2015 Broken Hill Resources Investment Symposium - University of Sydney - Peter Gunn
Transcript of 2015 Broken Hill Resources Investment Symposium - University of Sydney - Peter Gunn
An explanation for the distribution of Broken Hill style mineralisation invoking dense rift-related igneous intrusions.
This talk will suggest explanations for: - the gravity field of the Broken Hill area - the distribution and origin of the Ag-Pb-Zn mineralisation - the distribution and origin of the Cu mineralisation - the magnetic field of the Broken Hill area - the original structure of the Broken Hill area - the present structure of the Broken Hill area - the topography of the Broken Hill area
And - give guidelines for exploring for Broken Hill type deposits Presented by – Peter Gunn MSc, PhD, FAIG Bohuon Resources Pty. Ltd. 16 Plunkett Road, Mosman, NSW 2088 Australia Tel: +612 9968 1209 Email: [email protected]
Gravity field of the Broken Hill area showing loca8ons of Ag-‐Pb-‐Zn mineralisa8on. Many authors (e.g. Stevens et al., 1988) have iden8fied the Broken hill area as having a riI related origin. This talk will present an explana8on for the remarkable correla8on of the mineral occurrences with the edges of the gravity anomalies based on situa8ons the author has observed in during studies of numerous riI systems.
Red indicates high gravity. High gravity can indicates dense rocks, such as mafic or ultramafic intrusions. Major structures can also cause gravity highs. Main roads are shown. The main Broken Hill orebody centred on Broken Hill contained over 200 Mt of ore and is 7.5 km long.
47.3 KM
Broken Hill
From Forbes et al. (2008)
Geology with occurrences of Pb, Zn and Ag
The Thackaringa, Broken Hill and Sundown Groups have been interpreted as syn-‐riI sediments. The Paragon group has been interpreted as post riI.
From Gunn (1997)
CLASSICAL STAGES OF RIFTING The black bodies are igneous intrusions that result from decompression of the mantle during extension and crustal thinning. These intrusions cause significant gravity highs. The author has concluded that the situa8on at Broken Hill probably corresponded to Situa8on (ii) or (more probably) Situa8on (iii).
From Gunn (1997)
Several stages of riNing can exist in the same riN System. Note transfer faults allowing differenRal extension.
The author has done detailed studies of the following riIs where the model presented appears to be confirmed: The Midland Valley of Scotland and Northern Ireland (Gunn, 1972) The Joseph Bonaparte Gulf RiI (Petrel Sub-‐basin) (Gunn, 1988) The Otway Basin (Gunn et al., 1995) The Bass Basin (Gunn et al., 1996) The Sydney-‐Bowen Basin The Cuve]e Centrale of the Democra8c Republic of Congo The East African RiIs in Northern Kenya and Ethiopia The Western Desert area of Egypt The Etosha Basin In Namibia and Angola Par8al studies of other riIs and ar8cles in the literature appear to confirm the widespread occurrence of riIs conforming to the model although it must be admi]ed that not all riIs conform to the model presented. Studies of the Joseph Bonaparte Gulf riI will be presented to establish possible analogues with the Broken Hill area.
The Joseph Bonaparte Gulf contains an extremely well studied riI system. Gunn (1988) published the first detailed interpreta8on of this area in terms of modern riI theory. The riI is characterised by a striking axial gravity anomaly that bifurcates halfway along its length. Recent gravity modelling, deep seismic reflec8on surveys and tomographic inversions of a sea floor seismic refrac8on survey give results that agree almost exactly with the previously presented model of riIing processes.
Gravity field of the Bonaparte Gulf
Afar Triangle topography and structure
Seismic tomographic results imaging axial intrusions along the riI axis
From Bastow et al. (2011)
TOTAL MAGNETIC INTENSITY PROFILE
Gravity
INVERSION OF OCEAN BOTTOM SEISMIC REFRACTION DATA Tomographic inversion of sea floor seismic refrac8on data from Goncharov et al. (2004) and gravity modelling from Baldwin et al. (2003). Note posi8ons of magne8c highs.
MAGNETIC INTENSITY PROFILE
Gunn (1988) section
From Goncharov et al. (2004)
Gravity
BONAPARTE GULF RIFT
Goncharov et a. (2004) have overlain deep seismic reflec8on data on their tomographic inversion results. Their sec8on closely resembles the sec8on predicted by Gunn (1988)
Generalised gravity model showing that the two gravity highs at Broken Hill can be explained by two dense intrusions similar to those imaged in the Joseph Bonaparte Gulf RiI.
Country rock density 2.83 gm/cc Intrusion density 3.0 gm/cc (average for gabbro)
Observed gravity
Calculated gravity
So, are the Broken Hill gravity anomalies Indica8ng a similar situa8on as exists in The Joseph Bonaparte Gulf RiI? Could deep penetra8ng faults such as those that flank the intrusions in the Joseph Bonaparte Gulf RiI have channelled mineralising solu8ons so that mineralisa8on was precipitated along the edges of the gravity anomalies?
