Metallogeny of Georgetown Region

Transcription

1 Metallogeny of Georgetown Region Gregg Morrison, Harry Mustard, Ashley Cody Focused project: Update existing model 1987 (AMIRA) Limited coverage: Etheridge-Gilberton-Einasleigh, + Woolgar (no Croydon, Chillagoe West) Exploration data update Samples for quartz textures (133 mines) Samples for ME geochem (120 assays) Radiometric dating: K-Ar, MoS 2 (zircon U-Pb) (36 new) metallogenic GIS

2 Geological Setting of Georgetown Province In North Australian Craton Meso-Proterozoic. Thomson Neo-Proterozoic basement Both have Silurian Pama Province granitoids stitching And these extend south in Thomson and likely underlie Mossman Early Devonian thrust deformation in both Tabberabberan orogeny? Overprinted by Carb-Permian Kennedy Province CT TMIB

3 Georgetown in context of NQ Au Georgetown & Charters Towers both have Early Devonian granite hosted orogenic gold In the domains with Silurian granitoids and the Early Devonian thrusts Etheridge & Lolworth have mainly Early Carboniferous Kennedy sparse deposits Ravenswood & Chillagoe have mainly Late Carboniferous

4 Georgetown Camps Endowment 5Moz Kidston IRGD breccia 2Moz Woolgar plutonic & epithermal resource ~1Moz Croydon plutonic 0.7Moz Agate Creek epithermal resource ~1Moz Historic and recent production Etheridge But oxide & shallow dominant Endowment >1tonne Au CAMP Kidston endowment gold only kg Producti on: Metal (kg fine gold) Producti on Grade g/t Au Gold Resources Remaining/ original Age Minz. Environm ent ECARB IM BX,VN Woolgar mesozonal Mt at 2 g/t EDEV PLM LD Minz. Style Woolgar Epithermal g/t EPERM? EPB VN, SW Agate Creek g/t EPERM EPB VN,SW,DS Gilberton Alluvial 4000? RECENT Red Dam g/t LCARB/EDEV? PLE LD Mt Hogan g/t EDEV PLM VN Durham EDEV? PLM LD, VN Cumberland Mine EPERM IM LD Electric Light g/t EPERM IE SW, BX Queenslander EDEV? PLM LD International EDEV PLH LD Big Wonder EDEV? PLM VN

5 CRUSTAL SCALE FLUID-MINERALISATION ENVIRONMENTS Orogenic MS Intrusion-Related MS SLATE BELT PLUTONIC

6 Georgetown Geology Camps

7 Deposit Ages

8 MINERALISED ENVIRONMENTS DEFINED BY QUARTZ TEXTURES BRECCIATED EUHEDRAL BUCK PLUTONIC HYPOZONAL COARSE COMB QUARTZ PLUTONIC MESOZONAL INTRUSION RELATED MESOZONAL INTRUSION RELATED EPIZONAL FINE COMB SACCHAROIDAL AND SILICA-SULFIDE

9 DISTRIBUTION OF MINE CAMPS BY QUARTZ TEXTURE Mining districts of Georgetown and Forsayth are dominated by Devonian Plutonic Hypozonal and Mesozonal style deposits, lesser Intrusion Related and few Epithermal. Zoning from deeper to shallower level away from centres of each plutonic districts. The Carboniferous Intrusion-Related and Permian Epithermal Camps are partly overprinting but mainly peripheral to the Plutonic camps and localised by volcanic to sub-volcanic complexes and regional lineaments Quartz texture assemblages are a useful tool for deposit classification, especially in conjunction with multielement geochemistry

10 Geochemical classification Based on enrichment relative to granite For our samples and any others with ME data Tag for most enriched 3-4 elements Bi-Te dominant for the cores of the plutonic districts Au Ag As Te for the periphery of the plutonic districts Ag>Au As Te for Permo-Carb veins Te-Bi centres of Permo-Carb systems e.g. Phyllis May Most surprising is high Bi-Te throughout This is a magmatic fluid indicator For IRGD and the Plutonic deposits Unlike Charters Towers (Au-Ag-basemetals)

11 Metal Zoning in Plutonic Districts Plotting the most enriched basemetal for each camp produces a distinct pattern There are three centres: Georgetown, Forsayth & Percyvale Each with: Cu core, Pb intermediate zone As Sb periphery Au is in all the zones but best with Pb Where it is free on sulfide grain boundaries i.e. amenable to leaching Whereas As zones have more refractory gold & have mostly been mined in the oxide zone Best exploration potential for sulfide ore is in the Pb areas on surface or underneath the As zone

12 Devonian Camps on Silurian granitoids Silurian granites strong NE-SW orientation Parallel to regional faults Thrust and steep fault contacts Some intrusive contacts roof zone at Jubilee Plunger Early Devonian Au deposits Distinct corridor with no obvious control In mapped geology or structure So what controls their distribution & origin?

