Evolution of hydrothermal alteration facies at the Cerro Corona Cu-Au porphyry deposit, Northern

329 Pages·2017·21.11 MB·English

Evolution of hydrothermal
alteration facies at the
Cerro Corona Cu-Au
porphyry deposit,

Northern Peru

by

Jacob Longridge

Submitted in fulfilment of the requirements for
the degree of Doctor of Philosophy

Imperial College London
Department of Earth Science and Engineering

Submitted: November 2016
Declaration of originality
This thesis comprises of work that is my own except where due referencing and
acknowledgement is made.

Copyright Declaration
The copyright of this thesis rests with the author and is made available under a
Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers
are free to copy, distribute or transmit the thesis on the condition that they attribute it,
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Abstract
The Cerro Corona deposit is an active copper gold porphyry deposit, located in the
Hualgayoc District, Northern Peru. Its magmatic evolution is conformable with the current
regional model of crustal thickening, flat slab subduction and uplift. The deposit comprises
of multiple mineralogically and chemically indistinguishable biotite quartz diorite
porphyry intrusions, followed by intrusions of hornblende granodiorite and andesite
dykes. Igneous amphibole chemistry of these later intrusions suggests an increasing input
of deeper, mafic hydrous melts into a shallower magma chamber, similar to the paragenesis
at the nearby Yanacocha district.
Hydrothermal alteration is similar to other porphyry-type deposits, however the argillic
alteration telescopes onto the entire deposit, with pervasive argillic alteration extending to
a depth of 400m from the current surface. This suggests a significant uplift during
hydrothermal alteration.
Error is propagated through the mass transfer calculations, allowing for a more confident
interpretation of mass transfer models. These models show large net transfer of major and
minor components during potassic, propylitic and sericitic alteration, but less drastic
changes during argillic alteration. A 3D net mass transfer models is done for the first time
on a porphyry deposit, which reveals a previously unidentified zone of potassic alteration
in the north.
Infrared spectroscopy revealed the dominant clay mineral is smectite, with mixture of illite-
and/or kaolinite. Kaolinite-dominant argillic alteration appears to be related to sulphide
oxidation near the surface and telescopes down along preferentially permeable zones,
whereas smectite-dominant argillic alteration occurs at near neutral conditions, deeper
and peripheral to zones of sulphide mineralisation.
Intracrystalline oxygen isotope fractionation indicates illite formed at temperatures of
~300°C and kaolinite >150°C. The calculated fluid compositions suggests illite-formation
fluids are composed of >70% magmatic-derived fluids; kaolinite-formational fluids being
an equal mix of meteoric and magmatic fluids and smectite-formational fluid being
predominantly heated meteoric fluid.

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Acknowledgements

I would like to thank NERC and Goldfields La Cima for providing the funding that allowed
for this research, under the Collaborative awards in science and engineering studentship
(NERC CASE no.: NE/I018409/1).
Thank you Andy Wurst and Jamie Wilkinson for setting up this project, ensuring that I could
hit the ground running, helping to keep me on track and patiently reviewing my work.
Thank you to the Gold Fields team at Cerro Corona, especially Eddie Garcia, Angel Uzategui,
Edwin Ayala, Luis Armas, Lucia Torres, Roberto Rocha and Regina Baumgartner for your
help-from organising the logistics of my visits to patiently reviewing my reports.
Thank you to the staff at NHM, especially Jens Najorka for improving my understanding of
the intricacies involved in XRD. Thank you to the staff in the Imaging and Analysis Centre
of the NHM, especially Anton Kearsley and John Spratt for your unrivalled assistance in
answering all my questions related to electron microscopy.
At SUERC, I would like to thank Alison McDonald and Terry Donnelly for teaching me the
fundamentals of O and H isotope geochemistry. Thank you to Adrian Boyce for allowing me
the freedom to try new techniques and patiently editing and reviewing my isotope work.
I would like to thank my family for their strong encouragement that helped keep me going
through the tough days. To my friends and colleagues at Imperial College and the NHM -
cheers for keeping my enthusiasm in check-Will, Ed, Thilo, Jorge, Irina, Matt, Rob, Carmen,
Rachael, Adam, Simon, Han, Sarah and Yvette.
Thank you to my family as well as the Sykes family! Your patience, support, humour, roof
over my head and great craft beer helped get me through the darkest days of this write up.
I am indebted to the unwavering love and support from Meredith. I could not have done
this without you, thank you!

