Magnetic properties related to hydrothermal alteration ...

Miner Deposita DOI 10.1007/s00126-014-0514-7

ARTICLE

Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit, northern Chile K. Riveros & E. Veloso & E. Campos & A. Menzies & W. Véliz

Received: 23 February 2013 / Accepted: 5 March 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (Kbulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest Kbulk values (>3.93×10−3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low Kbulk values (60 μm in size, which are frequently intergrown with secondary biotite (Fig. 5a). Some of the magnetite crystals are in contact with titanohematite that is altered to sphene and rutile (Fig. 5b–g). Similar amounts of magnetite were observed in chloritized andesites, although strong oxidation has occurred along microfractures (Fig. 5h–j). Fe–Ti oxide minerals and their relationship to each of the hydrothermally altered rock units are summarized in Table 1. The potassic–chloritic-altered monzodiorite contains 1–2 vol.% of Fe–Ti oxide minerals, disseminated throughout the groundmass. Optical and EDS analyses indicate that these Fe– Ti oxide minerals are hematite, with some exsolved ilmenite (~1 vol.%) and homogeneous magnetite grains (~1 vol.%) that are typically >100 μm in size (Fig. 6a). The homogeneous magnetite occurs as isolated crystals in contact with hematite– ilmenite association (Fig. 6a). In some cases, the hematite– ilmenite association is completely occluded by magnetite (Fig. 6a), suggesting that the magnetite crystallized later than the hematite–ilmenite. The hematite–ilmenite association displays mostly graphic microtextures (Fig. 6b) and lamellar and trellis-like microtextures. EDS analyses reveal that these microtextures are formed by a strong partitioning of Fe and Ti between the host crystal and the exsolved regions (Fig. 6c–d), suggesting a high temperature oxy-exsolution process for their formation (Haggerty 1991). The homogeneous magnetite compositions are similar to those of the altered andesites (Fig. 6e–f), which could indicate a common origin for these mineral phases; however, no optical evidence of maghemitization processes was observed within these crystals. Based on the high Ti content detected in some exsolved ilmenite regions, it is postulated that ilmenite was partially altered to ferri-rutile (metailmenite), whereas high concentrations of Si, Ca, and Ti in other exsolved regions confirm that alteration of ilmenite to sphene also occurred (Fig. 6g–i). In contrast, most of the Fe–Ti oxide minerals cannot be distinguished by optical methods where the monzodiorite is affected by both an intense quartz–sericite and argillic alteration. In these rocks, paramagnetic rutile is the most common relict Fe–Ti oxide phase, representing 20 μm in size, is disseminated throughout the groundmass or hosted by completely altered phyllosilicates. The rutile crystals are typically bordered by undifferentiated and poorly crystallized iron hydroxides

(“limonites”). Some of the argillic-altered samples have been affected by moderate to intense supergene oxidation, resulting in pyrite crystals being completely altered to goethite. These grains are commonly associated with hematite and rutile

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ƒFig. 4

Representative photographs and photomicrographs of various altered rock units sampled within the Escondida deposit. Images were taken using both binocular lens (left, a–i) and light microscopy (right, b–j). a–b Potassic-altered andesite showing a well preserved porphyritic texture and groundmass intensely altered to a fine-grained biotite (sampling site E-1). c–d Chloritic-altered andesite displaying a partially obliterated texture. Ferromagnesian minerals have been intensely altered to ferric chlorite whereas plagioclase has been moderately altered to sericite (sampling site E-12). e–f Potassic-chloritic-altered monzodiorite displaying moderate alteration of plagioclase to K-feldspar and slight chloritization of the ferromagnesian minerals (sampling site E-4). g–h Quartz–sericite-altered monzodiorite in which the original texture has been completely obliterated by intense alteration of plagioclase to sericite and recrystallization and grain size reduction of quartz (sampling site E-9). i–j Argillic-altered monzodiorite exhibiting complete replacement of plagioclase by kaolinite (sampling site E-17). Photomicrographs b, d, and f were taken under plane-polarized light; h and j were taken under cross-polarized light

(Fig. 7c, e, f). Supergene oxidation of pyrite crystals forms characteristic nanometer-scale Fe-rich structures along the cleavage (Fig. 7d).

Fig. 5 Electron and reflected-light photomicrographs and semiquantitative chemical maps of Fe–Ti oxide mineral assemblages in andesite host rock. a Homogeneous magnetite crystal intergrown with secondary biotite (sampling site E-1). b Magnetite crystal associated with titanohematite altered to sphene and rutile (sampling site E-1). c Chemical distribution in b showing Fe (red), Ti (blue), and Ca (green) (sampling site

Rock magnetic properties Bulk magnetic susceptibility Bulk magnetic susceptibility (Kbulk) of rocks is the sum of the contributions of all the ferromagnetic, paramagnetic, and diamagnetic mineral phases present in the rock (Tarling and Hrouda 1993). Hydrothermally altered rocks from the Escondida porphyry Cu deposit display a wide range of Kbulk values. However, each altered lithological unit shows restricted and distinctive ranges of Kbulk, which are presented diagrammatically as box-plots (Fig. 8). The highest Kbulk values correspond to andesites affected by potassic and chloritic alteration, indicating that magnetite is the main magnetic carrier in these rocks. Slightly lower Kbulk values were measured for potassic–chloritic-altered monzodiorite, which is consistent with the observed low abundance of magnetite in these rocks. Significantly, samples from monzodiorite that

E-1). d–g Fe, Ti, Ca, and Si distribution in zone b (bright intensity indicates major element concentration) (sampling site E-1). h–i Homogeneous magnetite crystals oxidized to maghemite along microcracks (from chloritic alteration zone) (sampling site E-12). j Chemical distribution in i showing Fe (red), Ti (blue), and Si (green) (sampling site E-12)

Miner Deposita Table 1 Summary of lithological, alteration, and Fe–Ti oxide mineralogy data for all samples analyzed in this study Sampling site

No. of specimens

Lithology

Alteration

Main Fe–Ti oxide minerals

E-1, E-3

22

Andesite

Potassic (biotitization)

E-12

14

Andesite

Chloritic

E-2, E-4, E-5, E-6, E-7, E-8, E-10, E-13, E-14 E-9, E-15, E-20, E-22, E-23

102

Monzodiorite

Potassic-chloritic

54

Monzodiorite

Quartz-sericite

83

Monzodiorite

Argillic

Homogeneous magnetite (~5–10 %) and scarce titanohematite exsolved to rutile-sphene Strongly fractured and dissolved magnetite (~5–10 %) altered to maghemite along microcracks Homogeneous magnetite (~1 %) and hematite exsolved to ilmenite/ferri-rutile (~1 %) Rutile (