University of Nevada, Reno
Geology, Alteration, Paragenesis, and Geochemistry of the Vortex Zone of the Hycroft Gold-Silver Deposit, Humboldt County, Nevada
A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science in Geology
by
Karl Lowry
Dr. Tommy Thompson/Thesis Advisor
December, 2013
THE GRADUATE SCHOOL
We recommend that the thesis prepared under our supervision by
KARL LOWRY
entitled
Geology, Alteration, Paragenesis, And Geochemistry Of The Vortex Zone Of The Hycroft Gold-Silver Deposit, Humboldt County, Nevada
be accepted in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Tommy Thompson, Ph. D., Advisor
Christopher Henry, Ph. D., Committee Member
Thom Seal, Ph. D, Graduate School Representative
Marsha H. Read, Ph. D., Dean, Graduate School
December, 2013
i
Abstract
The Hycroft gold-silver mine is a low sulfidation epithermal hot spring deposit located 55 miles west of Winnemucca, NV near the Blackrock desert. It is located in the historic Sulphur district, which has had mining on and off since the late 1800’s. Sulphur was the main commodity initially, with the later discovery and mining of silver, alunite, and mercury through the first half of the 20th century. Gold was discovered in the district in 1974 by the Duvall Corporation. The first gold mining and recovery by heap leach was conducted in 1983 by Standard Slag. Allied Nevada acquired the property in 2008 and discovered the Vortex zone through induced polarization and resistivity surveys.
The oldest rocks in the region are the Permian Happy Creek Volcanic Series.
These are overlain by the Auld Lang Syne Group of metamorphosed argillaceous to sandy sedimentary rock. Tertiary volcanic and volcaniclastic rocks overlie the basement rocks. The region underwent folding and regional metamorphism in the Jurassic. In the
Late Cenozoic extension was the primary tectonic movement giving rise to the development of normal faults in the basin and range province..
The Auld Lang Syne rocks make up the basement in the Vortex area and are mostly in fault contact with overlying Kamma Mountains volcanic and volcaniclastic rocks. Previously undifferentiated, the Kamma Mountains rocks consists, from bottom to top, of (1) a lower flow-banded rhyolite, (2) ash-fall and lithic-rich tuffs, (3) a massive rhyolite flow, and (4), a clast-to-matrix-supported angular clastic unit. The youngest
Tertiary unit is the Sulphur rocks, which consist only of a rounded to subangular clast-to- matrix-supported conglomerate in the Vortex area. The upper parts of the Kamma
Mountains and the Sulphur rocks are lithified only where hydrothermally altered. All ii rocks are cut by a series of north-northeast-striking normal faults, the most important of which is the East fault.
Hydrothermal alteration in the Vortex zone is extensive and focused in layers due to the high permeability of most rock types. There are five types of alteration. An argillic alteration made up of kaolinite + smectite + anhedral quartz + sericite + marcasite + pyrite dominates the deposit. Argillic alteration is distinctly zoned from lower, kaolinite- dominated levels to upper smectite-dominated levels. Argillic alteration has been dated to
4.0 ± 0.1 Ma. Argillic alteration interfingers with propylitic alteration that consists of chalcedony + chlorite + pyrite + sericite + smectite + marcasite ± carbonates and occurs in veinlets and flooded into groundmass. Silicic alteration that consists of chalcedony ± granular quartz + pyrite + marcasite ± sericite formed above propylitic alteration in the middle part of the Kamma Mountains rocks. Opal or chalcedony + adularia + pyrite alteration locally occurs above the smectite alteration. Adularia from this alteration has been dated to 3.8 ± 0.09 Ma. Steam-heated acid-sulfate alteration overprints all alteration types at the top of the system. Most elements, except immobile elements were leached and native sulfur added to the upper part of the steam-heated zone. Alunite from this alteration type has been dated to 2.4 ± 0.1 Ma. The lower part is silica-cemented and has accumulated iron oxides leached from the upper part.
Paragenetic study shows that pervasive hydrothermal alteration occurred early in the system. Pervasive argillic alteration was overprinted with propylitic in the lower parts of the deposit, then silicic in the middle portion, then opal – adularia and acid leach in the upper portions. This was followed by several events of brecciation and veining. Silver iii mineralization occurs late in brecciation events and locally in veins as pyrargyrite, proustite, tennantite-tetrahedrite, and acanthite.
Geochemistry in the zone shows some typical epithermal zonation. Mercury and antimony show classical volatile zonation, occurring in the upper portions of the system.
Arsenic appears to have reverse zoning with higher levels lower in the system, due to inclusion in silver sulfosalts rather than in arsenic sulfides. The base metals occur at very low levels overall and do not show clear zonation, except copper, which has a bi-modal zonation with a horizon of copper occurring in chalcopyrite lower in the system, and one occurring in tennantite-tetrahedrite higher in the system. Correlation of elements shows that gold and silver mineralization are commonly associated with arsenic, selenium, and antimony deposition, though this is variable throughout different levels of the system.
The volcanic rocks in the system were likely deposited between 28 and ~16 Ma and cut by Basin and Range normal faulting at around 16 Ma. Normal faulting created the necessary conditions to form the volcaniclastic and Camel Conglomerates near the top of the deposit. Hydrothermal alteration began around 4 Ma and lithified these rocks, partially sealing the system. This led to the widespread creation of breccia dikes which roughly coincide with the boundary between the upper rhyolite and volcaniclastic units.
Precious metal mineralization occurred in these breccia dikes and later veining.
Hydrothermal activity continued after precious metal deposition with late overprints of acid leach. iv
Acknowledgements
I would like to thank all of those who have helped me along the way in my education, but in particular those who played a direct role in this thesis. Special thanks to
Dr. Tommy Thompson for accepting me into the CREG program and mentoring me both in classes and through numerous questions about how to proceed with this thesis. Thanks to Dr. Chris Henry for the immeasurable help in not only content editing, but in helping to make this document more concise. Thanks also go to Thom Seal for the outside perspective he was able to give on what would be interesting information to know about this system metallurgically.
Special thanks must also go out to the fine folks at Allied Nevada Gold for financial support and who were instrumental in helping me understand how to proceed with, up to this point, has been the most daunting undertaking of my career. Dave Flint in particular was always willing to take time out of his busy schedule whenever he could.
Don Harris was also very helpful in getting me the information I needed to keep the work moving forward. There were also some very helpful people at the Hycroft mine I would like to mention by name: Matt Hoffer for taking the time to show me around and help familiarize me with my study area, and Jeff Spence for taking the time to help familiarize me with the different rock types and alteration, as well as all the other mine geologists and geotechnicians.
Lastly this page would not be complete without thanks given to my lovely wife
Crystal for her continual support and undying optimism that I could accomplish my goals. v
Table of Contents
Abstract I
Acknowledgements IV
Table of Contents V
List of Tables VII
List of Figures VII
Introduction 1 Purpose 1 Location 1 History 1 Previous Work 6 Methodology 7 Regional Setting 8 Regional Lithology 7 Regional Structure 10 Deposit Geology 12 Methods 12 Cross Section Descriptions 15 Lithologic Unit Descriptions 20 Auld Lang Syne Formation 20 Kamma Mountains Group 21 Sulphur Group 29 Deposit Structure 29 Alteration 32 Methods 32 Distribution 32 Assemblage Descriptions 40 Argillic 40 Propylitic 43 Silicic 45 Opal – Adularia 47 Acid Leach 49
vi
Paragenesis 51 Hydrothermal Alteration 51 Brecciation 58 Veining 63 Early Veins 64 Quartz-Adularia 64 Iron Sulfide-Quartz 64 Banded Quartz-Chalcedony 69 Bladed Calcite 69 Intermediate Veins 70 Drusy Quartz-Chalcedony 70 Coarse Alunite 70 Late Veins 72 Quartz after Bladed Carbonate 72 Fine Grained Alunite 69 Thin Chalcedony 74 Geochemistry 75 Zonation 75 Volatiles 76 Precious Metals 81 Base Metals 85 Correlation 90 Formation Model, Discussion, and Future Work 95 Summary 100 Discussion 101 Future Work 102
References 104 vii
List of Tables
Table Page
1. Analysis of Whole Rock and REE from Rhyolite 28
List of Figures
Figure Page
1. Location Map of Hycroft Property 2
2. Location Map of Vortex Zone within Hycroft 3
3. Photograph of Historic Working in Silver Camel Hill 4
4. IP Survey Map of Vortex 6
5. Simplified Regional Geology Map 9
6. Geologic Map of Vortex Zone 13
7. Section A-A’ 14
8. Section B-B’ 16
9. Section C-C’ 18
10. Section D-D’ 19
11. Core Box Photograph of Auld Lang Syne Formation 21
12. Stratigraphic column of Kamma Mountains Group 22
13. Core Box Photograph of Banded Rhyolite 23
14. Photograph of Rhyolite at Surface 24
15. Core Box Photograph of Unaltered Volcaniclastics 25
16. Chondrite Normalized REE plot of Rhyolite Flows 25
17. TAS Diagram of Unaltered Rhyolite 27
18. Photograph of slickenlines from East Fault 31
19. Alteration Map of Vortex 33 viii
20. Paragenetic Diagram of Hydrothermal Alteration 34
21. Section A-A’ Hydrothermal Alteration 35
22. Section B-B’ Hydrothermal Alteration 36
23. Section C-C’ Hydrothermal Alteration 38
24. Section D-D’ Hydrothermal Alteration 39
25. Photomicrographs of Argillic Alteration Assemblage 41
26. Photomicrographs of Propylitic Alteration Assemblage 42
27. Photomicrographs of Silicic Alteration Assemblage 44
28. Core Box Photograph of Pervasive Silicic Alteration 45
29. Photographs of Opal - Adularia Alteration 46
30. Photographs of Acid Leach Alteration 48
31. Paragenetic Diagram of Hydrothermal Minerals 52
32. Photomicrograph Showing Argillic Alteration Paragenesis 54
33. Photomicrograph Showing Marcasite in Argillic Alteration 54
34. Photomicrograph Showing Galena, Marcasite, and Pyrrhotite 55
35. Photomicrograph Showing Euhedral Pyrite in Argillic Alteration 55
36. Photomicrograph Showing Paragenesis of Propylitic Alteration 56
37. Photomicrograph Showing Paragenesis of Propylitic Alteration 56
38. Photomicrograph Showing Paragenesis of Silicic Alteration 57
39. Photograph Showing Paragenesis of Sulfur in Acid Leach 57
40. Annotated Photograph of Core Sample 3477-406 60
41. Annotated Photograph of Core Sample 3475-1092 60
42. Photomicrograph of Paragenesis in Breccia #1 61
43. Photomicrograph of Pyrite and Marcasite in Breccia #1 61 ix
44. Photomicrograph of Paragenesis in Breccia #2 62
45. Photomicrograph of Pyrargyrite and Pyrite in Breccia #2 62
46. Photomicrograph of Quartz-Adularia Vein 65
47. Photomicrograph of Adularia Rhombs in Quartz-Adularia Vein 65
48. Photomicrograph of Marcasite-Quartz Vein 66
49. Photomicrograph of Drusy Quartz on Quartz-Chalcedony Vein Margin 66
50. Photomicrograph of Pyrite Aggregates in Iron-Sulfide Quartz Vein 67
51. Photomicrograph of Previous FOV with Crossed Polars 67
52. Photomicrograph of Bladed Calcite Vein 68
53. Photomicrograph of Tennantite-Tetrahedrite in Vug 68
54. Photomicrograph of Coarse Alunite Vein 71
55. Photomicrograph of Proustite in Vug 71
56. Photomicrograph of Chalcedony-Alunite Vein 73
57. Photomicrograph of Acanthite in Chalcedony-Alunite Vein 73
58. Anomalous Occurrences of Arsenic on Sections 78
59. Distribution of Arsenic Values within Vortex 78
60. Anomalous Occurrences of Mercury Values on Sections 79
61. Distribution of Mercury Values within Vortex 79
62. Anomalous Occurrences of Antimony on Sections 80
63. Distribution of Antimony Values within Vortex 80
64. Occurrences of Gold on Sections 82
65. Occurrence of Silver on Sections 83
66. Occurrences of Au Equivalent Values on Sections 84
67. Ag:Au in High Grade Zone on Section A-A’ 84 x
68. Anomalous Occurrences of Copper Values on Sections 86
69. Distribution of Copper Values within Vortex 86
70. Anomalous Occurrences of Lead on Sections 87
71. Distribution of Lead Values within Vortex 87
72. Anomalous Occurrences of Zinc on Sections 88
73. Distribution of Zinc Values within Vortex 88
74. Correlation Matrixes of Geochemistry Data from Section A-A’ 91-92
75. Early Formation Model of Vortex 96
76. Model of Beginning of Hydrothermal Alteration within Vortex 97
77. Model of Later Formational Processes within Vortex 98
78. Summary Diagram 100
79. Schematic Cross Section of Hot-Spring Type Deposit 102
1
Chapter 1: Introduction
Purpose
This study is being conducted to characterize the lithology, structure, alteration, mineralization, and geochemistry of the Vortex zone of the Hycroft mine. It is hoped that the knowledge gained will be of use in the future mining of Vortex, exploration throughout the Hycroft mining property, and to the general knowledge of low sulfidation hot-spring type epithermal deposits.
