The Giant Pebble Cu-Au-Mo Deposit and Surrounding Region, Southwest Alaska: Introduction

Kelley, Karen D.; Lang, James R.; Eppinger, Robert G.

doi:10.2113/econgeo.108.3.397

Cited by 8 publications

(4 citation statements)

References 28 publications

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“…The eastern part of the deposit (Pebble East zone) is covered by an eastward-thickening wedge up to 600 m thick of interbedded, postmineralization sedimentary and volcanic rocks, which are, in turn, overlain by glacial sediments, whereas the western part of the deposit (Pebble West zone) is almost completely concealed by glacial sediments. An extensive drilling program from 1998 to the present has allowed 3D characterization of subsurface geology and alteration history that is rarely available at such scale Kelley et al, 2013;Lang et al, 2013). The Pebble deposit has also been surveyed using magnetic, gravity, and various electrical methods including induced polarization (IP), magnetotellurics (MT), and airborne electromagnetics (Rebagliati and Lang, 2009;Anderson et al, 2011;Pare et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

et al. 2013

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We combined aeromagnetic, induced polarization, magnetotelluric, and gravity surveys as well as drillhole geologic, alteration, magnetic susceptibility, and density data for exploration and characterization of the Cu-Au-Mo Pebble porphyry deposit. This undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks, presenting an exploration challenge. Individual geophysical methods primarily assist regional characterization. Positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit, likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration. The mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit, and mafic rocks also exhibit strong chargeability. Conductivity anomalies similarly reflect widespread sulfides as well as water-saturated glacial sediments. Mineralogical and magnetic susceptibility data indicate magnetite destruction primarily within the Cu-Au-Mo mineralized area. The magnetic field does not show a corresponding anomaly low but the analytic signal does in areas where the deposit is not covered by postmineralization igneous rocks. The analytic signal shows similar lows over sedimentary rocks outside of the mineralized area, however, and cannot uniquely distinguish the deposit. We find that the intersection of positive chargeability anomalies with analytic signal lows, indicating elevated sulfide concentrations but low magnetite at shallow depths, roughly delineates the deposit where it is covered only by glacial sediments. Neither chargeability highs nor analytic signal lows are present where the deposit is covered by several hundred meters of sedimentary and volcanic rocks, but a 3D resistivity model derived from magnetotelluric data shows a corresponding zone of higher conductivity. Gravity data highlight geologic features within the deposit, including shallow diorite sills that locally contain higher-grade mineralization. The results thus show ways in which an integrated survey approach might be used to distinguish zones of potentially economic mineralization.

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Section: Introductionmentioning

confidence: 99%

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

et al. 2013

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“…Two recent examples illustrate the point. Large tonnages of high-grade mineralization were discovered in 2005 at the Pebble porphyry Cu-Au-Mo deposit in Alaska, with a measured and indicated resource of 5942 Mt at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo (Kelley et al, 2013); however, development has been blocked on environmental grounds. Similarly, a large high-grade resource was delineated in 2011 at the Reko Diq porphyry Cu-Au deposit in western Pakistan (Razique et al, 2014), with a mineable resource of 2200 Mt at 0.53% Cu and 0.30 g/t Au (Tethyan Copper Company, http:// www .tethyan .com /the-reko-diq-project /reko-diq-resource/, accessed 1 November 2018), but development has been blocked on political and legal grounds.…”

Section: Non-geological Factorsmentioning

confidence: 99%

Porphyry copper deposit formation in arcs: What are the odds?

Richards

2021

Geosphere

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Arc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to +2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their relatively high oxidation state limits the extent to which sulfide phases separate from the magma, which would otherwise tend to deplete the melt in chalcophile elements such as Cu (highly siderophile elements such as Au and especially platinum-group elements are depleted by even small amounts of sulfide segregation). As these magmas rise into the crust and begin to crystallize, they will reach volatile saturation, and a hydrous, saline, S-rich, moderately oxidized fluid is released, into which chalcophile and any remaining siderophile metals (as well as many other water-soluble elements) will strongly partition. This magmatic-hydrothermal fluid phase has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au deposits) if its metal load is precipitated in economic concentrations, but there are many steps along the way that must be successfully negotiated before this can occur. This paper seeks to identify the main steps along the path from magma genesis to hydrothermal mineral precipitation that affect the chances of forming an ore deposit (defined as an economically mineable resource) and attempts to estimate the probability of achieving each step. The cumulative probability of forming a large porphyry Cu deposit at any given time in an arc magmatic system (i.e., a single batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and less than 1/10 of these deposits will be uplifted and exposed at shallow enough depths to mine economically (0.0001%). Continued uplift and erosion in active convergent tectonic regimes rapidly remove these upper-crustal deposits from the geological record, such that the probability of finding them in older arc systems decreases further with age, to the point that porphyry Cu deposits are almost nonexistent in Precambrian rocks. A key finding of this paper is that most volcanoplutonic arcs above subduction zones are prospective for porphyry ore formation, with probabilities only falling to low values at late stages of magmatic-hydrothermal fluid exsolution, focusing, and metal deposition. This is in part because of the high threshold required in terms of grade and tonnage for a deposit to be considered economic. Thus, the probability of forming a porphyry-type system in any given arc segment is relatively high, but the probability that it will be a large economic deposit is low, dictated to a large extent by mineral economics and metal prices.

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“…Anderson provided the first case history of the session and examined regional and detailed aeromagnetic data around the Pebble Cu-Au deposit Alaska. Pebble is one of the largest underdeveloped copper and gold deposits in the world and was the subject of an extensive geological, geochemical and geophysical assessment by the USGS (Kelley et al 2013). Pebble itself lacks a strong direct magnetic signature but modelling shows that there is a very large magnetic high underlying the deposit area.…”

Section: Bumps Workhopmentioning

confidence: 99%

More than bumps: ASEG-PESA 2015 post conference workshop summary

2015

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The Giant Pebble Cu-Au-Mo Deposit and Surrounding Region, Southwest Alaska: Introduction

Cited by 8 publications

References 28 publications

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

Porphyry copper deposit formation in arcs: What are the odds?

More than bumps: ASEG-PESA 2015 post conference workshop summary

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