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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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….


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)


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…



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



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



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).


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


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


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


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

Exploration in the House 2015: The Broken Hill 1:250 000 Metallogenic Special by Joel Fitzherbert

Science

Transcript of Exploration in the House 2015: The Broken Hill 1:250 000 Metallogenic Special by Joel Fitzherbert