Location
The Hycroft deposit lies within the Basin and Range physiographic province of northwestern Nevada, approximately 55 miles west of the town of Winnemucca along
Nevada State Route 49 (Jungo Road) near the Blackrock desert next to the now defunct town of Sulphur (Fig. 1). The Vortex zone is located at the southern extent of active mining within the Hycroft property between and south of the Brimstone and South
Central pits (Fig. 2).
History
The earliest recorded mining in the Sulphur district began in the late 1800’s following the discovery of significant native sulfur deposits (Couch et al., 1943). Native sulfur was mined sporadically throughout the early 1900s with the last significant episode of mining during the 1950s. Over 180,000 tons of ore were mined and processed with an average grade of about 20-35% sulfur (McLean, 1991). The district received a boost in 2
Figure 1: Location map of Hycroft property (Flint et al., 2012).
3
Figure 2: Generalized map of Hycroft Mine modified to show Vortex Area (Moore et al., 2013). 4 production from the discovery in 1908 of nearly pure seams of cerargyrite (AgCl) plus alunite. These were found in the area presently referred to as the Silver Camel (Fig. 3).
Assays up to 117.9 kg/t Ag and 12.4 g/t Au were reported (Jones, 1921). Silver mining ceased by 1912 with a total estimated production of 5,670 kg Ag (Vanderburg, 1938).
During World War I, three 1.8 – 2.4 m wide veins of nearly pure alunite were mined in the southern part of the Sulphur district (Clark, 1918). In 1931 several hundred tons of alunite were mined and sold as a soil additive (Smith, 1932). From 1941 to 1943, cinnabar was mined with total mercury production from this time estimated to have reached 862 kg (McLean, 1991).
Figure 3: Historic workings in Silver Camel Hill 5
In 1966, the Great American Company began extensive exploration for native sulfur. Approximately 200 shallow holes were drilled and numerous trenches dug
(Friberg, 1980). In 1974, Duval Corporation drilled 18 holes on the property in search of a Frasch-type sulfur deposit (Wallace, 1980). Duval Corporation found no evidence for a sulfur deposit at depth, but did report elevated Au and Ag values. In 1977, Cordex
Syndicate recognized the potential for a large tonnage, low-grade precious metal deposit based on mapping and rock chip sampling. In 1978, Homestake Mining Company became interested in the property because of similarities with the McLaughlin hot-spring deposit in California. Numerous surface samples were collected and 112 holes were drilled, but the option was dropped because of low grades and limited extent (Friberg,
1980).
In 1983, Standard Slag Company acquired the Lewis part of the North Pit, which contained 1.2 million tonnes at 1.20 g/t Au. Vista acquired the Crofoot claims in 1985.
Discovery of the Crofoot deposit by Vista in 1985 was followed by development and construction in 1986 and 1987. The Lewis Mine was acquired from Standard Slag
Company by Vista in 1986 (Ebert, 1995). In 1998, mining was suspended due to low gold prices. In May 2007, the Nevada based holdings of Vista were spun out into Allied
Nevada Gold Corporation, which re-opened the mine in November 2008 (Flint et al.,
2012).
Indication of the Vortex zone was first recognized in 2007 through an induced polarization/resistivity geophysical survey (Fig. 4) (Wright, 2007). Drilling, commencing in 2008 with 10 holes and continuing through early 2012, identified an oxide resource to 6
Figure 4: IP survey results of Vortex (Modified from Wright, 2007). depths of 500 feet and sulfide mineralization to depths of 2500 feet (Flint et al., 2009).
Previous Work
The most prominent published work on Hycroft is a doctoral dissertation by Ebert
(1995) and derivative papers that focus on the effects of paleoclimate on the deposit
(Ebert and Rye, 1995) and on the geology and alteration (Ebert, Groves, and Jones,
1996). Wallace (1980) described the geology of the Sulphur district. Several unpublished company reports describe alteration, vein petrology, lithology, mineralogy including
XRD/XRF analysis, and geology of the southern part of the deposit.
7
Methodology
Field work for my study was carried out between May and August 2012 and began with a field geologic map of the study area. Because most of the field area is covered with alluvium, this map was combined with a sub-crop map created from drill hole data. The combined data give a better picture of rock types and alteration. Four diamond drill core holes were relogged to familiarize the author with the geology and alteration of the deposit and to collect samples for thin/polished section work. Core photos were used to complete four cross sections through the deposit to document the geology, structure, alteration, and mineralization of the Vortex zone. From September
2012, work focused on the laboratory analyses including thin and polished section petrographic descriptions of forty seven sections and geochemical correlation of elements and their zonation within the deposit.
8
Chapter 2: Regional Setting
Regional Lithology
The oldest exposed rocks in the region are the Happy Creek volcanic series (Fig.
5), which consist of an intrusive-extrusive complex of basaltic andesite, andesite, diorite, and quartz diorite dikes, lavas, and flow breccias. These rocks, which are probably
Permian based on overlying rocks that contain Permian fusulinids (Sherlock, 1989), make up most of the northern half of the Jackson Mountains. The volcanic rocks of this group are massive and lack primary structures to determine attitude. Therefore, it is difficult to determine conformity with overlying units (Willden, 1964). Undifferentiated Permian to
Triassic metavolcanic and metasedimentary rocks also crop out in the Jackson Mountains north of the property (Ebert, 1995).
The Auld Lang Syne Group overlies the Happy Creek volcanic series. This group is up to 25,000 feet thick and laterally extensive, but is much thinner in the area surrounding the deposit. The Auld Land Syne Group consists of nine formations that constitute a continuous succession of variably metamorphosed, interlensing, argillaceous and sandy strata (Burke et al., 1973), mostly phyllite, slate, metasiltstone, fine grained quartzite, and local hornfels (Ebert, 1995). Limestone and dolostone recrystallized to calcite and dolomite, respectively, are also present within the group (Burke et al., 1973).
Cretaceous granodiorite crops out erratically throughout the map area. Typical outcrops are round and hummocky on mountainsides, or rugged and bluff like near the crests of the high ranges. The outcrops on the lower relief areas have a thoroughly weathered rind about 6 inches thick. A sample from an outcrop to the east-southeast of 9
Figure 5: Simplified regional geologic map modified from Willden (1964) and Johnson (1977). 10 the mine site (see Fig. 5 for location) has K-Ar ages of 94.2±3.9 Ma (biotite) and
97.5±6.3 Ma (hornblende) (Smith et al, 1971).
Tertiary rocks consisting of volcanic and intrusive rocks ranging from olivine basalt to rhyolite as well as fresh water sedimentary rocks including conglomerate, sandstone, siltstone, shale, tuff, and diatomite are abundant throughout the region.
Tertiary olivine basalts are common throughout the area, and are considered to be related to Basin and Range faulting (Ebert, 1995). Of the felsic volcanic rocks in the area, the lava flows tend to have higher silica content and are classified in the rhyolite and quartz latite fields, while the tuffs are dacites and rhyodacites with less common quartz latites
(Willden, 1964). Tertiary rhyolites in the area have K-Ar dates from 28 to 12Ma
(Johnson, 1977).
Quaternary rocks consist mainly of alluvium and colluvium shed from the local mountain ranges, but lacustrine sediments from pre-historic Lake Lahontan occupy the
Black Rock Desert (Ebert, 1995).
Regional Structure
Two major episodes of deformation have affected the rocks of the Sulphur area
(Willden, 1964). Pre-Tertiary rocks underwent low-grade regional metamorphism, variably directed folding, and thrust faulting during the Jurassic(?) Nevadan Orogeny.
This deformation preceded emplacement of the granodiorite, which is believed to have been emplaced as a result of this event. Basin and Range extension is manifest by high angle, range-front normal faults with sub-parallel step-off structures that flatten at depth 11
(Willden, 1964). Basin and Range faulting is believed to have started in the region about
16 Ma as evidenced by dates taken from basalts associated with the earliest faults and continues through today (Johnson, 1977).
12
Chapter 3: Geology of the Vortex Zone
Previous studies divided the rocks into three main groups: the Jurassic Auld Lang
Syne Group, which occurs only in the subsurface, the exposed, informally named
Tertiary Kamma Mountains group, and the exposed, informally named Tertiary to
Pleistocene Sulphur group (Ebert, 1995). While previous reports have described these units (Wallace, 1980, Ebert, 1995), this study is the first to differentiate the units within the Kamma Mountains group. Some help was derived from unpublished internal reports meant to aid in core logging. The studies for this report include polished thin section work and geochemistry. The geochemistry work for this study included whole rock, and rare earth element analysis for the purpose of correlating surface units with those that had been dropped due to faulting and subsequently hydrothermally altered. This section of the report will start with a description of the field work including generation of the maps and cross sections, followed by descriptions of the lithologic units, followed by a discussion of the structural geology.
Methods
The combined outcrop/subcrop map produced from field work is shown in Figure
6. Most outcrops are found in the Kamma Mountains, east of the Vortex ore zone, with local outcrops found in road cuts and gulches. The majority of the map area is covered with colluvium. In order to fill in the information gathered from field mapping, core photographs from every diamond drill hole within the map area were observed. The first occurrence of solid rock was plotted on the map. Once all this information was recorded, outlines were drawn around the different lithologic units. 13
Figure 6: Geologic combined outcrop/subcrop map of Vortex area 14
Figure 7: A-A' Cross Section with lithologic units controlled from drill holes. (See Fig. 6 for location of cross section.)
15
After a geologic map of the area was created, cross sections were generated. This began with logging of four core holes - 3502, 3477, 3475, and 3408 – that align along section A-A’. Core box photographs were utilized to fill in the rest of the information along the section. Four sections were completed using this methodology from the northern portions of Vortex through to the south in order to determine how the geology changed throughout the zone.
Samples were taken from the rhyolite flows of the Kamma Mountains Group for whole rock and rare earth element analysis. These were delivered to ALS Chemex in
Reno, NV where they underwent sample preparation of crushing and dividing before being sent to Vancouver, BC for whole rock geochemistry and rare earth element analysis. Five samples were sent including one sample of rhyolite from the surface east of the East fault which is labeled on the subcrop/surface lithologic map (Fig. 6) as RY-3, and four samples were taken from core. The core samples are listed with hole number and footage as sample numbers so that sample location can be found on cross sections.
Cross Section Descriptions
The first section completed was A-A’ (Fig. 7). This section showed the continued presence of similar rocks to those that were found in the Kamma Mountains. Though it was evident that the rhyolite flows had been down-dropped, there was not enough control to determine the amount of displacement. What also became apparent was the presence of a series of sub-parallel normal faults with lesser offset than the East fault. These faults have been recognized in previous works, and are named from east to west the Fire, 16
Figure 8: B-B' Cross Section with lithologic units controlled by drill holes. (See Fig. 6 for location of the cross section.) 17
Albert, and Break faults.
