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Mineralogy and Geochemistry of Carbonaceous Mudstone As a Vector to Ore: a Case Study at the Lagunas Norte High-Sulfidation Gold Deposit, Peru

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Mineralogy and Geochemistry of Carbonaceous Mudstone As a Vector to Ore: a Case Study at the Lagunas Norte High-Sulfidation Gold Deposit, Peru MINERALOGY AND GEOCHEMISTRY OF CARBONACEOUS MUDSTONE AS A VECTOR TO ORE: A CASE STUDY AT THE LAGUNAS NORTE HIGH-SULFIDATION GOLD DEPOSIT, PERU by Harry Hanneman A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date ________________ Signed: ________________________ Harry Hanneman Signed: ________________________ Dr. Thomas Monecke Thesis Advisor Golden, Colorado Date ________________ Signed: ________________________ Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering ii ABSTRACT The Lagunas Norte Au deposit in the Alto Chicama district of Peru is a ~14 Moz high- sulfidation epithermal deposit that is hosted by an atypical host-rock succession for this deposit type. Approximately 80% of the ore body is contained in the Lower Cretaceous Chimu Formation, which is composed of quartz arenite with interbedded carbonaceous mudstone, siltstone, and coal seams. The remainder of the ore is hosted by the Miocene volcanic rocks of the Calipuy Group, forming an irregular and thin veneer on the deformed sedimentary rocks of the basement. The host rock succession of the Lagunas Norte deposit has been affected by widespread hydrothermal alteration. The alteration is cryptic within most of sedimentary rocks as the quartz arenite was largely inert to alteration by the strongly acidic fluids. Vuggy textures associated with residual quartz alteration can only be recognized in the overlying Miocene volcanic rocks. However, the present study shows that mudstone of the Chimu Formation records acid-type alteration due to its originally high clay mineral content. The mineralogy of the mudstone broadly changes from residual quartz to quartz-alunite-pyrophyllite-kaolinite/dickite to illite-smectite away from the deposit. Mineralogical changes are paralleled by notable variations in the whole-rock major and trace element composition of the mudstone, allowing the definition of a set of alteration vectors to ore. Textural evidence suggests that alteration of the host rocks at Lagunas Norte predated precious metal mineralization. Residual quartz formed during the early acidic alteration is overgrown by later euhedral quartz that is texturally associated with the ore minerals. Fluid inclusion data suggest that the euhedral quartz and associated ore minerals formed from a low-temperature, low-salinity liquid. This liquid infiltrated the previously altered host rock succession, forming the sulfide ore zones of the epithermal deposit. Subsequent supergene weathering resulted in deep oxidation of the ore body. iii TABLE OF CONTENTS CHAPTER 1: INTRODUCTION ...................................................................................................1 CHAPTER 2: REGIONAL GEOLOGY .........................................................................................6 2.1 Introduction ..............................................................................................................6 2.2 Late Jurassic-Cretaceous Mariana-Type Subduction Regime .................................6 2.2.1 Jurassic Siliciclastic Rocks ......................................................................................7 2.2.2 Cretaceous Siliciclastic Rocks .................................................................................7 2.2.3 Cretaceous Volcanic Rocks of the Casma Group ....................................................9 2.2.4 Cretaceous to Tertiary Coastal Batholith ...............................................................10 2.3 Late Cretaceous to Modern Andean-Type Subduction Regime ............................10 2.3.1 Tertiary Incaic Thrust and Fold Belt ......................................................................11 2.3.2 Tertiary Volcanic Rocks of the Calipuy Group .....................................................11 2.4 The Alto Chicama District .....................................................................................13 2.5 Regional Metallogeny ............................................................................................18 CHAPTER 3: DEPOSIT GEOLOGY ...........................................................................................21 3.1 Introduction ............................................................................................................21 3.2 Chicama Group ......................................................................................................21 3.3 Chimu Formation ...................................................................................................23 3.4 Santa-Carhuaz Formation ......................................................................................24 3.5 Volcanic Rocks of the Calipuy Group ...................................................................24 3.6 Ore Zones and Associated Alteration Halo ...........................................................28 CHAPTER 4: SEDIMENTOLOGY OF THE CHIMU FORMATION .......................................33 4.1 Introduction ............................................................................................................33 4.2 Methodology ..........................................................................................................33 4.3 Lithofacies Descriptions ........................................................................................36 4.3.1 Interbedded Quartz Arenite....................................................................................36 4.3.2 Massive Quartz Arenite .........................................................................................36 4.3.3 Interbedded Quartz Arenite and Thin Mudstone ...................................................38 4.3.4 Thinly Bedded Mudstone and Siltstone .................................................................38 4.3.5 Noncarbonaceous Mudstone ..................................................................................39 iv 4.3.6 Carbonaceous Mudstone ........................................................................................40 4.3.7 Coal ........................................................................................................................40 4.4 Stratigraphic Relationships ....................................................................................40 4.5 Lateral Facies Variations .......................................................................................43 4.6 Facies Interpretation...............................................................................................45 CHAPTER 5: ALTERATION MINERALOGY ..........................................................................46 5.1 Introduction ............................................................................................................46 5.2 Methodology ..........................................................................................................46 5.3 Alteration Facies Developed in Mudstone .............................................................47 5.3.1 Residual Quartz Alteration in Mudstone ...............................................................48 5.3.2 Advanced Argillic Alteration in Mudstone ............................................................51 5.3.3 Intermediate Argillic Alteration in Mudstone ........................................................54 5.4 Alteration Facies Developed in Quartz Arenite .....................................................57 5.4.1 Residual Quartz Alteration Facies in Quartz Arenite ............................................57 5.4.2 Advanced Argillic Alteration Facies in Quartz Arenite ........................................59 5.5 Distribution of Alteration Facies Across Mudstone Beds .....................................59 5.6 Distribution of Alteration Facies Across the Deposit ............................................61 5.7 Distribution of Alteration Facies Away From the Deposit ....................................66 CHAPTER 6: ALTERATION GEOCHEMISTRY .....................................................................69 6.1 Introduction ............................................................................................................69 6.2 Methodology ..........................................................................................................69 6.3 Major Element Geochemistry ................................................................................71 6.4 Trace Element Geochemistry .................................................................................76 6.5 Rare Earth Element Geochemistry .........................................................................84 6.6 Alteration Vectors ..................................................................................................87 CHAPTER 7: CONSTRAINTS ON THE NATURE OF THE MINERALIZING FLUIDS .......90 7.1 Introduction ............................................................................................................90 7.2 Materials and Methods ...........................................................................................90 7.3 Paragenetic Relationships ......................................................................................92 7.4 Fluid Inclusions Hosted by Quartz ......................................................................101 7.5 Fluid Inclusions Hosted by Sphalerite .................................................................104 v 7.6 Fluid Inclusions
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