Dr. Holly Stein, AIRIE Program, Colorado State University
Re-Os Dating: Ore Geology and Beyond
In the last ten years, the Re-Os method has permitted major advances in our understanding of ore genesis. The Re-Os method is the only means to directly date mineralization, as these two elements have strong chalcophile-siderophile properties, and take up residency directly in the sulfide structure.
In particular, the mineral molybdenite (MoS2), an accessory mineral in many magmatic-hydrothermal systems, offers single mineral dating using Re-Os. In addition to its presence in mineralizing systems, molybdenite is also an accessory mineral in some granites, may be found in skarn environments, and is known to form in shear zones, thereby permitting the direct dating of magmatic, metamorphic, and deformation events as well.
Re-Os ages are demonstrably robust, even enduring granulite facies metamorphism. Re-Os ages for molybdenite compare favorably with U-Pb ages on zircon. In the magmatic-hydrothermal ore-forming environment, with its complex temporal history of overprinting thermal events, argon-based geochronology generally does not produce geologically accurate ages for discrete mineralizing events, although argon-based ages are quoted at high precision.
In contrast, Re-Os dating provides geologically accurate results and at high precision. This introductory level talk will comparatively explore examples from around the world, in a demonstration of the success of this new-wave chronometer in ore geology.
Selective Extractions and their application to Mineral Exploration
Mary E. Doherty
International Geochemical Consultants, L.L.C.
Selective extraction geochemistry has received increasing application and success in mineral exploration surveys in the last 30 years. Initial development focused on acid digestions which could dissolve only specific components of a sample from which a signature would otherwise be overwhelmed by chemistry of the other sample components. Many of the early partial extractions were applied directly in the field. In the past 10 years, introduction of sensitive and multi-element determinations from ICP-MS into commercial labs servicing the mining industry has renewed application of selective extraction geochemistry. Surveys have proven effective using rock, lag, soil and stream sediment samples, in settings from which only weakly bound components are of interest. Surveys have been effective in mineral and petroleum discoveries, and in environmental applications.
In the case of buried mineral targets, soil composition may be related to volcanic or pediment gravel cover, onto which a relatively weak signature related to mineralization has been superimposed. If only this weaker, later material is dissolved, the geochemical signature has higher contrast and is more readily evident.
As with the introduction of all new technologies, a period of learning and misapplication occurs from which the technology implementation is improved. Application of selective extraction geochemistry with ICP-MS instrumentation necessarily requires more quality control measures and evaluation of soil chemistry. Without these, it is easy to generate anomalies related to changes in the original sample composition (geomorphology or climate changes) or lab procedures rather than to potential mineralization. Unfortunately, these potential pitfalls often lead explorationists to reject the tool and "throw the baby out with the bathwater".
Selective extraction geochemistry has regenerated interest in soil science, discussion of the processes by which soil composition is developed and how anomalies related to mineralization are developed. As these research programs develop, we will likely better understand soil processes starting with liberation of the metals, mobilization of these through unrelated materials to the surface, and element capture at the surface, from which we routinely sample. Dominant transport processes vary by environment (arid, tropical), age or type of cover, and structural/tectonic setting. In some areas water and electrochemical mechanisms may dominate metal transport. In others, movement of elements along faults by water, evapo-transpiration and/or gaseous diffusion dominate, such that the strongest signatures are evident around faults. In yet other areas hyper-saline groundwater contributes significantly to element mobility through formation of highly mobile metal anion complexes. Variable roles in metal transportation and fixation may be played by micro-organisms, cycling of metals by vegetation, seismic or barometric pumping of water and/or gases.
Surficial environments vary immensely on "Mother" earth, and as such the type of mechanism for trapping mobile elements at the surface also varies. In arid environments with abundant carbonate and prevailing soil pH of 7-9, most metals of interest to exploration do not travel far and are easily captured on the charged Mn-Al-Fe oxide sites in soils. In hyperarid climates, metal partitioning may be more related to loss of humidity at a given horizon rather than actual trapping. Heavily vegetated environments are controlled by metal cycling and organic acids in the surface soils. In response to these varying conditions, a selection of extractions have been developed, some of which are more effective in a given surficial environment than they would be in other environments. Again, application of the incorrect extraction can lead to inconclusive or random anomalies which has also led geologists to reject selective extraction application.
Effective integration of selective extraction geochemistry with other geologic information has led to a number of effective exploration programs and offers potential to provide geoscientists with an effective tool in detecting the subtle geochemical responses that occur with buried mineralization.
DONLIN CREEK, Southwest Alaska
Phil St. George, VP - Exploration for NovaGold Resources, Inc.
In July 2001, NovaGold completed an agreement with Placer Dome and Calista Corporation to acquire a 70% interest in the 13 million ounce Donlin Creek Gold Deposit. The deposit is located in southwestern Alaska on part of Calista Native Corporation's 6.5 million acres of private patented lands. NovaGold will earn a 70% interest in the project by spending $US10 million in exploration and developement within a ten year period. The current first phase drill program is anticipated to be $3 million in 2001.
