Downloaded from http://geea.lyellcollection.org/ by guest on October 1, 2021 Accepted Manuscript Geochemistry: Exploration, Environment, Analysis Identification of the expression of earthquake-induced surface flooding by groundwater using detailed regolith mapping at the buried Atlántida Deposit, northern Chile Alexandra Brown, Peter A. Winterburn & Thomas Bissig DOI: https://doi.org/10.1144/geochem2018-064 Received 26 August 2018 Revised 13 February 2019 Accepted 14 February 2019 © 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London for GSL and AAG. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics To cite this article, please follow the guidance at http://www.geolsoc.org.uk/onlinefirst#cit_journal Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. 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Downloaded from http://geea.lyellcollection.org/ by guest on October 1, 2021 Identification of the expression of earthquake-induced surface flooding by groundwater using detailed regolith mapping at the buried Atlántida Deposit, northern Chile Short title: Surface expression of seismic pumping, Chile A. E. BROWN1*, P. A. WINTERBURN1,2 & T. BISSIG1, 3 1Mineral Deposit Research Unit, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, B.C., V6T 1Z4, Canada 2Current address: Vale Exploraciones Chile, Rosario Norte 615 #1202, Las Condes, Santiago, Chile 3Current address: Goldcorp Inc., 666 Burrard St #3400, Vancouver, B.C., V6C 2X8, Canada *Corresponding author (e-mail: [email protected]) Abstract This paper describes saline pockets (10 cm – 3 m in diameter) of fine-grained material distributed on alluvial surfaces. These saline pockets are localized along structural trends at the Atlántida buried porphyry-skarn Cu-Au-(Mo) deposit in the Atacama Region of Chile. The distribution and highly saline nature of the material suggests formation by the pooling and evaporation of groundwater forced through fractures to the surface during seismic activity. These saline pockets are a surface expression of the hydrological effects of seismic activity along faults. Saline pockets with similar distribution and characteristics were also identified at three additional alluvium-covered areas, all located in the Antofagasta Region of Chile. Identification and mapping of these saline pockets relies on the ability to identify the continuation of structures through overlying gravels. Regolith mapping using high- resolution drone imagery and digital elevation modelling identified geomorphic markers of faulting which aided in the mapping of the distribution of saline pockets. Saline pockets provide a unique opportunity to sample the direct expression of transported groundwater reaching the surface from depth and provide a prime target medium for mineralMANUSCRIPT exploration through transported gravels. Keywords: buried deposits, exploration, geochemistry, geomorphology, earthquake-induced surface flooding, seismic pumping, regolith mapping, Chile Chile annually produces about 36 % of the world’s copper (U.S. Geological Survey, 2016), and the Atacama Desert hosts some of the largest copper deposits in the world, including the Chuquicamata, Escondida and Collahuasi porphyry clusters. However, discovery of economically viable deposits exposed at or near surface has become increasingly difficult in the last decade (Kelley et al. 2010). Exploration is presently focused on copper mineralization buried under transported gravels which cover muchACCEPTED of the Atacama Desert (Fig. 1). Ongoing exploration for buried deposits includes geochemical methods able to detect mineralization through transported cover, along with other techniques such as alteration mapping and geophysical techniques. Geochemical signatures of buried deposits in the Atacama Desert have previously been reported in areas where faults extend to surface (Cameron et al. 2002). These faults may act as highly permeable pathways through gravel cover by which groundwater that has interacted with buried mineralization can reach the surface during seismic activity. Groundwater can be transported to the surface by dilation-compaction pumping in faults and fractures (Cameron et al. 2004), and by pore-space Downloaded from http://geea.lyellcollection.org/ by guest on October 1, 2021 collapse in unconsolidated sediments. This process can transport copper porphyry indicator elements as ions or oxyanions (Mo, Re, Se, As, Cu) to the surface (Cameron et al. 2002; Cameron & Leybourne 2005; Leybourne & Cameron 2008). Salt efflorescence above faults identified at the Mantos Blancos deposit in Chile (Palacios et al. 2005) is an example of a surface expression of the hydrological effects of seismic activity that occur in the Atacama Desert. The distribution and internal structure of the efflorescence suggests formation as a result of a high pressured air-water mixture being injected at depth into fractures that reach the surface (Cameron et al. 2010). The hyper-aridity of the Atacama Desert allows these features to be preserved on the surface. At the Atlántida deposit (Fig. 1), highly saline pockets of material distributed along structural trends on the alluvial surface were identified using detailed regolith mapping aided by high- resolution drone imagery (8 cm horizontal resolution) and digital elevation models (1 m resolution). Saline pockets were also identified over three additional alluvium covered prospects in the Antofagasta Region of the Atacama Desert (Fig. 1). Evidence from the distribution and characteristics of the saline pockets supports the hypothesis that they are a widespread surface expression of earthquake-induced surface flooding of groundwater. This process has been detected geochemically by Cameron et al. (2002) in the Atacama Desert, where groundwater transported along structures in the gravels to the alluvial/colluvial surface during seismic activity (Fig. 2) produces geochemical anomalies on the surface. Geological Setting Atlántida, Atacama Region Atlántida is a porphyry-skarn Cu-Au-(Mo) deposit located approximately 55 km NE of Copiapó, Atacama Region, Chile (Fig. 1). The property (~12 km2) was previously explored by First Quantum Minerals Ltd., and is currently owned by Minera Atlántida. The porphyry-style mineralization has no surface outcrop. Mineralization (defined as > 0.1 % Cu and > 0.1 g/t Au) at Atlántida is reported as 100% hypogene and occurs 200 m below the gravel-bedrock contact, extending to at least 600 m below the surface (Hope & Andersson 2015). Bedrock is covered by 25 m – 80 m of transported alluvial gravels over the entire mineralized zone. TheMANUSCRIPT water table is variable across the property, though generally located at 40 – 50 m below the surface. Atlántida lies between the Atacama and the Domeyko Fault Systems within the southern extent of the Central Depression (Bissig & Riquelme 2010). The porphyry is part of the Cretaceous porphyry copper belt (Maksaev et al. 2010). Two main intrusive phases are host to the bulk of mineralization and intrude into volcanic and limestone units. The earliest intrusive phase has a tonalitic composition and is interpreted as pre-mineralization (Hope & Andersson 2015; Pavez 2016). This phase is intruded by a granodiorite, feldspar-rich porphyry, characterized by brecciated contact zones with the tonalite intrusion (Hope & Andersson 2015; Pavez 2016). Local skarn and carbonate replacement polymetallic veins are exposed in limestone outcrops east and west of the buried porphyry. Atlántida has average grades of 0.2 % Cu and 0.34 g/t Au inclusive of the local skarn with a preliminary resource estimate of 427.1 M tonnes (Hope & Andersson 2015). Gravel cover at Atlántida is deepest in the south, where a large NE trending valley cuts through the propertyACCEPTED area (Fig. 3). The alluvial surfaces lack the development of a true desert pavement (Laity 2009), a common characteristic of gravel surfaces in the driest parts of the Atacama Desert. The unconsolidated gravels are dominated by clastic material below the cobble surface with varying occurrences of hard or soft calcrete starting at a depth of 20 cm – 40 cm. The gypcrete and soluble nitrate horizons commonly found in northern Chile, for example the gypcrete horizons within coalesced alluvial fan deposits spanning over 35 km in the Calama Basin, northern Chile (Hartley & May 1998), are not present at Atlántida. Geomorphological relationships between surfaces indicate there have been several erosional events causing incision of alluvial deposits by younger, fluvial or alluvial material. Older alluvial surfaces at Atlántida are dominated