Abstract December 9, 2013
Technical Presentation

Geology of the Van Dyke Copper Deposit, Miami, Arizona
Dr. Tim Marsh
Consulting Geologist

Beneath the streets of downtown Miami, Arizona, is a historical inferred copper resource of 157 million tons grading 0.44% copper at a 0.15% cutoff as estimated in 1976 by Occidental Minerals Company. Thisestimate is historical in nature, does not comply with NI 43-101 standards, and should not be relied upon. The deposit was originally discovered in 1916 by Cleve Van Dyke, founder of the town of Miami. While drilling a 1,900 foot churn drill hole on a ridge top at the north edge of town, he cut copper oxides beginning at 1,200 feet. A second 2,000-foot churn drill hole almost one mile to the east demonstrated he was on to something big. Van Dyke's mine produced 12 million pounds of 5% copper ore between 1929 and 1945. A modern effort by Occidental Minerals Company to develop the resource between 1966 and 1984 came to an end just when a favorable federal court decision ruled that Occidental had the right to extract ore from beneath the town. Copper prices, however, had fallen too low to finance further work. In 1989, Kocide Chemical Company operated an in-situ mining operation on the site, producing 4 million pounds of cement copper for use as a fungicide in the Casa Grande cotton fields. Bell Copper began negotiations with the heirs of Cleve Van Dyke in March 2010. They signed a purchase agreement in February 2012, and sold the property to Copper Fox Metals in April 2013. Copper Fox Van Dyke Company is now reassessing Occidental's work as a step toward turning Van Dyke into a current mineral resource.
 

Abstract November 4, 2013
Technical Presentation

Tectonics and Gold Metallogeny

 
R.J. Goldfarb
United States Geological Survey, Denver Federal Center, Denver, Colorado 80225-0046, USA

The temporal pattern for different types of gold deposits will vary with evolving global tectonic geodynamics, such that a particular deposit type will tend to have a characteristic time-bound nature. Factors bearing on the age distribution of a particular type of gold deposit include uneven preservation, data gaps, and long-term secular changes in the Earth System.

The distribution of gold-rich porphyry and epithermal deposits is skewed towards the late Cenozoic (Fig. 1). The ores are associated with subvolcanic plutonic complexes and shallower parts of oceanic and continental arcs in the convergent margins of the circum-Pacific and Tethyan of southern Europe. Most deposits that formed in the upper few km of crust before ca. 20-30 Ma, were uplifted and eroded, and thus lost from the geologic record, although significant exceptions date back through all Phanerozoic orogens, and even to the Archean. Carlin-type deposits are only widely recognized in Nevada (Tertiary) and perhaps along the SW edge of the Yangtze craton (Jurassic), so knowledge about these remains too limited to confidently relate the ores to major global tectonic patterns.

Orogenic gold deposit formed in medium-grade metamorphic belts tens of millions of years subsequent to host rock deposition. The deposits in both eastern China and Sonora are hosted in high-grade rocks and provide global anomalies where deposits post-date host rock metamorphism by billions of years, leading to revisions in the ore genesis model. Preserved orogenic gold deposits correlate in time with addition of new oceanic lithosphere to craton margins during supercontinent growth at ca. 2.8-2.55 Ga, 2.1-1.75 Ga, and 650-35 Ma (Fig. 2). Major lithospheric instabilities controlling ore formation include thickening by terrane accretion, subduction of a spreading ridge, rollback or delamination of subducted oceanic lithosphere, or Precambrian plume events. The ca. 3.0 Ga timing of stabilization of subcontinental lithospheric mantle (SCLM) below the Kaapvaal craton indicates that the Witwatersrand gold ores cannot be Late Archean orogenic deposits.

The IOCG deposits represent the one group of gold ores in intracratonic settings, typically 100-200 km inland from the craton margins, where extension and anorogenic magmatism occur between areas of Archean and Proterozoic SCLM. The partial melting of metasomatized SCML, either by mantle underplating or plume episodes, leads to IOCG development in buoyant and refractory Precambrian cratons, such that even shallowly formed deposits have been preserved.

Fig. 1 Temporal distribution of major gold deposit types.

Fig. 2 Orogenic gold vs. crustal growth.

