The exploration of concealed porphyry Cu Au deposits is challenging as their footprints in aeromagnetic surveys can be rather complex depending on the respective magnetic susceptibilities of their wallrocks and possible postmineral tectonic overprints or structural offsets. Unfortunately, magnetic anomalies of porphyry Cu deposits are poorly documented (and illustrated) in the literature. However, locally their magnetic responses consist of two rather distinct subtypes: 1) magnetic bulls eye and 2) magnetic doughnut anomalies.
The most common magnetic response of a hydrothermal porphyry system is a distinct magnetic high anomaly, typically measuring several hundred meters in diameter, and reflecting the magnetite-rich potassic altered core. A well documented example is the Bajo de la Alumbrera porphyry Cu Au deposit, Catamarca Province, Argentina (Fig. 1). During the waning and cooling stages of the hydrothermal system, cooler late-stage fluids can overprint the stockwork mineralization and its associated potassic alteration assemblage. This may lead to the subsequent oxidation (i.e. martitization) of hydrothermal magnetite to hematite. This process may cause the de-magnetization of the magnetic high into a “doughnut-shaped” or “torus-like” magnetic anomaly as recorded at the Northparkes and Cadia porphyry Cu Au clusters in the Lachlan Fold Belt in N.S.W., Australia (Fig. 2). The latter type of magnetic anomalies appears to be more common at alkalic porphyry Cu Au deposits that are hosted by high potassic and shoshonitic intrusions.
I should be grateful for any comments on this topic! Additional illustrations of magnetic anomalies of porphyry Cu deposits are most welcome! Many thanks.
Dear Dr. Muller,
Here is the list of references about magnetic responses of porphyry deposits (most of them only include a brief paragraph about magnetic data). The best ones are the works by Dr. Clark. I hope it helps.
Regards,
Reza
References:
Gunn, P.J., Dentith, M., (1997): Magnetic responses associated with mineral deposits, AGSO Journal of Autralian Geology & Geophysics, Vol. 17, No. 2, pp. 145-158.
Clark, 2014, Magnetic effects of hydrothermal alteration in porphyry copper and iron-oxide copper-gold systems: A review, Tectonophysics, Vol. 624.
Clark, D.A., French, D.H., Lackie, M.A., Schmidt, P.W., (1992): Magnetic petrology: Application of integrated rock magnetic and petrological techniques to geological interpretation of magnetic surveys, Exploration Geophysics, Vol. 23, pp. 65-68.
Oldenburg, D.W., Li, Y., Ellis, R.G., (1997): Inversion of geophysical data over a copper gold porphyry deposit: A case history for Mt. Milligan, Geophysics, Vol. 62, No. 5, pp. 1419-1431.
Hildenbrand, T.G., Berger, B., Jachens, R.C., Ludington, S., (2000): Regional Crustal Structures and Their Relationship to the Distribution of Ore Deposits in the Western United States, Based on Magnetic and Gravity Data, Economic Geology, Vol. 95, pp. 1583-1603.
Behn, G., Camus, F., Carrasco, P., Ware, H., (2001): Aeromagnetic Signature of Porphyry Copper Systems in Northern Chile and Its Geologic Implications, Economic Geology, Vol. 96, pp. 239-248.
Clark, D.A., Schmidt, P.W., (2001): Petrophysical Properties of the Goonumbla Volcanic Complex, NSW: Implications for Magnetic and Gravity Signatures of Porphyry Cu-Au Mineralisation, Exploration Geophysics, Vol. 32, pp. 171-175.
Clark, D.A., Guena, S., Schmidt, P.W., (2004): PREDICTIVE MAGNETIC EXPLORATION MODELS FOR PORPHYRY, EPITHERMAL AND IRON OXIDE COPPER-GOLD DEPOSITS: IMPLICATIONS FOR EXPLORATION, Exploration and Mining Report 1073R, CSIRO Exploration & Mining / Amira International Limited.
Ford, K., Keating, P., Thomas, M.D., (2007): OVERVIEW OF GEOPHYSICAL SIGNATURES ASSOCIATED WITH CANADIAN ORE DEPOSITS. (Available at: http://www.openground.co.za/wp-content/plugins/jb_fileUploader/files/Overview%20of% 20Geophysical%20Signatures%20Associated%20with%20Canadian%20.pdf)
Berger, B., Ayuso, R.A., Wynn, J., Seal, R.R., (2008): Preliminary Model of Porphyry Copper Deposits.
Purucker, M.E., Clarck, D.A., (2010): Mapping and Interpretation of the Lithospheric Magnetic Field, Geomagnetic Observations and Models, pp. 311-337.
Chang, Z., Hedenquist, J.W., White, N.C., Cooke, D.R., ROACH, M., Deyell, C.L., Garcia, J., Gemmell, J.B., McKnight, S., Cuison, A.L., (2011): Exploration Tools for Linked Porphyry and Epithermal Deposits: Example from the Mankayan Intrusion-Centered Cu-Au District, Luzon, Philippines, Economic Geology, Vol. 106, pp. 1365-1398.