Geometry of lithologic units with this section was relatively tightly controlled with drilling. The upper rhyolite flow (Tr) dips west. This rhyolite also thins towards the west, possibly showing the lateral extent of this unit. The wedge shape also indicates a source for this flow somewhere to the east of the mine site. The lower banded rhyolite
(Trb) is truncated by the East fault, so exact geometry cannot be determined. The volcaniclastic (Tvc) unit does not exist east of the East fault and appears to thin with distance from the Kamma Mountains. This geometry suggests that this unit is an alluvial clastic unit shed off the Kamma Mountains. The Camel Conglomerate (Tcc) occurs only in the western half of the section and thickens toward the west.
The next section to the south, B-B’ (Fig. 8) was created sub-parallel to A-A’. This section revealed very similar lithologic unit geometries to those revealed by A-A’. What is unique about this section is the geometry of the East fault, which decreases dip from
65° above ~1000 feet to 45° below. Offsets on the Fire and Albert fault were also less pronounced than in the northern zone. The same gentle dip to the west and thinning of the rhyolite section are apparent.
This section also shows the first appearance of an ash-fall tuff unit. This unit appears towards the west and between the upper rhyolite and lower rhyolite/lithic-rich tuff. This unit is less than 50 feet thick, but has a lateral extent of at least 300 feet within this section.
The next section to the south, C-C’ (Fig. 9), was also created sub-parallel to the 18
Figure 9: C-C' Cross Section with lithologic units controlled by drill holes. (See Fig. 6 for location of the cross section).
19
Figure 10: D-D' Cross Section with lithologic units controlled by drill holes. (See Fig. 6 for location of the cross section).
20 first two sections. The shape of the East fault returns to the standard listric geometry seen in the first section. The most notable difference between this and all other sections was the amount of brecciation observed with the Tr. Core logging showed that this unit was extensively brecciated along horizontal layers, which initially were confused with interlayered rhyolite and lithic rich tuff. The ash-fall tuff also has a much broader exposure within this section. The placement is the same as in B-B’, but thickness is more than 300 feet and lateral extent is at least 1200 feet.
The final section, D-D’ (Fig. 10) is of limited extent and at about a 45° angle to previous sections, because drill holes in the southern part of Vortex are sparse and align along this trend. The few drill holes make interpretation of faulting difficult. The Fire fault is probably present in section D-D’ as indicated by the major change in elevation of the top of the massive rhyolite between drill holes 4172 and 4173 on the eastern part of the section. Due to small change in elevation of the upper rhyolite between holes 4172 and 4138, it is interpreted that offset along the Albert fault ends somewhere between section C-C’ and section D-D’. This section does not extend to where the Break fault would be expected, so no interpretation is made here on its extent.
Lithologic Unit Descriptions
Auld Lang Syne Group
In the area of this study, the Auld Lang Syne (Jals) consists of black to gray finely laminated argillite with abundant graphite giving sheen to cleavage planes (Fig. 12). The unit contains minor disseminated pyrite and localized 0.5 to 1cm veinlets of calcite + 21
Figure 11: Core box photograph of Auld Lang Syne Formation quartz + pyrite of limited continuity. Although grades of up to 14 g/t Ag (Ebert, 1995) have been reported in these veinlets, little to no hydrothermal alteration has taken place in this unit. Where encountered by drilling, this unit is in fault contact with the overlying
Kamma Mountains unit (Figs. 7-10). To the east of the mine, this contact is a simple unconformity.
Kamma Mountains Group
The Kamma Mountains group is a series of volcanic and volcaniclastic units (Fig.
12). The lowermost unit is a flow banded rhyolite (Trb). Overlying the Trb are two tuff units. The lower of these two units is a lithic-rich tuff (Ttl) while the upper is an ash-fall tuff (Tta). Above the tuff units is another rhyolite flow (Tr). This rhyolite has local flow
22
Figure 12: Stratigraphic column of Kamma Mountains Group
23
Figure 13: Core box photograph of Banded Rhyolite banding, and is highly jointed. The uppermost unit is a volcaniclastic unit (Tvc) composed of rhyolite and sediment shed off the Kamma Mountains.
The oldest unit of the group is a flow-banded rhyolite, known as the Trb in mine nomenclature, found in the western, and more dominantly in the southern part of Vortex directly above the fault contact with the ALS (Figs. 7-10). The rock is characterized by alternating light gray and gray flow bands (Fig. 13) that locally have asymmetric several centimeters to meter scale folds. Crystal content in this lava flow unit is very low (< 3%) and consists of euhedral sanidine and quartz. Sanidine has an average size of 0.5-1mm.
Quartz phenocrysts occur with similar size to sanidine. A smaller size range of quartz crystals with an average size 0.1mm occurs in wispy accumulations associated with flow banding. These are likely secondary, probably vapor-phase crystallization. The matrix is aphanitic with no visible pore space. This unit is generally fractured with most fractures parallel to flow banding with the remainder crossing banding at a wide range of angles. 24
Immediately overlying the banded rhyolite is a group of tuffs including lithic-rich tuff known as TTL and a unit of finely laminated ash-fall with local interbeds of lithic rich layers known as TTA. The most common of these is the lithic-rich tuff. This unit is non-welded and contains from 10 – 60% lithic fragments of banded rhyolite and lesser
ALS fragments. Clasts range from 3mm up to 2cm and are angular to subrounded. No pumice fragments were noted in this unit. The ash-fall unit is not noted in section A-A’
(Fig. 7), and is most prominent in C-C’ (Fig. 10). It is finely laminated, dark brown to black ash that appears in the southern portion of the zone.
Figure 14: Rhyolite at surface showing highly jointed nature.
25
Figure 15: Core Box Photograph of unaltered TVC from Hole 4186.
Figure 16: Chondrite normalized plot for REE analysis of rhyolite samples from within the Kamma Mountains unit. Sample numbers are hole number followed by footage for location on cross sections. RY-3 is a surface sample with location noted on field map. 26
Overlying and locally interbedded with the tuff units is an upper rhyolite unit (Tr).
This unit is mostly massive with only local flow bands. The crystal content of this unit is the same as the flow banded unit with quartz >> sanidine with disseminated larger crystals and accumulations of smaller quartz in microscopic flow bands. Where found at the surface, east of the East fault, this unit weathers to a medium brown, and is dark navy blue on fresh surfaces. The unit is brittle (Fig. 14) and highly jointed at the surface, which is manifest at depth in core by a network of sericite-, alunite-, marcasite-, and pyrite- filled veinlets. The uppermost unit of the Kamma Mountains Formation is a clastic sedimentary unit referred to in mine terminology as the Tertiary Volcaniclastic, or TVC, and is found only west of the East fault. This unit is a clast- to matrix-supported conglomerate of angular to subangular rhyolite, banded rhyolite, and minor ALS fragments. The angularity of the rhyolite clasts in this unit appears to be due to the brittle and jointed nature of the rhyolite (Fig. 14) flows rather than an active brecciation event.
Clasts range from a few mm up to 10+ cm size but average 1-2cm. No discernible grading or sedimentary structures are present within this unit. This unit is very poorly sorted except in local interbeds of sand and silt that range from a few centimeters to a meter in thickness and are more common towards the upper part of the unit. Outside the zone of alteration, this unit is not cemented and is highly porous with a reddish-brown, silty-clay matrix (Fig. 15). Within the zone of hydrothermal alteration, this unit has a light green to greenish gray illite-smectite altered matrix more common in the upper parts of the unit, and a silicified matrix more common in the lower parts of the unit.
27
TAS (Le Bas et al. 1986)
Ultrabasic Basic Intermediate Acid 15
Alkaline Phonolite
Foidite Tephri- Trachyte phonolite
Trachydacite 10 Phono-
O tephrite 2 Trachy-
K andesite Rhyolite O
2 Basaltic a Tephrite N trachy- Basanite andesite Trachy-
basalt
5
Dacite
Basaltic
Basalt andesite Andesite
Subalkaline/Tholeiitic
Picrobasalt 0
40 50 60 70 80
SiO2 RY 3
Figure 17: TAS diagram with unaltered sample TR-3 plotted.
28
29
The five rhyolite samples are plotted in Figure 16. It was also determined from
RY-3, which is the only sample that had not undergone extensive hydrothermal alteration, that these flows were indeed rhyolite of the subalkaline/tholiitic series (Le Bas,
1986) (Fig. 17, Table 1).
Sulphur Group
The only unit present from the Sulphur group is the informally named Camel
Conglomerate, abbreviated TCC in mine nomenclature, which overlies the TVC in the western part of the zone. This unit has a gradational contact with the TVC. Within the zone of gradational contact, distinct beds of TVC are interlayered with distinct beds of
TCC. In addition, intermediate horizons show characteristics of both units. The Camel
Conglomerate is a clast-supported conglomerate with local matrix-supported interbeds.
Clasts locally comprise from 15 to 85% of the unit and range in size from medium sand through 10 cm and an average size of 5 to 10 mm. Clasts tend to be moderately sorted with localized grading from coarse to fine over the length of a meter of core and are rounded to subangular. Clasts are 50 to 80% meta-sedimentary with the remainder of volcanic origin, mostly rhyolite with lesser dacites and basalt. The meta-sedimentary fragments make up most of the smaller size fraction, and have a higher degree of rounding while the volcanic derived fragments are larger and more angular.
Deposit Structure
The main structures in the area are a series of sub-parallel, north- to northeast- striking, steeply-westward dipping normal faults. The most prominent of these is the East 30
fault which is the only fault directly observed (Fig. 7), and the range bounding fault of the
Kamma Mountains. The main part of this fault extends north to northeast and, as indicated by drilling, the dip angle shallows with depth. The other north-striking faults sole into the East fault, which is the main contact between the ALS and the Kamma
Mountains Group. It is believed that this fault serves as the main conduit for ore fluids to the Vortex area. This normal fault separates the Kamma Mountains to the east from alluvium on the west. This fault is manifested both as a linear divide of steep hillsides from shallow alluvium, and as silicified fault gouge. A small splay of this fault is noted in the map area that traverses into the foothills of the Kamma Mountains. This splay is noted because of a fin of silicified fault gouge which resembles a dike that was measured for strike, dip, and rake of fault striae (Fig. 18). This splay strikes northeast with a dip of
45 to 55° northwest, and fault striae rake 65 to 75° to the south, indicating dextral movement.
As can be seen in the sections, the surfaces of the flow units in the Kamma
Mountains Group are irregular, which made exact placement and offset of the less prominent faults in the area somewhat difficult. Attempts to find marker beds to aid correlation of units were unsuccessful. Therefore, the offsets shown in section (Figs. 7-
10) are the best interpretations based on previous work, alteration, geochemical, and of course, kinematic indicators in core logging. The less prominent normal faults in the area from east to west are the Fire, Albert, and Break. Of these, the Fire is most prominent with notable offset in the A-A’ section (Fig. 7). This is the only fault that has noted offset through all four cross sections. The Albert fault, the next fault to the west, has notable 31
Figure 18: Photograph of slickenlines on silicified fault gouge along the East fault in the Kamma Mountains offset in the northern three sections, but has no noticeable offset in the D-D’ section (Fig.
10), though it should be noted that the geology is not well constrained through drilling in this section. The Break fault shows offset only in the A-A’ section (Fig 7), but as will be shown later, significant geochemical indication is given for the continued existence of this fault through to the southern portion of the zone. 32
Chapter 4: Alteration
Methods
The study of alteration within the Vortex zone started during the field season when cross sections showing the distribution of alteration were made. This was accomplished by taking the alteration observed while logging core, comparing it to the core photographs taken of the same holes logged to verify what alteration looked like in photograph, and then looking at the core photographs of the other holes that were used to complete the four cross sections in the Vortex area.
A subcrop/outcrop alteration map (Fig. 19) was completed using the same techniques as described in the previous section for the lithologic map. The same holes that were used to create the geologic cross sections were used to create alteration sections
(Figs. 21-24).
