Pluton Vectoring

‘Pluton vectoring’ refers to the method of mineralogically, geochemically, and geographically tracking a given magma-metal series through a staged rock system/porphyry metal sequence with a level of specificity and predictivity not previously attainable (Keith, 1994).

Beginning in 1994, field application of the magma-metal series approach to specific client portfolios for copper or gold in Peru, Chile, and Nevada led to the discovery that intrusive sequences showed specific and predictive differentiation geography with respect to a sequence of copper- and/or gold-rich fluid releases. The differentiation geography can be delineated by the use of alteration resistant, compatible trace elements, such as scandium and titanium. When integrated with metal dispersion information for the various fluid releases, three-dimensional models can be constructed from existing map and drill-controlled sampling. Analysis of this information leads to very specific and ultimately predictive drill hole targeting strategies. The overall application of this analysis is known as ‘pluton vectoring’. As can be seen from the successful contribution list (see back of text volume), pluton vectoring has been instrumental in the discovery of over nine gold and/or copper deposits. Some of these deposits have approached or will approach ‘giant’ size, especially Pascua in central Chile and Espanola in north-central Chile.

A number of key observations have arisen from the successful application of pluton vectoring that have revolutionary implications for porphyry metal exploration:

  1. Porphyry metal deposits typically represent the economic portions of a sequence of separate fluid releases that accompany a sequence of intrusives.
  2. The serial nature of a given fractional differentiation sequence allows the general construction of a paragenetic model that can be repeated in other cases where the serial parameters can be reliably established.
  3. Maps of these fractional differentiation sequences have revealed a laterality that allows specific and predictive map delineation of drill targets. To some extent, map views of the sequence constitute cross-sectional views of the entire differentiation process. Consequently, many porphyry metal systems have yielded new exploration targets by considering the laterality of the process.
  4. The lateral nature of the process implies an association with lateral structural features, especially strike-slip faults. An intimate association with strike-slip faulting has been established in all of the case histories outlined in the back of the text volume.

Probably the most important and revolutionary observation about pluton vectoring is that in most cases an exposed plutonic phase can be metal enriched as demonstrated by compositional and textural/geography as the physical source of a given fluid release. By establishing where the fluid actually originated in three-dimensional space, a fluid path can be worked out from the pluton source to the trap environment when combined with metal and element dispersion determined through collection of metallized and altered samples. The explorations can now work forward or backward in the magmato-hydrothermal process to identify the economically interesting portions of the system. Pluton vectoring is now routinely used in any project where the pluton component can be identified in surface outcrops or in subsurface geophysical expressions (especially with the aid of magnetics or gravity).

Some of the philosophical implications of the above observations for petrology in general and exploration in particular have been discussed by Keith (1998). An expanded abstract on ‘Porphyry Metallogeny, Bowen’s Yoke and Lateralism’ is included in the model book (Keith, 2002).