trace element geochemistry and carbonate alteration as indicators for gold mineralization at the ...

128 Pages·2011·7.27 MB·English

TRACE ELEMENT GEOCHEMISTRY AND CARBONATE ALTERATION AS
INDICATORS FOR GOLD MINERALIZATION AT THE CORTEZ
HILLS DEPOSIT, LANDER COUNTY, NEVADA
by
James F. Venendaal
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 fZ-J - ÛC7-
Signed:
ames F. Venendaal
Approved:
Dr. L. Graham Closs
Thesis Advisor
Golden, Colorado
0
Date , - 3 - o
Signed: W on SON
Dr. ohn D. Humphrey
Interim Department Head
Department of Geology and
Geological Engineering
ii
ABSTRACT
The application of trace element geochemistry and carbonate alteration
mapping has the potential to identify and target gold mineralized rock in the Carlin -trend
as well as in other trends in northern Nevada. At the Cortez Hills deposit, located in the
Battle Mountain- Eureka trend, trace element geochemistry and carbonate zonation
mapping was done using drill core from areas within, proximal, and outside of
mineralization to determine if a "footprint" for mineralization exists.
1018 composite samples from 23 selected drillholes within the Cortez Hills
deposit, as well as numerous surface and outcrop samples representing the Wenban
Formation were analyzed and interpreted for statistical and spatial significance and
distribution of trace elements around gold mineralization. The 1018 composite samples
from the 28600 section line were analyzed using the ALS CHEMEX ME -MS41 package.
This approach was based upon an aqua regia digestion followed by element estimation
via ICP -MS and ICP -AES instrumentation. Detailed geochemical overlays of
statistically and spatially coherent elements were combined into suites of elements
representing original stratigraphy, W event, base -metal (skarn),Carlin -type
mineralization, and supergene alteration.
Trace element geochemistry highlighted anomalous concentrations of elements
associated with black shales. These elements (Mo, Ni, Fe, P, V, and U) have strong
spatial and statistical correlation to the silty -shaley horizons that host gold mineralization
at the Cortez Hills deposit.
The statistical analysis of the composite data illustrates that the highest correlation
for gold mineralization is the Carlin -type mineralization suite (Tl, Hg, As, Sb, and Ag).
Specifically thallium and mercury have the strongest correlation followed by arsenic,
antimony and silver. Outside of the Carlin -type mineralization suite of elements,
tungsten, tellurium, bismuth, and tin have the strongest correlation to gold mineralization.
The tungsten mineralization suite has strong correlation and spatial coherence whereas
the base -metal suite does not. Within the supergene suite of elements, some of the
elements have distributions that coincide with mineralization as well as oxidation
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boundary and supergene alteration effects. The iron -aluminum ratio of the data set along
the 28600 section line illustrates strong association to elevated gold values.
A carbonate staining technique was applied to asses carbonate mineral zonation at
the Cortez Hills deposit. Carbonate speciation other than calcite (ferroan calcite, ferroan
dolomite, and dolomite) at the Cortez Hills deposit was not identified using this
technique.
Interpretation of the results of the study supports the hypothesis that typical
Carlin -type mineralization elements as well as other elements associated with skarn (Ag,
Bi, and Cu), and tungsten mineralization have an expression that outlines mineralization
and that alteration element zonation (Al, Ga, Ca, Mg, and Li) illustrates the effects of
mineralization as well as supergene alteration and that no carbonate mineral zonation was
detected using staining techniques.