Gravity
Normally deep riN related intrusions such as those in the Joseph Bonaparte Gulf cause significant magneRc anomalies. (see for example the magneRc anomalies shown on the secRon in Slide 11). No such anomalies exist over the immediate Broken Hill area. In fact the Broken Hill area corresponds to a regional magneRc low. This regional low shows very liVle correlaRon with surface geology. This fact suggests that it is reflecRng a superimposed event such as a metamorphic episode (eclogite facies at depth?). It is possible that such a metamorphic effect destroyed any original magneRc properRes of the inferred gravity related intrusions. It is also possible that the mafic magmas that produced the inferred intrusions must have been relaRvely reduced and were derived from primiRve mantle. Many ancient mafic magmas are reduced and produce gabbroic intrusions that are weakly or not magneRc. The ultramafic cumulates beneath them also contain no primary magneRte (D. Clark, CSIRO – personal communicaRon).
Total magne8c intensity (TMI) Ver8cal deriva8ve of TMI
Cu occurrences at Broken Hill superimposed on gravity. Note that the Cu occurrences are on the gravity anomalies
Gravity of the Joseph Bonaparte Gulf riI
RiI axis intrusions oIen underlie volcanic centres such as those indicated by black triangles in the above figure of the northern por8on of the East African RiI system. It seems likely that in submarine situa8ons that such volcanic centres would be the source of “black smokers” and massive sulphide precipita8on. Could it be that the gravity anomalies at Broken Hill are due to fragments of such axial intrusions and the copper occurrences occurring on the gravity anomalies are due to such hydrothermal ac8vity, in this case, with similari8es to Cyprus type Cu deposits?
From Daly et al.(2008)
Outcrops of amphibolites. Some clustering over inferred intrusions. Are these evidence of igneous rocks sourced from deeper intrusions?
From Binns (2004)
Sec8on across a black smoker volcanic centre on the Pual Ridge in the Manus Basin RiI According to Binns: Seismic evidence exists for a magma chamber 2.5-‐3 km below, Cu and Au are likely derived from an ascending magma8c component, and various juvenile sites of ven8ng occur in the area Are the Cu occurrences at Broken Hill the result of similar ven8ng?
Transfer fault
Normal fault
Generalised major transfer zones as heavy black lines, generalised normal faults as dashed lines ?
It is possible that the NW trending shears at Broken Hill are relicts of transfer faults and the NE trending shear zones are relicts of normal faults? AN IDEA OF THE ORIGINAL RIFT GEOMETRY?
Egyp8an Qatara Depression analogue?
Mundi Mundi Plains
Mundi Mundi Fault a thrust?
ElevaRon – is it reflecRng an inverted riN?
Is this a thrust caused by bu]ressing against a dense igneous body c.f. the Beliat An8cline in Brunei and Sarawak?
Libyan Plateau Analogue?
Darling Range Thrust zone
GRAVITY The thrust of Thorndale Composite Gneiss appears to be wrapped around the southeastern side of the gravity high interpreted as due to a mafic intrusion. Has a buVressing effect of the intrusion controlled the geometry of the thrust?
The Thorndale Composite Gneiss is magneRc and appears to be a detached thin slice of magneRc material thrust over Thackaringa Group units.
High resolu8on seismic reflec8on data across the Broken Hill area (Fomin and Korsh, 2007) does not clearly image deep structures.
Possible intrusion at depth flanked by fault zone
Line drawing of an interpreta8on of regional seismic reflec8on data from Gibson et al. (1998) Some of the events could be mul8ples, reflected refrac8ons and side swipe reflec8ons. Some possible doming is indicated between Li]le Broken Hill and the Broken Hill Synform. There appears to be a possible change in character of the crust beneath the Broken Hill Synform area. Large igneous intrusions almost never image well in seismic reflec8on data due to the lack of internal velocity contrasts. The shallow thrus8ng that has been interpreted is probably real. Many riIs get compressed and “inverted” during which the syn-‐riI sediments are squashed to the extent that they no longer fit in the original riI depression. This causes folding and thrus8ng of the syn-‐riI sediments. Such a process could explain much of the structuring at Broken Hill. The Olarian Orogeny probably had an influence on post riI structuring.
Reduced to pole magnetic intensity over the Yalata Layered Intrusion. Are such features remnants of rift axis intrusions?
Gravity
Gravity
SA
WA
From Gunn (1984)
Bonaparte gravity
Gunn (1984) hypothised that a riI margin situa8on with associated gravity and magne8c highs exists along the western margin of the Adelaide Geosyncline (the Torrens Hinge Zone). This zone could have similar prospec8vity to the Broken Hill area.
From Gunn (1984)
MagneRc intensity of South Australia MagneRc intensity of South Australia
ACCORDING TO WIKIPEDIA THERE HAVE BEEN OVER 1500 PUBLICATIONS ON BROKEN HILL GEOLOGY. YOU HAVE JUST HEARD ANOTHER ONE. DID YOU BELIEVE IT? THANK YOU Acknowledgements: The images and the mineral occurrences locaRons were obtained from Geological Survey of NSW databases. Kate Hine of Mitre Geophysics Pty. Ltd. helped by allowing access to her gravity modelling soNware and by explaining how to run it. Dave Clarke of the CSIRO helped with discussions on magneRc properRes.