Passive seismic and other geophysical methods identify a ...

Passive seismic and other geophysical methods identify a young meteorite crater in Archaean greenstone of the

Coolgardie Goldfield, WA

Phone: +61 (8) 9289 9100 Email: [email protected] www.respot.com.au

Dr Jayson Meyers Principal Consultant

AIG – WORKSHOP 2 MAY2017

Focus Minerals Limited Co-conspirators: • Sharna Riley – Resource Potentials • Wesley Groome – Focus Minerals • Michael Guo – Focus Minerals • John Sinnott – Resource Potentials • Leon Matthews – Atlas Geophysical • Grant Coopes – Atlas Geophysical

The 3 main reasons why we became geoscientists:

• Small circular area of deep regolith cover in centre of amphibolite facies greenstone belt surround by very thin regolith cover.

• Very conductive and circular disk shaped anomaly 800m wide in VTEM helicopter EM survey (cannot detect base of conductive layer), transported cover in mapping, sparse drilling indicates thicker cover, all suggesting deeply “eroded” zone filled with Cainozoic clays and hypersaline groundwater.

• Recent detailed gravity surveying using 100m (N-S) x 50m (E-W) station spacing shows very sharp and circular gravity low anomaly coinciding to VTEM anomaly.

• Aeromagnetic anomaly patterns from underlying greenstone does not indicate the presence of an internal granitoid or porphyry intrusive stock.

• Recent HVSR passive seismic survey defines the base of an asymmetric shaped crater, and gravity modelling indicates breccia zone below it.

• Recent drilling by Focus Minerals shows highly brecciated rocks at crater base that are very unusual for this area and support an impact theory.

• Archaean amphibolite facies greenstone belt

• Thin to stripped regolith profile

• Mature gold mining province

• RAB and AC generally ineffective, therefore targeting using outcrop Au, soil geochem, and geophysics – interp features and sulphide anomalies

CRATER

BEDROCK GEOLOGICAL DOMAINS GRAVITY OVER MAGNETICS

(Geology from Hunter, 1993)

• Gravity anomaly patterns provide additional layers of useful information about the regolith, bedrock units, underlying intrusive bodies and structures

• Very strong gravity low corresponding to VTEM conductor

VTEM conductivity inversion (red) and gravity low inversion (blue) showing coincident body diameters, but gravity extends to greater depth forming a “fang shaped” body

Crater – not granite

Gravity

VTEM

Passive

Too conductive!

800m

LIKELY LOW ANGLE IMPACT TRAJECTORY

FROM THE SE

1,200m diameter (most raised rim preserved), 180m deep, 30-50m projectile, ca 50Ka

(Geology from Kring, 2007)

(Geological cross-section from Shoemaker, 1974)

(Gravity model from Regan and Hintze, 1975) Fang shaped low density zone

Found in Google Earth by A. Hickman 2008, 260m diameter (most raised rim preserved), 80m deep, 10m projectile, ca 10-100Ka?

NEW?

(Peter Haines, GSWA, 2014)

Brecciated rhyolite

Vesicular glass

Crater alone cannot explain gravity anomaly, and requires steep breccia lens similar to Barringer Crater – further evidence!

Mafic Amphibolite

Cainozoic crater fill

Fang shaped low density zone

3D view of gravity anomaly draped on passive seismic crater depth with drilling and access tracks, view to the SW – raised rim must be eroded

Leading pressure zone Shadow zone

serpentine or chlorite matrix with triangular clasts

Angular breccia with “glassy” siliceous matrix.

Silicified angular “shatter” breccia.

Voids in breccia with glassy, blue botryoidal secondary silica.

Hole still being drilled, breccia zone continuing at depth

• Isolated crater in a weakly weathered Archaean greenstone belt, crater diameter of 800m and depth of 150m, but erosion of raised rim suggests slightly wider diameter and depth at time of impact, likely during the Cainozoic from a low angle SE to NW trajectory.

• Similar in size and depth to Barringer Crater, therefore 30-50m sized

projectile likely.

• Possible shatter effects logged in core: mafic breccia along crater side showing cone shaped clasts with divergent foliation of clays (serpentine or chlorite?) and downward pressure shadowing around breccia clasts, hydrous silica alteration filling cavities in shallow parts of the breccia zone. Shows similar breccia textures to the Hickman Crater in the Hamersley Group of the Pilbara region, but in that case goethite was a secondary hydrous mineral due to BIF layers mixed with the rhyolite host rock.

• Very high-resolution ground magnetic surveying using 10m line spacing (or less) for detecting iron meteorite at base of crater.

• Dedicated diamond drillhole in centre of crater, possibly on ground magnetic target, to prove meteorite origin.

• Detailed core logging for lithology, weathering, mineralogy, alteration and deformation textures.

• Detailed geochemical sampling of core at sub-metre intervals for Ni and PGEs (especially Ir).

• Petrographic analysis (optical, microprobe, QEMSCAN, etc) for shocked quartz, shattered silicate minerals, vesicular glass (if young enough), shatter cone-in-cone textures, high P quartz polymorph minerals coesite and stichovite, impact diamonds, etc.

• Mineral separates at base of crater deposits for meteorite fragments, melt droplets in and around the crater, pollen spore fossils, etc.

• Age dating of impact. • Modelling of impact and subsequent erosion of raised rim for original size.

THANK YOU!