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Table of Contents
1 Chapter One: Literature review ........................................................................................................... 1
1.1 Porphyry ore-forming processes ............................................................................................... 1
1.1.1 Tectonic setting of porphyry deposits ........................................................................... 1
1.1.2 Metal zoning in porphyry systems................................................................................... 5
1.1.3 Alteration zoning in porphyry systems ......................................................................... 6
1.2 Techniques used in this thesis ................................................................................................. 11
1.2.1 Infrared spectroscopy ........................................................................................................ 11
1.2.2 Stable isotopes ...................................................................................................................... 14
1.3 References ........................................................................................................................................ 15
2 Chapter Two: Introduction.................................................................................................................. 19
2.1 Generic model for porphyry copper formation ................................................................ 19
2.2 The importance of argillic alteration ..................................................................................... 21
2.3 Case study of the Cerro Corona deposit ............................................................................... 22
2.4 References ........................................................................................................................................ 26
3 Chapter Three: Regional Geology ..................................................................................................... 29
3.1 Introduction ..................................................................................................................................... 29
3.2 Geological setting of the Cajamarca district ....................................................................... 32
3.2.1 Stratigraphy ........................................................................................................................... 32
3.2.2 Principal structures and tectonic development ...................................................... 35
3.3 Igneous geochemistry of Cajamarca District ...................................................................... 42
3.4 Uplift and erosion .......................................................................................................................... 50
3.5 Discussion ......................................................................................................................................... 50
3.6 References ........................................................................................................................................ 51
4 Chapter Four: Geology of Cerro Corona ......................................................................................... 58
4.1 Introduction ..................................................................................................................................... 58
4.1.1 The Hualgayoc mining district ....................................................................................... 58
4.1.2 Local stratigraphy and intrusive ages ......................................................................... 58
4.1.3 Structural geology ............................................................................................................... 65
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4.1.4 The Cerro Corona intrusive complex ........................................................................... 66
4.1.5 Hydrothermal alteration ................................................................................................... 68
4.2 New insights into the intrusive history at Cerro Corona .............................................. 70
4.2.1 Petrography............................................................................................................................ 72
4.2.2 Mineral chemistry ................................................................................................................ 74
4.2.2.1 Feldspar chemistry .............................................................................................................. 74
4.2.3 Geochemistry of igneous rocks ...................................................................................... 81
4.2.4 General alteration facies ................................................................................................... 86
4.2.5 Vein paragenesis ................................................................................................................ 101
4.2.6 Geological modelling of Cerro Corona ....................................................................... 105
4.3 Summary ......................................................................................................................................... 110
4.4 References ...................................................................................................................................... 115
5 Chapter Five: Alteration geochemistry ........................................................................................ 119
5.1 Introduction ................................................................................................................................... 119
5.2 Methods ........................................................................................................................................... 119
5.2.1 Geochemistry ....................................................................................................................... 119
5.2.2 Sample selection for geochemical mass transfer analyses ............................... 119
5.2.3 The isocon method ............................................................................................................ 125
5.2.4 Error estimates ................................................................................................................... 128
5.3 Results ............................................................................................................................................. 129
5.3.1 Potassic alteration ............................................................................................................. 130
5.3.1.2 High Field Strength Elements ....................................................................................... 131
5.3.1.3 Metals ...................................................................................................................................... 135
5.3.2 Propylitic alteration .......................................................................................................... 137
5.3.2.1 High Field Strength Elements ....................................................................................... 140
5.3.3 Sericitic alteration ............................................................................................................. 145
5.3.3.1 High Field Strength Elements ....................................................................................... 148
