geochemical Signs

• Coincident Cu & Au or Mo is a good sign of a porphyry system and fringing Zn-Pb, even at the roots - with very minor Zn-Pb sulphides mixing sporadically

• But not widespread Cu-Au (or Mo) coincident w/ Zn, Pb, ± As

• The above may actually be - - Intermediate sulphidation epithermal, - Sub-epithermal base metals-Au-Ag, or - Base metal skarns in volcanics

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Discussion

• Companies loves to speculate on porphyry Cu potential in their portfolio • Whenever Cu mineralization is weak, first reaction – - prospect is at the top or fringe of the porphyry system

• If there is no lithocap & quartz stockworks does not exist - - the prospect would most likely be at the roots of the system • Root parts of the porphyry is never published or researched in detail

• Now you are aware

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Example of Roots

• Cu sulphides in fracture fills & disseminations in propylitic batholith

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Example of Roots

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Example of Roots

• Barren sheeted quartz veins – orogenic and not porphyry-related

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Back to Square One?!?