Daniel T. Maughan and Dr. Jeffrey D. Keith, Geology
Mafic alkaline magmas are unusually oxidized, volatile-rich, primitive magmas that are derived from the earth’s mantle when portions of the crust are being extended (stretched). The occurrence of alkaline magmatism is beginning to be recognized as one of the main criteria necessary for the formation of some types of metallic ore deposits through out the world. Alkaline magmas likely deliver metals and volatile gasses to the base of magma chambers, both of which are requisite for the mineralization of ore deposits. The Bingham district, Tintic district, Park City district, Alta district, and other smaller mining districts in north-central Utah all have alkaline rocks that are spatially and genetically related to their ore deposits.
Thirty three samples were collected from seven areas in northern Utah. The collected samples have several things in common. The collection areas all occur along the Uinta- Cortez axis, a crustal weakness defined by an Archaen-Proterozoic plate boundary. The Uinta-Cortez axis extends along the Uinta mountains, and then passes through Park City, Brighton, Point of the mountain, Bingham, and then out into the west desert. Chronologically, the samples all have an age of 39 ± 1.5 Ma. Whole rock analyses indicate that the samples are all alkaline but with mafic to intermediate compositions, in other words they have high K+ Na contents and variable SiO , contents. Because many of the samples were collected in mining districts, considerable hydrothermal alteration has taken place, (like what is happening to the rocks of Yellowstone Park).
Apatite and mica mineral compositions from eight samples were analyzed by an electron microprobe. Funding for the analysis was provided by the ORCA grant. Of particular interest were the contents of Cr and volatiles ( Cl, F, OH, and S) within the minerals. Cr is an element whose contents are unaffected by alteration, and large amounts indicate a primitive nature to the minerals (high Cr is associated with minerals formed in the mantle).
Mica volatile compositions proved to be dependent upon hydrothermal alteration. Mica Cr amounts were stable in hydrothermally altered and unaltered minerals and reflect a primitive nature to their formation. The mica compositions were compared to those probed by Waite (in prep).
Apatite compositions reflect the relative volatile contents of their magmas at different pressures. Those rocks that were emplaced close to, or at the surface of the earth had little or no volatiles left in their apatites, whereas those emplaced deeper in the crust retained significant amounts of volatiles. The relative amount of volatiles in apatite is much less than the amount of volatiles in its magma, which is much less than the amount of volatiles released by a magma into the atmosphere. Because S has a different molecular location in crystals (this location is less susceptible to alteration) than do other volatiles in apatites, a linear trend is seen on unaltered rocks whereas altered rocks have enriched Cl and variable S. This research is the first to use S compositions in alkaline apatites to determine volatile compositions, and five more samples are now being probed so as to set standards to be used.
References
- Waite, K.A., in preparation, The petrogenesis of the intrusive and volcanic rocks in the vicinity of the Bingham porphyry copper deposit, Utah: Unpublished M.S. Thesis, Brigham Young University, Provo, Utah.