13 Mt Hogan early Devonian flat lode Early Devonian deposit in granite close to contact. Shallow dome shaped lode distribution pxrf surveys highlight the extent of these key metals correlated with Au in poorly exposed areas, helping redirect the survey in real-time thus directly vectoring towards mineralisation. Mineralisation does not extend laterally far away from the vein(s), but sericitic alteration forms broader haloes often >10x the thickness of the vein. This can be mapped with the pxrf using Rb/Sr ratios: Courtesy Activex Ltd 13 13

14 Mt Hogan Early Devonian flat lode Thrust fault zone Brecciated buck quartz veins Sulfide-gold infill Te Au Bi Ag Pb Sb signature Shallow part only mined 5.5g/tAu Extent of structure undefined

15 Carboniferous Camps on Carboniferous Geology Mt Turner Carboniferous with isolated dikes and plugs not in intrusive or volcanic complexes Early Carboniferous on east side of Newcastle range in sub-volcanic domain Early & Late Carboniferous complexes west side similar Kidston

16 Kidston Pipe sheet vein zones dikes & ore bodies KLONDIKE

17 SHEET VEINS Comb texture Qz-py-sp-gn+/-cp, BiS Free gold, 100 micron degC 2 phase fluid magmatic KLONDIKE

18 Metal Zoning within the Kidston breccia pipe Porphyry Mo-W-Bi system Gold basemetal top Confined in pipe Classic zoning on a thermal gradient Mo-py deep veins KLONDIKE

19 KIDSTON ORE Ore in vein sets bound by pipe margin, pipe roof and a sill in the upper part of the pipe +3g/t shoots at vein intersections Deeper parallel vein set poorly Au mineralised Mineralisation related to withdrawal of sill/dike KLONDIKE

20 Permian ~285Ma Camps & Geology Electric light Epithermal and Ag bearing porphyry Cu Phyllis May Ag Cu Related to dikes and plugs Part of but peripheral to mapped intrusions & volcanics Agate Creek Percyvale

21 Woolgar Goldfield ~100km SW Gilberton Basement window on edge Mesozoic Proterozoic high grade metamorphics c.f. Einasleigh NE shear zone >12km long localises with plutonic hypozonal lodes BVS deposit global resource 2g/t Au for 1.173Moz Au ESE trending vein swarm Sandy Creek with Permian? Rhyolite dikes Low-sulfidation epithermal veins 0.97 g/t Au for 21887kg Resource data:

22 Early Devonian Hypozonal Plutonic lode Early-mid Devonian c.f. Etheridge, Gilberton Au Ag As Pb Sb Zn enrichment Au Ag Pb Zn correlation Euhedral buck quartz Shatter breccia Stylolites & spider veinlets host sulfides Ribbon texture recrystallised buck quartz Fine comb fill with sulfide infill & overprint

23 Sandy Creek Epithermal Texture Zoning Digweed, 1990

24 Shallow Zones bladed calcite blade/colloform crustiform-colloform crustiform-colloform

25 Deeper zones banded adularia crystalline

26 Metallogenic Summary Epithermal deposits are Carboniferous and Early Permian All have rhyolites related to or of the same age as mineralisation chemistry notably Te significant and Ag>Au most common. The implication is that the epithermal deposits are part of the Intrusion- Related Epizonal system and predominantly of Permian age. The Intrusion-Related deposits have Early Permian and Early Carboniferous groups with two possible Late Carboniferous examples (Mt Turner, Log Creek). All the deposits are polymetallic with prominent Te-Bi and As-Sb and are mesozonal to epizonal. The Early Carboniferous deposits in the in the Kidston region are Au-rich mesozonal hydrothermal breccias whereas the deposits west of Georgetown are Early Permian-Late Carboniferous Au>Ag lode deposits (Electric light, Cumberland Mine, Beverley, Double Z) or Ag>Au epizonal-mesozonal lodes, stockworks and breccia deposits (Ironhurst, Phyllis May, Mt Turner, Bald Mountain). The Plutonic deposits are those in which there is no demonstrated spatial or temporal link to intrusions. These deposits are Early Devonian shear-hosted lodes that may have steep or shallow dipping orientation and be extensive on km scale along the structure. There are three districts-georgetown, Forsayth & Gilberton each with a distinct Au>Ag, Bi-Te, As-Sb, basemetal signature and zoning on km scale in both quartz textures and metal geochemistry. The geochemistry especially the zoning patterns suggest magmatic fluid input