Alteration mineralogy was determined from 47 polished and thin sections of samples collected while core logging. Samples were photographed, cut to size, marked, cataloged, and shipped to Spectrum Petrographics in Vancouver, WA for section preparation. Five samples were determined to be in an oxide zone, and were prepared as standard thin sections. The rest were prepared as polished thin sections in order to facilitate both transmitted and reflected light petrographic work. Slides were analyzed and described between first receiving them in November, 2012 and January, 2013.
Distribution
Alteration within the Vortex zone is pervasive and tends to form in 33
Figure 19: Alteration map of Vortex Area at the same stratigraphic level as outcrop/subcrop map (Fig. 6). 34
horizontal layers rather than halos to faults. This is believed to be due to the high permeability of both the TVC and TCM host rocks, which are both only lithified within the zones of hydrothermal alteration. The most intense alteration occurs near fault zones, where it is believed that hydrothermal fluids fed from lower levels of the system into the higher levels.
Five types of alteration are present within the zone. Listed in paragenetic order
(Fig. 20), these include (1): an argillic alteration that is pervasive throughout the zone with a distinct zonation from kaolinite dominant to smectite dominant; (2): a propylitic zone of limited extent that occurs locally in the banded rhyolite, rhyolite, and tuffs of the
Kamma Mountains unit; (3): a pervasive silicic alteration that follows fault zones, and is present in the contact zone between the upper rhyolite and the lower TVC, extending several tens of meters in to the TVC; (4): an opal + adularia zone that has formed a blanket alteration near surface in the northern portions of the zone; and (5): a blanket acid leach alteration which includes the upper pervasively leached zone and a lower basal zone where some of the elements leached from the upper zone accumulate.
Figure 20: Paragenetic diagram for hydrothermal alteration sequence. Thickness of lines represents relative abundance. 35
Figure 21: Alteration through cross section A-A'. Lithology contacts are shown with dashed lines (see Fig. 7).
36
Figure 22: Alteration of section B-B'. Lithology contacts are shown with dashed lines (see Fig. 8).
37
Section A-A’ (Fig. 21) shows the basic pattern that is followed throughout much of the zone. Rock type seems to be somewhat important in controlling alteration. Argillic alteration occurs throughout the zone, except where overprinted by later alteration types.
The dividing line between smectite dominant and kaolinite dominant types is roughly the top of the massive rhyolite. This boundary coincides with silicic alteration. The upper lobe of propylitic alteration occurs directly below the silicic alteration, with a lower lobe coincident with top of the banded rhyolite. A second type of control on alteration is elevation within the system rather than lithologic control. This type includes opal – adularia and acid leach. The opal – adularia alteration occurs at depths from 150 to 300 feet with thicknesses reaching 450 feet. The acid leach alteration is in contact with the top of the opal – adularia and continues to very near the surface, where covered by recent alluvium.
The next section to the south, B-B’ (Fig. 22), has a similar alteration pattern to A-
A’. The most striking difference is the absence of opal – adularia, which is also absent throughout the rest of the Vortex zone. Although patterns remain similar, all alteration types shift to higher elevations and higher stratigraphically. The boundary between smectite- and kaolinite-dominant argillic alteration occurs around the middle of the TVC, rather than the top of the massive rhyolite. The silicic alteration also occurs here, with a significantly thicker blanket. This blanket reaches its greatest thickness coincident with the Fire fault.
The next section to the south, C-C’ (Fig. 23), has similar alteration distribution to the previous sections. The silicic alteration within this section is thinner than the sections 38
Figure 23: Alteration of Section C-C'. Lithology contacts are shown with dashed lines (see Fig. 9).
39
Figure 24: Alteration of D-D' Section. Lithology contacts are shown with dashed lines (see Fig. 10).
40 to the north, and is absent in the eastern part. Silicic alteration drops significantly between the Fire and Albert faults. Propylitic alteration is also less prevalent within this zone occurring only west of the Albert fault.
The final section, D-D’ (Fig. 24) shows a marked decrease in hydrothermal alteration within Vortex. Propylitic alteration is absent and basal acid leach alteration is restricted to the middle of the section; argillic alteration only occurs to the midway point of the TVC. The limited faulting in this section provided fewer conduits for hydrothermal fluids than in other sections, contributing to less alteration.
Assemblage Descriptions
Argillic Alteration
The argillic alteration zone is a pervasive alteration to clays + anhedral quartz + sericite + marcasite + pyrite ± chlorite (Fig. 25). The clay minerals within this alteration type are zoned with smectite the dominant clay above the contact with the TVC-TR and kaolinite the dominant clay mineral beneath the contact. The rhyolite sections are altered to kaolinite + anhedral, very fine grained (< 0.001mm) quartz that appears to have been precipitated during the alteration process with fibrous and locally massive marcasite overgrown by euhedral pyrite. Fractures locally have euhedral quartz up to 0.5mm coating edges and are filled with very fine grained sericite with local blebs of marcasite.
In the tuff sequence, clasts have the same alteration as groundmass in rhyolite, but the matrix also has abundant sericite. Pyrite dominates the sulfide assemblage in the matrix here, while marcasite is more common in the clasts. The majority of the banded rhyolite, 41
A B
C D
Figure 25: Argillic alteration assemblage. E Photomicrographs from Slide 3477-1859: A-C, 3477-704: D-E A: Marcasite and pyrite in a matrix of quartz and kaolinite. 0.85mm FOV Plane polarized and reflected light. B: Same FOV with polarized light. C: Sericite in kaolinite with anhedral quartz. 0.43mm FOV. D: Smectite replacing rhyolite clast. 0.85mm FOV. E: Chlorite (center) in clays with euhedral disseminated pyrite. Plane and reflected light, 0.85mm FOV.
42
A B
C D
E Figure 26: Propylitic alteration assemblage. Photomicrographs from slide 3475-1830. A: Pyrite with chlorite cut by sericite and chalcedony. Plane polarized and reflected light, 0.43mm FOV. B: Same FOV polarized light. C: Smectite in chalcedony and chlorite. 0.85mm FOV. D: Chlorite with sericite and chalcedony. 0.85mm FOV. E: Marcasite in chlorite and chalcedony. 0.85mm FOV.
43 tuff sequence, and rhyolite are within this alteration type. Though not noted on cross section, there is a thin band of argillic alteration that affects the rhyolite bounding the east side of the East fault. The upper zones where smectite is the dominant clay mineral are equivalent to the illite-smectite alteration classification of Ebert (1995). The TVC and
TCM are the most commonly affected units by this alteration type. Rhyolite clasts are altered to mottled 0.5mm smectite grains with 1 – 5% disseminated fine-grained marcasite ± fine-grained reticulate textured sericite. Matrix consists of aggregates of granular, subhedral quartz in fine grained smectite and sericite with disseminated pyrite.
Illite from an illite + quartz + pyrite altered felsic volcanic rock was 40K/39Ar dated in
Ebert’s study (1995) to 4.0 ± 0.1 Ma.
Propylitic Alteration
Propylitic alteration is focused mostly in veins cutting through the lower portions of the Kamma Mountains Group and consists of chalcedony, chlorite, sericite, smectite, marcasite, pyrite and trace amounts of carbonates (Fig. 26). Locally, chlorite is found to flood into wall rock, but tends to be very fine-grained and anhedral. A faint to dark green color is associated with areas of propylitic alteration, but as shown in thin section, smectite group clays are the most common mineral in these areas with only minor chlorite. Propylitic alteration is most common in sections B-B’ and C-C’, is present in A-
A’, and absent in D-D’. Where present, it occurs in two separate lobes: one in the upper portions of the TTL and lower portions of TR, and one in the upper portions of the TRB and lower portions of the TTL. These lobes pinch out towards the eastern part of the zone. 44
A B
C D
E Figure 27: Silicic alteration assemblage. Photomicrographs from 3502-1132: A-B, 3502-1357: C, 3502-938: D, and 3502- 1035: E. A: Pyrite in chalcedony cut by quartz. Plane polarized and reflected light, 0.43mm FOV. B: Same FOV with polarized light. C: Chalcedony and quartz around previously argillized clast. 0.85mm FOV. D: Sericite between quartz grains. 0.43mm FOV. E: Marcasite in quartz. Plane polarized and reflected light, 0.85mm FOV.
45
Figure 28: Core box photograph from drill hole 4326 at footage 1222-1232. This interval has pervasive silicic alteration. Note the fragmental banded vein below the 1228 mark. The interval from 1225 to 1230 assayed 0.174 opt Au and 1.843 opt Ag.
Silicic Alteration
Silica flooding is a common alteration associated with fault zones, breccias, and the boundary between the TR and TVC. Silicified breccias occur on or near fault zones and throughout the lower Kamma Mountains Group, but are rare in the upper portions of the TVC. In most cases, clasts are angular to subrounded and range in size from 0.5mm to 20mm and are either altered to the argillic, smectite, or rarely propylitic assemblages.
These clasts are encapsulated in a matrix of chalcedonic to subhedral granular quartz ranging in grain size from less than 0.01mm to 15mm. Pervasive silicic alteration is also present as a blanket flooding and brecciated vein alteration type coincident with the lower 46
A
B C
Figure 29: Photographs of Opal – Adularia alteration. A: Core photograph of typical opal – adularia altered TVC. B: Sodium cobaltinitrite staining shows that adularia is a late alteration product along fractures and filling vugs. C: Vug on right side of previous picture with late pyrite infill.
47
portions of the TVC. In the pervasive zones this alteration occurs as chalcedony + granular quartz + sericite + pyrite + marcasite in the matrix (Fig. 27) clasts are incompletely replaced, and commonly are smectite altered. The brecciated vein zone consists of discontinuous banded veins up to 0.5m thick that have been brecciated.
Locally these brecciated veins have been encapsulated in the pervasive silica, but commonly are the only silica alteration in the area and enveloped in argillic alteration.
These fragmented banded veins commonly host gold (Fig. 28). Not noted in the cross sections is a five to ten meter thick band of silica associated with the East Fault. This silicification is noted both in drill core, and as outcrops of resistant rock that have a dike- like appearance, cropping out on the eastern splay of the East Fault.
Opal – Adularia Alteration
Opal – adularia alteration is present only in the northernmost part of the system
(Figs. 21, 22). Where present, this alteration type forms a pervasive blanket of silicification/adularia (Fig. 29). Minerals found within this alteration type include quartz
+ chalcedony + adularia + pyrite. Within the district, Ebert (1995) also reports opal + marcasite ± stibnite ± leucoxene and/or rutile within this alteration type. Ebert (1995) also did extensive SEM work that revealed that the matrix of opal + adularia-altered
Camel Conglomerate and TVC consists dominantly of opal or chalcedony with between
15 and 50% adularia. The presence of this alteration type was confirmed by staining of a sample taken from hole 3502 at a depth of 503 feet (Fig. 29).
48
A B
C
Figure 30: Photographs of acid leach alteration. A: Moderately altered acid leach from drill hole 3502 at 200 feet depth. B: Acid leach alteration around historic mine portal near East fault. C: Core box photo of basal acid leach with limonite staining.
49
Ebert (1995) dated adularia from this alteration using a laser microprobe 40Ar/39Ar method. This method was used because SEM analysis showed that adularia in the matrix of this alteration type was pure KAlSi3O8 whereas adularia altered clasts had traces of
CaO and Na2O, indicating incomplete replacement. Age dates from two different locations within the matrix of this alteration yielded ages of 3.8 ± 0.09 Ma and 3.9 ±0.3
Ma. Ages from the incompletely replaced clasts yielded ages of 10.6 ± 0.17 Ma and 14.9
± 1.6 Ma. The dates from the matrix as well as the illite date mentioned earlier constrain the age of the main body of alteration to approximately 4.0 Ma. The incompletely replaced clast dates are also of interest, because they could possibly give a minimum age for the felsic volcanic of the Kamma Mountains at 14.9 ± 1.6 Ma (Ebert, 1995).
