Download - Exploration in the House 2015: The Broken Hill 1:250 000 Metallogenic Special by Joel Fitzherbert
- The Broken Hill 1:250 000 Metallogenic Special - New insights into understanding mineralisation and mineral potential
Joel Fitzherbert Peter Downes, Phil Blevin and David Forster (Read the map and quarterly notes for the full list of contributors)
• Broken Hill in 3 slides (Background geology)
• Broken Hill 1:250 000 metallogenic special
– Concept and components
• Mineralisation in the Broken Hill region • Which fire is smoking?
• Orogeny in Broken Hill: Applying metamorphic facies/isograd
maps to high-grade orogenic mineral systems
Willyama Supergroup & Curnamona rift basin
• Palaeoproterozoic Willyama Supergroup
• Deposition 1720-1640 Ma
• Epicontinental rift basin
• Olary Domain SA palaeo-basin flank
• Broken Hill Domain NSW closer to the rift axis
• More diverse sequences • Deposition of Broken Hill
Group and ~300 Mt Broken Hill ore body.
Connor (2004)
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2
3
Palaeoproterozoic in the Broken Hill Domain
• 1) Early-rift (fluviatile to shallow marine sedimentation) ~1720 to ~1690 Ma
• 2) Rift-deepening (marine transgression and magmatic escalation) ~1690 to ~1670 Ma
– Volcaniclastic/volcanic Potosi Gneisses – Silver City Suite high-level granite sills
• 3) Post-rift (deeper water, sag phase deposition followed by marine regression) ~1670 to ~1640 Ma
East West Ma
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3
Willyama Supergroup
• Olarian Orogeny
• 1620-1580 Ma
• High T/P field array – Greenschist to granulite
Post-depositional modification of the Willyama Supergroup
Delamerian Orogeny
520-500 Ma
Moderate T/P field array Staurolite to chloritoid zones
Neoproterozoic rifting
Mesoproterozoic orogeny
Cambrian orogeny
Broken Hill 1:250 000 Concept and components
Broken Hill 1:250 000 Stratigraphic map face and mineral systems
Complex geology at 1:250 000 Highlights important stratigraphy
Purnamoota Subgroup Hores Gneiss Silver City Suite
Post-Palaeoproterozoic intrusions excluded from basement for stratigraphic clarity
Simplified version of: Broken Hill Stratigraphic (Willis 1989) Interpretation of Broken Hill 1:25 000 map
series Stevens (2009) 1:100 000 scale interpretive Proterozoic
geology GSNSW (2009) Includes undercover interpretation Full Neoproterozoic geology Cover sequences GA 1 million
Transparent, stippled
Mineral Systems of NSW Local mineral deposit types have been
reassigned to statewide mineral system models Downes et al (2001)
First complete time-space plot
1720 Ma to present
Includes updated geology and dating Stevens (2008)
Dutch et al (2006)
GSNSW (2009)
Addition of mineral deposits against time
Addition of tectonic interpretation against time
Broken Hill 1:250 000 Components (Time-Space)
Addition of two 1:500 000 scale insets for Olarian and Delamerian Isograd/facies maps
Commodity plots for mineral occurrences interpreted to be associated with each orogenic event
Olarian inset includes Olarian intrusive phases
Delamerian Post Olarian dykes and intrusions Delamerian structure
Broken Hill 1:250 000 Components (metamorphic maps)
Broken Hill mineralising events
• Palaeochannel-hosted U?
• Cambrian, Delamerian orogeny – Hydrothermal metamorphic base
metal • Pb-Ag +/-Cu veins
• Neoproterozoic rifting – Ultramafic Ni-Cu-PGE – MVT
• Mesoproterozoic Olarian orogeny – Magmatic Sn-W – IOCG?
• Palaeoproterozoic rifting – Broken Hill Type Pb-Zn-Ag – Pyrite Hill Py-Co
Smoke and Fire
• Palaeoproterozoic – Raging fire, very little smoke – Pronounced stratigraphic & tectonic block control
• Mesoproterozoic – More smoke – Apparent STRATIGRAPHIC OR THERMAL Control
• Neoproterozoic – Very little smoke. Some recent interest
• Cambrian – Widespread smoke – No apparent stratigraphic or thermal control
THE POINT:
• Most of the smoke is OROGENIC – Poorly describe mineral system in Broken Hill
• Focus on our current understanding of orogenic
mineral systems in Broken Hill
Mineralisation associated with orogeny
• Mineral occurrence data plotted over metamorphic grade
• A key aspect of assessing any relationship is the need for spatially correct maps of metamorphic grade
Olarian Orogeny Delamerian Orogeny
Drill Core
Field Traverses
Previous work
Petrographic re-evaluation
Metamorphic facies/isograd maps Construction
Petrographic database
Ready to be applied to the orogenic mineral systems of the area
Isograds/facies broadly parallel to stratigraphy Stratabound
In a broad sense isograds are rotated around regional F2 structures.
Facies maps: Upper-greenschist Amphibolite Granulite Isograds: Biotite Sillimanite Sillimanite+K-feldspar Othopyroxene
Olarian Orogeny (1600-1580 Ma)
Key features
Occurrence over Olarian C.1600 Ma metamorphic grade
Addition of Olarian intrusive phases - Pegmatite ~1600-1590 Ma - Mundi Mundi granites ~1590-1580 Ma
Due to the stratabound nature of the metamorphic field gradient there is also an association with metamorphic facies zones and isograds
Apparent stratigraphic control on mineral deposits and pegmatite
But….
