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Linked to Mineralisation Episode Are Included i Exploration and Mining Report 1073R PREDICTIVE MAGNETIC EXPLORATION MODELS FOR PORPHYRY, EPITHERMAL AND IRON OXIDE COPPER-GOLD DEPOSITS: IMPLICATIONS FOR EXPLORATION D.A. Clark, S. Geuna and P.W.Schmidt SHORT COURSE MANUAL FOR AMIRA P700 April 2003 RESTRICTED CIRCULATION This report is not to be cited in other documents without the consent of CSIRO Exploration and Mining ______________________ ii CSIRO Exploration and Mining PO Box 136, North Ryde, NSW, Australia iii iv Distribution List Copy No. AMIRA 1-8 Dr N. Phillips, Chief CSIRO Exploration and Mining 9 Mr D.A. Clark 10 Dr S. Geuna 11 Dr P.W. Schmidt 12 Records Section (North Ryde) 13 Copy no. ........................ of 19 copies v Page no. TABLE OF CONTENTS 1. INTRODUCTION 1 2. MAGNETIC PETROPHYSICS AND MAGNETIC PETROLOGY: 4 APPLICATION TO INTERPRETATION OF MAGNETIC SURVEYS AND IMPLICATIONS FOR EXPLORATION 3. MAGNETIC PETROLOGY OF IGNEOUS INTRUSIONS: 46 IMPLICATIONS FOR EXPLORATION AND MAGNETIC INTERPRETATION 4. METHODS OF DETERMINING MAGNETIC PROPERTIES WITHIN 95 MINERALISED SYSTEMS 5. GEOLOGICAL MODELS OF PORPHYRY COPPER (Mo, Au), 122 VOLCANIC-HOSTED EPITHERMAL AND IRON OXIDE COPPER- GOLD SYSTEMS 6. GEOLOGICAL AND GEOMAGNETIC FACTORS THAT 192 CONTROL MAGNETIC SIGNATURES OF PORPHYRY, EPITHERMAL AND IRON OXIDE COPPER-GOLD DEPOSITS 7. PETROPHYSICAL PROPERTIES OF VOLCANO-PLUTONIC 212 TERRAINS, PORPHYRY SYSTEMS, IRON OXIDE COPPER-GOLD SYSTEMS AND VOLCANIC-HOSTED EPITHERMAL SYSTEMS – P700 CASE STUDIES 8. MAGNETIC PROPERTIES OF PORPHYRY, EPITHERMAL AND 226 IRON OXIDE COPPER-GOLD DEPOSITS SYSTEMS - SYNTHESIS 9. MAGNETIC AND OTHER GEOPHYSICAL SIGNATURES OF 238 P700 STUDY AREAS 10. MAGNETIC SIGNATURES OF PORPHYRY, EPITHERMAL AND 274 IRON OXIDE COPPER-GOLD MINERALISATION - REVIEW AND SYNTHESIS 11. EXPLANATORY NOTES FOR THE P700 DATABASE 300 12. EXPLANATORY NOTES FOR THE P700 ATLAS OF 314 PREDICTIVE MAGNETIC EXPLORATION MODELS 13. GEOPHYSICAL EXPLORATION CRITERIA FOR PORPHYRY, 331 EPITHERMAL AND IRON OXIDE COPPER GOLD DEPOSITS 14. ACKNOWLEDGMENTS 334 15. REFERENCES AND BIBLIOGRAPHY 335 APPENDIX A - PRODUCTS DISTRIBUTED TO SPONSORS 382 INTRODUCTION 1 1. INTRODUCTION AMIRA project P700 was aimed at improving the utility of magnetic surveys in exploration for a number of deposit types: porphyry Cu (Au, Mo) and related deposits, the newly recognised category of reduced porphyry deposits, precious metal and Cu-Au volcanic-hosted epithermal deposits, and iron oxide Cu-Au deposits. In the course of the project predictive magnetic exploration models were developed for these types of deposit. The magnetic models are based on realistic geological models, a comprehensive magnetic property database, and magnetic petrological principles. Outcomes of the project include an Atlas of Predictive Magnetic Exploration Models for these types of deposit, in a wide range of geological settings, and a set of practical and easily understandable criteria for exploration using magnetics for porphyry and epithermal deposits. Magnetics is the most widely used geophysical method in hard rock exploration and magnetic surveys are an integral part of exploration programmes for porphyry and epithermal deposits. However, the magnetic signatures of porphyry and epithermal deposits are extremely variable and exploration that is based simply on searching for signatures that resemble those of known deposits is rarely successful. A number of well-known geological models of porphyry and epithermal deposits are routinely used in exploration, even though most deposits fail to match the idealised models closely, due to post-emplacement tectonic disruption and rotations, asymmetric alteration zoning due to emplacement along a contact between contrasting country rock types, and so on. These complications are taken into account by exploration geologists as geological information about a prospect accumulates. The variability of magnetic signatures of these deposits reflects: i. strong dependence of magnetic signatures on local geological setting, ii. departures of real mineralised systems from idealised geological models, iii. the direction and intensity of the geomagnetic field, which varies over the Earth, iv. differing magnetic environments (host rock magnetisation, regional gradients, interference from neighbouring anomalies etc.). Points (i)-(ii) are the most important reasons for variability of magnetic signatures of mineralised systems. Point (i) covers a number of significant geological factors, the most important of which are: tectonic setting and its influence on magma composition and mode of emplacement; influence of pre-existing structures on the geometry and depth of emplacement; and the crucial influence of host rock composition on alteration