5.3.4 Argillic alteration ............................................................................................................... 153
5.3.4.1 High Field Strength Elements ....................................................................................... 155
5.3.5 Model of chemical mass transfer ................................................................................. 159
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5.4 References ...................................................................................................................................... 182
6 Chapter Six: Infrared spectroscopy of clay minerals .............................................................. 183
6.1 Introduction ................................................................................................................................... 183
6.2 Methods ........................................................................................................................................... 183
6.2.1 IR spectroscopy .................................................................................................................. 183
6.3 Results .............................................................................................................................................. 196
6.3.1 Major minerals in infrared spectroscopy ................................................................. 199
6.3.2 Mineral variance ................................................................................................................. 199
6.3.3 Future work.......................................................................................................................... 219
6.4 References ...................................................................................................................................... 219
7 Chapter Seven: Oxygen and hydrogen isotope analyses of argillic alteration ............. 223
7.1 Introduction ................................................................................................................................... 223
7.2 Methods ........................................................................................................................................... 223
7.2.1 Clay mineral separation and X-ray Diffraction analysis .................................... 223
7.2.2 Sample homogeneity ........................................................................................................ 227
7.2.3 Single mineral dehydroxylation temperatures ..................................................... 228
7.2.4 Oxygen isotope analysis .................................................................................................. 229
7.2.5 Hydrogen isotope analysis ............................................................................................. 232
7.2.6 Local meteoric waters ...................................................................................................... 234
7.3 Results .............................................................................................................................................. 234
7.3.1 Bulk mineral oxygen isotope results ......................................................................... 234
7.3.2 De-hydroxylated oxygen isotope results ................................................................. 235
7.3.3 Hydrogen isotope results ............................................................................................... 240
7.3.4 Local meteoric waters ...................................................................................................... 241
7.4 Discussion ....................................................................................................................................... 242
7.4.1 Estimation of clay formation temperatures ............................................................ 242
7.4.2 Calculation of fluid temperatures and compositions .......................................... 245
7.4.3 Modelling of exchanged meteoric and magmatic waters and mixing curves
250
7.4.4 Results of modelling exchanged fluids ...................................................................... 252
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7.5 Summary ......................................................................................................................................... 260
7.6 References ...................................................................................................................................... 261
8 Chapter Eight: Discussion .................................................................................................................. 265
8.1 Geodynamic setting of Cerro Corona and source magmas ......................................... 265
8.2 Mineralogical controls of chemical mass transfer effects ........................................... 267
8.2.1 Potassic alteration ............................................................................................................. 269
8.2.2 Propylitic alteration .......................................................................................................... 272
8.2.3 Sericitic alteration ............................................................................................................. 273
8.2.4 Argillic alteration ............................................................................................................... 276
8.2.5 3D Mass transfer modelling ........................................................................................... 279
8.3 The origin of clay alteration at Cerro Corona .................................................................. 283
8.4 Fluid flow model for the genesis of argillic alteration at Cerro Corona ................ 285
8.5 References ...................................................................................................................................... 285
9 Chapter Nine: Conclusions ................................................................................................................ 289
9.1 Development of mineral-bearing arc volcanism in northern Peru ......................... 289
9.2 Implications from mass transfer models of hydrothermal alteration in a porphyry
deposit ............................................................................................................................................................ 290
9.3 Inferences from stable isotope and infrared spectroscopy of clay minerals ...... 293
9.4 Future work ................................................................................................................................... 296
9.4.1 Regional and local geology ............................................................................................. 296
9.4.2 Alteration geochemistry ................................................................................................. 297
9.4.3 Infrared spectroscopy ...................................................................................................... 297
9.4.4 Oxygen and hydrogen isotopes .................................................................................... 297
9.4.5 Generic model...................................................................................................................... 298
9.5 References ...................................................................................................................................... 298