Donlin Creek is one of the largest undeveloped gold resources in the world with a Measured and Indicated Resource that is estimated to be 6.9 million ounces of gold grading 3.06 grams per tonne with an additional Inferred Resource of 6.0 million ounces of gold grading 2.83 grams per tonne using a 1.5 gram per tonne gold cut-off grade (see Table 1). This resource remains open both at both ends and at depth, with potential to define a resource of over 13 million ounces through higher density drilling which would elevate the current Inferred Resources to the higher Measured and Indicated Categories. The current total potential higher-grade resource is estimated at 5.5 million ounces of gold with an average gold grade of 5.1 grams per tonne (0.149 ounces per ton) using a 3.5 gram per tonne cut-off. This total potential resource is comprised of a Measured and Indicated Resource of 3.1 million ounces of gold grading 5.20 grams per tonne with an additional Inferred Resource of 2.4 million ounces grading 4.96 grams per tonne.(see footnote 1,2). The above resources were estimated by Placer Dome in year 2000 and do not include results from the current 2001 drilling program. Following the completion of this first phase drill program, a new higher-grade resource will be estimated in early 2002 using a qualified, independent third party.
|Total M & I||
||6,895,000||Total M & I||
|Total MI & I||
||12,904,000||Total MI & I||
The Donlin Creek project is located in southwest Alaska approximately 450 kilometers (280 miles) west of Anchorage and 19 kilometers (12 miles) from barge access on the Kuskokwim River. The project is directly serviced by commercial air services out of both Anchorage and Aniak 80 km (50 miles) to the west. A state designated winter road provides access from the barge site. The 109 square kilometer property (42 sq. miles) is owned by Calista Corporation and Kuskokwim Corporation, the regional and village Native Corporations of the lower Kuskokwim region. The project has a 75 person all-season camp and an adjacent high quality 1,500 meter (5000 ft) long airstrip that is capable of handling large commercial aircraft (up to C-130 Hercules) for efficient shipment of personnel, large equipment and supplies.
NovaGold's exploration program at Donlin commenced in early June, when NovaGold personnel began on-site work including geologic mapping, sampling and trenching. This work was focused on defining and expanding the higher-grade gold mineralization within the deposit. In August, drilling began on one of several higher- grade targets within the Donlin resource area. Using two diamond drills, NovaGold intends to complete in-fill and step-out drilling to better define the higher-grade resource in preparation for detailed engineering and pre-feasibility studies to begin in early 2002.
Gold mineralization at Donlin is associated with gold-bearing fine-grained sulfides. A majority of the gold occurs within intrusive dikes and sills, but also as high-grade stockwork-vein zones in the surrounding sedimentary rocks. The Donlin Creek property has had a total of US$37-million dollars in exploration expenditures, including over 110,000 meters (360,000 feet)of drilling, and 25,800 meters (84,000 feet) of trenching, as well as comprehensive surface and airborne geophysics. The exploration work completed by NovaGold since June has been integrated with the extensive exploration database collected on the property by previous workers to further refine the new 3D geologic model used in the current program.
In 2001, NovaGold has received outstanding results from the drill holes in the first phase drill program. These include some of the best results seen on the property to date. The majority of the drill holes in this first phase program are offset and step-out holes that will expand the existing high-grade Acma resource area to the west, south and east. The in-fill drill holes will upgrade Inferred category resources to the higher Measured and Indicated category resources. There is excellent potential tocontinue to expand the extent of the higher-grade Acma Zone, as well as to discover new higher-grade zones.
For more information see www.novagold.net
These 2 talks are jointly sponsored by DREGS and the Colorado School Of Mines student chapter of SEG. The lst presentation was on March 4 and the second was given on March 5, at the Colorado school of Mines
by Dave Leach
Mississippi Valley-Type Lead-Zinc Deposits through Geological Time: Implications from Recent Age-Dating Research
Remarkable advances in dating Mississippi Valley-type (MVT) lead-zinc deposits provide a newopportunity to understand how and why these deposits form in the Earth's crust. These dates are summarized and examined in a framework of global tectonics, paleogeography, fluid migration, and paleoclimate. Nineteen districts have been dated by paleomagnetic and/or radiometric methods. Ofthe districts which have both paleomagnetic and radiometric dates, only the Pine Point and East Tennessee districts have significant disagreements. This broad agreement between paleomagnetic and radiometric dates provides added confidence in the dating techniques used.
The new age dates confirm the direct connection between the genesis of MVT lead-zinc ores with global-scale tectonic events. The age dates show that MVT deposits formed mainly during large contractional tectonic events at restricted times in the history of the Earth. Only the deposits in the Lennard Shelf of Australia and Nanisivik in Canada have dates that correspond toextensional tectonic events. The most important period for MVT genesis was the Devonian to Permian time that corresponds to a series of intense tectonic events during the assimilation of Pangea. The second most important period for MVT genesis was Cretaceous to Tertiary timewhen microplate assimilation affected the western margin of North America and Africa-Eurasia. There is a notable paucity of MVT lead-zinc ore formation following the breakup of Rodinia and Pangea. Of the five MVT deposits hosted in Proterozoic rocks, only the Nanisivik deposit has been dated as Proterozoic. The contrast in abundance between SEDEX and MVT lead-zinc deposits in the Proterozoic questions the frequently suggested notion that the two types of oresshare similar genetic paths.
The ages of MVT deposits, when viewed with respect to the orogenic cycle in theadjacent orogen suggest that no single hydrologic model can be universally applied to the migration of the ore fluids. However, topographically-driven models best explain most MVT districts. The migration of MVT ore fluids is not a natural consequence of basin evolution; rather, MVT districts formed mainly where platform carbonates had some hydrological connection to orogenic belts.
