Abstracts2000

January 2000
Orogenic Gold and Geologic Time: A Global Synthesis
Richard Goldfarb
U. S. Geological Survey , MS-973, Box 25096, Denver, CO 80225

Orogenic gold deposits have formed over more than 3 b.y. of Earth's history, episodically during the Middle Archean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristically associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustal structures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts and fluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which are commonly intimately associated with this mineral deposit type, recognized production and resources from economic Phanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversial Witwatersrand ores, known Precambrian gold concentrations are about half this amount. The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most of the world's major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in space and time.

The important periods of Precambrian orogenic gold-deposit formation, at ca. 2.8-2.55 Ga and 2.1-1.8 Ga, correlate well with episodes of growth of juvenile continental crust. Similar characteristics of the Precambrian orogenic gold ores to those of Phanerozoic age have led to arguments that "Cordilleran-style" plate tectonics were also ultimately responsible for the older lodes. However, the episodic nature of ore formation prior to ca. 650 Ma also suggests significant differences in overall tectonic controls. The two broad episodes of Precambrian continental growth, and associated orogenic gold-veining, are presently best explained by major mantle overturning in the hotter early Earth, with associated plumes causing extreme heating at the base of the crust. This subsequently led to massive melting, granitoid emplacement, depleted lower crust and resultant extensive buoyant continental crust. Resulting Late Archean and Paleoproterozoic crustal blocks are large and relatively equi-dimensional stable continental masses that are thermally and geometrically most suitable for the long-term preservation of auriferous mid-crustal orogens, particularly distal to their margins.

More than fifty percent of the preserved Precambrian crust formed between 1.8 and 0.6 Ga, yet these rocks contain few orogenic gold deposits, therefore indicating that more than volume of preserved crust controls the distribution of these ores. Despite much of this appearing to have been a time of worldwide extension and anorogenic magmatism in cratonic interiors, significant continental growth was still occurring along cratonic margins culminating with the formation of Rodinia by ca 1.0 Ga. Beginning at the end of the Paleoproterozoic, however, there was a change in crustal growth patterns, such that juvenile crust began to be added as long narrow microcontinents and accretionary complexes around the margins of older cratons. This probably reflects the gradual change from strongly plume-influenced plate tectonics to a less-episodic, more-continuous present-day style of slab subduction and plate tectonics as a more homogeneous mantle was evolved. The long and narrow strips of juvenile crust younger than 1.8 Ga would have been relatively susceptible to continual reactivation and reworking during Mesoproterozoic through Phanerozoic collisions, and the high metamorphic-grade of most 1.8-0.6 Ga crustal sequences indicates unroofing of core zones to the orogens. These schist and gneiss sequences would have been beneath the levels of most-productive orogenic gold-vein formation within most orogens.

The distribution of orogenic gold ores formed during the last 650 m.y. of Earth's history is well-correlated with exposures of the greenschist-facies mobile belts surrounding 1.8 Ga cratonic masses. Reworking of cratonic margins has eroded away most indications of orogenic gold older than ca. 650 Ma in these crustal belts, whereas younger lode systems are preserved especially from the last 450 m.y. The immense circum-Pacific placer goldfields collectively suggest a short lifespan for many of the lode systems; veins are apparently recycled into the sedimentary rock reservoir within 100-150 m.y. after their initial emplacement if continental margins remain active. Where continent-continent collisions preserved Phanerozoic orogens in a "craton-like" stable continental block (e.g. central Asia) during supercontinent growth, gold lodes (e.g. Muruntau) could be better preserved. The lack of any exposed, large orogenic gold-systems younger than about 55 Ma indicates that, typically, at least 50 m.y. are required before these mid-crustal ores are unroofed and exposed at the Earth's surface.

February 2000
SUPERIMPOSED LARAMIDE AND MIDDLE TERTIARY DEFORMATIONS IN THE ROSEMONT-HELVETIA DISTRICT, SOUTHEASTERN ARIZONA
James J. Hardy, Jr.
Applied Geologic Studies, Inc.
2875 W. Oxford, Ste. 3
Englewood, CO 80110

Copper deposits of the Rosemont-Helvetia mining district lie in the northern Santa Rita Mountains approximately 35-40 miles south-southeast of Tucson, Arizona. Within the district, four separate areas of copper mineralization occur - Rosemont, Peach Elgin, Broad Top Butte, and Copper World. However, only the Rosemont and Peach Elgin deposits have drill-defined reserves. These various mineral reserves are now the property of Grupo Mexico after their purchase of Asarco, Inc.

