Emily Smyk's MSc Thesis Abstract

Thesis Title: 
A Comparison of the Hydrothermal Alteration Systems Around the Mo-hosting White Pine Intrusion, Utah, and the Buckingham Porphyry, Nevada

The White Pine Fork Mo porphyry has an estimated resource of 16 Mt of Mo at 0.1% Mo.  The host intrusions are K-feldspar- and quartz-porphyritic monzo- to syeno-granites characterised by LREE enrichment, fractionated HREEs and negative Nb, Ta, Sr and Ti anomalies, consistent with a subarc mantle melt source.  The granites also have an adakitic geochemical signature.  A U-Pb age for zircon from a clast in the mineralized breccia pipe at White Pine yielded an age of 26.52±0.42 Ma, which falls within the error of the White Pine intrusion age of 26.61±0.24 Ma.  Re-Os ages for duplicate samples of the Mo-mineralised quartz breccia are 30.21±0.14 and 29.84±0.15 Ma, which correlate with the age of the Little Cottonwood stock rather than the host intrusion and may represent Re-Os inheritance.  In contrast, Buckingham Mo (-Cu) porphyry has an estimated resource of 1,000 Mt of Mo at 0.1% MO and is hosted in Cretaceous K-feldspar- and quartz-porphyritic granites.  Four feldspar- and quartz-porphyritic granites in the area were dated using LA-ICP-MS U-Pb of zircon and yielded ages of 38.68±0.53, 39.28±0.58, 40.76±0.41, and 40.81±0.51 Ma and therefore unrelated to the Buckingham deposit, and instead are correlated with Tertiary magmatism associated with Au skarns in the nearby Battle Mountain district.  The Tertiary intrusions are feldspar- and quartz porphyritic granites.  Primitive mantle-normalized geochemistry of both suites of intrusive rocks have LREE enrichment, fractionated HREE, negative Nb, Ta and Ti anomalies and a slight enrichment of Zr and Hf, consistent with a subarc mantle source for both suites.

The most prominent alteration in both systems is phyllic alteration comprising an assemblage of white micas, quartz and pyrite. 
Potassic alteration was also observed at the White Pine Fork Mo breccia pipe and kaolinite and chlorite observed in SWIR data suggest advanced argillic alteration around the Buckingham system.  These petrographic observations are substantiated by the whole rock geochemistry.  The potassic, phyllic, and possible advanced argillic alteration were mapped out by the absolute values of trace elements.  The trace element geochemistry of quartz and pyrite can be used to fingerprint deposit types and as vectors toward mineralisation in alteration systems around ore deposits.  At White Pine Fork, the hydrothermal quartz is characterised by higher Ti and As than the igneous quartz.  The Li content of hydrothermal quartz is greater near the centre of the White Pine Fork deposit than in its margins.  At Buckingham, quartz in the breccia cement at the centre of the deposit shows the highest concentration of Al, Li, K, Ca, As, and Sb, and metals (i.e. Cu, Fe, Zn, and Pb), whereas the igneous and sedimentary quartz shows the highest Ti values.  The high values indicate that the primary Ti contents were not subjected to recalibration during hydrothermal alteration.  The Al and Sb contents of quartz decrease away from the centre of deposit at Buckingham.  This trend was not observed at White Pine Fork.

At White Pine Fork, pyrite occurs in a domain extending more than 1.5 km from the breccia pipe and is partially weathered to Fe-oxides and Fe-hydroxides.  LA-ICP-MS analyses showed Ni and Co compositional zoning within the pyrite grains.  The Ni content in the pyrite increases away from the centre of the White Pine Fork deposit.  At Buckingham, the pyrite was almost completely altered to Fe-oxides and Fe-hydroxides in most of the surface samples studied here.  The trace element concentrations of Au, Cu, and Cd in pyrite decreases away from the centre of the deposit.  Ni, Co and Pb zonations were also seen in the cores of the pyrite grains at Buckingham.  LA-ICP-MS element maps in most of the surface samples studied here showed that the pyrite at Buckingham is depleted in trace elements relative to the weathering rinds which are enriched in Au, Ag, Cu, As, Sb and Mo.  The increased Au concentration in the weathering rind suggests either an overprinting mineralised rind similar to the proximal skarns or Carlin-style sediment hosted Au.  The rind was then weathered during supergene alteration.  Another possibility is that the cores of the sulphides acted as traps for the precipitation of precious metals from the hydrothermal event associated with the proximal Eocene granite intrusions which were subsequently concentrated in the weathering rind during supergene alteration.

The comparison the White Pine Fork and Buckingham Mo porphyries has refined the processes associated with hydrothermal alteration around Mo porphyry deposits and the applicability of trace element chemistry of alteration minerals as exploration tools.