Geophysical signatures of the gold rich porphyry copper deposits: A case study at the Dalli Cu-Au porphyry deposit

Introduction
Geophysical exploration is an inexpensive, fast and efficient tool to provide valuable information about the sub-surface geological complications (Dentith and Mudge, 2014). Modern geophysical methods are widely used to identify and characterize porphyry copper deposits on various scales (Holden et al., 2011; Hoschke, 2011; Clark, 2014). It is often an indirect exploration method; therefore, an accurate data interpretation is required to extract the proper information associated with mineralization (Clark, 2014). For efficient interpretation of geophysical data in mineral exploration, it is initially important to understand the geological properties of a deposit (i.e., host rock, hydrothermal alteration system, mineralogical characteristics, texture, structural controls, zones of outcropping mineralization, etc.). Then, according to these properties and other genetic information, a conceptual model is defined to choose the proper exploration criteria and geophysical exploration methods to identify real anomalies associated with mineralization. Finally, the geophysical data are interpreted by considering the physical properties of the conceptual model. The conceptual model and the interpretation of geophysical data could be updated by using the new information acquired from the exploratory boreholes.
This paper discusses the effectiveness of several geophysical methods in exploration of gold-rich porphyry copper deposits, and presents the exploration models related to the geophysical features of such deposits. We mostly used the related papers published in the same field to prepare these models.  Then, on the basis of the defined geophysical signatures of the porphyry deposits, the IP&RS and magnetic data of the Dalli Cu-Au porphyry deposit were interpreted.
 
Materials and methods
Porphyry deposits are the most important source of copper, molybdenum and rhenium (Sillitoe, 2010) and provide significant amount of gold, silver and some other metals (Cooke et al., 2014). These are intrusion- related deposits which are geometrically symmetrical and are affected by different hydrothermal potassic, phyllic, argillic and propylitic alterations that often show a spatial zonation. Copper-gold mineralization mostly occur in the potassic alteration zone within the contact of the intrusive body and its adjacent wall rock.
The physical properties of minerals and hydrothermal alterations associated with porphyry deposits near the surface are very variable, and therefore allow the use of various geophysical methods for exploration of such deposits. In porphyry deposits, sulfide minerals are present in different alteration zones with varying abundance, which could provide the use of electrical resistivity (RS) and induction polarization (IP) surveys to detect them. In gold-rich porphyry copper deposits, phyllic alteration zone often contain sulfide mineralization, therefore, this zone could be identified by high chargeability anomalies and low resistivities in the induced polarization surveys. The potassic alteration zone also contains sulfide minerals and is characterized in IP data with moderate to high-chargeability values. The IP method is the most extensively used geophysical approach in exploration of porphyry deposits.
Magnetic minerals are enriched and destroyed respectively in potassic and phyllic alteration zones. Therefore, a high circular or elliptical magnetic anomaly is detected at the potassic alteration zone and is surrounded by a low magnetic anomaly related to the phyllic alteration zone. Hence, the airborne and ground magnetic surveys are useful for targeting the copper-gold porphyry deposits. The potassic alteration zone consists of the radiometric K element facilitating the application of the radiometric survey for targeting this zone. Nevertheless, the investigation depth of the radiometric approach is less than a few centimeters, and therefore, it is suitable only for mapping the deeply eroded deposits in which the mineralization occurred in the potassic alteration zone.
 
Result
The ground magnetic and IP-RS geophysical data of the Dalli Cu-Au porphyry deposit were interpreted based on the proposed conceptual model of the geophysical signature of Cu-Au porphyry systems. Integrating and evaluating the geophysical processes with the result of preliminary drillings indicated that in the Dalli Cu-Au porphyry deposit, the zones with positive and strong magnetic anomalies, high to moderate chargeability and high conductivity, are associated with copper and gold mineralization. Therefore, these criteria should be considered in designing the additional/infill boreholes in further exploration plans for this deposit.
 
Discussion
The magnetic, IP and RS surveys are the most important and common geophysical methods for targeting the porphyry copper and gold deposits. In particular, implementation and integration of these three methods can be more effective. Other geophysical approaches such as gravity, electromagnetic and seismic methods are also applicable for this purpose, but they are more expensive and complicated than the afore-mentioned approaches.
For proper analysis of the geophysical data, first, it is necessary to recognize the geological model, hydrothermal alteration and mineralization systems of the studied deposits and the geophysical signatures of each alteration zone.  Then, an appropriate interpretation of geophysical data is provided through combining the geological information of the deposit with the geophysical data.
 
References
Clark, D.A., 2014. Magnetic effects of hydrothermal alteration in porphyry copper and iron-oxide copper–gold systems: A review. Tectonophysics, 624–625 (1): 46–65.
Cooke D.R., Hollings P., Wilkinson J.J. and Tosdal, R.M., 2014. Geochemistry of Porphyry Deposits. In: H.D. Holland and K.K. Turekian (Editors), Treatise on Geochemistry. Elsevier, USA, pp. 357–381.
Dentith, M. and Mudge, S., 2014. Geophysics for the Mineral Exploration Geoscientist. Cambridge University Press, New York, 454 pp.
Holden, E.J, Fu, S.‌C., Kovesi, P., Dentith, M., Bourne, B. and Hope, M., 2011. Automatic identification of responses from porphyry intrusive systems within magnetic data using image analysis. Journal of Applied Geophysics, 74‌(4): 255–262.
Hoschke, T.‌G., 2011. Geophysical Signatures of Copper-gold Porphyry and Epithermal Gold Deposits, and Implications for Exploration. ARC Centre of Excellence in Ore Deposits, University of Tasmania, Tasmania, 47 pp.
Sillitoe, R.H., 2010. Porphyry-copper systems. Economic Geology, 105‌(1): 3–41.