Most of the mineralization in the district occurs in skarned Paleozoic carbonate strata adjacent to Laramide age quartz latite porphyry stocks. The location and emplacement of these pyometasomatic deposits and associated intrusions appear to have been influenced by the location of Laramide folds and thrust (reverse) faults. Middle Tertiary extensional normal faults have subsequently cut, offset and disrupted the once contiguous mineral system.

The main purpose of this paper is to demonstrate the complex structural relationships that can occur when highly folded and faulted rocks are again deformed by younger events. The Rosemont-Helvetia area has special relevance to the exploration geologist because the large, once contiguous porphyry copper/skarn system was later rotated, truncated and fragmented by middle Tertiary low-angle normal faults that have structurally expanded the original system into several distinct and separate mineralized areas.

Structural relationships in the Rosemont-Helvetia district of the northern Santa Rita Mountains clearly indicate the occurrence of both Laramide compressional deformation and younger middle Tertiary extensional faults. Laramide structures were tilted to the east during middle Tertiary time as evidenced by moderate plunging Laramide folds and southeast-dipping Oligocene-Miocene strata. This west-directed extension was accommodated by a master detachment fault at depth and a structurally higher, west-dipping low-angle normal fault that underlies the Helvetia klippe. This configuration of normal faults has, in essence, segmented the northern Santa Rita Mountains into a lower, middle and upper plate. The lower plate is not exposed in the northern Santa Rita Mountains, but lies at depth below the master detachment surface and is composed of rocks of unknown character. Rocks above this master detachment fault and below the Helvetia klippe form the middle plate and are composed of Precambrian crystalline rocks through Tertiary Pantano Formation. Rocks of the middle plate are extensively exposed throughout the northern Santa Rita Mountains. Paleozoic, Mesozoic and Laramide age rocks of the Helvetia klippe form the upper plate in this scenario. Porphyry copper-related skarn mineralization is present in both the middle and upper plates, but separated across a fault with approximately 10,000 feet of displacement.

This structural interpretation of the Rosemont-Helvetia district shows the importance of faults in controlling both the original site of plutonism and mineralization, but also later dismemberment and truncation of the older mineral system. Some would argue that both the Rosemont and Peach-Elgin deposits were discovered without intimate knowledge of the structural complexities of the district; they would be correct. However, both deposits were exposed, or partly exposed, at the surface. The problem now becomes where to search for additional orebodies. This is especially true in areas of pediment or volcanic cover that is so common to the southwestern North America porphyry copper province. Here is where knowledge of the structural system and its implications can play an important role. Understanding that the orebodies at both Rosemont and Peach-Elgin are essentially lying on their sides has important implications about where to drill (or more importantly, where not to drill) for extensions of known mineralization. The structural model presented here for the Rosemont-Helvetia district can serve as an example to be used in other districts that may have superimposed Laramide and middle Tertiary deformations.

March 2000
Mineralization, Alteration, and Strain of the Gold-Bearing L1 and L2 Liese Quartz Zones, Pogo Deposit, East Central Alaska
Keri Moore (CSM), Moira Smith (Teck Exploration), and Murray Hitzman (CSM)

Gold at the Pogo deposit (estimated 10 million tons, average grade 0.52 opt) is hosted in the subhorizontal, subparallel L1 and L2 Liese quartz zones. These zones crosscut Late Proterozoic (?) to Middle Paleozoic amphibolite-grade paragneiss and orthogneiss of the Yukon-Tanana terrane. In the Pogo area, these gneisses have been intruded by a Cretaceous granitic unit (107 Ma); similar, though slightly younger (91-94.5 Ma), intrusions are considered to be related to mineralization along the "Tintina Gold Belt" in Alaska and the Yukon.