There may be a connection between paleoclimate and the formation of some MVT deposits. This possible relationship is suggested by the dominance of evaporated seawater in fluid inclusions in MVT ores, by hydrological considerations that include the need for multiple-basin volumes of ore fluid to form most MVT districts, and the need for adequate precipitation to provide sufficient topographic head for topographically-driven fluid migration. Paleoclimatic conditions that lead to formation of evaporite conditions but yet have adequate precipitation to form large hydrological systems are most commonly present in low latitudes.
For the MVT deposits and districts that have been dated, more than 75 % of the combined metal produced are from deposits that have dates that correspond to assembly of Pangea in Devonian through Permian time. The exceptional endowment of Pangea and especially, North America with MVT lead-zinc deposits may be explained by the following: 1) Laurentia, that formed the core of North America, stayed in low latitudes during the Paleozoic that allowed the development of vast carbonate platforms. 2) Intense orogenic activity during the assembly of Pangea created ground preparation for many MVT districts through far-field deformation of the craton. 3) Uplifted orogenic belts along Pangean suture zones established large-scale migration of basin fluids. 4) The location of Pangea in low latitudes with paleoclimates with high evaporation rates led to the formation of brines by the evaporation of seawater and infiltration of these brines into deep basin aquifers during Pangean orogenic events.
The Giant Red Dog Shale-Hosted Zinc-Lead Deposit, Western Brooks
Discovery, Geology, and Genesis.
The Red Dog shale-hosted zinc-lead-silver deposit is located
in the DeLong Mountains of the western Brooks Range in northwestern
Alaska. Mississippian to Pennsylvanian black shales of the Kuna
Formation are hosts to the ore. In addition to
sphalerite and galena, the ores contain enormous amounts of barite and silica. Excluding the recently discovered deposits located several kilometers from the Red Dog deposit, current resources are in excess of 150 million tons of ore that grade about 17% Zn, 4% Pb, and 83 g/t Ag. Red Dog is the largest Zn mine in the world, and with the recent discoveries of ore near the deposit, Red Dog and the surrounding area will continue to be one of the most significant resources of Pb and Zn known in the Earth's crust.
An overview of the discovery, the geological setting, and discussion of the types of ore and alteration at the Red Dog deposit will be presented. Examples of the typical ore at Red Dog and from the new discoveries will be presented. Insights into the processes that formed the ore and alteration at Red Dog will be summarized from the results of ongoing studies by geologists from the USGS and TeckCominco.
The Red Dog ores have traditionally been described as SEDEX or synsedimentary replacement ores. New data obtained for the Red Dog ores show that the extensive silicification that overprints the deposit was produced at depths of between 2 and 6 km of burial during Mesozoic thrusting, and unrelated to sulfide deposition. Furthermore, observations from the new discoveries clearly show that these ores selectively replaced carbonate turbidites. However, it is uncertain how far these observations can be extrapolated to Red Dog. Nevertheless, the absence of unequivocal evidence for seafloor exhalites or syngenetic deposition of the sulfides on the sea floor, either at Red Dog or the new discoveries questions the importance of strictly SEDEX or synsedimentary seafloor mineralization in the district. Rather, most evidence points to ore formation in a syndiagentic environment.
Gary A. Parkison, Project Manager, and Greg A. Hahn, President and CEO, Summo Minerals Corporation
The Terrazas Copper-Zinc Oxide Deposit, Chihuahua, Mexico:
Geology, Mineralization and
The Terrazas deposit is located about 40 km. northwest of Chihuahua City in north-central Mexico in an area with atypically well developed infrastructure. The deposit lies along a northwest trending zone that separates the Laramide-aged Mexican Thrust Belt to the east from the Tertiary volcanic plateau of the Sierra Madre Occidental to the west. The Terrazas deposit has some similarities to larger deposits in the vicinity such as Naica and Santa Eulalia. A sequence of Cretaceous limestones in the deposit area was subjected to low angle faulting, erosion and development of karst features. The erosion led to the local deposition of basin-filling, Tertiary-aged conglomerate. Contemporaneous with the deposition of the conglomerate was the onset of predominantly rhyolitic volcanic activity in the area manifested by flows, dikes, sills other subvolcanic intrusions.
Significant alteration in the area affected both the limestone and the overlying conglomerate and resulted in the development of several areas of extensive skarn, two of which host copper and zinc mineralization. Skarn development is most closely related in time and space to the area of the rhyolitic volcanics, but a non-outcropping intrusive is inferred. The mineralized skarn forms two half-mooned shaped areas separated by the north-trending Verde Fault and with both areas of skarn truncated at depth by a low angle fault from an underlying intrusive. Mineralogy of the skarn is mostly green, andradite garnet with lesser quartz, and with some retrograde(?) calcite.
Copper and zinc mineralization is generally confined to the skarn, with higher grades, and most past mining, often in close association with the marble front. The skarn is generally completely oxidized, with only minor remnants of the sulfides pyrite, chalcopyrite and sphalerite. Oxidation has led to only minor remobilization of copper and zinc and the generation of malachite, chrysocolla, hemimorphite, willemite and Cu- and Zn-bearing iron oxides. Areas of higher copper and zinc grades are not generally coincident. The deposit has been subject to several periods of drilling, with the latest conducted by Summo in 2000. Based on the drilling results, a geologic resource exceeding 110 million tonnes has been estimated, grading 0.26% Cu and 0.36% Zn, and which is open to expansion.
