Geochemistry of Kalgoorlie Gold Deposits. A seminar presented by;

Sponsored by;

Low temperature alteration signatures in Yilgarn Gold Deposits

26th July 2017

Epithermal vs Archean signatures In epithermal environments Cesium is an indicator of low temperature clay alteration.

Cesium map from an epithermal district.

KB

Kundana

Mt Pleasant

Golden Mile

Cesium map from Kalgoorlie.

Hypothesis; Fimiston vs Charlotte-style gold

Observations. 1. Geological relationships show that there are multiple gold events that are

spatially superimposed. 2. Many orogenic gold deposits sit inside the footprint of pathfinder element cells

that are normally associated with low temperature, clay-rich alteration.

Hypothesis 1. Early (Fimiston-age) alteration systems are relatively common. 2. Fimiston systems were formed at very shallow crustal levels, with proximal

sericite-carbonate and distal smectite. 3. Syn-volcanic alteration systems are overprinted by at least one

deformation/metamorphic event. 4. The clay alteration systems provide a localized source of water, metal, sulfur

and CO2 which is liberated at lower to middle greenschist metamorphism.

Red Hill

Post foliation orogenic flat veins

Kanowna Belle Pyrite

Kanowna Belle Camp – At least 3 different ages of mineralization. Qtz-Mo-Au in apophyses of Scotia-Kanowna dome (2668, Re-Os) Deformed ore-stage pyrite at KB Post-foliation veins at Red Hill

(from Gerard Tripp)

Breccia

Host Granite

Breccia Pipe

3 g/t Au Grade Shell

Temperature-controlled metal zoning patterns Mt Wright Breccia pipe; pipe geometry 50m by 120m by 1500m.

Mo Te Bi Au As Sb

Temperature-controlled metal zoning patterns • Hydrothermal systems have a very consistent zoning pattern in pathfinder elements. • Zoned from Mo to Sn to Se to Bi to Te to As to Sb to Tl. • These metals substitute into the lattice of pyrite. • Indicative of the temperature control on metal substitution in the pyrite lattice. • Many Yilgarn systems show these patterns

Scotia-Kanowna Dome, Molybdenum assays Magmatic hydrothermal system, 2670my, tilted and eroded prior to KB mineralization.

Low grade porphyry Mo mineralization with Gold! These have been intentionally plotted upside down, because this is exactly what a cross section through a porphyry Cu system

would look like.

Porphyry Mo occurrence

Clasts of porphyry Mo min deposited in Golden Valley conglomerate

Scotia-Kanowna Dome, Bismuth assays Magmatic hydrothermal system, 2670my, tilted and eroded prior to KB mineralization.

Low grade porphyry Mo mineralization with Gold!

Scotia-Kanowna Dome, Arsenic assays Magmatic hydrothermal system, 2670my, tilted and eroded prior to KB mineralization.

Low grade porphyry Mo mineralization with Gold!

Scotia-Kanowna Dome, Antimony assays Magmatic hydrothermal system, 2670my, tilted and eroded prior to KB mineralization.

Low grade porphyry Mo mineralization with Gold!

Scotia-Kanowna Dome, White mica compositions This is the pathfinder/mica chemistry pattern that we would expect in a porphyry Cu

systems. There are many hydrothermal systems in the Eastern Goldfields that are zoned like this.

Porphyry

Komatiite

Ora Banda Geology

Enterprise

Gimlet South

Enterprise

Gimlet South

Ora Banda RTP_1VD

Enterprise

Gimlet South

Ora Banda Gravity

Enterprise

Gimlet South

Ora Banda Gridded Au assays from all de-surveyed points

Bi

Enterprise

Gimlet South

Ora Banda Bismuth assays

W

Enterprise

Gimlet South

Ora Banda Tungsten assays

As

Enterprise

Gimlet South

Ora Banda Arsenic assays

Sb

Enterprise

Gimlet South

Ora Banda Antimony assays

Gimlet South Mine ~ 1Mozs

Enterprise Mine ~ 1.3Mozs

Anomaly One Laterite Pit

Looking NNW

2km

Ora Banda Long Section in the plane of Gimlet South

Gimlet South Mine ~ 1Mozs

Enterprise Mine ~ 1.3Mozs

Anomaly One Laterite Pit

2km

Ora Banda Long Section in the plane of Gimlet South Metals show a temperature zoning away from the Lone Hand Monzogranite.

The age and metal zoning suggests that the Ora Banda system was tilted after formation.

Sb As W Bi Mo

Enterprise quartz-moly veins; Rh-Os age 2670 Ma

Chambefort, I., Lewis, B., Simpson, M. P., Bignall, G., Rae, A. J., & Ganefianto, N. (2017). Ngatamariki Geothermal System: Magmatic to Epithermal Transition in the Taupo Volcanic Zone, New Zealand. Economic Geology, 112(2), 319-346.

Geothermal Systems form regionally extensive clay blankets

Chlorite Zone

Illite Zone

Smectite Zone

Epithermal Vein

Clay Blankets on epithermal systems The transition from sericite to smectite is lithologically controlled, at the interface between

coherent facies and porous/permeable volcaniclastics.

Host Minerals for Cesium? Epithermal Au geochem data set

K/Al versus Na/Al molar ratio plot coloured by Cs assays. Smectite alteration is a hydrolysis process; feldspar + clay = water

Clay-rich alteration commonly does not change the bulk composition of the rock.

Host Minerals for Lithium? Epithermal Au geochem data set

K/Al versus Na/Al molar ratio plot coloured by Li assays. Li is usually hosted by Mg smectite or interlayered chlorite-smectite.

Arsenic in Epithermal systems Epithermal Au geochem data set

K/Al versus Na/Al molar ratio plot coloured by As assays. The highest arsenic concentrations are in the rocks with the most intense illite/adularia.

Antimony in Epithermal systems Epithermal Au geochem data set

K/Al versus Na/Al molar ratio plot coloured by Sb assays. Antimony in contrast to As has high values in Al-clay and smectite zones.

As-Sb-Cs-Li patterns in Epithermal systems Arsenic is most proximal, Sb at shallow levels Li and Cs above and outboard from LSE veins.

Blind LSE vein

Outcropping LSE vein

Kanowna Belle; Sb and Cs well into the hangingwall This is an epithermal pattern consistent with smectitic hangingwall alteration; now metamorphosed to

sericite-albite-ankerite-pyrite.

KB

Kundana

Mt Pleasant

KB

Kundana

Mt Pleasant

Regional Scale Low Temperature footprints Cesium is a proxy for low temperature clay alteration. Sb (decoupled from arsenic) is a

proxy for low temperature pyrite. Low temp, pre-metamorphic alteration signatures overprinted by middle greenschist to low amphibolite metamorphism.

Inclusion-

free rim

Euhedral zonationand inclusion-rich

core

Pyrite growth zoning Is this hydrothermal pyrite overgrown by metamorphic pyrite?

Biotite

Illite-Smectite

Chlorite-Smectite

Smectite

Paragonite-Fe chlorite +/- Chloritoid

Middle greenschist

Clinozoisite -Albite

Upper greenschist

2670 to 2660Ma shallow level alteration. Clay alteration in mafic rocks is metamorphosed to paragonite-Fe chlorite-chloritoid at

middle greenschist, clinozoisite-albite at upper greenschist