Mineralization and timing of the Lover's Lane Breccia, Santa Rita Porphyry Cu-Mo Deposit, Grant County, New Mexico
AuthorHannink, Randall Lewis
AdvisorThompson, Tommy B
Geological Sciences and Engineering
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The Lover's Lane breccia at Santa Rita is a magmatic-hydrothermal breccia with several distinct stages of mineral development. Mineralized clasts of stockwork and skarn ore are present and truncated by the matrix of the breccia. The matrix of the breccia hosts locally abundant molybdenite and chalcopyrite, associated with primary matrix-forming quartz, orthoclase, and biotite, and also with hydrothermal alteration of the matrix. Fluid inclusion and stable isotope analyses suggest meteoric water infiltrated the breccia shortly after the matrix had formed, and accompanied much of the chalcopyrite and pyrite. Molybdenite is intimately associated with the matrix-forming assemblage, as a late breccia feature which uses the matrix as a fluid pathway, with hydrous alteration of breccia in zones of Mg-rich skarn, and in post-breccia veinlets that cross-cut the matrix. Documentation of cross cutting relationships during logging of drill core has led to the development of a model, in which the formation of the Lover's Lane breccia is a later phase of the 60 Ma Santa Rita system as a whole. Following intrusion of the Santa Rita stock and associated hypogene mineralization including the Cu- and Mo- mineralized Whim Hill breccia, a second stage of vapor bubble generation led to the brecciation in the Lover's Lane area. Magmatic fluid from this second event precipitated quartz, orthoclase, biotite, and sulfides, predominantly molybdenite, in the matrix. Meteoric influx into the breccia influenced various later alteration assemblages including chlorite-chalcopyrite and retrograde, hydrous alteration. Upon lithification of the breccia, a volumetrically minor breccia phase with an igneous matrix rebrecciated portions of the Lover's Lane breccia. Further magmatic crystallization produced fluids which entered the breccia, mixed with meteoric fluids present in the breccia, and precipitated quartz±molybdenite±anhydrite±pyrite veinlets. The Hanover Hole diatreme breccia, a deep, post-mineral infill breccia, cross-cuts mineralization in the area, and is only intruded by an unaltered, barren rhyodacite dike. Uranium/lead dating of zircons on selected intrusive phases that constrain the relative ages of the Lover's Lane breccia and Hanover Hole diatreme yielded ages which are several million years younger than previously determined Re-Os radiometric ages.