Acid Leach Alteration
The acid leach alteration is a steam-heated acid-sulfate alteration that occurred late (Ebert, 1995) in the system development and overprints all alteration in the upper portions of the deposit. As indicated by the name, this alteration type is characterized by an intense leaching of mobile elements. A zoning to the acid leach alteration is apparent with the upper, more intense areas being zones where elements are leached and only local native sulfur is added (Fig. 30). This zone is characterized by its bleached white color, powdery nature, and the mineral assemblage of nearly 100% quartz with trace amounts of alunite, native sulfur, and jarosite. This alteration type is more common in the northern parts of Vortex, and generally extends to greater depths in fault zones, particularly along 50 the East fault. The basal acid leach zone is far more pervasive in the Vortex area. It is characterized by an accumulation of some of the less mobile elements, particularly iron in the form of hematite. This zone is cemented with silica in the form of opal or chalcedony
(Ebert, 1995) and while commonly gray or white in color, is more generally stained red by hematite, and locally yellow to orange by a combination of goethite and jarosite.
Alunite from the acid leach alteration had ages ranging from 2.4 ± 0.1 Ma to 2.0 ± 0.1 Ma based on 40K/39Ar dating. Jarosite from late veins within this alteration was dated at 0.7 ±
0.2 Ma (Ebert, 1995). This agrees with field evidence indicating that acid leach alteration was a late overprint on all other alteration types. In addition, the jarosite date indicates prolonged hydrothermal alteration.
51
Chapter 5: Paragenesis
The earliest alteration type present within the Vortex zone is the argillic assemblage. This is evidenced by argillically-altered clasts present within the silicic zone, and propylitic veins cutting argillically altered groundmass. The only timing that remains unclear is that between silicification and propylitic assemblages. There is inconclusive evidence to suggest that propylitic was earlier than silicic. It is apparent that opal - adularia alteration occurred after the silicic alteration. This is indicated due to the nature of the silica in each. The silicic alteration contains high amounts of quartz and chalcedony. The silica in opal – adularia alteration occurs as opal and chalcedony which have been recrystallized to quartz. Since quartz forms at higher temperatures than chalcedony, which forms at higher temperatures than opal, it is assumed that the silicic alteration occurred earlier in the alteration process. The opal – adularia formed before the acid leach alteration as evidenced by pods of opal – adularia mineralization encapsulated in acid leach alteration (Ebert, 1995). In summary, a pervasive argillic overprint happened first followed by propylitic, then silicic which was followed by opal – adularia then acid leach. Hydrothermal alteration was followed by hydrothermal brecciation and several episodes of veining (Figs. 20 and 31).
Hydrothermal Alteration
The paragenesis within the argillic alteration started with early clay minerals. In zones where there is a clast and matrix type host rock (TVC, TTL) early clay plus quartz replaces the clasts (Fig. 32) followed by marcasite and pyrrhotite (Fig. 33). Found only in 52
Figure 31: Paragenetic Diagram of hydrothermal minerals found with the Vortex zone. Cross cutting relationships for all events could not be determined. Length of line is used only schematically to show when one mineral deposition encapsulated other minerals. Relative abundance is shown by thickness of lines.
53
one thin polished section, galena replaces this marcasite (Fig. 34). This is cut by matrix alteration to sericite + quartz + pyrite (Fig. 35).
Propylitic alteration locally veins the argillic alteration. Within these veins, chlorite is the first mineral deposited (Fig. 36) followed by a mix of chalcedony, chlorite, ferroan carbonates and marcasite (Fig. 37) with late stage quartz + marcasite veins. These could possibly be evidence for silica alteration cutting propylitic, but since these veins are not very laterally continuous, it is assumed that they are a late stage of propylitic alteration. There are propylitically altered clasts within some of the silicified breccias coincident with the silicic alteration zone.
Silicic alteration follows argillic alteration as evidenced by remnant argillic clasts encapsulated in silica. These clasts are rimmed by granular euhedral quartz which then grades into chalcedony. Locally sericite occurs within the chalcedony with a late overprint of marcasite (Fig. 38).
The opal – adularia alteration follows silicic alteration. Since these two alteration types do not overlap on any of the samples collected, the evidence that is used for this is the relative abundance of opal/chalcedony in the opal – adularia alteration type compared to the silicic alteration. The silicic assemblage has more crystalline quartz, which forms at a higher temperature, so it is assumed that it formed earlier. Within the samples recovered for opal – adularia alteration, chalcedony/opal, quartz, adularia, and pyrite are the only minerals identified by this study. Chalcedony/opal were the first deposited in a pervasive replacement of the matrix and some of the clasts 54
Figure 32: Photomicrograph from 3389-1701 in polarized light. This from a TTL sequence showing that clasts (top and bottom) with early clay + quartz alteration cut by later sericite + quartz + pyrite alteration of the matrix. FOV 0.43mm
Figure 33: Photomicrograph from 3389-1701 in reflected light. Marcasite is shown here within a clast with abundant clay/quartz inclusions. FOV 0.43mm 55
Figure 34: Photomicrograph from 3389-1701 in reflected light. Galena (gray) replacing marcasite (creamy yellow) and pyrrhotite (Brownish yellow). 0.18mm FOV
Figure 35: Photomicrograph from 3389-1701 in reflected light. Late euhedral pyrite (center yellow) in sericite + quartz matrix material. 0.18mm FOV 56
Figure 36: Photomicrograph from 3475-1830 in plane polarized plus reflected light. Wallrock is on the left side of the vein. The first mineral deposited is chlorite (dark green) followed by chlorite + chalcedony. The vein is cut by late stage quartz + marcasite. 1.7mm FOV. Following photograph taken to right of this FOV.
Figure 37: Photomicrograph from 3475-1830 in plane polarized plus reflected light. Central portion of veinlet in previous figure showing late chlorite + chalcedony + marcasite with local ferroan carbonates (gray mineral in bottom left portion above quartz veinlet). 1.7mm FOV 57
Figure 38: Photomicrograph from 3502-938 in polarized light. This section is from the TVC where an argillically altered clast (top right) is overprinted by silicic alteration starting with coarse granular quartz grading out into chalcedonic quartz with interstitial sericite and marcasite. FOV= 1.7mm
Figure 39: Photograph of acid leach sample with late sulfur as a vug infill. 58
along with minor amounts of pyrite. Adularia forms as a later vug/fracture fill (Fig. 29) after crystalline quartz. Small euhedral pyrite grains formed after adularia.
Acid leach was the latest hydrothermal alteration event. As discussed in the section describing this alteration, the upper portion is characterized by mineralogy being depleted due to the intense acidity of the fluids. The only minerals found in this zone during this study were quartz, alunite, and native sulfur. Quartz was the first deposited followed by alunite then native sulfur as a late vug fill/fracture coating (Fig. 39). A similar paragenesis is followed in the lower zone where chalcedony was first deposited followed by hematite, goethite, and jarosite which occur as fracture coatings.
Brecciation
At least five breccia events followed pervasive hydrothermal alteration. These have been broken down into three classes: those with bladed carbonates, those which have been completely silicified and one sample which had a smectite-rock flour matrix.
Cross cutting relationships are rare in breccias, and are only recorded in one sample.
Some breccias include fragments which have clearly been created in earlier events, but where these are absent, timing has been inferred on silica form with the assumption that the forms from higher temperatures occur earlier in the system.
The earliest breccia is matrix-supported with 10% clasts of moderately sorted subangular rhyolite ranging from sub-mm size up to 1cm found in sample 3389-1490 and
3389-1112. The matrix is composed of 0.5mm jigsaw quartz with relict bladed carbonate and explosion textures. About 1% of the sample is made up of late 0.01mm rounded 59 ferroan carbonates, which are commonly rimmed by subhedral to euhedral 0.01mm pyrite grains and aggregates. Trace amounts of burrowed, anhedral rhodochrosite are found as late vug fill. Local quartz fragments without any boiling textures are present. Local feathery alunite-replaced clasts of rhyolite make up 1-2% of the breccia.
The second breccia to occur is recorded only in sample 3475-1092 (Fig. 41). This breccia is matrix-supported with about 25% clasts which are poorly sorted, rounded to subangular, range in size from <1mm to 2cm, and are silicified or argillized rhyolite.
This breccia is characterized by an abundance of bladed carbonate fragments with later infill of chalcedony followed by granular quartz (Fig. 42). These are overprinted by fibrous marcasite with later euhedral pyrite (Fig. 43). Included within this breccia are fragments of earlier clasts, which show quartz replacement of bladed carbonate textures believed to be from the earliest breccia.
The third event is recorded in at least two samples: 3477-460 (Fig. 40) and 3475-
1092 (Fig. 44). This breccia is matrix-supported with only 10-15% rounded fragment content, moderately sorted ranging in size form 2-10mm, and silicified rhyolite clasts. In the sample 3475-460, the breccia consists of early chalcedony with local blebs of included alunite (Fig. 44) followed by granular quartz. Included within vugs of the granular quartz is marcasite and pyrite with later overprints of pyrargyrite (Fig. 45), which also overprint earlier chalcedony. This event clearly cross cuts the previous breccia in sample 3475-1092.
The fourth event observed is found only in sample 3475-1131. This breccia is very poorly sorted, clast-supported, sub-rounded, and cemented by quartz and 60
Figure 40: Annotated photograph of core sample from 3477-406 to show relationship of host rock, vein, and breccia. Rectangular outline shows where thin section was taken.
Figure 41: Annotated photograph of sample 3475-1092 showing TR being cut by veining, then brecciation which is subsequently cut by a second veining event and a second brecciation.
61
Figure 42: Photomicrograph of 3475-1092 #2 in polarized light. Taken within Breccia #1. Fragments of bladed carbonate with chalcedony and granular quartz infill. FOV 1.7mm
Figure 43: Photomicrograph of 3475-1092 #2 in reflected light. Taken within Breccia #1. Euhedral pyrite overgrows earlier marcasite. FOV 0.18mm 62
Figure 44: Photomicrograph of 3477-460 within Breccia #2 in polarized light. Early chalcedony with later granular quartz and interstitial alunite. FOV 0.43mm
Figure 45: Photomicrograph of 3477-460 within Breccia #2 in reflected light. Pyrite forms in vugs left by earlier granular quartz and is overprinted by massive pyrargyrite. FOV 0.43mm 63
chalcedony. Clasts make up 60% of this breccia and derived from smectite altered rhyolite. Some clasts contain earlier marcasite/quartz veins.
The final event observed is found in sample 3475-570. This breccia is clast- supported, moderately sorted, subangular to subrounded, and has little to no silica cementation. Matrix is a combination of rock flour and smectite. About 20% fragments of jigsaw quartz after bladed calcite are included within this event. Fragmental/subhedral pyrite averaging 0.01mm in size is disseminated in aggregates throughout this sample.
Veining
Since cross cutting relationships could not be found for all veins, they are divided into three categories: early, middle and late. Veins are placed in each of these categories based on their relationships within a given sample. Placement on the paragenetic diagram can only be taken as within the group of early, intermediate, or late, and the veins within each group are placed in order arbitrarily. There are several types of veins within the
Vortex system. As previously discussed there were at least two early bladed carbonate veining events and one iron sulfide-silica vein before the final brecciation event. In the iron sulfide-silica vein type, iron sulfides occur most commonly as marcasite with lesser pyrite. This was followed by several other vein types which are categorized as intermediate timing or late. Intermediate veins consist of a two types including coarse alunite and banded silica. Banded silica veins are characterized by drusy quartz followed by massive silica, usually chalcedony with lesser quartz. Late veining comes in several different types. Thin, 2-3mm wide chalcedony veins which have a white, massive appearance cut all other veins in several brecciated/vein areas. The largest vein types are 64
massive quartz after bladed calcite veins, which have been logged up to 100 feet thick in hole 3477, though margins were brecciated, so true thickness was indeterminate. A third type of late veining consists of chalcedony + alunite ± acanthite.
Early Veins
As was previously mentioned, cross cutting relationships within vein groups were rare, so these vein types are discussed in an arbitrary order.