Economic related criteria, including the extraction of copper and zinc from numerous column leach tests, were developed and used to generate a potentially mineable resource estimated at 58.3 million tonnes grading 0.35% Cu and 0.57% Zn with a strip ratio of 0.62/1.0. A hydrometallurgical flowsheet was also developed utilizing heap leach extraction with recovery of copper via conventional SX-EW and zinc via solution reduction and direct EW. A recently completed pre-feasibility study proposes an operation with an 11 year life with average co-product basis cash operating costs of $0.35 per pound copper and $0.21 per pound zinc.
Richard L. Nielsen
COPPER DEPOSITS OF THE KUPERSCHIEFER IN SOUTHWEST POLAND
This talk is a report of a field trip in Poland, August 2001, sponsored by the Society for Geology Applied to Mineral Deposits.
Exploration and development of the ores commenced in 1956 and initial production in 1959 was based on a reserve estimate of 1.37 billion tonnes containing 19 million tonnes of copper metal. Present reserves are estimated at 4 billion tonnes at 2% copper, containing 70 to 80 million tonnes of copper metal. Cutoff grade is 0.7% copper.
Three separate mining complexes exploit the mineral field, each with underground workings 400 to 1,300 meters deep, and each has a surface mill and concentrator. Total capacity of all three is about 100,000 tonnes per day. Production in 1999 was 470,000 tonnes of copper and 1000 tonnes of silver with minor gold, platinum, palladium, lead and cobalt.
The deposits are sediment-hosted stratabound sulfide deposits hosted in gently dipping Permian sandstone, shale, dolomite and anhydrite. Chalcocite is the main ore mineral with lesser amounts of chalcopyrite, digenite, covellite, sphalerite, pyrite, tennorite and tetrahedrite. Copper mineralization is concentrated along a bed of organic-rich shale (Kuperschiefer) but also cuts across beds and is present in underlying sandstone or in overlying dolomite. Mineralization is crudely zoned: gold is enriched in oxidized sandstone below the copper zone; lead and zinc values are relatively high in dolomite above the copper zone.
Copper mineralization is located along an oxidation-reduction boundary located at or near the organic rich shale. Isotopic measurements indicate sulfide sulfur is of biologic origin. Copper moved upwards and laterally through oxidized sandstone and was fixed as sulfides near the overlying shale by life process of sulfate-reducing bacteria feeding off organic material in shale and dolomite.
Some features of the ore zone suggest roll-fronts and other characteristics familiar to workers in the uranium deposits of the Rocky Mountain region.
Department of Geological Sciences,
University of Colorado, Boulder
El Teniente breccia deposit: new perspective on hypogene copper distribution and emplacement
El Teniente, located in the Andes of central Chile, is the world's largest known copper deposit. In the past El Teniente has been considered a porphyry copper, centered on a felsic intrusive, but recent geological work indicates that, like other giant Miocene and Pliocene copper deposits in central Chile, it is actually best classified as a megabreccia deposit.
Most of the high- grade hypogene copper at El Teniente occurs in and surrounding multiple magmatic-hydothermal breccia pipes that developed over a time span greater than 2 million years as the result of changing geometry of plate convergence due to ridge subduction. The new information suggests that exploration strategies for copper deposits in the Andes, where some of the world's largest deposits occur, should be re-evaluated.
CRIS CARMAN1, IAIN GROVES2, MURRAY HITZMAN1
1Colorado School of Mines, Golden, Colorado 80401-1887,
2Perilya Limited, Australia
THE HYPOGENE WILLEMITE DEPOSITS OF THE AROONA TREND, SOUTH AUSTRALIA
The Aroona trend, South Australia is a six kilometer long narrow zone containing zinc deposits. Zinc occurs predominantly as willemite (Zn2SiO4) and is associated with a distinctive hydrothermal zincian and hematitic dolomitization. The host lithologies along the trend are Lower Cambrian carbonate rocks, caught in a sliver bounded by the Aroona and Norwest faults. Mineralization occurred dominantly along the Aroona fault, which acted as the major fluid pathway, but is also localized within particular lithologic units. Age of mineralization is not known, but has been attributed to the Cambro-Ordovician Delamarian orogeny. The Aroona willemite ore bodies formed by precipitation from a low temperature, oxidizing hydrothermal fluid rich in metals. Optical, cathodoluminescent, and scanning electron microscopy have detailed the paragenesis as commencing with alteration dolomitization, then willemite and hematite precipitation. Willemite and hematite are brecciated and replaced by a lead and manganese rich assemblage, dominated by coronadite (MnPbMn5O14), hedyphane ((Zn,Ca,Pb)5(AsO4)3Cl), and a second generation of willemite. Calcite and dolomite then filled remaining open space and locally replaced willemite. Microscopy and previous stable isotope (carbon and oxygen) studies indicate these late carbonates are not in equilibrium with either willemite bearing assemblage as previously assumed.
There are only four known major deposits of willemite ore in the world. Study of these deposits has led to contradicting genetic theories. The Aroona trend is located in the North Flinders Ranges, South Australia. The mineralized rocks are within the Lower Cambrian section of the Adelaide basin. This basin is interpreted to have accumulated more than 15 kilometers of sediments, dominated by siliciclastic rocks in a passive margin setting. Sedimentation began in the Willouran, (roughly 900 Ma) and ceased in the Middle Cambrian.
