The giant Antamina deposit, Peru: intrusive sequence, skarn formation, and mineralisation

Mrozek, Stephanie A. (2018) The giant Antamina deposit, Peru: intrusive sequence, skarn formation, and mineralisation. PhD thesis, James Cook University.

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View at Publisher Website: https://doi.org/10.25903/y152-2p45
 
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Abstract

The Antamina Cu-Zn skarn, in the central Andes of Peru, is the largest skarn in the world with resources of ~2,968 Mt averaging 0.89% Cu, 0.77% Zn, 11 g/t Ag and 0.02% Mo as of 2015. The deposit measures ~2.5 km long by ~1.5 km wide with a vertical extension > 2.2 km and remains open at depth. The host rocks are structurally stacked limestones and marls of the Cretaceous Jumasha and Celendín formations. Skarn and mineralisation occur in and around the Antamina Porphyry Complex (APC), which consists of 4 porphyry centres emplaced between 10.95 ± 0.20 Ma and 10.24 ± 0.23 Ma (U-Pb zircon). The core of the main ore zone is composed of 3 contiguous porphyry centres (Oscarina, Taco-Bornita, Usupallares) oriented lengthwise from NE to SW; the fourth centre (Condorcocha) is located ~1 km north of Taco. This study combines field observations of cross-cutting relationships with U-Pb (zircon) and Re-Os (molybdenite) geochronology into a spatio-temporal model that explains how Antamina became a giant ore deposit.

At least 11 intrusive phases were identified at the Antamina deposit. The major porphyry phases documented in each intrusive centre at Antamina are classified as P1 (early), P2 (inter-mineralisation), and P3 (late inter-mineralisation). Sub-phases are denoted in alphabetical order as P2a, P2b, etc. General characteristics of the major phases are as follows: P1 is the causative skarn-forming intrusion in each porphyry centre. P1 ages range from 10.95 ± 0.20 Ma (Taco) to 10.24 ± 0.23 Ma (Usupallares). Endoskarn and exoskarn are genetically related to P1 porphyries. Stockwork quartz veins (≤ 40% volume) and hydrothermal biotite (potassic alteration) are locally abundant in the central porphyry complex. P2 intrusions cut P1 intrusions, related skarns, and early quartz veins. Quartz stockwork veins (± pyrite ± chalcopyrite ± molybdenite) extend outward from P2 porphyries and cross-cut P1 skarns. Locally, P2 contains xenoliths (up to several m long) of P1 exoskarn, P1 endoskarn, and refractory quartz vein fragments. P3 cross-cuts P1, P2, and early veins. Neither P2 nor P3 contain endoskarn alteration, although they do contain veins and disseminations of secondary biotite overprinted by sericite-chlorite alteration.

Molybdenite mineralisation occurs in two stages across the Taco-Bornita and Usupallares zones. Stage I molybdenite occurs in skarns; Re-Os ages include 10.58 ± 0.07 Ma and 10.44 ± 0.05 to 10.39 ± 0.05 Ma. Stage II molybdenite + quartz veins cut across P2 and P3 porphyries and Re-Os ages include 9.99 ± 0.04 and 9.68 ± 0.05 Ma. In general, U-Pb zircon and Re-Os molybdenite ages decrease from northeast to southwest along the structurally-controlled axis of the deposit. Together, these ages constrain the duration of magmatic-hydrothermal activity at Antamina to ~1.1 Ma.

The skarns formed in sequence with porphyry emplacement, then coalesced to form a continuous body of skarn that comprises the giant Antamina deposit. Systematic patterns of alteration and mineralisation are centred on the multi-phase porphyry centre; outward from this centre, alteration consists of hydrothermal biotite (i.e., potassic alteration) transitioning into endoskarn, exoskarn, bleached marble, and fluid escape structures in the most distal reaches of the deposit. Skarn garnet colour changes from pink to red to brown in endoskarn and from red to brown and green (from proximal to distal) in exoskarn. Garnet becomes more andradite-rich and grossularite-poor from proximal to distal skarns, and clinopyroxene becomes more hedenbergite-rich and diopside-poor along the same trend. Mineralisation and metal zoning shows an outward progression from Mo ± Cu in the central porphyry, to Cu (± Ag, Bi)-Zn-Pb from proximal to distal exoskarns. The ore mineralogy is dominated by molybdenite, chalcopyrite, bornite, and sphalerite with lesser galena and minor Bi-Ag-S minerals. Fluid inclusions in a unidirectional solidification texture (UST) sample consist of four types assigned to primary and secondary assemblages. Primary inclusions are highly saline with liquid-vapour-solid phases and contain up to 5 translucent daughters plus a triangular opaque daughter. Secondary inclusions display variable proportions of liquid-vapour-solid phases, and are generally less saline and more vapour-rich than primary inclusions. Primary inclusions homogenise by halite disappearance, whereas secondary inclusions homogenise to the liquid state. Lithostatic pressure estimates from primary fluid inclusions range from 1.2 to 0.95 kbar, which equates to formation depth range from ~4.6 to 3.5 km.

The formation of the giant Antamina skarn deposit is attributed to the emplacement of multiple fertile porphyries along a NE-trending fault zone into reactive wall rocks between 10.95 ± 0.20 Ma (oldest U-Pb zircon age) and 9.68 ± 0.05 Ma (youngest Re-Os molybdenite age). Miocene surface uplift (on the order of 3.5 km), coupled with Pleistocene glaciation, exposed the top of the deposit, preserving the ore body at a favourable erosional level.

Item ID: 64498
Item Type: Thesis (PhD)
Keywords: skarn deposits, copper, zinc, Peru, Antamina, mineralisation, ore
Copyright Information: Copyright © 2018 Stephanie A. Mrozek.
Date Deposited: 29 Sep 2020 00:28
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 100%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840108 Zinc Ore Exploration @ 50%
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