Geology, Chemostratigraphy, and Alteration Geochemistry of the Rosemont Cu-Mo-Ag Skarn Deposit, Southern Arizona

In 2014, Hudbay acquired control of Augusta Resource Corporation and its Rosemont project, an open-pit copper-molybdenum-silver skarn deposit located in the Laramide porphyry belt of Arizona, 40 km to the southeast of Tucson. The deposit contains over a billion tons of mineralized rocks hosted dominantly within a Paleozoic chemical sedimentary sequence comprising limestone, dolostone, marlstone, siltstone, mudstone, chert, and quartzite. Mineralized Tertiary quartz-feldspar porphyries, small relative to the deposit size, intrude the Paleozoic sequence and are controversially thought to be the source of mineralization and calc-silicate metasomatism. The Paleozoic sedimentary sequence forms an east-dipping homoclinal sequence which is truncated and non-conformably overlain by Mesozoic and Cenozoic siliciclastic and volcanosedimentary rocks which include arkose, siltstone, andesite flows, and localized basal limestone conglomerate. The Paleozoic and Meso-Cenozoic sequences are grouped into two panels: Rosemont lower and upper plates, respectively. Calc-silicate alteration in the lower plate is characterized by fine to very fine-grained skarn facies such as garnet-, pyroxene-, wollastonite-, and serpentine-skarn. Alteration in the upper plate is dominated by massive fine-grained epidote alteration.

Although the stratigraphy of the area is very well constrained, the deposit-scale alteration and recrystallization complicate the visual identification of rock types. These problems result in inconsistent lithological and stratigraphic observations within and between drill holes introducing uncertainties in the reconstruction of the deposit stratigraphic domains. In addition, the characteristic fine-grained calc-silicate alteration of the deposit renders visual estimations of skarn alteration highly uncertain. To solve problems arising from uncertain geological observations, this study presents a geochemical data analysis applied to 4-acids multi-element geochemical data obtained from drill core samples of the Rosemont deposit. Geochemistry covers the full length of 90 diamond drill holes drilled by Hudbay distributed representatively along the mineralized area.

The deposit lithogeochemical and chemostratigraphic attributes were reconstructed using hierarchical cluster analysis on compositional variables, principal component analysis, and centred ternary diagrams. In addition, a predictive model for calc-silicate alteration facies was built using geochemical variables as inputs and quantitative mineralogy, QEMSCAN and XRD, as training outputs. The geochemical modelling indicates that ore grades are controlled by the relative proportion of chemical to siliciclastic component of the mineralized rocks. Accordingly, higher grades are expected in relatively pure chemical sedimentary rocks with low content of siliciclastic component. In addition, the predictive skarn model reveals that calc-silicate alteration is controlled by a few small porphyries. Applying data analytics to multi-element geochemistry resolved issues related to uncertainties with visual geological observations and resulted in a proposed exploration model in which small porphyritic intrusions and relatively pure chemical sedimentary rocks are the key ingredients for the generation of large skarn deposits.

Sergio Gelcich

Sergio Gelcich is the Exploration Director at Hudbay Minerals.

He is in charge of global exploration with special emphasis in technical geological aspects. Sergio is part of a highly skilled geoscientific team that deals with green to brown field exploration and contributes also to the geological understanding of Hudbay’s current operations.,

Sergio has over 20 years of experience; he started his career as a project Geologist in the Geological Survey of Chile and after completing his PhD at the University of Toronto in 2006 has advanced his career through a number of positions in Toronto based junior and mid-tier mining companies. Prior to joining Hudbay, Sergio was the Regional (Americas) Exploration Manager for GlobeStar Mining. Sergio’s field experience includes almost every country in the Americas, his current focus is in Canada, US, Peru and Chile.

Sergio has a Honours Bachelor of Science in Geology and a Master of Science in Geology from Universidad de Chile; and a PhD in Geology from the University of Toronto.

Juan Carlos Ordonez

Juan Carlos Ordonez is as a geochemist with Hudbay Minerals and an adjuct professor at the Harquail School of Earth Sciences at Laurentian University. His key expertise includes applied geohemistry and field geology of mineral deposits. Juan Carlos approaches geochemistry from a compositional data analysis and data analytics perspective to better integrate geochemical data with geological, mineralogical, and geometallurgical data. He has an M.Sc and a Ph.D in petrology and geochemistry from, respectively, Shimane University in Japan and the University of Windsor in Canada. He has over 15 years of experience dealing with geochemical data in diverse mineral systems such as porphyry, skarn, VMS, IOCG, sedimentary hosted, and epithermal deposits in 10 different countries spanning Precambrian shields to Cordilleran settings.