The oldest rocks exposed are the Callana Beds which consist of interlayered siltstone, shale and dolomiteand are interpreted as an early rift sequence. The Rawnsley Quartzite, a mature white and red quartz sandstone of latest Neoproterozoic age, unconformably overlies the Callana Beds. The Cambrian section begins over a basin-wide unconformity with the onlapping Parachilina Formation. These bioturbated sandstones and glauconitic shales grade upward into the Woodendinna Dolomite which contains numerous parasequences of microbial laminate, stromatolitic, oolitic units, and mudcracked siltstones (Drexel and Preiss 1995). Dolomitization of the Woodendinna is interpreted to be penecontemporaneous with deposition (James and Gravestock 1990). Above this is the deeper water Wilkawillina Limestone, a thickly bedded fossiliferous and reef limestone. The Wilkawillina is the highest stratigraphic layer preserved along the Aroona Trend. A prominent ridge of Copley Quartzite, a cross bedded mature white quartz, is exposed north of the Norwest fault. It was deposited above the Callana Beds and well below the Rawnsley Quartzite.
The mineralized section is caught in a narrow (1 to 300 m wide) fault wedge between two major faults with reverse offset, the Aroona, and Norwest faults (Figure 1). From drilling results, the Norwest fault truncates the Aroona fault at a shallow depth. Oblique movement of the Norwest fault is interpreted by clockwise rotation of blocks between the faults. The only age constraint on mineralization is its occurrence along faults of Delamarian age, cut by later, possibly Tertiary faults with little throw. There are no minerals associated with mineralization or alteration that could provide a radiogenic age.
Two minerals characterize alteration: dolomite and hematite. The hematite is most often found as inclusions within dolomite growth planes, but also between host rock grains and alteration dolomite crystals. The hematite is typically the earthy form, however specular hematite also formed, primarily (but not exclusively) along veins and within the ore. Zinc and manganese are found in the dolomite crystal lattice. They covary with iron enrichment in the alteration assemblage indicating coprecipitation from onefluid. Cathodoluminescent examination of the alteration dolomite reveals strong luminescence throughout, as well as a direct positive relationship between hematite and intensity of luminescence (indicating iron is found as Fe3+).
Alteration style depends on host lithology. Alteration in the
host carbonate rocks primarily involved dolomitization, while
in carbonate deficient rocks, the alteration was less pervasive.
Alteration of the Woodendinna dolomite can be pervasive, but was
clearly controlled by veins. The alteration progressed outward
from veins, imparting a brecciated appearance to the host dolomite,
until recrystallization was complete. Alteration intensity grades
lower away from the Aroona fault. In adjacent sandstones, alteration
is primarily confined to veins of hematite-zoned dolomite. Where
porosity permitted, intergranular space was filled with hematite
zoned dolomite crystals. Some fine-grained feldspathic siltstones
of the Callana Beds evidence dolomite replacement of fine-grained
The mined resource at the main Aroona ore body was 0.15 Mt at 34.4% Zn, and 1.6% Pb. This high grade mineralization was dominated by willemite, with minor hedyphane, coronadite, and hetaerolite. Gangue minerals are hematite, calcite, dolomite, and minor quartz. Most willemite occurs as fine grained radial clusters (<1 mm diameter) colored red by earthy hematite. The remaining willemite forms white crustiform bands of prismatic pseudoradial willemite. Minor white botryoidal willemite postdates the hematite bearing assemblages ore. Lead is found primarily in the ore footwall (adjacent to the Aroona fault) in coronadite and hedyphane which occur as black amorphous veins and cavity fill.
The paragenesis, is summarized in Figure 2. The earliest ore stage event was the alteration of the host rocks by hematitic zincian dolomite. This was followed by precipitation of willemite with hematite inclusions, first as fine grained, then as coarse grained prismatic crustiform bands with a radial habit. Willemite is commonly brecciated by lead and manganese minerals (hedyphane, coronadite, hetaerolite). The lead-rich stage is predominantly found in the footwall of the ore bodies. The final event was cavity fill, brecciation, and replacement of willemite by manganoan calcite and dolomite, with later crosscutting veins of ferroan calcite. This calcite stage postdated the zinc minerals; carbon isotope data (C. Johnson pers. comm.) could indicate derivation from wall rock. Additionally, minor quartz is also found in this late stage cavity fill. No sulphide or sulfate minerals have been identified from the Aroona deposit.
| Figure 1: Schematic cross section for
mineralization along the Aroona trend.
|Figure 2: Paragenesis of the Aroona willemite deposit depicting relative timing of mineralizing events.|
Little work has been done with stable isotopes on willemite systems. A previous study (Burdett, 2000 concentrated on post-ore dolomite and calcite as well as the alteration dolomite. Oxygen isotope fractionation was used to determine a temperature of alteration of roughly 200°C. Unpublished data on willemite oxygen isotope composition from the associated nearby Beltana deposit corroborates petrography by demonstrating disequilibria between the late carbonate species (Johnson pers comm) and the willemite. In the paragenetic sequence two separate ore stage mineral pairs have been identified as likely being in equilibrium: zincian dolomite and hematite, willemite and hematite. Work is in progress to determine oxygen isotope values on the ore minerals.
MPERATURE OF FORMATION
The formation temperature of willemite from the Aroona Trend has not been conclusively determined. Though there are abundant fluid inclusions in the willemite, no previous fluid inclusion study has identified two-phase inclusions. Thus previous studies (Hallam, 2000; Waterhouse, 1975) concentrated on the late stage dolomite and calcite fill, and determined a temperature of formation between 70 and 150°C. Though this was interpreted as a minimum temperature for the ore forming fluids, petrographic evidence demonstrates the calcite is a later event that replaced willemite. Additionally, the willemite from the area was described as only having single phase gas filled inclusions, supporting formation at low temperature via supergene enrichment. Through comparison to other localities worldwide, radial growth willemite was found to not preserve two-phase inclusions; it can be found coprecipitated with quartz with numerous twophase fluid inclusions. However, rare related blocky willemite grains do preserve inclusions and these have been located in the Aroona deposit. Work is in progress on these inclusions.