 

Abstract October 7, 2013
Technical Presentation

Shale Development and Hydraulic Fracturing

 
Dr. William Fleckenstein, PE.
Interim Department Head and BP Adjunct Professor in Petroleum Engineering, Colorado School of Mines,
US Geological Survey

There have been over 1 million hydraulic fracturing treatments since 1947 in the US, long before the shale boom began. Currently, nearly 1200 rigs are drilling horizontal shale wells in the US, more than twice as many horizontal wells as vertical wells. Shale development combines horizontal drilling and multistage, hydraulic fracturing or "fracking". The US is just beginning to get an understanding of how widespread these shale plays are, and can now, with growing confidence, predict reaching true energy independence. The resource potential is that great.

I have traveled a lot in Europe talking about Shale Development and Hydraulic Fracturing in the last two years - Brussels, Bucharest, Sofia, Warsaw, Vilnius and three cities in Ukraine and now the Middle East. There are tremendous shale resources globally; the US does not have a corner on the geology. Shale reservoirs occur anywhere there is conventional production, since the shales are the sources of the hydrocarbons for those reservoirs. There is tremendous potential for unconventional oil and gas development throughout the world.
 

 


September 9, 2013
Technical Presentation

Arc Volcanoes: Nature’s Cu-Au Refineries

 

 
Byron R. Berger
US Geological Survey
Crustal Geophysics and Geochemistry Science Center MS964 Denver, CO 80225

Active magmatic-arc volcanoes have long been recognized as the hearths within which many base and precious metal deposits are born. What we have lacked is a comprehensive understanding of just how magmatic, high-enthalpy fluid flow systems work from their melt regime source areas up through a volcanic complex to the discharge vents. Through the study of sub-volcanic mineralizing systems such as high-sulfidation Cu-Au and porphyry Cu deposits, we now know - in wonkish tones - that these deposits consistently record the sustained histories of magmatic gas streaming through the volcanic systems interspersed with continuing intrusive and eruptive activity. The fluxes of metals and semi-metals are controlled by the maintenance of fracture permeability in stressed crust within and below volcanoes and consequently chemical process are triggered as magmatic gas, initially undersaturated with metals and semi-metals, expands from lithostatic to very low pressure conditions through fracture arrays. Throttles in the fracture array guide the expansion of the gas to the surface. As the gas expands to the surface by abrupt pressure drops, solvated molecular metal and semi-metal species in the magmatic gas mixtures are destabilized and changes in their redox conditions are triggered. Low electronegativity metals such as Cu and Fe deposit rapidly in response to expansion followed more slowly by As with Sb as sulfosalts. Heavy, large radius elements such Bi, Pb, Tl, and Te are strongly fractionated to the vapor along the way, eventually venting their excess or possibly reaching sufficient saturation to deposit in the upper reaches of the volcanic systems.

By way of illustration, I will first present a conceptual image of the architecture of a volcanic complex, introduce you to images from the numerical modeling of magmatic vapor plumes which I then superimpose on the volcanic construct, make a few comments about chemical transport in magmatic gases including the variation in metal concentration as a function of pressure during magmatic-gas expansion, and then illustrate some of the processes going on in the ore-formation process in high-sulfidation deposits by means of field-emission scanning electron microscope images of sulfide-sulfosalt ores from principally the El Indio, Chile, Goldfield, Nevada, Chinkuashih, Taiwan, and Lepanto, Philippines, deposits. Along the way, you’ll see evidence of the very high-temperature origin of these deposits, witness the chemical engineering of a deposit as tennantite dissolves and replaces pyrite and enargite in most unkind ways and heavy metals and semi-metals diffuse along vapor-solid interfaces of enargite, tennantite, and subsequent sulfosalts, in the process enriching the vapor in the heavier elements, including gold, that we find in the shallower parts of high-sulfidation deposits and in the discharge wastes at Earth’s surface.


January 2013 Technical Presentation
Geology of the Cerro Jumil Gold Skarn, Mexico
Jeffrey Edelen

Cerro Jumil exhibits distinctive characteristics from other regional gold skarns. This study explores and characterizes these differences placing the deposit in a unique position along the continuum of Au-bearing skarn deposit styles found in southern Mexico.