Three distinct, mappable "types" of quartz make up the Liese zones; these types are based on color, mineralogy, textures, and strain features (Figure 1). Type 1 is "granular" quartz, which is defined by a unique texture and mineralogy consisting of at least 90% sub- to anhedral quartz grains included within larger (>3 mm) microcline crystals. Alteration minerals (sericite and dolomite), sulfides, and gold occur interstitial to the quartz.

Types 2a and 2b are both coarse-grained (0.5-1.5 mm), massive quartz. Type 2a is white, with sulfides dominated by pyrrhotite and pyrite as blebs and along fractures. Discontinuous fine-grained reddish biotitic alteration occurs preferentially in host rocks in contact with this type. Type 2b is gray due to increased strain of the quartz and contains primarily fracture-controlled arsenopyrite and pyrite. Host rocks in contact with this type have commonly been affected by sericite/ferroan dolomite alteration.

Type 3 is a thin, relatively continuous zone of fine-grained (0.05-0.5 mm), highly strained to recrystallized quartz to a quartz- and/or arsenopyrite-healed breccia which follows either or both the hangingwall or footwall contacts of the Liese zones. Significant amounts of sericite and ferroan dolomite occur both with arsenopyrite and pyrite within this quartz type and also as pervasive alteration products within the adjoining wallrocks. Gold occurs as blebs with or within various bismuth minerals (e.g., native bismuth, bismuthinite, maldonite) and preferentially near sulfides. Although gold does occur in all of the quartz types, higher grades are found in the altered type 1 and in type 3.Gold at the Pogo deposit (estimated 10 million tons, average grade 0.52 opt) is hosted in the subhorizontal, subparallel L1 and L2 Liese quartz zones. These zones crosscut Late Proterozoic (?) to Middle Paleozoic amphibolite-grade paragneiss and orthogneiss of the Yukon-Tanana terrane. In the Pogo area, these gneisses have been intruded by a Cretaceous granitic unit (107 Ma); similar, though slightly younger (91-94.5 Ma), intrusions are considered to be related to mineralization along the "Tintina Gold Belt" in Alaska and the Yukon.

Three distinct, mappable "types" of quartz make up the Liese zones; these types are based on color, mineralogy, textures, and strain features (Figure 1). Type 1 is "granular" quartz, which is defined by a unique texture and mineralogy consisting of at least 90% sub- to anhedral quartz grains included within larger (>3 mm) microcline crystals. Alteration minerals (sericite and dolomite), sulfides, and gold occur interstitial to the quartz.

Types 2a and 2b are both coarse-grained (0.5-1.5 mm), massive quartz. Type 2a is white, with sulfides dominated by pyrrhotite and pyrite as blebs and along fractures. Discontinuous fine-grained reddish biotitic alteration occurs preferentially in host rocks in contact with this type. Type 2b is gray due to increased strain of the quartz and contains primarily fracture-controlled arsenopyrite and pyrite. Host rocks in contact with this type have commonly been affected by sericite/ferroan dolomite alteration.

Type 3 is a thin, relatively continuous zone of fine-grained (0.05-0.5 mm), highly strained to recrystallized quartz to a quartz- and/or arsenopyrite-healed breccia which follows either or both the hangingwall or footwall contacts of the Liese zones. Significant amounts of sericite and ferroan dolomite occur both with arsenopyrite and pyrite within this quartz type and also as pervasive alteration products within the adjoining wallrocks. Gold occurs as blebs with or within various bismuth minerals (e.g., native bismuth, bismuthinite, maldonite) and preferentially near sulfides. Although gold does occur in all of the quartz types, higher grades are found in the altered type 1 and in type 3.

April 2000
Regional Metallogenesis of Central Alaska
T.K. Bundtzen,
B.A. Bouley,
H. J. Noyes, North Star Exploration Inc.
and W.J. Nokleberg, USGS

Central Alaska is a 474,000 km2 region regarded as a geological frontier well endowed with a wide variety of natural resources. A recurrent theme in the mining history of the area dates back to the late 19th century, when a series of more than 20 rushes (stampedes) for mainly gold took place throughout the region over a 30 year period. From 1886 to 1999 an estimated 482 t (15 Moz) gold or 45 percent of the total mined in the 49th State has been recovered from placer and lode mines in interior Alaska. Significant byproducts of antimony, tin, mercury, lead, copper, and zinc have also been recovered from the region's historic mining districts.