The Aroona willemite deposit is a structurally controlled hypogene deposit formed by low temperature oxidizing hydrothermal fluids rich in zinc, iron, manganese, and lead, but deficient in sulfur. Though temperature of formation for the willemite has not been determined, the presence of two-phase inclusions, the elevated temperature derived from post ore minerals, the strong structural control of the ore body, and widespread zincian dolomitization indicate this deposit is hypogene and not supergene, as previously reported.
Burdett, M., 2000; Origin of the Beltana and Aroona Willemite Deposits, Flinders Ranges, South Australia. unpublished B.Sc. thesis, University of Melbourne.
Drexel, J.F., and Preiss, W.V. ed, 1995, The geology of South Australia, Volume 2: the Phanerozoic. Bulletin No.54, Geol. Survey of South Australia.
James, N.P. and Gravestock, D.I. 1990; Lower Cambrian shelf and shelf margin buildups, Flinders Ranges, South Australia. Sedimentology 37, 455-480.
Preiss, W.V. compiler, 1987 The Adelaide Geosyncline; late Proterozoic stratigraphy, sedimentation, paleontology and tectonics. Bulletin No.53, Geol. Survey of South Australia.
Waterhouse, 1975; A Fluid Inclusion Study of the Beltana Zinc Deposit, North Flinders Ranges, South Australia. unpublished B.Sc. thesis, University of Adelaide.
Exploration for and assessment of epithermal precious-metal
Critical characteristics, and their variations
Jeffrey W. Hedenquist, Colorado School of Mines, Hedenquist@aol.com
Antonio Arribas R., Placer Dome Exploration, Reno
Marco T. Einaudi and E. Esra Inan, Stanford University
Richard H. Sillitoe, consultant, London
Epithermal gold and silver deposits of both vein and bulk-tonnage
styles are known by a variety of largely synonymous terms (Table
1; Sillitoe and Hedenquist). They may be broadly grouped into
high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation
(LS) types based on the sulfidation states of their primary sulfide
assemblages (Hedenquist et al., 2000). Sillitoe (1989) subdivided
low-sulfidation epithermal gold deposits of the western Pacific
region into sulfide- and base metal-rich types in normal andesitic-dacitic
arcs and sulfide- and base metal-poor types in rhyolite-bearing
extensional arcs. More recently, John (2001) noted the volcanotectonic
distinction between sulfide-rich epithermal deposits of the western
andesite arc of Nevada and the sulfide-poor, high-grade gold deposits
of the northern Nevada rift, hosted by bimodal volcanic rocks.
He also highlighted the differences in oxidation state of the
associated magmas, and the apparent reflection in oxidation state
of the ore assemblages.
These observations led Sillitoe and Hedenquist (2003) to compile
the volcanotectonic affiliations of different types of epithermal
deposit. Most HS deposits are generated in mildly extensional
to neutral calc-alkaline andesitic-dacitic arcs, although a few
major deposits also occur in compressive arcs characterized by
the of suppression volcanic activity. Rhyolitic rocks lack appreciable
HS deposits. Highly acidic fluids produced the advanced argillic
lithocaps that presage HS mineralization, which itself is due
to higher-pH, moderate- to low-salinity fluids. IS epithermal
deposits occur in a broadly similar spectrum of andesitic-dacitic
arcs, but commonly do not possess such a close connection with
porphyry copper deposits as does the HS type. IS deposits form
from fluids of broadly similar salinities to those responsible
for the HS type, although gold-silver, silver-gold, and base-metal-rich
silver-(gold) subtypes reveal progressively higher salinities
(Albinson et al., 2001). Most LS deposits, including a large proportion
of the world's bonanza veins, are associated with bimodal (basalt-rhyolite)
volcanic suites in a broad spectrum of extensional tectonic settings,
including intra-, near-, and backarc as well as postcollisional
rifts. Some LS deposits, however, accompany extension-related
alkaline magmatism, which unlike the bimodal suites, is capable
of generating porphyry copper deposits. LS deposits linked to
bimodal volcanism are formed from extremely dilute fluids, with
H2S content, possibly of mafic origin, being a major
factor in gold transport.
Vuggy quartz is typical, but not a determining characteristic,
of HS epithermal deposits. Zones of residual, vuggy quartz have
halos of advanced argillic quartz-alunite alteration and roots
of pyrophyllite and/or sericite. These zones typically contain
disseminated pyrite and >95 wt% SiO2, and form bodies
that flare out upwards and/or preferentially replace a lithologic
unit. In many cases, these bodies lack base- and precious-metals
and constitute a barren lithocap of advanced argillic alteration
(Sillitoe, 1995). In other cases, after the leaching stage, copper
and gold were introduced to form epithermal Au-(Cu) deposits with
abundant sulfides, e.g., the Lepanto HS deposit that overlies
the Far Southeast porphyry deposit in Luzon, Philippines (Hedenquist
et al., 1998). The principal copper minerals are enargite, luzonite
and/or famatinite, indicating a high sulfidation state. A typical
sequence of mineral deposition is pyrite + enargite luzonite,
followed by chalcopyrite tennantite sphalerite galena + pyrite.