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TABLE OF CONTENTS
ABSTRACT iii
LIST OF FIGURES vii
LIST OF TABLES x
ACKNOWLEDGEMENTS xi
DEDICATION xii
CHAPTER INTRODUCTION
1 1
1.0. Introduction 1
1.1. Purpose
1
1.2. Exploration History 5
CHAPTER 2 METHODOLOGY 6
2.0. Introduction 6
2.1. Sampling 7
2.2. 28600 Type Section 7
2.3. Logging 10
2.4. Carbonate Staining 11
2.5. Geochemical Data 11
CHAPTER 3 GENERAL GEOLOGY 14
3.0. Carlin -type Deposit Geology 14
3.1. Carlin Geologic Model 16
3.2. Carlin -type Mineralization 17
3.3. Geochemical Zoning 19
3.4. Carbonate Alteration and Zonation 19
CHAPTER 4 CORTEZ HILLS GEOLOGY 21
4.0. Regional Geology of Northern Nevada 21
4.1. Paleozoic Geology 22
4.2. Mesozoic Geology 25
4.3. Cretaceous Geology 25
4.4. Tertiary Geology 27
4.5. Cortez Hills Stratigraphy 27
4.6. Faults 36
v
4.7. Metamorphism, Oxidation Boundary, and Brecciation 37
4.8. Mineralization and Alteration at Cortez Hills 39
4.9. Carbonate Alteration and Staining 45
CHAPTER 5 GEOCHEMICAL PATTERNS AT CORTEZ HILLS 49
5.0. Trace Element Geochemistry 49
5.1. Original Lithology 51
5.2. Tungsten Event 55
5.3. Base -Metal Associations 60
5.4. Carlin -type Mineralization Pathfinder Elements 65
5.5. Supergene / Mineralization Related Alteration Elements 69
5.6. Relationships Between Geochemical Suites 74
5.7. Substitution Elements 76
5.8. Carbonate Mineralogy and Ratios 77
CHAPTER 6 SUMMARY AND CONCLUSIONS 82
6.0. Discussion 82
REFERENCES CITED 87
APPENDIX 92
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LIST OF FIGURES
Figure 1.1 Location maps of the Cortez Hills deposit relative to major cities
and gold deposit trends 2
Figure 1.2 The location of the Cortez Hills deposit relative to other
deposits in the Cortez district 3
Figure 1.3 Regional geology map of the Cortez Hills area 4
Figure 2.1 28600 section line showing location of drill holes 9
Figure 2.2 Carbonate stained core and hand sample 13
Figure 4.1 Basin and range province map 21
Figure 4.2 Antler Orogeny, Roberts Mountains Thrust map 23
Figure 4.3 Sonoma Orogeny, Golconda Thrust map 24
Figure 4.4 Simplified stratigraphic column of the Cortez district 28
Figure 4.5 Stratigraphic type section (28600) 29
Figure 4.6 Authigenic cubic pyrite photomicrograph 33
Figure 4.7 Anhedral pyrite in sulfide vein photomicrograph 33
Figure 4.8 Resorbed quartz texture photomicrograph 36
Figure 4.9 Arsenic rich pyrite vein photomicrograph 36
Figure 4.10 Metamorphism overlay map 38
Figure 4.11 Oxidation boundary overlay map 38
Figure 4.12 Brecciation overlay map 39
Figure 4.13 Clay zone outside of silica halo photomicrograph 41
Figure 4.14 Reflected light photomicrograph of clay outside of silica halo 41
Figure 4.15 Silica replacement of calc- silicate minerals photomicrograph 41
Figure 4.16 Silica replacement of carbonates photomicrograph 41
Figure 4.17 Carbonate and calcite vein crystals photomicrograph 42
Figure 4.18 Silica halo around sulfide in carbonate rock photomicrograph 42
Figure 4.19 Carlin -type silica front in carbonate rock photomicrograph 42
Figure 4.20 Silicification in refractory carbonate rock photomicrograph 42
Figure 4.21 Hematite with pyrite photomicrograph 43
Figure 4.22 Oxidized pyrite cubes photomicrograph 43
Figure 4.23 Silica halo around sulfide (skarn) photomicrograph 43
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Figure 4.24 Reflected light image of silica halo around sulfide 43
Figure 4.25 Clay alteration with silicification and replacement 44
Figure 4.26 Clay alteration proximal to sulfides photomicrograph 44