Quartz-Adularia
The earliest deposition in this vein type was chalcedony, which formed around rounded clasts of jigsaw texture quartz. Later, very fine grained anhedral pyrite formed interstitial to quartz and chalcedony and was replaced by ferroan carbonates. Explosion texture in quartz is pervasive throughout this portion of the vein. Outboard of this in a sharp contact is coarser quartz with a pseudo-reticulate texture. Sericite inclusions are common in the grains closest to the inner boundary (Fig. 46), but become less common towards the middle of the vein. Outboard from the most abundant sericite zones, small adularia rhombs appear, locally forming aggregates interstitially to quartz (Fig. 47).
Iron Sulfide-Quartz
This vein type was observed in three separate samples and averages about 5mm in width. In two samples, the vein type is cross cut by later quartz veins. In another, it is included within a clast that was later brecciated giving clear indication that this was an early vein type. In two cases, the first mineral deposited was marcasite followed by quartz and chalcedony. In one sample marcasite is banded along the vein margin with 65
Figure 46: Photomicrograph of granular quartz in quartz-adularia vein with included sericite and carbonates. Crossed polars, FOV 0.85mm.
Figure 47: Photomicrograph of adularia rhombs in quartz-adularia vein. Crossed polars, FOV 0.18mm. 66
Figure 48: Marcasite-Quartz vein with banded and fibrous marcasite at the vein margins (right of picture). Euhedral marcasite follows the first quartz deposition (center). Plane polarized and reflected light, FOV 0.85mm
Figure 49: Banded Quartz-Chalcedony vein margin showing drusy quartz (left) followed by marcasite and pyrite (opaques) deposition and then chalcedony. Polarized light, FOV 1.7mm. 67
Figure 50: Photomicrograph of subhedral pyrite aggregates in interior of iron sulfide- quartz vein. Reflected light, 1.7mm FOV
Figure 51: Same FOV as previous figure showing drusy quartz vein margin with interstitial sericite and chalcedony filling vugs left by subhedral pyrite. 68
Figure 52: Photomicrograph of 3475-1092 #1 showing vein margin (left) with chalcedony as the first deposition followed by bladed calcite with quartz infill and local replacement. 0.85mm FOV
Figure 53: Photomicrograph of 3475-1092 #2 showing tennantite-tetrahedrite infilling vug replacing pyrite. Reflected light, 0.43mm FOV. 69
fibrous crystals reaching into the interior of the vein. This was followed by the first quartz deposition and euhedral to fractured/subhedral marcasite (Fig. 48). The central part of the vein is composed of mottled quartz and chalcedony. The second marcasite vein follows this general pattern of deposition, but marcasite only appears as fibrous crystals. The third vein of this classification is pyrite dominant with drusy quartz along vein margins followed by large aggregates of subhedral pyrite that compose most of the interior of the vein with local vugs filled with chalcedony (Figs. 49-51).
Banded Quartz-Chalcedony
There are several examples of this type of vein throughout the system. They generally are a few mm to a cm wide and appear white to gray in color. The first mineral deposited was drusy quartz along the vein margins followed by chalcedony with fibrous marcasite followed by pyrite. The sulfide content is highest just outboard of the drusy quartz.
Bladed Calcite
These vein types are only found in sample 3475-1092 (Fig. 41). The cross cutting relationships outlined in Figure 41 illustrate that these veins preceded the end of brecciation. The first mineral deposited at the margin of these veins is a 0.1mm band of chalcedony. This is followed by a bladed carbonate with quartz infill (Fig. 52). The bladed carbonate is an indication of boiling hydrothermal fluids. Euhedral pyrite occurs in vugs. Locally the carbonate has been replaced by quartz. In the latter of the two veining events, tennantite-tetrahedrite fills vugs and replaces earlier pyrite (Fig. 53). 70
Intermediate Veins
Drusy Quartz-Chalcedony
This is a relatively common vein type, and appears in several samples. The first mineral deposited is drusy quartz that varies in size and texture from sample to sample, but generally is 0.25-1mm in length and has a feathery extinction. Some samples have pyramidal terminations to the early quartz while others are truncated. The centers of these veins are filled with chalcedony, and local drusy quartz surrounding vugs. Sample 3389-
1112 is the largest example of this vein type and shows explosion texture on the outer edges of the drusy quartz. This sample is also the largest vein of this type, being at least
2cm wide, but exposure is limited due to core width. It is also the only sample of this vein type that hosts late subhedral proustite in vugs created by quartz (Fig. 55).
Coarse Alunite
The hypogene alunite is assumed to be later in the system development, but no cross cutting relationships were found, and no silica is present to give an estimate on temperature of formation. This vein has euhedral alunite up to 1mm in size forming an irregular band along earlier supergene alunite wall rock (Fig. 54). A thin band of explosion texture is present along the outboard edge of coarse alunite. Crystals of 0.1mm euhedral pyrite form along the edges of coarse alunite crystals. Outboard of this is massive euhedral to subhedral 0.1-0.5mm alunite with trace amounts of 0.1mm euhedral pyrite.
71
Figure 54: Photomicrograph of section 3475-454 showing euhedral alunite at vein margin with a line of explosion texture along edge of coarse alunite. 1.7mm FOV
Figure 55: Photomicrograph of section 3389-112 showing anhedral proustite as a late vug fill. 0.85mm FOV 72
Late Veins
Quartz after Bladed Carbonate
The most prominent of the late veins is massive quartz after bladed carbonate veins. These veins can reach apparent thicknesses up to 100 feet in drill core, but orientation of vein margins was indeterminate, making true thickness immeasurable.
Most of these vein types usually average only a few feet thick and consist only of quartz/chalcedony and remnant carbonates within the thin sections studied. Individual blades reach up to 2 cm in length. Vugs created by blades are commonly only partially filled with quartz druse, leaving a vuggy silica texture. Locally pyrite replaces margins in anhedral blebs. Wall rock is coated with chalcedony followed by carbonates then quartz.
Local assay values indicate slightly elevated silver, but not of ore grade. These veins are never cross cut by other veins, indicating that they were very late in the system. The abundant bladed texture indicates boiling late in the system
Fine Grained Alunite
The fine grained alunite veins come in a couple of varieties. The most common of which is a mixture of alunite with chalcedony with no other minerals (Fig. 56). These veins occur higher up in the system, with no samples found lower than 500 feet from the surface. Macroscopically, they are massive and pink with irregularly shaped contacts with wall rock and are relatively hard. The fine grained nature and hardness make the mineralogy of these veins difficult to identify in core. 73
Figure 56: Photomicrograph of chalcedony/alunite mixture in the central portion of pink alunite veins. FOV 0.43mm
Figure 57: Photomicrograph of acanthite crystals growing on edge of vein. Wallrock (right half) is argillically altered and acanthite is encapsulated in chalcedony (left). FOV 0.43mm 74
The other identified fine grained alunite occurrence is as a central portion to a vein found in sample 3477-406 (Fig. 41 between TVC and Breccia #2). In this sample, the vein margin is intermittently lined with <0.5mm sized euhedral acanthite crystals, which are surrounded by a 1mm band of chalcedony (Fig. 57). The central portion of this vein is then filled with a similar chalcedony/alunite mixture to the other fine grained alunite, but is a grayish brown color instead of pink.
Thin Chalcedony
One of the last vein types to be deposited is a 1-2mm thick chalcedony vein. This vein is often observed cutting other veins, but no contact between this vein and bladed quartz-carbonate veins was found. It is assumed that the lower temperature silica indicates that this vein formed later.
75
Chapter 6: Geochemistry
Geochemical data have been gathered on the Vortex zone since shallow drilling commenced in 2007 using Inductively Coupled Plasma Mass Spectroscopy conducted by
ALS Chemex in Reno, Nevada. A total of 132 drill holes within the Vortex zone have multi-element geochemistry data. However, not all elements were analyzed consistently through the time span of testing. This inconsistency made it necessary to select only those elements that have enough samples to create a representative population. The elements that were dismissed due to incomplete sampling were B, Bi, Cd, La, Sc, Se, Th, Tl, U, V, and W. Those elements that had sufficient sampling for analysis were Al, As, Ba, Ca, Co,
Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Sr, Ti, and Zn. The holes completed in the years 2007-2008 had ICP data completed on the same sample interval as
Au and Ag assay data, most commonly 5 foot intervals. All holes after 2008 tested composites of 5 continuous Au/Ag sample intervals. Since Au and Ag were not included in the ICP data, they were added using fire assay data. This was relatively simple where sample intervals were the consistent, but with the later holes, a weighted average had to be taken in order to transfer data from fire assay intervals to ICP intervals. Once Au/Ag values were added to the data base, it was determined that earlier holes that had ICP data on 5 foot intervals would need to be averaged to 25 foot intervals so that valid descriptive statistics could be generated.
Zonation
In order to determine the zonation of elements commonly associated with low sulfidation epithermal deposits, the ICP data were loaded into Leapfrog, a three 76
dimensional modeling software. Geologic logs and original Au – Ag assay data were also loaded. A model was built using the geologic logs, though this model was of limited use due to inconsistencies in logging codes when the area was first being drilled, and current usage. This affected mainly the distinction between TVC and TCM, but also the upper rhyolite (TR) and the lower banded rhyolite (TRB). In order to keep this report consistent, the geochemical shapes were generated within Leapfrog, but then using the elevation and east-west mine grid were transferred to the alteration cross sections that have already been created. Shapes were limited to within 300 feet of an existing drill hole, and were assumed not to extend into the ALS. In order to determine where the threshold of anomalous concentration lies, descriptive statistics were generated for each element. Values that fell outside the main body of frequency distributions were discarded.
The mean value plus two times the standard deviation was used for threshold between background and anomalous values. These data are included with each individual element description.
Volatiles
The volatile elements that have been studied for this report include Hg, Sb, As, and S. Though sulfur is not generally included within this group, it is included for the purpose of demonstrating the timing of Hg deposition and is included on the same figures as Hg.
In a classically zoned epithermal deposit, arsenic would be expected high in the system and be included in arsenic sulfide minerals such as realgar or orpiment, which were not found, however. During the petrographic study it was noted that pyrite and 77
marcasite had a slightly more yellow appearance than is characteristic of non-ore pyrite, and it is a strong possibility that arsenic is included within these minerals. Another strong possibility is that arsenic is deposited in proustite or tennantite. Previous reports (Flint et al., 2012) have documented pyrargyrite, miargyrite, and naumannite as silver sulfosalts, but proustite has been absent. Unfortunately, during time spent core logging no samples were taken from zones of very high silver concentration that also coincided with high arsenic values due to most of these intervals being consumed for metallurgical purposes.
Some geochemical correlation was conducted that indicates the presence of proustite, which is discussed later in this section. It was determined that the threshold cutoff value for background arsenic to anomalous arsenic was 397 ppm (Fig. 59). In order to help show distribution, the Leapfrog model for As was set to 300, 400, and 500 ppm in the different zones (Fig. 58). According to this method, it can be seen that arsenic does not fit the classical geochemical model, but shows a distribution within the lower to mid-level part of the system, somewhat coincident with where precious metal minerals occur. One notable exception occurs in the B-B’ section in the eastern portion of the deposit, but it is of note that this anomalous occurrence is coincident with precious metals. No anomalous arsenic was detected in the D-D’ section.
Mercury in the form of cinnabar has been reported within Sulphur (Ebert, 1995), as well as being previously mined from other areas within the Sulphur district (McLean,
1991). No cinnabar was noted during this study. The threshold value for anomalous mercury was determined to be 29 ppm (Fig. 60-61). Mercury anomalies are associated with the acid leach alteration type. This conforms to the classical epithermal model of 78
Figure 58: Anomalous occurrences of arsenic within previously defined cross sections. The outermost shell which is colored green, represents 300 ppm, the middle yellow shell represents 400 ppm, and the innermost red shell represents 500 ppm As. No grades above 300 ppm were detected in the D-D’ section
1200 1000 Mean: 129.9 Threshold: 397.6 800 600 400 200
0
0-25
25-50 50-75
75-100
125-150 500-525 100-125 150-175 175-200 200-225 225-250 250-275 275-300 300-325 325-350 350-375 375-400 400-425 425-450 450-475 475-500 525-550 550-575 575-600 600-625 625-650 650-675 675-700
Figure 59: Histogram from geochemical database collected in Vortex area showing the distribution of As in ppm. The mean value is 129.9 ppm, the standard deviation 133.8, and a threshold value of 397.6 ppm. 79
Figure 60: Anomalous occurrences of mercury and sulfur within previously defined cross sections. The outermost shell which is colored green, represents 15 ppm, the middle yellow shell represents 30 ppm, and the innermost red shell represents 50 ppm As. The outer brown shell for sulfur is 3 pct, and the orange 5 pct.