The Central Alaska region contains a variety of pre-, syn-, and post-accretionary mineral deposits that are related to the growth and evolution of the northwest North American continental margin. Within pre-accretionary terranes, the Devonian-Carboniferous, carbonate-hosted MVT zinc-lead deposits of the Kandik and Reef Ridge districts in Central Alaska, and two Devonian-Mississippian volcanogenic massive sulfide (VMS) polymetallic districts in the north-central Alaska Range have received the most exploration interest. The Alaskan VMS districts are probably analogous to Devonian-Mississippian VMS deposits of the Finlayson district in SE Yukon Territory, Canada, where important resources of zinc, copper, and cobalt occur at the Kudz ze Kayah and Wolverine Lake deposits. More than 1,200 km separate the Alaskan examples from those in Yukon, which suggests that Devonian-Mississippian, rift related magmatism is widespread along the ancestral NW continental margin of North America.

Syn-accretionary rift-related, mafic and ultramafic sill-form intrusions of the 1200-km-long Kluane-Nikolai igneous belt represents a newly recognized PGE-nickel-copper province that is being explored by several companies in Central Alaska and South-central Yukon, Canada. In addition 100-115 Ma syn-accretionary granitic plutons in the Purcell Mountains, Illinois creek, and Sithylemenkat areas have been explored for uranium-REE-gold, gold-polymetallic, and tin-REE deposits respectively.

Post-accretionary granitoid intrusions and associated volcanic fields that range in age from 55-110 Ma contain significant gold-silver and polymetallic resources in the East-Central Alaska, Hogatza, and Kuskokwim Mineral Belts. Collectively the three mainly post-accretionary plutonic/volcanic belts are part of the Tintina Gold Province, where exploration of 10 selected deposits since 1990 has identified about 850 t (26.44 Moz) gold. Included in the plutonic-related series is the high grade Pogo and large, moderate grade, Donlin Creek deposits, which contain 161 t gold and 368 t gold respectively.

The search for high sulfidation, epithermal gold-silver deposits has just begun with the 1999 North Star Exploration, Inc., middle Tertiary Kaiyah discovery in the Lower Yukon River basin. Examination of Cretaceous-Tertiary calderas throughout the Central Alaska region should yield additional, epithermal, precious metal discoveries in the future

May 2000
Milestones in Exploration History
A look at the role of key past events in shaping tomorrow's exploration industry
Douglas B. Silver

If one reviews the history of exploration discoveries, it is immediately apparent that key events led to massive strings of mineral discoveries. These events, such as the gold price in 1975, led to twenty years of worldwide gold exploration at levels unprecedented in the history of Mankind. There are about ten of these events which have shaped the industry.

The question is whether these events are related and, more importantly, predictable into the future. If they are, then we can see where our business is headed and how we must position our careers and companies to take advantage of this foresight.>

This presentation will present the facts related to this debate and provide insights into the New Economy and the role of mineral exploration.

September 2000
Geology and Genesis of the Irish Zn-Pb-Ag Orefield
Murray W. Hitzman
Dept. of Geology and Geological Engineering,
Colorado School of Mines,
Golden, CO 80401-1887
mhitzman@mines.edu

Carbonate rocks in the Irish Midlands host one of the world's major zinc-lead orefields. This orefield includes the world-class Navan orebody (>70 million tonnes; 10% Zn, 2.6% Pb), four significant deposits (Lisheen, Silvermines, Galmoy, and Tynagh), and a number of smaller prospects that are currently either marginally economic or sub-economic. The mineralized area covers approximately 8,000 square kilometers. The deposits occur in a transgressive sequence of Lower Carboniferous marine carbonate rocks lying above a wedge of Upper Devonian continental red beds. The orefield is regionally zoned; deposits in the southern portion of the orefield have elevated copper and silver contents while those in the north are dominated by zinc.

