OCGC Seminar - Dr. Julian Pearce

   

Immobile Element Fingerprinting of Igneous Rocks:

Applications to the Development of Ore Deposit Models

 

Dr. Julian Pearce 
School of Earth and Ocean Sciences, Cardiff University

Thursday, April 30th, 2015
11:30 a.m.  

3120 HP
Carleton University 

 

Abstract

Geochemical fingerprinting of igneous rocks in the Geological Record is now a well-established procedure. In whole-rocks, methodologies are based on the so-called immobile elements, elements which are relatively unaffected by weathering, metamorphism and (around hydrothermal ore deposits) wall-rock alteration. These are typically elements in III to V oxidation states with intermediate ionic radii. Fingerprinting normally involves three stages: 1) classification of rock type using an immobile element proxy diagram (e.g. Zr/Ti-Nb/Y as proxy for the TAS diagram); 2) identification of tectonically defined magma type (e.g. Th/Yb-Nb/Yb which distinguishes MORB and OIB from subduction-related and continental magma types); and 3) petrogenetic diagrams which provide more detail on the precise tectonic setting. For VMS-type deposits, it has long been apparent that mineralization may be associated with almost any magma type, but immobile element fingerprinting does have uses in 1) placing the district in a tectonic context, 2) giving a name to highly altered rocks, 3) quantifying alteration through immobile-mobile element relationships; and 4) identifying lava unit boundaries, which are often the preferential sites of mineralization. Interestingly, the immobile element-based tectonic results are not in agreement with the widely-used lithotectonic interpretations of VMS deposits by Franklin et al. (2005) or Mosier et al. (2009), notably demonstrating that, with the obvious exception of Kuroko deposits (sensu stricto), subduction is much less significant overall than these authors claim. Fingerprinting can also work well on plutonic rock types, where it usually easier to find rocks and/or minerals that have escaped alteration. As with lavas, the methods have their applications and limitations in contributing to mineral deposit models. For example, the well-known granite discrimination diagrams based on Y, Nb and Rb have been used to predict that type of mineralization likely to be associated with agranite of given composition. For each deposit type, however, more detailed petrogenetic diagrams are usually needed: for example, granites associated with porphyry copper deposits have been shown not just to have volcanic arc settings, but also a low-HREE (adakite) character, which may relate to a particular, if debated, stage of subduction zone evolution; apparently fresh rocks can also have low-MnO, which may relate to its history of fluid loss.