5000 4000 Mean: 4.9 3000 Threshold: 29.1 2000 1000 0
Figure 61: Histogram from geochemical database collected in Vortex area showing the distribution of Hg in ppm. The mean value is 4.9 ppm, the standard deviation 12.1, and a threshold value of 29.1 ppm. 80
Figure 62: Anomalous occurrences of antimony within previously defined cross sections. The outermost shell which is colored green, represents the 100 ppm cutoff, the yellow shell represents 150 ppm, the innermost red shell represents 300 ppm. No grades above 100 ppm were detected in the D-D’ section
1800 1600 1400 Mean: 40.2 Threshold: 136.0 1200 1000 800 600 400 200 0
Figure 63: Histogram from geochemical database collected in Vortex area showing the distribution of Sb in ppm. The mean value was 40.2 ppm, the standard deviation 47.8, and a threshold value of 136.0 ppm. 81
mercury occurrence high in the system, but also indicates a later occurrence as any mercury in the zone before acid leach alteration would have been displaced. To illustrate this point, total sulfur was also plotted on the same cross sections. Arbitrary cutoffs of
3% and 5% were used based on the Leapfrog model. As was discussed in the mining history, large amounts of native sulfur were deposited in the system and occurred as vug and open space infill in the acid leach alteration (Ebert, 1995). High grades of sulfur occur intermittently just below the levels of anomalous mercury. This is an indication that both of these elements could have been deposited simultaneously or at least due to similar emplacement mechanisms.
Antimony in the form of stibnite, pyrargyrite, and miargyrite has been reported in the Sulphur district, but only pyrargyrite and tennantite-tetrahedrite were found in this study, in several different events within Vortex (see section on paragenesis). The Sb cutoff grade was determined to by 136 ppm (Fig. 63). To help show the shape of the anomaly, grades of 100, 150, and 300 ppm were used in Leapfrog (Fig. 62). Antimony conforms to classical zonation with most of the anomalous values occurring in a range from 500 to 800 feet below the surface, and above the zone of precious metal development.
Precious Metals
Silver and gold distribution within the Vortex zone conforms to classical zonation within an epithermal system. Mineralization occurs below the level of both mercury and antimony, but due to its unusual nature, occurs coincident with and slightly higher in the system than arsenic (Fig. 64-66). This distribution is also spatially coincident with the 82
Figure 64: Gold distribution on previously defined cross sections within Vortex. The outermost green shell represents 0.1 ppm. The middle yellow shell represents 0.35 ppm. The innermost red shell represents 0.7 ppm. 83
Figure 65: Ag distribution within previously defined cross sections. The outermost green shell represents 5ppm. The middle yellow shell represents 20 ppm. The innermost red shell represents 40 ppm.
84
Figure 66: Au equivalent distributions on previously defined alteration cross sections. The purple shell represents 0.15 ppm the green shell represents 0.4ppm, the yellow 0.6ppm, and the red 1.0ppm. The area outlined in section A-A’ is the area shown in fig. 64.
Figure 67: Outlined area from fig. 63. Black numbers represent silver to gold ratios taken from 100 foot intervals of assay data from the drill hole on which it is placed. Numbers are placed in the central portion of the interval. 85
upper massive rhyolite and lower TVC for the highest grade intervals, except in the B-B’ section where the highest grade mineralization tends to be just above the East fault. This distribution is continuous from east to west throughout the deposit, independent of depth.
Zonation of precious metals is shown using Au, Ag, and gold equivalent (AuEq). Gold equivalent is defined as the concentration of gold plus the concentration of silver divided by a value that equates the amount of gold to that of silver in a monetary sense. At time of writing, this number was approximately 57. Therefore AuEq = Au + Ag/57.
Within the zone of precious metal mineralization, there is a fairly distinct zonation of silver and gold. Ag:Au ratios for 100 foot intervals on the drill holes used to create the original cross section were calculated and plotted for one ore zone on the A-A’
AuEq section (Fig. 67). Towards the upper, and to a lesser extent, peripheral and lower portions of ore grade mineralization there is a higher gold content shown with a lower ratio. The more central areas of high grade mineralization tend to have higher Ag:Au ratios with one 100 foot interval in hole 3408 reaching 293:1. Unlike the other elements in this section, cutoffs for zonation were based on ore grades instead of determining anomalous cutoffs. The two lower shells are the cutoffs for oxide heap leachable material and sulfide material that will need to be milled.
Base Metals
Anomalous base metal occurs only erratically in Vortex with a very minor base metal signature overall. Even though little to no hydrothermal alteration has occurred in the Auld Lang Syne Group, base metal signatures are generally above those considered 86
Figure 68: Anomalous occurrences of copper within previously defined cross sections. The outermost shell which has been colored green represents 35 ppm Cu, the yellow shell represents 50 ppm, and the red shell represents 100ppm. No grades above 35 ppm were detected in the D-D’ section.
3500 3000 2500 Mean: 15.0 Threshold: 55.6 2000 1500 1000 500
0
0-10
10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90
90-100
110-120 100-110 120-130 130-140 140-150 150-160 160-170 170-180 180-190 190-200 200-210 210-220 220-230 230-240 240-250
Figure 69: Histogram from geochemical database collected in Vortex area showing the distribution of Cu in ppm. The mean value was 15.0 ppm, the standard deviation 20.3, and a threshold value of 55.6 ppm. 87
Figure 70: Anomalous occurrences of lead within previously defined cross sections. The outermost shell which has been colored green, represents 30 ppm, the yellow shell represents 55 ppm, and the red shell represents 70 ppm. No grades above 30 ppm were detected in the D-D’ section.
1800 1600 1400 Mean: 16.1 1200 1000 Threshold: 31.0 800 600 400 200
0
0-5
5-10
75-80 80-85 85-90 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 90-95
95-100
105-110 110-115 115-120 120-125 100-105 Figure 71: Histogram from geochemical database collected in Vortex area showing the distribution of Pb in ppm. The mean value was 7.5 ppm, the standard deviation 19.1, and a threshold value of 31.0 ppm. 88
Figure 72: Anomalous occurrences of zinc within previously defined cross sections. The outermost shell which has been colored green, represents the 115 ppm, the yellow shell represents 150 ppm, and the red shell represents 200 ppm.
1200 1000 Mean: 52.3 Threshold: 139.2 800 600 400 200
0
0-10
10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90
90-100
150-160 110-120 120-130 130-140 140-150 160-170 170-180 180-190 190-200 200-210 210-220 220-230 230-240 240-250 100-110 Figure 73: Histogram from geochemical database collected in Vortex area showing the distribution of Zn in ppm. The mean value was 52.3 ppm, the standard deviation 43.4, and a threshold value of 139.2 ppm. 89
anomalous for the deposit. This is believed to be related to depositional setting, a deep marine sedimentary rock.
Copper is the most widely distributed of the base metals (Fig. 68). It has a distinctly bi-modal zonation with anomalous values usually appearing in the lowermost western part of the system and along a horizon centered around 600 feet below the surface. The lower copper forms in a sparse chalcopyrite-bearing interval, which was only found in one hand sample, while the upper copper forms in the tennantite- tetrahedrite solid solution series, which is also likely a host for silver. The cutoff grade for copper was determined to be 55 ppm (Fig. 69). Copper is also deposited centrally to where precious metal deposition occurs with the greatest concentrations localized along the A-A’ and B-B’ sections a hundred or so feet above the main body of gold-silver.
Lead is the least prominent of the base metals with very few anomalous occurrences within the system (Fig. 70). Only one grain of galena was found through petrographic study (Fig. 34). The cutoff grade for lead was found to be 55 ppm (Fig 71).
The distribution is limited to one occurrence in the B-B’ and coincides with the lower smectite/upper silicic alteration type (Fig. 70).
Zinc is only slightly more prominent than lead. No zinc minerals were observed, but background values were higher than other base metals. Cutoff grade was 140 ppm
(Fig. 73). Anomalous grades of zinc were only found in the C-C’ section (Fig. 72). Both occurrences coincided with Fire fault, giving further evidence that faults were conduits for hydrothermal fluids. Having the only occurrence in C-C’ suggest that zinc is deposited peripheral to the main body of precious metal mineralization. 90
Correlation
In order to help define mineralization within Vortex, geochemical data were analyzed using Microsoft Excel correlation matrixes to help determine which elements had the greatest tendency to occur together. This can be a rough proxy for mineralogy where not observed. Caution should be taken when doing such, since there could be several factors which influence why elements occur proximally to each other.
In order to give a smaller scale view of what is happening in the system, three holes that were used in the creation of section A-A’ which also had geochemical data were put into a single spread sheet for analysis (Fig. 74). With the addition of Se, K, Fe, and Ba, only those elements that are discussed in the zoning section are included.
Although Se was not sampled widely enough to be shown in zoning, all three of the holes on section A-A’ had Se data. A master correlation matrix was created in order to show a broad picture of what was happening in this section. The data were then broken down based on the depth from surface from which they were collected in 400 foot intervals, with all the data from 2000 feet below surface to total depth lumped into the final group.
Correlation matrixes were run on all of these subsets in order to see the variation that occurs vertically within the system. It should be noted that since sampling intervals were on average 25 feet, these matrixes represent somewhat broad deposition intervals, and while they can be used to imply mineralogy, they should not be taken as true mineralogy.
This could also contribute to the moderate correlation values. 91
A:
B:
C:
92
D:
E:
F:
Figure 74: Correlation matrixes from holes 3502, 3501, and 3475. Matrixes were created on 400 foot intervals from surface down. A: 0-400ft covering mostly oxide mineralization n=31. B: 400-800ft n=43. C: 800-1200ft n=36. D:1200-1600ft n=48. E: 1600-2000ft n=40. F: 2000ft-TD n=15. Only hole 3501 extended this far. Cells with values above 0.9 were highlighted with red, 0.7-0.9 with orange, 0.5-0.7 with yellow, 0.3- 0.5 with green, 0.1-0.3 with blue, and the bottom 10 values for each matrix were highlighted with light red and red numbering.
93
Since no gold was observed during the petrographic study, these correlation values are the only evidence as to the mineral assemblages in which gold occurs.
Throughout the section, there is a relatively high correlation between gold and silver, ranging from 0.51 in the 1600-2000 foot range to 0.81 in the 0-400 interval. The 0-400 interval is roughly corollary to the oxide zone. This high correlation could indicate a higher relative abundance of electrum in this part of the system due to the leaching of silver sulfosalts. The only other notable correlations of gold in this interval are with iron
(0.57) and selenium (0.51). The gold – selenium correlation continues at depth, increasing to a value of 0.79 in the 1600-2000 foot interval, whereas the gold – iron correlation abruptly drops off in the unoxidized portion of the section. This suggests that while gold and iron were deposited at different places in the hypogene system, overprint leaching in the upper portions deposited both in the same intervals.
Hypogene mineralization of gold shows a variable correlation between gold, arsenic, antimony, and selenium. The last three elements have their highest correlation in the 1600-2000 foot interval, which also roughly coincides with anomalous arsenic in the section, and their lowest correlation in the 800-1200 foot interval. As indicated by other studies, the abundance of arsenic, selenium, and antimony as substitutions in iron sulfides could allow gold inclusion in these sulfides.