Olarian Orogeny (1600-1580 Ma)
Mesoproterozoic orogenic mineralisation
Yanco Glen (W)
Euriowie (Sn) Kantappa (Sn)
Apparent stratigraphic control, particularly W
Lower sill complex (W) Hores Gneiss Upper pegmatite field (Sn) Paragon Group
Lower layered sill complex (W) Amphbolite facies
Upper pegmatite field (Sn) Greenschist facies
Pegmatite/quartz vein hosted
Waukeroo (Sn)
Olarian Orogeny (1600-1580 Ma)
A Mesoproterozoic orogenic mineral system: Sn-W-bearing pegmatite
Syn to post D1 Lower sill complex in amphibolite facies
Upper pegmatite field (upper greenschist facies)
Metamorphic field gradient is essentially stratabound
Intrusive phases are essentially stratabound
Yanco Glen (W) Waukeroo (Sn) Euriowie (Sn) Kantappa (Sn)
BUT…
Olarian Orogeny (1600-1580 Ma)
A Mesoproterozoic orogenic mineral system: Sn-W-bearing pegmatite
Stevens 2006
Sillimanite + K-feldspar (+ partial melt) isograd Thermal death at or close to wet solidus (minimum melting curve) Helps to explain lower sill complex, what about upper pegmatite field
Olarian Orogeny (1600-1580 Ma)
A Mesoproterozoic orogenic mineral system: Sn-W-bearing pegmatite
Sawyer (1998) Fitzherbert (1998)
Lower sill complex = pooling small volume melt batches at or below solidus. Level of the Hores Gneiss and sill-Kspar-partial melt isograd
Small fractionated melt batches, enriched in fluxing elements (F, Li, B) Amblygonite (Li, F) Flouroapatite (F) Lepidolite (Li) Tourmaline (B) Depressed solidus promotes intrusion at considerably higher levels in the Paragon Group
Olarian Orogeny (1600-1580 Ma)
A Mesoproterozoic orogenic mineral system: Sn-W-bearing pegmatite
Mundi Mundi Suite (1590-1580 Ma) Equivalents of Bimbowrie Supersuite rocks Iron Duke Type breccias Fe +/- U Amphibolite facies and shallower Larger plutons greenschist facies In Broken Hill, just see the eastern edge of the extensive late syn- to post-Olarian magmatic event (1590-1580 Ma).
Olarian Orogeny (1600-1580 Ma)
Broader application of a model
Isograds: Webb and Crooks (2005) Fitzherbert (2015)
Mundi Mundi equivalent Bimbowrie Supersuite
Located in the amphibolite facies and above. Main zone between Sillimanite and sillimanite-K-feldspar isograds
IOCG association
Olarian Orogeny (1600-1580 Ma)
Curnamona-wide application of a model Burtt et al (2004)
Done partial melting, migration and metal transport
Possible prograde metamorphic fluids higher in the pile, also volatile release from partial melts.
Retrograde metamorphic fluids require rehydration of the pile.
Partial melting Retrograde H2O addition
Olarian Orogeny (1600-1580 Ma)
Mesoproterozoic orogenic: what’s possible?
Dehydration
Calc-silicate Amphibolite
If we have: Reactive rocks, competency contrast, thermal contrast, fluid mixing…..
Retrograde fluids? Alteration/hydration of a granulite facies fabric In margin of Amphibolite Around crosscutting veins Associated base metal mineralisation at mafic metasediment boundary
Olarian Orogeny (1600-1580 Ma)
Mesoproterozoic orogenic: Retrograde fluids?
Commodities over Delamerian isograds. Thackaringa Type siderite-quartz veins Ag-rich base metal veins Arcuate zone of shear zone hosted veins centered around Broken Hill Broadly parallel, but transgress Delamerian field gradient to south Alteration and fluid inclusions 150-200°C (Dong et al (1987) Reid et al (2009)
Not stratabound
Low metamorphic grade
Distribution implies late formation during exhumation of deeper levels to the south-west
Delamerian Orogeny (515-500 Ma) Cambrian orogenic
Summary
• The combined parts of the new metallogenic map provide a holistic view of the mineralising processes/mineral systems in the Broken Hill region
• Palaeoproterozoic strataform, hydrothermal inferred magmatic input • Olarian magmatic • Neopeoterozoic magamatic/strataform • Delamerian orogenic hydrothermal metamorphic
• This work can be combined with the previous editions of detailed lithology/deposit type studies in an attempt to generate some new ideas on mineralisation in the region
• Understanding metamorphism is important in Broken Hill
• New metamorphic maps further highlight the stratabound nature of the Olarian metamorphic field gradient
– Some peripheral deposits previously though of as stratabound/?strataform are much better explained by metamorphic processes (thermally bound).
• Early Sn/W-bearing pegmatite systems
– Facies/isograd maps useful in tracking areas of potential influence from 1590-1580 Ma thermal event
• Many more of the peripheral deposits to the Broken Hill main line need to be scrutinised with respect to their relationship to metamorphism
Broken Hill 1:250 000 Supporting info