Magma-wallrock interaction in crustal magma chambers (a process known as crustal assimilation) is critical to the thermodynamic and chemical evolution of a magmatic system and formation of many of the most economically important base and precious metal deposits on Earth. Although such generalized model is largely accepted, details on how these interactions take place are relatively poorly characterized. One of the major issues has been the lack of models that integrate mass and energy exchange, thermodynamics and geochemistry. The widely used assimilation-fractional crystallization (AFC) model does not provide any hint on whether its results are thermodynamically feasible or not. These limitations may significantly impact the mass balance of crustal and magma sources in the models, and thus obscure the constraints on the generation and identification of valuable metal deposits.
We propose to explore the petrologic and geochemical impact of magma-wallrock interaction at major intrusive complexes in, for example, Antarctica, United States, and Finland in a multidisciplinary study. Its central part is computational modeling using recently developed energy-constrained equations (Magma Chamber Simulator = MCS). The models add thermodynamic constraints for a multicomponent + multiphase magma body that crystallizes in contact with a crustal wallrock and is recharged with batches of fresh magma. The results of the models will be tested against existing and potentially new geochemical data and state-of-the-art wallrock partial melting experiments. In the experiments, the goal is to melt the wall rock alone, and to melt the wall rock together with the resident magma.
The outlined research plan is first of its kind, combines world-class expertise of different aspects of the issue, and is expected to provide unprecedented insight into the relative contributions of magma and crust to the formation of layered intrusions and associated ore deposits. The results should be of great interest to both academic and non-academic institutions and companies. An expected innovative outcome of the proposed project is that the results of research in the field of petrology and geochemistry are implemented in ore deposit models applicable to ore exploration. For example, mappings of “thermodynamically feasible” magma-wallrock pairs can potentially lead to new discoveries. The project is funded by the Academy of Finland.
The objectives will be approached in four research packages, last of which is composed of three case studies.
1. Sampling and whole-rock geochemical analyses
Drill core and sample archives of the case-study intrusions will be reviewed. Key samples (e.g., representative wallrock, the most primitive portions of the intrusions or related dikes) will be sent to a geoanalytical laboratory for major and trace element and isotopic analysis if necessary (i.e. no previous data available). Representative samples for wallrock melting experiments will be collected.
2. MCS learning
Learning and applying MCS (Bohrson et al., 2014) are the most important activities for the proposed research. MCS will address significant petrological questions such as:
- What magma temperature and volume are required to melt wallrock?
- How do multiple recharge and assimilation events affect mineral formation and elemental chemistry of the magma?
- What are the relative contributions from magma and wallrock to important ore-forming elements?
MCS, which utilizes Mac-based software developed by Drs. Bohrson and Spera, is labor-intensive to learn and thus must be learned under supervision of the developers. After the learning period at United States, during which one of the case studies is used as a testing environment, Dr. Heinonen will be one of a small number of highly experienced users able to install and teach the code.
3. Experimental petrology and analysis of resulting phases
The MCS constraints that are usually the most difficult to estimate, are the trace element partition coefficients and partial melting behavior of the wallrock. The planned partial melting experiments can be assessed using the wallrock melting function in MCS. That is, we can directly compare the partial melt compositions from the experiments to those predicted by MCS. This “cross-checking” would be the first of its kind for MCS, and is both innovative and will have impact on our understanding of how MCS does in melting certain kinds of wallrock.
To achieve this aim, melting experiments on representative wallrock samples will be performed at crustal pressures. Our goal is to melt the wall rock alone, and to melt the wall rock together with the resident magma. The latter approach has never been done before either and the peritectic reactions involved may lead to a very different mineralogy that may further improve our understanding of assimilation mechanisms. The resulting glass (and residual mineral phases) will be analyzed for major and trace elements.
4. MCS modeling
MCS modeling and partial melting experiments are utilized to understand the magma-wallrock interaction, magmatic evolution ± ore formation in three different case studies which are described here.