Hypogene silver mineralization also shows interesting correlations with arsenic, antimony, and selenium. This is to be expected since this and other studies have found pyrargyrite, miargyrite, proustite, and naumannite as the silver sulfosalts. The data are also evidence for a mineralogical zonation of silver sulfosalts. Within the hypogene zone, 94
silver has a low to negative correlation with most elements. The except is selenium in the
1200-1600 and the 1600-2000 foot interval where correlation values are high with arsenic, antimony, and selenium suggesting a co-occurrence of all previously reported silver sulfosalts. In the 1200-1600 foot interval the arsenic and antimony correlation values decrease while the selenium value increases, as well as the highest correlation value with copper. This suggests an increase in naumannite and tennantite-tetrahedrite in this zone with decreases in miargyrite, pyrargyrite, and proustite. In the 800-1200 interval, correlation values drop off substantially, with only selenium retaining any significant value at 0.49.
95
Chapter 7: Formational Model, Discussion, and Future Work
The formational model that best fits the Vortex zone can be broken down into four separate stages: the formation and faulting of the host rocks, an early pervasive alteration stage, a middle stage where the system became over pressured creating breccia dikes and small quartz veins, and a final veining stage that consists mostly of small chalcedony/opal veining as well as very thick quartz after bladed calcite veins.
The volcanic units of the Kamma Mountains were deposited unconformably on the Auld Lang Syne Group between 28 and 16 (Fig. 75 - A). While there are no dates on the Kamma Mountains rocks, Ebert (1995) suggests a minimum age of 14.9 ± 1.6 Ma based on dating of an incompletely replaced volcanic clast. Other Tertiary rhyolites within the region have been dated from 28-12 Ma (Johnson, 1977). Basin and range extension starting around 16 Ma creating the series of sub-parallel normal faults running throughout Vortex (Fig 75 - B). As faulting occurred, the rhyolite of the Kamma
Mountains group was fractured and shed along with other sediment from the upthrown blocks onto the downthrown blocks, creating the TVC. This is evident because of the lack of any TVC within the Kamma Mountains.
Hydrothermal fluids are believed to have originated from the Black Rock Desert basin based on stable isotope work completed by Ebert (1995) and were heated due to a high geothermal gradient in the area to the south of the deposit (Ebert, 1995). The East fault was the primary conduit for hydrothermal fluids entering the Vortex system (Fig.
76). From there, early hydrothermal fluids moved easily through the highly permeable host rocks. The tuffs and the initially unlithified volcaniclastic units had primary 96
Figure 75: (A) First deposition of Kamma Mountains Group in Vortex and (B) beginning of basin and range faulting and volcaniclastic unit deposition. 97
Figure 76: Beginning of hydrothermal alteration within Vortex. Fluid flow was pervasive throughout the lithic tuff and volcaniclastic units due to high primary porosity and permeability. Rhyolite units were fractured due to faulting creating a high secondary permeability leading to early pervasive argillic alteration.
98
Figure 77: After pervasive alteration, breccia dikes and acid leaching, late quartz-bladed carbonate veins and chalcedony-alunite veins occurred.
99
permeability, and faulting and fracturing of the rhyolite flows generated secondary permeability. In addition to the general permeability, the major faults acted as fluid conduits. This led to early pervasive argillic alteration under acidic conditions as evidenced by the high kaolinite content in the lower zone and the marcasite throughout the system. The fluids naturally cooled closer to the surface creating opal – adularia alteration near the top of the system which was dated by Ebert (1995) at 4 Ma.
The pervasive argillization decreased the permeability of the system, lithified volcaniclastics and tuffs and led to an increase in the fluid pressures (Fig. 77). These changes allowed for fracturing within the rock types that previously would have been too unlithified to fracture. Banded veins began to form at the top of the massive rhyolite. The interpretation for this placement is that fluids were under less confining pressure, or could mix with non-ore groundwater in order to start deposition. After the formation of these early banded veins, the pressures within the system built to the point of inflation and creation of very low angle breccia dikes that coincided with the top of the massive rhyolite and lower portion of the volcaniclastic unit. Most silver mineralization occurred during this phase. Introduction of silica with this event also caused pervasive silicification, which allowed brittle fracturing within the volcaniclastic units. Fractures associated with tectonic movement had greatly reduced confining pressures, which allowed hydrothermal fluids to boil and deposit late bladed carbonate veins.
100
Summary
The Vortex system is complex in many ways, but very simple in many others. A summary diagram (Fig. 78) schematically shows the basic geochemical, lithologic, alteration, and ore mineralization patterns. This diagram can be thought of as a series of columns showing idealized occurrences within Vortex. The lithology and alteration columns can be thought of as idealized cross sections. The trace elements and precious
Figure 78: Summary diagram of lithology, alteration, geochemistry, and ore intercepts within the study area. 101
metals columns can be thought of as graphs with depth on the Y axis and abundance on the X axis. In order to keep the precious metals diagram cleaner, only AuEq and Au PPM were plotted, the implication being that the area between the two is occupied by Ag.
Therefore, where AuEq is high, but Au is low, there is an abundance of Ag in the system.
Discussion
This system has been referred to as low-sulfidation epithermal, however, that classification only tells half of the story. The geometry of mineralization, associated sinter to the north of the Vortex area (Ebert, 1995), pervasive acid leaching, and host rocks all put this in the category of a low-sulfidation hot-spring deposit; the key difference between these two types being the host rock and an early event which sealed the system (Berger, 1985). These factors are important in this type of system for two reasons: a permeable sedimentary host rock allows for pervasive alteration, in this case argillization with some opal – adularia towards the northern part of Vortex, and, once sealed, allows for extensive brecciation with increased pressures (Berger, 1985).
Although Nelson and Giles (1985) suggest hydrothermal eruption is needed to deposit gold in hot spring systems, this is not widely observed in Vortex. While the geometry of gold mineralization towards the East fault suggests the possibility of breccia vents (Fig. 68), none were observed. Instead, precious metal mineralization tends to have coincided with what have been termed breccia dikes (Fig. 79). These form when hydrostatic pressure exceeds rock tensile strength, but not lithostatic load (Nelson and
Giles, 1985). This could explain why breccia diking coincides with the transition from rhyolite flows to volcaniclastic sedimentary rock. Pressure could also be slowly 102
dissipated into the volcaniclastics during breccia formation, preventing most breccias
from venting to the surface.
This system also varies from
many other low-sulfidation systems in
the apparent pH of the ore-forming
fluids. Marcasite is generally accepted
to be an indicator of pH below 5. In
addition, the presence of kaolinite,
particularly in the lower sections of
the deposit, is an indicator that initial Figure 79: Schematic cross section illustrating geologic features found in hot-spring precious- alteration fluids were low pH. These metal deposits from Berger (1985) conditions varied widely throughout the history of formation as evidenced by pseudomorphs of bladed carbonates formed through much of the history of the deposit. As the paragenetic diagram shows, there were also several episodes of marcasite formation. Clearly, pH of fluids was variable throughout the system. In addition, there is only sparse adularia within Vortex.
Future Work
Though zonation of precious metals was documented in this study, no visible gold was found in thin section. The location of gold in the system is based only on assay data.
The geochemistry discussed shows that gold formation is correlated with As, Ag, Sb, and
Se. This suggests that gold forms during the same events that deposited silver sulfosalts, possibly within these sulfosalts, or possibly in iron sulfides associated with these events. 103
The substitution of As in iron sulfides has been well documented and allows for the inclusion of gold in Carlin systems. SEM backscatter could be useful in determining the exact occurrence of gold.
104
References
Berger, B.R., 1985, Geologic-Geochemical Features of Hot-Spring Precious-Metal Deposits in Tooker, E.T., US Geological Survey Bulletin 1646: Geologic Characteristics of Sediment- and Volcanic-Hosted Disseminated Gold Deposits – Search for an Occurrence Model, p. 47-53
Burke D.B. and Silberling, N.J., 1973, The Auld Lange Syne Group of Late Triassic and Jurassic(?) Age: North Central Nevada, US Geological Survey Bulletin 1394-E, 24 p.
Clark I.C. 1918, Recently Recognized Alunite Deposits at Sulphur, Humboldt County, Nevada: Engineering and Mining Journal, Vol. 106, no. 4
Couch B.F. and Carpenter, 1943 J.A. Nevada’s Metal and Mineral Production (1859- 1940 Inclusive): University of Nevada Bulletin, Vol. 37, no. 4
Ebert, S., 1995, The Anatomy and Origin of the Crofoot/Lewis Mine, A Low Sulfidation Hot-Spring Type Gold-Silver Deposit Located in Northwest Nevada: University of Western Australia Ph.D. Thesis, 228 p.
Flint, D.C., Gorman, M., Harris, D. Moore, D.B., and Wilson, S.E., 2012, Technical Report, Allied Nevada Gold Corp. Hycroft Mine, Winnemucca, Nevada, USA: Effective April 9, 2012, 163 p.
Friberg, R.S., 1980, Detailed Evaluation Report of the Sulphur Gold-Silver Prospect, Humboldt and Pershing Counties, Nevada: Unpublished Homestake Mining Corporation Report, 32 p.
Fulton and Smith, 1932, Nevada Bureau of Mines and Geology File Manuscript.
Henley, R.W. and Livingston, W.K., 1990, Evaluation of the Sulphur District, Nevada. Deep Exploration Potential: Unpublished Hycroft Resource and Development Incorporated Report, 42 p.
Johnson, M.G., 1977, Geology and Mineral Deposits of Pershing County, Nevada: Nevada Bureau of Mines and Geology Bulletin 89, 115 p.
Jones, J.C., 1921, Report on the Property of the Silver Camel Mining and Development Corporation, Sulphur, Nevada: Unpublished Silver Camel Mining and Development Corporation Report, 6 p.
Lebas, M.J., Lemaitre, R.W., Streckeisen, A., and Zanettin, B., 1986, A Chemical Classification of Volcanic-Rocks Based on the Total Alkali Silica Diagram: Journal of Petrology, v. 27, p. 745-750 105
McLean, D.A., 1991, Geology of the Crofoot/Lewis Mine: Unpublished Hycroft Resource and Development Report, 11 p.
Moore, D.B., Gorman, M. Harris, D., Peterson, A.T., Reeves, and M., Wilson, S.E., 2013, Technical Report, Allied Nevada Gold Corp. Hycroft Mine, Winnemucca, Nevada, USA: Effective March 6, 2013, 145 p.
Nelson, C.E. and Giles, D.L., 1985, Hydrothermal Eruption Mechanisms of Hot Spring Gold Deposits: Economic Geology, v. 80, p. 1633-1639
Schaefer, D.H., Welch, A.H, and Maurer, D., 1983, Geothermal Resources of the Western Arm of the Blackrock Desert, Northwestern Nevada, Part 1, Geology and Geophysics: United States Geologic Survey Open File Report 81-918, 37 p.
Sherlock, M.G., 1989, Metallogenic Map of Volcanogenic Massive-Sulfide Deposits in Pre-Tertiary Island Arc and Ocean Basin Environments in Nevada: US Geological Survey Miscellaneous Field Studies Map: 1853-E, 17 p.
Smith, J.G., Mckee, E.H., Tatlock, D.B., and Marvin, R.F., 1971, Mesozoic Granitic Rocks in Northwestern Nevada: A Link Between the Sierra Nevada and Idaho Batholiths: Geological Society of America Bulletin, v. 82, p. 2933-2944
Vanderburg, W.O., 1938, Reconnaisance of Mining Districts in Humboldt County, Nevada: United States Bureau of Mines and Information Circular 6995, 47 p.
Wallace, A.B., 1980, Geology of the Sulphur District, Southwestern Humboldt County, Nevada: Society of Economic Geologist Field Trip Guide, p. 80-91
Willden, R., 1964, Geology and Mineral Deposits of Humboldt County, Nevada: Nevada Bureau of Mines and Geology Bulletin 59, 154 p.
Willden, R., 1961, Major Westward Thrusting of Post-Middle Triassic Age in Northwestern Nevada, US Geological Survey Professional Paper 424-C, 192 p.
Wright, J.L., 2007, Hycroft Property, Induced Polarization Survey, GIS Database: Unpublished Allied Nevada Gold Corp Report