The Cerro Jumil deposit is a recently discovered gold skarn located in the state of Morelos, Mexico. The deposit is comprised of 1.47 million ounces Au and 16.01 million ounces Ag in the measured and indicated category and 0.17 million ounces Au and 2.17 million ounces Ag in the inferred category at a 0.3 g/t gold equivalent cut-off. Mineralized skarn zones have been developed in limestone of the Mesozoic Morelos-Guerrero Platform in response to the intrusion of a feldspar porphyritic granodiorite. The calc-silicate alteration association with the granodiorite intrusion shows a pronounced mineralogical zonation that developed around zones of increased fluid flow and thermal perturbation. The zonation from distal to proximal is defined by marble to low-temperature calc-silicate alteration to high-temperature, pervasive calc-silicate alteration. This zoning occurs along the flanks of the granodiorite and above the cupola of the intrusion, but is also developed along fault planes and bedding planes away from the intrusive contact.

Petrographic and mineral chemical investigations permitted a detailed reconstruction of the skarn paragenetic sequence and also provided important constraints on the relative timing of gold mineralization. Textural evidence suggests that wollastonite formation preceded development of clinopyroxene, vesuvianite, and garnet during the prograde stage of skarn formation. Wollastonite skarn is well developed along former limestone bedding planes. Vesuvianite and garnet skarn occurs as massive zones, containing subordinate amounts of clinopyroxene. Formation of the grossular and andradite series garnet skarn is interpreted to record peak metamorphic conditions. Some pyrite is associated with the garnet skarn.

Garnet skarn formation was followed by the onset of retrograde metamorphism. Actinolite and tremolite are common and pervasive talc pseudomorphs prograde and retrograde alteration minerals. Minor plagioclase, calcite, quartz, epidote, and chlorite are observed to form interstitial grains between talc. A second generation of pyrite formed during the retrograde evolution of the exoskarn. Due to supergene weathering, this pyrite is now largely replaced by Fe oxides and hydroxides. These limonitic intervals commonly show elevated gold grades, suggesting that gold mineralization was primarily associated with this sulfidation event. Bismuth shows a moderate to strong correlation with Au.

The limonitic zones are spatially associated with epidote that occurs as disseminated grains and along fractures. These fractures are indicative of widespread faulting. The location of these faults appears to have controlled the subsequent formation of fine-grained, massive, hematite- and silica-rich jasperoid. Although jasperoids are commonly barren, high gold grades have been observed for at least some intersections of jasperoid in drill core, suggesting that fluid pathways played a role in the late remobilization and concentration of gold.

The formation of retrograde alteration assemblages was followed by the intrusion of a late quartz porphyritic granodiorite. The quartz porphyritic intrusion crosscuts the different skarn mineral assemblages and the marble. Skarn development around this intrusion was not observed though hydrothermal alteration of the igneous rock is common. Though weakly mineralized, this intrusive phase does not contribute to the overall metal content of the Cerro Jumil deposit. The intrusion of the quartz porphyritic granodiorite broadly coincided with widespread faulting and a second, hematite-rich though barren jasperoid at Cerro Jumil.

The prograde and retrograde skarn assemblages at Cerro Jumil are overprinted by pervasive supergene alteration, causing the observed extensive replacement of sulfides by limonite. Supergene alteration seems to have been important by redistributing gold on a local scale, making Cerro Jumil an attractive bulk-mineable deposit.

Fluid inclusion investigations revealed that the prograde and retrograde stages of skarn formation occurred from a single-phase, intermediate-density hydrothermal fluid, suggesting that intrusion of the granodiorite occurred at an intermediate crustal depth (3 to 4.1 km at lithostatic conditions). Due to the comparably deep environment of skarn formation, late-stage epithermal-style mineralization observed in many other skarn deposits could not develop at Cerro Jumil. For the same reason, mixing of the magmatic-hydrothermal fluids with meteoric water was probably insignificant.

Although Cerro Jumil shares a number of deposit characteristics with other gold skarns in the regional Morelos-Guerrero Gold Belt to the south, the results of the present study highlight some important differences such as chemistry and alteration state of the skarn-related intrusion, chemistry of metasomatism and resultant mineralogy, lack of exoskarn, depth of the intrusion and corresponding lack of pervasive retrograde alteration overprint, and age of formation. In particular, the age of Cerro Jumil has significant implications for regional exploration, as Tertiary intrusions into the Mesozoic Morelos-Guerrero Platform are not necessarily barren as previously suggested.