Holden, E., Fu, S.C., Kovesi, P., Dentith, M., Broune, B., Hope, M., (2011): Automatic identification of responses from porphyry intrusive systems within magnetic data using image analysis, Journal of Applied Geophysics, Vol. 74, pp. 255–262. (doi:10.1016/j.jappgeo.2011.06.016)
Hoschke, T., (2012): Geophysics of the Elang Cu-Au porphyry deposit, Indonesia, and comparison with other Cu-Au porphyry systems, 22nd International Geophysical Conference & Exhibition, Brisbane, Australia.(https://doi.org/10.1071/ASEG2012ab178)
Anderson, E.D., (2013): AEROMAGNETIC SIGNATURE OF THE GEOLOGY AND MINERAL RESOURCES NEAR THE PEBBLE PORPHYRY CU-AU-MO DEPOSIT, SOUTHWEST ALASKA, PhD thesis (Geology), Colorado School of Mines. (Available at: https://mountainscholar.org/handle/11124/11)
Mitchinson, D.E., Enkin, R.J., Hart, C.J.R., (2013): Linking Porphyry Deposit Geology to Geophysics via Physical Properties: Adding Value to Geoscience BC Geophysical Data, Geoscience BC Report 2013‐14. (Available at: http://www.geosciencebc.com/i/ project_ data/GBC_Report2013-14/GBC_Report2013-14.pdf)
Clark, D.A., (2014): Magnetic effects of hydrothermal alteration in porphyry copper and iron-oxide copper–gold systems: A review, Tectonophysics, Vol. 624-625, pp. 46-65. (https://doi.org/10.1016/j.tecto.2013.12.011)
Riveros, K., Veloso, E., Campos, E., Menzies, A., Véliz, W., (2014): Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit, northern Chile, Mineralium Deposita, Vol. 49, No. 6, pp. 693–707. (DOI 10.1007/s00126-014-0514-7)
Airo, M.L., (2015): GEOPHYSICAL SIGNATURES OF MINERAl DEPOSIT TYPES – SYNOPSIS, Geological Survey of Finland, Special Paper 58, pp. 9–70. (Available at: http://tupa.gtk.fi/julkaisu/specialpaper/sp_058_pages_009_070.pdf)
Hope, M., Anderson, S., (2016): The discovery and geophysical response of the Atlántida Cu–Au porphyry deposit, Chile, Exploration Geophysics, Vol. 47, No. 3, pp. 237-247. (http://dx.doi.org/10.1071/EG15094)
Shah, A.K., Bedrosian, P.A., Anderson, E.D., Kelley, K.D., Lang, J., (2013): Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska, Geophysics, Vol. 78, No. 5, pp. B317–B328. (10.1190/GEO2013-0046.1)
Mohebi, A., Mirnejad, H., Lentz, D., Behzadi, M., Dolati, A., Kani, A., Taghizadeh, H., (2015): Controls on porphyry Cu mineralization around Hanza Mountain, south-east of Iran: An analysis of structural evolution from remote sensing, geophysical, geochemical and geological data, Ore Geology Reviews, Vol. 69, pp. 187-198.
Kheyrollahi, H., Alinia, F., Ghods, A., (2016): Regional magnetic lithologies and structures as controls on porphyry copper deposits: evidence from Iran, Exploration Geophysics, Vol. 49, No. 1, pp. 98-110. (https://doi.org/10.1071/EG16042)
Anderson, E.D., Hitzman, M.W., Monecke, T., Bedrosian, P.A., Shah, A.K., Kelley, K.D., (2013): Geological Analysis of Aeromagnetic Data from Southwestern Alaska: Implications for Exploration in the Area of the Pebble Porphyry Cu-Au-Mo Deposit, Economic Geology, Vol. 108, pp. 421–436.
Anderson, E.D., Monecke, T., Hitzman, M.W., Zhou, W., Bedrosian, P.A., (2017): Mineral Potential Mapping in an Accreted Island-Arc Setting Using Aeromagnetic Data: An Example from Southwest Alaska, Economic Geology, Vol. 112, pp. 375–396.
Also, here is a typical dipolar anomaly from the known Masjed Daghi Cu-Au porphyry deposit in NW of Iran. The magnetite rich potassic core was accurately highlighted with 3D susceptibility model.
Dear Seyyed, many thanks for the excellent references and also for the Figures from the Masjed Daghi deposit! Best wishes Daniel
Dear Ioan, many thanks for your interesting attachment! Kind regards Daniel
Dear Ioan, many thanks for your interesting attachment.
If I may expound upon the article. It seems to explain the strange occurrence of hundreds of meters of disseminated very fine grained bornite/covellite at our porphyry target. We find concentrations between 1000 and 3000 ppm Cu in what seems to be an altered andesite pile at advanced argyllic alteration. Using the "geobattery" concept the dispersed migration of Cu+ from the deep metalliferous anode portrayed in the article and the strange abundance of anhydrite at depth are actually explained.
There had been little or no other significant sign of the necessary processes... hydrofracture/vein or relict potassic (high temperature) alteration that might be responsible for such significant amounts of "high sulfidation" copper mineralization. In fact, I rarely found pyrite or quartz in our best cores and the Cu grade was strangely uniform...about 2000 ppm. While that may be a common observation...how else to explain its presence except by "hand waving". Now we can hypothecate a fluid mediated diffusion of Cu+ following a weak voltage gradient directed by the respective field lines. Upon review of the very best "greenfields" intercept of the year for a major company I thought the material was junk. The highest grade core was dirty and barren at 10x but at higher magnification at 10x or even 200x the bornite appeared together with an intergrain microcockade. Why no pyrite? Where is the silica flooding?
The article explains it.
What the article does not explain is the source of oxygen at depth. I think the article's age fails to consider the advances in magmachem theory following the discovery of the "adakite" problem.
To that end...if I may...I suggest oxygen is introduced from depth and represents anomalous oxygen formation associated with garnet formation and destruction during the magnetite crisis at about 40km. It is my belief that the relatively shallow Iron oxide driven geo-battery "plug" at 3-4km wears out and the magnetic field density gets weaker...but the bornite should show a stronger geo-paramagnetic effect if the bornite is still within a conductive substrate...damp clay or within a salt saturated lithology.
Уважаемый Даниэль!
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