Lithogeochemistry and chemostratigraphy of the Rosemont Cu-Mo-Ag skarn deposit, SE Tucson Arizona: A simplicial geometry approach

Abstract

The Rosemont skarn deposit contains resources exceeding 1 billion tons of Cu-Mo-Ag mineralization. The deposit comprises three major structural-stratigraphic domains informally named Lower Plate, Upper Plate, and West Block. The Lower Plate is composed dominantly of Paleozoic chemical sedimentary rocks forming an east dipping homoclinal sequence. The Upper Plate overlies the Lower Plate and is composed dominantly of Mesozoic and Cenozoic siliciclastic sedimentary rocks. The West Block is dominated by Precambrian granitoids structurally interleaved with panels of Paleozoic chemical and siliciclastic sedimentary rocks. Most of the economic mineralization is hosted in the Lower Plate.Lithological variability, structural complications of the stratigraphy, and calc-silicate metasomatism complicates the visual identification of lithologies resulting in uncertain stratigraphic domains based on drill core observations. To circumvent this problem, a deposit-scale lithogeochemical and chemostratigraphic model was developed using multivariate statistics following a compositional data analysis approach to respect the relative scale and multivariate nature of geochemical compositions. Accordingly, hierarchical cluster analysis of variables using the variation matrix as a measure of similarity, and principal component analysis on centered logratio coefficients (clr) were used to explore relationships between variables and to reduce dimensionality. Simplicial projections including centered tetrahedral and ternary diagrams were used to develop a lithogeochemical classification for the sedimentary rocks of the Rosemont deposit. A ternary diagram, or 3-part simplex with vertices Ca, Mg, and a composite variable given by the geometric mean of Cr, Ni, Co, V, P, Hf, Zr, Th, Ti, Al, Nb, Sc, Ta, Y, Ce, and La contains a rich data structure for lithogeochemical classification.The lithological attributes of the Rosemont deposit can be subdivided into 7 lithogeochemical classes evident on the ternary diagram. Limestone, dolostone, and siliciclastic-crystalline classes are key end members. The remaining 4 classes represent mixed chemical-siliciclastic lithogeochemical attributes discriminating complex lithological variations induced by the incorporation of siliciclastic component in chemical sedimentary rocks. The lithogeochemical classification derived from the ternary diagram is supported by K-means clustering applied to two balances representing isometric logratio (ilr) coordinates of the 3-part simplex.The geospatial distribution of the lithogeochemical classes allows the simplification of the stratigraphy of the Lower Plate into three relevant chemostratigraphic units, namely a Lower Limestone Unit and an Upper Dolostone Unit separated by a Mixed Unit of chemical-siliciclastic sedimentary rocks. It is noteworthy that ore grades are controlled by the relative proportion of chemical to siliciclastic component of the mineralized rocks. Higher metal grades are characteristic of relatively pure chemical sedimentary rocks in the Lower Limestone and Upper Dolostone Units; whereas low metal grades characterize the Mixed Unit with a large proportion of siliciclastic component. As a corollary, the lithogeochemical classification can be used as a skarn fertility tool to predict the economic potential of chemical-siliciclastic sedimentary sequences in geological environments permissive of skarn mineralization.