February Technical Presentation
Mother Lode Type Gold Deposits Exemplified by the Lincoln Mine of Sutter Gold Mining Inc.
by Stephen Zahony, Consulting Mining Geolo
gist

The Mother Lode vein system follows a prominent complex shear zone situated along the western Sierra Nevada foothills. It has produced approximately 14 million ounces of gold with more than half that amount recovered from a relatively short segment of the system between the towns of Jackson and Plymouth in Amador County. Old-time mining was extensive in this portion of the Mother Lode. Gold mineralization occurs in fissure veins formed within the broad Melones Fault system with veins dipping at moderate to steep angles to the east, cutting across the metamorphic fabric at acute angles. Foliation of the host rocks is near vertical, generally dipping steeply east. Fissure veins are simple in mineralogy but very complex structurally. Veins are predominantly quartz with small amounts of carbonate and contain 1-2% sulfide minerals dominated by pyrite and arsenopyrite. Locally pyrrhotite is common in veins but especially in wall rock immediately surrounding veins. Sphalerite, galena, and chalcopyrite are insignificantly rare but are good indicators of gold. A strong but imperfect correlation exists between gold and arsenic; generally +5,000 ppm As correlates with Au values >0.2 opt. No zoning is noticeable in the veins over their several thousand feet of known vertical extent. Wall rock reactions are distinct and vary with host rock type. Erratically distributed and lower grade but locally mineable gold mineralization occurs in strongly carbonatized and sericitized quartz-veined wall rock called Gray Ore. By 1900 production of this type of low-grade ore was prevalent at some of the mines. Discontinuous axinite-containing quartz-calcite veins, without sulfides or gold, postdate the productive gold-bearing structures and appear to be the last hydrothermal event.

Faulting occurred before, during, and after vein formation. Documented fault movements are almost exclusively reverse, a true compressive regime, but wrench movement is also probable. Mother Lode orogenic fissure vein systems differ markedly from epithermal style veining by their lack of banding and the relatively simple massive quartz filling. Only a few phases of quartz formation are apparent: generally strained or crushed quartz cut by later massive milky quartz. Open vug-filling quartz crystals are uncommon, but where they occur show lower fluid inclusion temperatures than the 250 to 450 degree temperatures generally obtained from the CO2-rich fluid inclusions in quartz. Included altered wall rock slabs give veins a ribbon-like appearance. Sulfide minerals tend to congregate along and within the sericite-carbonate-altered included wall rock slabs. An enigmatic feature of Mother Lode quartz veins are crenulated styolite-like bands of pyrite, arsenopyrite, and muscovite, often the locus of gold, that may be due to pressure solution and deposition. Some call these features the true ribbon structure.

The Lincoln-Comet deposit was discovered by Callahan in 1983 while drilling an arsenic soil anomaly over a non-productive segment of the Mother Lode between the prolific Keystone and Eureka Mines. Since that time 221 drill hole have outlined a gold resource sufficient to support a 150 tpd mining operation for five to seven years with a diluted grade of +0.3 opt Au. Continued exploration hopes to significantly expand the life of the operation. Mine and mill development are almost complete with full production anticipated by midyear. Contrary to the surrounding mines of the Mother Lode, the northern Comet resource block of the Lincoln Mine is horizontally elongate, dips at steep angles to the west, and is hosted completely in greenstone, predominantly an augite-bearing pyroclastic meta-basalt. The southern Lincoln resource block is also horizontally elongate, near vertical in dip, and its locus is the contact zone between the greenstone meta-basalt and the Interbedded Series, a rock unit composed of interbedded tuffs, fine-grained volcaniclastic sediments, and beds of black carbon-bearing argillite.

Whatever the origin of the vein-producing fluids, wall rock reactions were dramatic, with silica removed and carbonate introduced into the enclosing rocks. Fluids must have been under tremendous pressure and may have buoyed fault openings. Unlike flow-through epithermal systems, it is believed that fluids were relatively static. Fissures were sealed at their top with probable internal circulation and chemical diffusion at work. Periodic rupture along structures caused wall rock incorporation and pressure drops which caused mineral precipitation. Orogenic is a good term for the Mother Lode type veins as they differ from true mesothermal veins that generally contain more sulfide minerals, are zoned, and have a direct and identifiable intrusive rock affiliation.