Also post-dating the enargite assemblage is the gold stage, consisting
of electrum and gold tellurides, as at Lepanto, Goldfield, Nevada,
and El Indio, Chile. The porphyry-related base-metal veins and
the HS epithermal deposits share many common features, and some
deposits appear to occupy the middle ground; they obviously have
a close relationship (Einaudi et al., 2003).
Intermediate sulfidation (IS) epithermal deposits that are
also sulfide-rich share many of the sulfide assemblages of HS
deposits, except that the enargite-bearing assemblage is lacking
and the Ag:Au ratios may be higher, at least 10 or 20 to 1, and
typically >100:1. The total sulfide content can be highly variable,
from 1 to > 10 percent, Mn carbonates and silicates are common
gangue minerals, and "sericite" is typically widespread
(Albinson et al., 2001). These features are characteristic of
the base-metal + silver veins of Pachuca and Fresnillo, Mexico,
Comstock Lode, Nevada and Creede, Colorado, plus deposits in Perú,
Romania and elsewhere. The veins have a halo of illite ±
adularia which grades downward to sericitic and outward to propylitic
assemblages. The major sulfide assemblage can be relatively simple,
composed invariably of three to five minerals, including sphalerite,
galena, pyrite, chalcopyrite, and tetrahedrite. Silver is present
as Ag sulfosalts, in some cases with a large variety of these
minerals in trace quantities, either relatively late in the sequence
(Pachuca) or down the flow path (Creede).
The third class of epithermal deposit is sulfide-poor and dominated
by gold, typically of bonanza grades, and can be distinguished
on the basis of the ore-mineral assemblage. Alteration halos are
narrow and consist of illite or chlorite, with chlorite dominant
in more mafic host rocks (John, 2001). These deposits appear to
form at relatively low temperature (< 220°C) and at shallow
depths (< 250 m), in places immediately beneath hot spring
sinters, as at McLaughlin, California. The very low sulfide content,
<1 %, is dominated by pyrite (in places with arsenian rims),
although marcasite also is common as a result of low temperatures.
Gold as electrum, in places dendritic, is closely associated with
naumannite (e.g., ginguro or black ore at Hishikari and other
bonanza deposits in Japan) or pyrite (Sleeper and Midas, Nevada),
and is typically present in bands of botryoidal quartz or chalcedony.
The gold dendrites and chalcedony suggest that gold and silica
deposited as colloids (Saunders, 1994), possibly as a result of
flashing and extremely rapid ascent of the hydrothermal fluid
that led to supersaturated conditions. Other sulfides are present
only in trace amounts, and include sphalerite, chalcopyrite, galena,
Ag sulfosalts, and minor occurrences of arsenopyrite and rare
pyrrhotite (e.g., Esquel, Argentina; Mule Canyon, Nevada; Rio
Blanco, Ecuador; El Limon, Nicaragua; Sillitoe et al., 2002).
The abundance of sphalerite, chalcopyrite, and galena increases
with depth in some deposits, below the gold ore zone (e.g., McLaughlin),
but within the gold ore zone the total base-metal content is typically
a few 100s ppm at most. Sharp transients in sulfidation state
during formation result in local inconsistencies in sulfide assemblage,
in these and IS plus HS deposits (Einaudi et al., 2003).
There are characteristic mineral textures and assemblages associated
with epithermal deposits (Lindgren, 1933; Sillitoe, 1993; Hedenquist
et al., 2000, and references therein), and coupled with fluid
inclusion data, they indicate that most epithermal deposits form
in a temperature range of about 160° to 270° C. This temperature
interval corresponds to a depth below the paleowater table of
about 50 to 700 m. Boiling is the process that most favors precipitation
of bisulfide-complexed metals such as gold. This process and the
concomitant rapid cooling also result in many related features,
such as gangue-mineral deposition of quartz with a colloform texture,
adularia and bladed calcite in LS deposits, and the formation
of steam-heated waters that typically create advanced argillic
alteration blankets over most epithermal deposits.
Epithermal deposits are extremely variable in form, and much of this variability is caused by strong permeability differences in the near-surface environment, resulting from lithologic, structural and hydrothermal controls. LS deposits typically vary from vein through stockwork to disseminated forms. Gold ore in LS deposits is commonly associated with quartz and adularia, plus calcite or sericite, as the major gangue minerals. The alteration halos to the zone of ore, particularly in vein deposits, include a variety of temperature-sensitive clay minerals that can help to indicate locations of paleofluid flow. The areal extent of such clay alteration, at least within IS districts of large and extensive veins, may be two orders of magnitude larger than the actual ore deposit, depending on the permeability of the wall rock. In contrast, the silicic core of of HS deposits is the principal host to ore. As noted above, outward from this vuggy quartz core is an advanced argillic zone consisting of hypogene quartz-alunite and kaolin minerals, in places with pyrophyllite, diaspore or zunyite. The deposit form varies from disseminations or replacements to veins, stockworks and hydrothermal breccia.
During initial assessment of a prospect, the first goal is
to determine if it is epithermal, and if so, its style, LS, IS,
or HS. Determination of the alteration mineralogy and zonation
is critical at this step. Other essential determinations are:
i) the origin of advanced argillic alteration, i.e., hypogene,
steam-heated or supergene, the latter two with blanket morphology,
ii) the origin of silicic alteration (e.g., residual silica or
silicification), and iii) the likely controls on grade, i.e.,
the potential form of the ore body, as this is one of the most
basic characteristics of any deposit. These determinations will
define in part the questions to be asked, such as the relationship
between alteration zoning and the potential ore zone, and will
guide further exploration and eventual drilling, if warranted.