The Irish Zn-Pb-Ag deposits share the following features:

Stratigraphy exerts a fundamental control on the location of Irish zinc deposits. Two stratigraphic intervals within the layer-cake Lower Carboniferous (Courceyan age) carbonate sequence host the vast majority of Irish zinc deposits and prospects: the Navan Group in the central and northern portion of the Irish Midlands and the Waulsortian Limestone in central and southern Ireland. These intervals are the stratigraphically lowest units of non-argillaceous carbonate rocks within the local Carboniferous sequence. The layer-cake Courceyan-age platform carbonate sequence gave way to a Chadian-age complex facies mosaic consisting of closely juxtaposed basinal and shallow marine sediments indicative of a strong structural control over facies development. Platformal sediments range from argillaceous ramp carbonates to weakly argillaceous, thick-bedded, shallow-water bioclastic, and locally oolitic, limestones. Basinal sediments consist of well-bedded, graded, moderately to highly argillaceous carbonate turbidites or distinctive bioturbated, unfossiliferous mudstones; the carbonate turbidites display well-developed flute molds, slumps, and ball and pillow structures as well as coarse grainstone beds containing reworked fossil and oolitic debris and local olistostromes which demonstrate significant local tectonic uplift.

The Irish zinc deposits formed along, or adjacent to, normal faults that were initiated during the Chadian-Arundian tectonic event. While mineralization at Lisheen, Silvermines, and several other deposits occurred along fault systems separating carbonate shelves from turbidite basins, other deposits, notably Navan, are located along fault systems controlling sub-basin edges within the major turbidite basins. Alteration and metal zoning within the deposits indicates that mineralizing fluids moved along narrow portions of the normal fault systems, commonly at points of maximum throw.

The Irish deposits commonly display well-developed alteration halos consisting primarily of pre- or syn-mineralization dolomitization. The hydrothermal dolomite is: 1) fine-grained and replacive; 2) generally not geochemically anomalous with respect to Zn, Pb, Cu; 3) mildly to significantly ferroan; and 4) generally slightly lighter in 18O relative to host carbonate rocks. Mineralized zones in limestone are commonly surrounded by massive hydrothermal dolomite which may display subsequent hydrothermal dolomite veining. Mineralized zones in dolostone are commonly surrounded by a distinctive stockwork of hydrothermal dolomite veins. Vein width ranges from microns to tens of centimeters. Where well developed, hydrothermal dolomite vein stockwork forms pseudo-breccia, informally termed black matrix breccia, produced by in-situ alteration of wall-rocks adjacent to the hydrothermal dolomite veins. Well-developed black matrix breccia consists of subrounded to angular remnants of wall-rock with delicately scalloped or deeply embayed edges in a dark, fine-grained dolomite matrix. Hydrothermal dolomitization in these systems extends laterally and vertically from centimeters to 500m beyond the edge of economically significant sulfide zones. Alteration zone geometry is controlled both by sedimentary and diagenetic porosity/permeability and structures. Because hydrothermal dolomite may be similar in color and texture to sedimentary or diagenetic host-rock dolomite, careful examination of rock, including staining with potassium ferricyanide for iron, may be necessary to discern hydrothermal alteration.

The age of mineralization within the Irish orefield is known with certainty only for the Navan deposit which formed during the Chadian-Arundian, several million years after deposition of its Courceyan-age host sediments (the Navan Group). Geologic relationships suggest that the other Irish deposits formed at approximately the same time as the Navan deposit. The structures which control mineralization and the Chadian-Arundian facies mosaic formed in an extensional tectonic environment. The apparent contemporaneity of mineralization and tectonism in Ireland, together with the regional zoning of metals and dolomitization, suggests that the Hercynian Orogeny was a fundamental driving force for mineralization in the Irish orefield. Topography-driven flow related to the uplift of Hercynian highlands to the south of Ireland produced a hydraulic head that drove formation waters northward through the confined Upper Devonian red bed aquifer which served as a source of metals. Fluids were focused into the area of present-day Ireland by a high-standing basement block to the east and by the northward thinning of the red bed aquifer. The Irish zinc deposits formed where normal faults tapped the confined red bed aquifer and focused flow of hydrothermal solutions upwards into the Lower Carboniferous carbonate sequence. This focusing allowed the development of discrete thermal anomalies capable of initiating thermal convection cells which mixed formation water from within the Carboniferous sequence with seawater from the overlying ocean.