March Technical Presentation
To reactivate or not to reactivate- Nature and varied behavior of structural inheritance in the Proterozoic basement of the eastern Colorado Mineral Belt over 1.7 billion years of earth history
Jonathan Saul Caine* U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, Colorado 80225-0046, USA
John Ridley
Zachary R. Wessel Colorado State University, Fort Collins, Colorado, USA

Faulting occurred before, during, and after vein formation. Documented fault movements are almost exclusively reverse, a true compressive regime, but wrench movement is also probable. Mother Lode orogenic fissure vein systems differ markedly from epithermal style veining by their lack of banding and the relatively simple massive quartz filling. Only a few phases of quartz formation are apparent: generally strained or crushed quartz cut by later massive milky quartz. Open vug-filling quartz crystals are uncommon, but where they occur show lower fluid inclusion temperatures than the 250 to 450 degree temperatures generally obtained from the CO2-rich fluid inclusions in quartz. Included altered wall rock slabs give veins a ribbon-like appearance. Sulfide minerals tend to congregate along and within the sericite-carbonate-altered included wall rock slabs. An enigmatic feature of Mother Lode quartz veins are crenulated styolite-like bands of pyrite, arsenopyrite, and muscovite, often the locus of gold, that may be due to pressure solution and deposition. Some call these features the true ribbon structure.

The Lincoln-Comet deposit was discovered by Callahan in 1983 while drilling an arsenic soil anomaly over a non-productive segment of the Mother Lode between the prolific Keystone and Eureka Mines. Since that time 221 drill hole have outlined a gold resource sufficient to support a 150 tpd mining operation for five to seven years with a diluted grade of +0.3 opt Au. Continued exploration hopes to significantly expand the life of the operation. Mine and mill development are almost complete with full production anticipated by midyear. Contrary to the surrounding mines of the Mother Lode, the northern Comet resource block of the Lincoln Mine is horizontally elongate, dips at steep angles to the west, and is hosted completely in greenstone, predominantly an augite-bearing pyroclastic meta-basalt. The southern Lincoln resource block is also horizontally elongate, near vertical in dip, and its locus is the contact zone between the greenstone meta-basalt and the Interbedded Series, a rock unit composed of interbedded tuffs, fine-grained volcaniclastic sediments, and beds of black carbon-bearing argillite.

Whatever the origin of the vein-producing fluids, wall rock reactions were dramatic, with silica removed and carbonate introduced into the enclosing rocks. Fluids must have been under tremendous pressure and may have buoyed fault openings. Unlike flow-through epithermal systems, it is believed that fluids were relatively static. Fissures were sealed at their top with probable internal circulation and chemical diffusion at work. Periodic rupture along structures caused wall rock incorporation and pressure drops which caused mineral precipitation. Orogenic is a good term for the Mother Lode type veins as they differ from true mesothermal veins that generally contain more sulfide minerals, are zoned, and have a direct and identifiable intrusive rock affiliation.


Copper Deposits of the Coast Ranges of Chile
A trip through time, space, and ore deposit nomenclature
John Dreier

The Coast Ranges of Chile, extending for about 4300 km along western South America from the Peruvian border (latitude 18 degrees S) to Tierra del Fuego (latitude 55 degrees S), are largely underlain by volcanic and plutonic rocks of a Jurassic-Lower-Mid Cretaceous subduction-related volcanic arc complex. For slightly less than half that distance, from 18 degrees S to ~35 degrees S, the Jurassic-Cretaceous arc hosts numerous copper deposits (perhaps well into the thousands), which contain variable amounts of specularite and/or magnetite plus the common wall rock alteration/gangue minerals, albite, K-feldspar, chlorite, epidote, calcite, apatite, sericite, biotite, and scapolite. Some of these deposits have Au as an important byproduct, others contain Ag; a few are Ag-dominant or lack Cu altogether, and some are Mn-dominant. In form, the deposits include veins, mantos, stockworks, and breccia pipes. A number of these deposits are porphyry-like in some aspects of their form and mineralization. The Coast Ranges deposits vary in structural control according to the age of formation and in form and mineralogy according to depth of exposure and host rock. Jurassic deposits are dominantly related to and/or controlled by ENE or WSW-trending strike slip faults of apparently small displacement, whereas the Cretaceous deposits tend to be controlled by northerly-trending high-angle transpressional strike-slip faults or reverse faults all related to the compression and uplift of the volcanic arc and the closure of the back arc basin to its east. Within intrusive rocks, all deposits are veins, but in volcanics and sediments veins, mantos, stockworks, and breccia pipes are common - sometimes within the same deposit. In many districts, mineralization is dispersed throughout 10s or hundreds of square km and in these areas it is common to trace vein systems for many km along strike within batholiths and to have them morph into manto systems when one enters the surrounding volcanics. The Coast Ranges deposits are enigmatic in their placement within the classification of mineral deposits. Historically they were simply classed as either veins or Chilean Manto-Type Cu Deposits, however, within the past 15 years or so, spurred by work at Candelaria, they were reclassified as IOCG deposits. It should be evident from this summary that the Coast Ranges ore systems extend beyond the bounds of the IOCG designation and for this reason their place within the world of ore deposits should be given further consideration.