assemblages, including secondary magnetic minerals, and on the stability of primary magnetic minerals. Many porphyry systems depart from idealised models due to emplacement along geological contacts with contrasting rocks on either side, producing asymmetric zoning patterns. Point (ii) reflects not only the local geological setting at the time of emplacement, but also post-emplacement modification of deposits. Post-emplacement tilting of porphyry and epithermal systems and dismemberment by faulting are very INTRODUCTION 2 common and drastically modify the geophysical signatures. Burial of a deposit by younger sedimentary or volcanic rocks also modifies the anomaly pattern. Conversely, exhumation and partial erosion of the system produces a very different magnetic signature. In older deposits, metamorphism can substantially modify the magnetic mineralogy of the deposits and host rocks, with concomitant changes in the magnetic anomaly pattern. Although the majority of porphyry and epithermal deposits are relatively young and, at most, weakly metamorphosed, some relatively ancient deposits in metamorphosed terrains are known. There is a strong possibility that some older porphyry and epithermal deposits occur that have not been recognised, because effects of metamorphism and deformation have obscured their true nature. Points (i)-(ii) can only be satisfactorily addressed by an approach that integrates a knowledge base of magnetic properties, based on detailed sampling of a variety of deposits and modelling of their magnetic anomalies, combined with magnetic petrological principles to produce predictive magnetic exploration models that are specific to each geological setting. Given the magnetic petrophysical database and the understanding of the geological factors that create and destroy magnetic minerals in porphyry systems, however, the magnetic effects of the above-mentioned geological complications are quite predictable. The CSIRO Rock Magnetism Group is uniquely qualified to tackle predictive magnetic modelling of mineral deposit models, as it combines expertise in magnetic interpretation with extensive knowledge of magnetic petrophysics and magnetic petrology, acquired over a succession of AMIRA projects, as well as a long history of case studies of mineralised areas carried out for exploration companies. Point (iii) above is routinely addressed by reduction-to-the-pole (RTP) processing of measured total magnetic intensity (TMI) data, which is generally successful in producing more intuitively interpretable magnetic signatures that are no longer skewed by the inclination of the geomagnetic field and can therefore be directly compared with signatures from other geomagnetic latitudes. Thus it is proposed to produce the predictive magnetic exploration models predominantly in the form of calculated RTP anomalies that can be compared with RTP processed surveys, although TMI signatures at other geomagnetic latitudes will be presented for selected models. Once a generic predictive magnetic model has been produced for a particular geological setting, point (iv) above can be addressed by using the grid of predicted values as a template that can be superimposed on the measured values anywhere in the survey area to produce the predicted signature for the local magnetic environment. For example, in the presence of a steep regional field gradient, the generic signature can be grossly distorted and may easily elude recognition. Superimposing the grid values for the generic model on this gradient produces the anomaly that is to be expected at that specific locality, which may be quite different from the signature of an identical source in a different regional field. The project provided the following products: INTRODUCTION 3 • An Atlas of Predictive Magnetic Exploration Models (and case histories), based on standard geological models for porphyry copper and copper-gold deposits (including associated skarn deposits and intrusive-related breccia-hosted Au deposits), volcanic-hosted epithermal precious metal and Au-Cu deposits, and iron oxide copper-gold deposits for a wide range of geological settings (RTP images, contour maps and profiles; supplemented by TMI equivalents at a range of latitudes, for selected models). • Calculated grids for each of the models, in formats convenient for sponsors. • An extensive review and classification of observed magnetic signatures of porphyry and epithermal deposits and an analysis of the geological factors that influence those signatures, • A relational database (Access and/or Paradox) of geological, magnetic petrophysical, magnetic petrological and magnetic anomaly characteristics along with other geophysical characteristics of well-documented porphyry and epithermal deposits worldwide. 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