September 2000
THE LISHEEN DEPOSIT, RATHDOWNEY TREND, IRELAND -
DISCOVERY AND DELINEATION OF A BLIND ZINC-LEAD-SILVER OREBODY
Murray W. Hitzman
Dept. of Geology and Geological Engineering,
Colorado School of Mines,
Golden, CO 80401-1887
mhitzman@mines.edu

The Lisheen zinc-lead-silver deposit in southcentral Ireland was discovered in early 1990 by the Chevron Mineral Corporation of Ireland/Ivernia West plc Joint Venture. Chevron entered Ireland in 1984 due to the perceived high potential for discovery of significant carbonate-hosted zinc-lead deposits, the favorable fiscal climate and the availability of attractive joint ventures. Chevron's entry coincided with the release of significant new data on the Irish Carboniferous and its zinc-lead deposits. In addition to exploration, early work in the country focused on the development of an new exploration model emphasizing the epigenetic nature of the mineralization and the significance of alteration dolomite halos. Application of this model to southcentral Ireland in 1986-1987 lead to the recognition of the Rathdowney Trend and definition of the Lisheen prospect. Budgetary cuts forced Chevron to reduce exploration in 1988-1989 and to seek a joint venture partner in 1990. A joint venture agreement signed in early 1990 with Ivernia West plc allowed exploration to resume. The Lisheen prospect was the initial drill target. Discovery of the Main zone took place within a month of the first drill hole being collared. Stepout drilling at 30 and 60m centers confirmed the significance of the discovery. Transient electromagnetic surveys conducted in early 1991 delineated a significant anomaly 1 km east of the Main zone, leading to the discovery of the Derryville zone. Geologically based exploration, especially elucidation of the area's structural architecture, resulted in the discovery of the North zone in 1991 and the Barnalisheen zone in 1993. The Lisheen Mine was officially opened in June, 2000.

October 2000
New Developments in Geological Modeling of the Cripple Creek Mining District, Colorado, USA<
David M.Vardiman
Exploration Manager
Anglo Gold (Colorado) Corporation
Cripple Creek & Victor Gold Mining Company

The Cripple Creek Mining District has produced +650 tonnes, (+21 MM troy ounces) of gold since its discovery in 1890, with the majority of this historical production having been obtained from underground mines ranging from near surface to 917 meters, (3,000 feet) deep. This historic production ranks the Cripple Creek Mining District as the United States of America's third largest lode gold producer behind the Carlin Trend, Nevada and the Homestake Mine in South Dakota. With the advent of modern heap leach processing technologies in the late 1970's, opportunities to extract large volume, low grade, production from the district have been successfully initiated. Recent evaluation of a portion of the district's ore controls through a compilation of historic mining data and accompanying geologic data indicate additional opportunities remain for discovery of near surface and underground minable resources.

Aggressive exploration programs initiated in 1998 targeting these opportunities have increased near surface ore reserves and resources by 114 MM ore tons containing 3.265 MM gold ounces, (1.997 MM recoverable gold ounces), point forward to January 1998. The implementation and development of computer hardware/software applications, newly applied drill techniques, and computer-modeling procedures have been significant contributors to this success.

In particular the application of a 3 dimensional district geological, structural, and geochemical model using Vulcan (Maptek, KRJA Inc., Denver, Colorado, USA) software has significantly improved specific exploration program drill targeting. Development of the Voice Logger (3DI Inc., Fort Collins, Colorado, USA) drill data capture software has led to reduced transcription errors, expanded data capture capabilities and resulted in faster data collection and interpretation in conjunction with the Vulcan software. The implementation of an Access database management system in conjunction with Microsoft SQL Server 7 as the Relational Data Base Management System (RDBMS) engine has allowed for quicker data compilation, evaluation, and result driven program adjustments and implementation while allowing utilization of data across the entire operation. All aspects of the exploration program from design, QA/QC, management, interpretation and modeling of results have been significantly improved allowing significant gains in efficiencies and better use of exploration funding.