Observations in the field must focus on the geologic setting and
structural controls, alteration mineralogy and textures, geochemical
anomalies, etc. Erosion and weathering must also be considered,
the latter masking ore in places but potentially improving the
ore quality through oxidation. As information is compiled, reconstruction
of the topography and hence hydraulic gradient during hydrothermal
activity, combined with identification of the zones of paleofluid
flow, will help to identify ore targets. Geophysical data, when
interpreted carefully in the appropriate geological and geochemical
context, may provide valuable information to aid drilling by identifying,
e.g., resistive and/or chargeable areas.
The potential for a variety of related deposits in epithermal
districts has exploration implications. For example, there is
clear evidence for a spatial, and in some cases genetic relationship
between HS epithermal deposits and underlying or adjacent porphyry
deposits. Similarly, there is increasing recognition of the potential
for economic IS base-metal ± Au-Ag veins adjacent to HS
deposits. By contrast, most LS deposits form in a distinctly different
volcanotectonic environment that is incompatible with porphyry
or HS deposits (Sillitoe, 2002; Sillitoe and Hedenquist, 2003).
The explanation for these empirical metallogenic relationships
may eventually be found in the characteristics of the magma, e.g.,
oxidation potential and volatile content, and of the magmatic
fluid genetically associated with the epithermal deposit.
Arribas, A., Jr., 1995, Characteristics of high-sulfidation epithermal deposits, and their relation to magmatic fluid, in Thompson, J.F.H., ed., Magmas, fluids, and ore deposits: Mineralogical Association of Canada Short Course, v. 23, p. 419-454.
Albinson, T., Norman, D.I., Cole, D., and Chomiak, B., 2001, Controls on formation of low-sulfidation epithermal deposits in Mexico: Constraints from fluid inclusion and stable isotope data: Society of Economic Geologist Special Publication 8, p. 1-32.
Einaudi, M.T., Hedenquist, J.W., and Inan, E.E., 2003, Sulfidation state of hydrothermal fluids: The porphyry-epithermal transition and beyond, in Simmons, S.F, ed., Understanding crustal fluids: Roles and witnesses of processes deep within the earth, Giggenbach memorial volume: Society of Economic Geologists and Geochemical Society, Special Publication, in revision.
John, D.A., 2001, Miocene and early Pliocene epithermal gold-silver deposits in the northern Great Basin, western USA: Characteristics, distribution, and relationship to magmatism: Economic Geology, v. 96, p. 1827-1853.
Hedenquist, J. W., Arribas, A., and Reynolds, T. J., 1998, Evolution of an intrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry and epithermal Cu-Au deposits, Philippines: Economic Geology, v. 93, p. 373-404.
Hedenquist, J. W., Arribas, A., Jr., and Gonzalez-Urien, E., 2000, Exploration for epithermal gold deposits: Reviews in Economic Geology, v. 13, p. 245-277.
Lindgren, W., 1933, Mineral deposits, 4th edition: New York and London, McGraw-Hill Book Company, 930 p.
Saunders, J. A., 1994, Silica and gold textures in bonanza ores of the Sleeper deposit, Humboldt County, Nevada: Evidence for colloids and implications for epithermal ore-forming processes: Economic Geology, v. 89, p. 628-638.
Sillitoe, R.H., 1989, Gold deposits in western Pacific island arcs: The magmatic connection: Economic Geology Monograph 6, p. 274-291.
Sillitoe, R.H., 1993, Epithermal models: Genetic types, geometrical controls and shallow features: Geological Association of Canada Special Paper 40, p. 403-417.
Sillitoe, R. H., 1995, Exploration of porphyry copper lithocaps, in Mauk, J.L., and St. George, J.D., eds., Pacific Rim Congress 1995, Auckland, Proceedings: Parkville, Victoria, Australasian Institute of Mining and Metallurgy, p. 527-532.
Sillitoe, R.H., 2002, Rifting, bimodal volcanism, and bonanza gold veins: Society of Economic Geologists Newsletter, no. 48, p. 24-26.
Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious-metal deposits, in Simmons, S.F, ed., Understanding crustal fluids: Roles and witnesses of processes deep within the earth, Giggenbach memorial volume: Society of Economic Geologists and Geochemical Society, Special Publication, submitted.
|Table 1: History of nomenclature for epithermal
deposit types, as summarized by Sillitoe and
Hedenquist (2003). For reference list, see this paper.
|Goldfield type||Ransome (1907)|
|Alunitic kaolinic gold veins||Sericitic zinc-silver veins||Gold-silver-adularia veins Fluoritic tellurium-adularia gold veins||Emmons (1918)|
|Argentite-gold quartz veins
Base metal veins
|Gold-quartz veins in rhyolite
Gold telluride veins
Gold selenide veins
|Secondary quartzite||Fedorov (1903); Nakovnik (1933)|
|Hot-spring type||Giles and Nelson (1982)|
||Bonham (1986, 1988)|
||Hayba et al. (1985),
Heald et al. (1987)
||Berger and Henley (1989)|
|Type 1 adularia-sericite||Type 2 adularia-sericite||Albino and Margolis (1991)|
|High sulfidation||High sulfide + base metal,
|Low sulfide + base metal,
|High sulfidation||Western andesite assemblage,
assemblage low sulfidation
|John et al. (1999); John (2001)|
|LOW SULFIDATION (LS)||Hedenquist et al. (2000)|
|Note: CAPITALIZED names used in this presentation|