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List of Figures

Figure 1.1: Global distribution of porphyry deposits (USGS, 2017) and subduction zones. 1
Figure 1.2: Tectono-magmatic model for the emplacement of porphyry deposits (Richards,
2003). Hot zones at the base of the lower crust cause initial homogenisation of magmas (as
described in the text). Within the shallow crust, magma ascent is controlled by local
stresses. The magma rises to a state of neutral buoyancy, where it pools to form a
subvolcanic pluton. Note the change in scale in the upper crust. .................................................... 2
Figure 1.3: Fluid composition versus temperature diagram illustrating that under full
equilibrium CO becomes the major control of fluid acidity below ~350°C (Giggenbach
2
1997). ........................................................................................................................................................................ 4
Figure 1.4: Schematic vertical cross section of metal distribution in porphyry deposits
(Halley et al. 2015). ............................................................................................................................................. 5
Figure 1.5: Idealised distribution of alteration assemblages associated with a porphyry
deposit (Seedorff et al. 2003). ........................................................................................................................ 6
Figure 1.6: Phase diagram for the system K O-Al O -SiO -H O-KCl-HCl at atmospheric
2 2 3 2 2
pressure. The labelled areas broadly correspond to alteration assemblages. Mineral
abbreviations are: Cord-cordierite; And-andalusite; Ksp-potassium feldspar; Bi-biotite;
Chl-chlorite; Pyro-pyrophyllite; Musc-muscovite; Kaol-kaolinite; Ill-illite; Mont-
montmorillonite. Note that chlorite, cordierite and biotite stability fields are based on a
simple solid solution model for their Mg end members (Seedorff et al. 2005). ........................ 7
Figure 1.7: The stability of aluminosilicates is primarily a function of m /m and
HCl KCl
temperature. The red arrow shows the path that a vapour phase released from a degassing
magma is likely to take (i.e. low P degassing) (Giggenbach, 1997). ............................................... 8
Figure 1.8: Fluid pathway for magmatic liquids and vapours involved in the formation of
argillic alteration (Hedenquist et al. 1998) projected onto a phase diagram showing the
stability of minerals in the K O-Na O-Al O -SiO -HCl system (Sverjensky et al. 1991).
2 2 2 3 2
Dashed grey lines are approximate boundaries between clay minerals. Abbreviations for
minerals are: And=andalusite, Pyro(ph)=pyrophyllite, Dick=dickite, Kaol=kaolinite,
Musc=muscovite, Ill=illite, Smec=smectite and Ksp=potassium feldspar. ................................ 10
Figure 1.9: An example of the “vuggy quartz” from the Cerro Corona deposit. Note the rock
is dominated by dark silica replacement. ............................................................................................... 10
Figure 1.10: Location of absorption features commonly found that are related to clay
minerals. These absorption features are due to the combination of symmetrical stretching
(v1), H-O-H bending (v2) and asymmetrical OH-stretching (v3) of the water molecule
(shown in blue) and the combination of hydroxyl stretching (vOH in yellow) with metal (Al,
Fe or Mg) hydroxyl bending (δXOH in peach). ..................................................................................... 12
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Figure 1.11: An example of physical mixtures between dickite and kaolinite in 20 wt.%
increments (this study). The ~1380 nm absorption feature of dickite is more intense than
that of the equivalent ~1395 nm absorption feature of kaolinite. Thus, when using a linear
mixing algorithm for quantification, the dickite proportion is overestimated. ...................... 13
Figure 2.1: Schematic of the typical hydrothermal alteration pattern encountered in
porphyry-type ore deposits. These deposits have large alteration footprints; however, the
distribution of each type may vary significantly in extent and form from deposit to deposit.
From Sillitoe (2010). ....................................................................................................................................... 20
Figure 2.2: Percentage of global copper production in relation to classified resource type
(Gerst 2008). ....................................................................................................................................................... 21
Figure 2.3: Copper recovery from porphyry ores with increasing clay content. Adapted from
Bulatovic et al. (1998). .................................................................................................................................... 22
Figure 2.4: Maps of polygons coloured by: A) clay content; and B) copper recovery.
Reproduced from quality control maps (Uzategui and Ayala 2012). .......................................... 24
Figure 3.1 Northern Peruvian margin during the late Cretaceous. The Western Peruvian
Trough is defined by the Paracas High to the West and Maranon Geanticline to the East. The
Normal faults controlled sedimentation within this miogeocline. These faults were
subsequently reactivated during subsequent orogenic events. Adapted from Scherrenberg
et al. 2012. ............................................................................................................................................................ 29
Figure 3.2: Geological terrains of Northern Peru. The Cajamarca region lies west of the
Marañon Fold Thrust Belt (MFTB), within the Western Cordillera. The Cajamarca curvature
is defined by the change of the trend of predominant fold axial planes from northwest to a
west-northwest. The Huancabamba deflection is a large-scale change in fault trend from
NW (in Peru) to NE (in Ecuador and Colombia). The inset box (expanded in Fig. 3.5) shows
the area of study. Adapted from Mégard (1984). ................................................................................ 31
Figure 3.3: Schematic stratigraphic column for the Cajamarca district. Cretaceous
stratigraphy adopted from Wilson (1984). Cenozoic volcanism is incorporated from Chavez
et al. (2012). Decollement horizons are inferred from Eude (2014) .......................................... 34
Figure 3.4: Average convergence rate between the Nazca and South American plate over
time and approximate plate motion, based on plate reconstruction (Pardo-Casas and
Molnar 1987). Note the periods of high convergence coincide with deformation stages. . 36
Figure 3.5: Map of the Cajamarca district. Maps used: Wilson 1984, Noble et al.1990,
Macfarlane et al. 1990, Davies 2002, Longo 2005, Navarro 2012 ................................................ 37
Figure 3.6: Sequence of magmatic events in the Cajamarca district. Adapted from Davies
2002 ........................................................................................................................................................................ 41
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