While exploration programs continue to maximize district surface production opportunities, compilation and interpretation of historical underground infrastructure, lithological, geophysical, structural and geochemical data have prompted the initiation of an underground-mine-target exploration program. This district-wide compilation and interpretation is an unprecedented opportunity to view the entire Cripple Creek Mining District's geological ore controls for the successful targeting of deeper sited exploration drill targets.

November 2000
THE ELKO OROGENY, NEVADA-UTAH A MAJOR TECTONIC EVENT WITH MINOR MINERALIZATION
PLUS THE SIGNIFICANCE OF THE SEVIER AND CENOZOIC OROGENIES TO MINERALIZATION
Chuck Thorman
CTGS International
Lakewood, Colorado 80225

The Elko orogeny, a widespread Middle Jurassic event, extended from central Nevada to central Utah. Its age and nature are based on direct and indirect evidence. The youngest units involved are early Middle Jurassic sandstone. Late- to post-tectonic late Middle Jurassic plutons cut structural and regional metamorphic features throughout the region. Central Nevada to central Utah has undergone a continuum of deformational pulses from the middle Paleozoic to early Tertiary. Each successive orogenic event resulted in the superposition of structures. This has made it difficult to determine the age of many structures because of their similarity in style where crosscutting relationships do not exist. These events commenced with the Antler orogeny (late Devonian-middle Mississippian) and continued with the Humboldt (late Pennsylvanian), Sonoma (late Permian-early Triassic), Elko, and the Sevier (middle Early Cretaceous-early Tertiary) orogenies.

The Elko orogeny includes extensional structures, making it somewhat different from the Cretaceous Sevier orogeny in the Paleozoic miogeocline of eastern Nevada-western Utah. Sevier structures typically are contractional with eastward vergence and included large-scale crustal shortening. In contrast, comparable large-scale crustal shortening of Middle Jurassic age has yet to be demonstrated, though many Elko structures indicate crustal shortening occurred. Low-angle and high-angle extensional faults have been documented in numerous ranges as being Middle Jurassic in age. This style of deformation is atypical of the Sevier orogeny. Both orogenies included younger-over-older low-angle and bedding-parallel faulting, and older-over-younger thrusting.

Late Middle Jurassic plutons (155 to 165 Ma) of the Nevada Jurassic Magmatic Province are distributed across the region and range from granite to granodioritic in composition. They intrude deformed eugeoclinal and miogeoclinal units ranging in age from Cambrian to Permian, including regionally metamorphosed miogeoclinal strata in northeastern Nevada.

A Middle Jurassic foredeep in western Utah along the leading eastern edge of the Elko orogenic belt is inferred from Middle Jurassic strata, which thicken westward towards the edge of the Colorado Plateau. Equivalent strata are not preserved in the Basin and Range of western Utah. Therefore, the depocenter of the westward expanding sedimentary sequence was in western Utah, and possibly easternmost Nevada, but was uplifted and eroded during the Sevier orogeny. The existence of a Middle Jurassic foredeep subsequently uplifted and eroded during the Sevier orogeny is compatible with an orogenic belt to the west and provides a plausible explanation for the missing westward thickening wedge.

Major mineral deposits do not appear to be of Elko age in eastern Nevada-western Utah. Rather, mineralization related to the plutons formed predominantly small to moderate polymetallic base-metal deposits with subordinate gold. A possible exception is the Bald Mountain gold deposit. Many of the plutons are elongate parallel to easterly-trending high-angle faults, interpreted as tear faults related to east-west contraction. Mineralization was strongly controlled by the tear faults and related structures.

The Cretaceous-early Cenozoic Sevier orogeny primarily formed polymetallic base-metal deposits. Eocene-Oligocene appears to have been the time for the formation of the majority of the gold-only deposits of the Carlin, Battle Mountain-Eureka, and Getchel Trends as well as those at Jerritt Canyon. Eocene-Oligocene volcanism and plutonism correlates with the formation of the gold-only systems and marks the transition from contractional deformation of the Mesozoic and the pronounced extensional tectonics of the Miocene. The easterly-trending high-angle faults formed during the contractional Mesozoic (and older) events were reactivated in the Cenozoic and were major controlling factors for both igneous events and the formation of mineral deposits.