Laboratory of Mineral Deposits Research

Laboratory for Mineral Deposits Research

This laboratory has a long and active history of allowing undergraduates and high school students to perform research alongside their research group. For information about high school, undergraduate, and graduate research opportunities and internships, please contact Phil Candela or Phil Piccoli.

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By a combination of physical and chemical modeling, we have attempted to explain why mineral deposits of certain types (e.g. high Mo/Cu porphyry deposits or deposits with high W/Mo ratios) are associated with particular intrusive types. Specifically, we (Phil Candela and Phil Piccoli) have been developing a chemical model for progressive devolatilization of both vapor and brine from a crystallizing silicate melt. The model is unique in its ability to simulate the partitioning of any number of elements among melt, vapor, brine and crystalline phases simultaneously, accounting for inter-element effects (e.g., how the concentration of HCl in the magmatic volatile phase affects the concentration the FeCl2 or CuCl concentration in a magnetite-saturated magmatic system at a given f(O2 ) ,T, and P). The modeling suggests chemical conditions that maximize the likelihood of ore formation for a given ore metal, and for a given set of extensive and intensive variables. Models of metal and HCl partitioning can be used to predict the concentration of non-sulfurous components in a geothermal (or ore-generative) magmatic - hydrothermal fluid at its source, and, by considering fugacities of sulfur and oxygen, can be used to predict model compositions of the hydrothermal fluids.

Critical experiments are necessary to allow the quantitative simulations discussed in Part 1.* The behavior of Cu, Au, Mo, W, S, HCl and Cl in melt/crystal/volatile phase systems* Experimentation on vapor/melt partitioning, and on brine alteration of minerals and other phases, has been performed by numerous workers in our lab, past and present (including, currently, Mark Frank, Matt Hall, Phil Piccoli and Phil Candela, and recently, Pedro Jugo and Tom Williams). (Philosophical note): The "partitioning" of chlorine between melt and magmatic volatile phases is the cumulative result of multiple equilibria, and chlorine partitioning must be examined in this light;* The behavior of elements of isotopic importance (e.g. Li) in melt-crystal-volatile systems* Lithium, and lithium isotope partitioning between chloride-bearing solutions and quartz, muscovite and other minerals is being performed in collaboration with the Isotope Geochemistry Lab at the University of Maryland. (LMDR: Steve Lynton, and Phil Candela).

For the past few years, my research group at Maryland has been studying granites and felsic volcanic rocks in the western US, and I have also examined granitic rocks in eastern Australia in collaboration with Phil Blevin and Bruce Chappell (ANU Geology Dept.). This work has included studies of the halogens in apatite of felsic plutonic and volcanic rocks as an indicator of halogen fugacities, and as a means to calculate model concentrations of Cl in melts. Phil Piccoli and I published on this technique in AJS in 1994. That paper documents the first estimate of magmatic Cl concentrations in the melt phase precursor to a granite suite (the Tuolumne Intrusive Suite of the Sierra Nevada Batholith). We have recently applied this technique to a shallower system, the Billy Lake - Rush Creek granite - aplite complex of the Ritter range, south of Yosemite National Park, with good results. A recent experimental study by Tom Williams yielded preliminary data for the equilibria controlling HCl in the magmatic aqueous phase. The equilibrium constants for these equilibria allow the concentration of HCl in a magmatic volatile phase of a given äCl concentration to be estimated if the Aluminum Saturation Index for the vapor-saturated melt is known. By using these results, we can calculate the HCl /äCl in a volatile phase associated with a given granitic melt. Therefore, by a combination of apatite chemistry (Piccoli and Candela, 1994) and aplite ASI, we can estimate parameters such as melt Cl and volatile phase HCl concentrations. These are important parameters for evaluating the ore potential of magmatic systems, and also for estimating HCl inputs to the deep levels of geothermal systems. It may be possible to characterize magmatic provinces in terms of parameters that are important in magmatic-hydrothermal ore formation, including initial magmatic chlorine and water concentrations, pressure, oxygen fugacity, and magmatic crystallization history. Field studies have also included the examination of granite textures. Phil Blevin (ANU) and I examined the relationship between granite textures and magmatic volatile phase (MVP) exsolution. We described (in a paper in press in Economic Geology) a texture that we refer to as an "Interconnected Miarolitic Texture ". Given that miarolitic cavities are accepted as evidence of MVP saturation in magmatic systems, the extension of the miarolitic texture in three dimensions, connecting miarolitic cavities, can be taken as evidence of an interconnected MVP. We have described this texture now from a number of localities in eastern Australia. This suggests further research, such as: 1. examining granitic textures experimentally, including crystallization experiments on brine- saturated melts (Bryan Stuller, Senior thesis project in progress, 1995); 2. modeling the physics of MVP exsolution from crystallizing and decompressing magmas; and, 3. performing further field work on granite texture in conjunction with field geochemical studies of granites and studies of any associated mineralization (including analysis of fluid inclusions therein). I am also interested in the relationships between mineralization and pluton shape (which will affect the physical aspects of volatile phase exsolution). 4.Piccoli and Candela have also been involved in studies of ore metals in accessory phases in granitic rocks, and in associated high temperature veins. This is an ongoing project that is still in progress. Related topics include estimates of oxygen fugacity, pressure, and initial water concentrations in granite rocks. Kent Ratajeski (M.S.,1995, LMDR, Geology, UMCP) and Phil Candela are preparing a paper on the estimation of intensive parameters for some shallow, ore- related granitic systems in Nevada, USA.

This research is supported by the National Science Foundation and the Department of Energy.