Figure 4.27 Late stage carbonate and realgar photomicrograph 44
Figure 4.28 Sulfides with silica and carbonate deposition photomicrograph 44
Figure 4.29 Supergene oxidation of sulfides photomicrograph 45
Figure 4.30 Chalcopyrite and pyrite photomicrograph 45
Figure 4.31 Sulfides from base metal vein photomicrograph 45
Figure 4.32 Additional sulfides from base metal vein photomicrograph 45
Figure 4.33 Stain colors on core 48
Figure 4.34 Calcite and silica deposition photomicrograph 48
Figure 4.35 Multiple depositional bands of calcite photomicrograph 48
Figure 5.1 Sample location map 50
Figure 5.2 Gold distribution map 50
Figure 5.3 Original Stratigraphy (Mo) map 52
Figure 5.4 Original Stratigraphy (Ni) map 53
Figure 5.5 Original Stratigraphy (Fe) map 53
Figure 5.6 Original Stratigraphy (P) map 54
Figure 5.7 Original Stratigraphy (U) map 54
Figure 5.8 Original Stratigraphy (V) map 55
Figure 5.9 Tungsten Event (Bi) map 57
Figure 5.10 Tungsten Event (Cu) map 57
Figure 5.11 Tungsten Event (Mo) map 58
Figure 5.12 Tungsten Event (Sn) map 58
Figure 5.13 Tungsten Event (Te) map 59
Figure 5.14 Tungsten Event (W) map 59
Figure 5.15 Base -metal (Ag) map 61
Figure 5.16 Base -metal (Bi) map 62
Figure 5.17 Base -metal (Cu) map 62
Figure 5.18 Base -metal (Fe) map 63
Figure 5.19 Base -metal (Ni) map 63
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Figure 5.20 Base metal (Pb) map 64
Figure 5.21 Base metal (Zn) map 64
Figure 5.22 Carlin -type mineralization Pathfinder (Tl) map 66
Figure 5.23 Carlin -type mineralization Pathfinder (Hg) map 67
Figure 5.24 Carlin -type mineralization Pathfinder (As) map 67
Figure 5.25 Carlin -type mineralization Pathfinder (Sb) map 68
Figure 5.26 Carlin -type mineralization Pathfinder (Ag) map 68
Figure 5.27 Supergene Alteration (Al) map 70
Figure 5.28 Supergene Alteration (Ga) map 71
Figure 5.29 Supergene Alteration (Ca) map 71
Figure 5.30 Supergene Alteration (Mg) map 72
Figure 5.31 Supergene Alteration (Mn) map 72
Figure 5.32 Supergene Alteration (Sr) map 73
Figure 5.33 Supergene Alteration (K) map 73
Figure 5.34 Supergene Alteration (Fe) map 74
Figure 5.35 Additional elements (Li) map 77
Figure 5.36 Carbonate microveins photomicrograph 79
Figure 5.37 Carbonate macroveins image 79
Figure 5.38 Wenban Formation, silty limestone photomicrograph 79
Figure 5.39 Carbonate breccia photomicrograph 79
Figure 5.40 Carbonate and sulfide replacement photomicrograph 80
Figure 5.41 Carbonate replacing phenocrysts photomicrograph 80
Figure 5.42 Carbonate crystals and veins photomicrograph 80
Figure 5.43 "Dog- tooth" calcite photomicrograph 80
Figure 5.44 Fe / Al Ratio map 81
Figure 5.45 S / Al Ratio map 81
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LIST OF TABLES
Table 2.1 Drill hole database 6
Table 2.2 Carbonate and Clay Samples Collected for Analysis 8
Table 2.3 ALS CHEMEX Detection Limits for the ICP -MS41 and ICP -AES
techniques 13
Table 5.1 Element correlations to gold 51
Table 5.2 Correlation coefficients for original stratigraphy suite 52
Table 5.3 Correlation coefficients for tungsten event suite 56
Table 5.4 Correlation coefficients for base -metal suite elements 61
Table 5.5 Correlation coefficients for Carlin -type mineralization pathfinder suite
elements 66
Table 5.6 Correlation coefficients for supergene / mineralization related alteration
suite elements 70
Table 5.7 Correlation coefficients for alteration and Carlin -type mineralization
pathfinder suite elements 76
Table 5.8 Correlation coefficients for alteration and base -metal suite elements 76