Deeper-level deposits are 1-5 m wide veins hosted by intermediate-composition plutonic rocks plus volcanics and sediments metamorphosed to greenschist or higher facies. Individual deep-level veins or vein clusters may contain up to 50,000 tonnes of Cu metal as chalcopyrite and lesser bornite plus 60,000 ounces of gold. The Coast Ranges batholiths and their metamorphic host rocks contain hundreds or perhaps thousands of such individual vein deposits. About 10 of these deposits are in production by milling operations, which process 2,000 - 3,000 TPD of ore grading ~1.5% Cu and 0.5 to 1 g/t Au and ship 4,000 - 5,000 TPM of concentrate to smelters.

Intermediate-level Cu deposits of the Coast Ranges have produced > 2 M tonnes of Cu metal, and many of these are (or were in the past) mined by bulk methods. Ore minerals for the intermediate-level deposits include chalcopyrite, bornite, and minor hypogene chalcocite; gangue/alteration minerals include magnetite, specularite, K-feldspar, albite, biotite, scapolite, chlorite, and calcite. The bulk minable deposits are hosted by volcanic and sedimentary rocks. In detail, mineralization in the intermediate level deposits is controlled by structures and certain favorable beds or stratigraphic units; vesicular lava flow tops and brecciated lava flow tops and bottoms being very common and important ore hosts. The deposits typically contain 0.2 - 0.3 g/t Au and up to 20 g/t Ag. Important intermediate-level deposits include Candelaria, Atacama Kozan, Santa, and others in the Tierra Amarilla district.

High-level deposits may assume a variety of forms, including veins, breccia pipes, or mantos and the breccias appear to be hydrothermal in origin. The stratigraphic control to the intermediate-level deposits is similar to that previously mentioned, especially in respect of control by lava flow tops and bottoms. Common ore minerals in the high-level deposits include hypogene chalcocite, digenite, bornite, lesser chalcopyrite, and minor native silver. In some high-level deposits, a portion of the mineralization is intimately related to hydrocarbons. The high-level deposits lack Au and may contain up to 150 g/t Ag. At least one high-level deposit resembles epithermal veins in form and ore textures and, like many epithermal veins, the vein terminates upward by pinching out. Near its top, this contains mantos that were mined for Mn. Common gangue/wall rock alteration minerals in the high-level deposits include quartz, calcite, chlorite, sericite, K-feldspar, albite, and epidote, plus galena and sphalerite. Rocks surrounding the high-level deposits are commonly affected by regional zeolite/pumpellyite alteration/metamorphism. High-level systems include Michilla, Buena Esperanza, Mantos Blancos, Frankenstein, El Soldado, and Cerro Negro (sur). Total production from these deposits is > 4 M t Cu metal. Recent exploration in some former Chilean Ag districts indicates that these represent upper halos above Cu-Fe systems.

Geological relationships evident throughout the Coast Ranges make it abundantly clear that the mineral systems are vertical zoned with respect to form, ore mineralization, and gangue/wall rock alteration mineralogy. Thus, IOCG-like characteristics predominate in the lower levels of the systems but these gradually give way upward first, to Cu-Au-Ag, then to Cu-Ag, then to Ag, and finally, in the upper-most regions, to Mn mantos. A full understanding of the vertical zoning and the principal structural and stratigraphic controls to mineralization is a requisite for successful exploration.

Copper mining in Chile began in the 1830's with small-scale operations focused on high-grade veins of oxide and secondary enriched sulfides, in which hand-sorted ore was smelted on site or shipped to Wales. Beginning in the 1960's, production from the small-scale operations was progressively supplanted by copper from increasingly larger-scale mines for leach operations, which to the present, have produced a total of ~8 M tonnes of Cu metal, mostly from mine for leach/SXEW operations. Large modern operations include Michilla, Mantos Blancos, Manto Verde, and El Soldado. Modern intermediate-scale operations include Ivan-Zar, Mantos de la Luna, Sierra Valenzuela (closed), Sierra Miranda, Las Luces, Frankenstein (recently failed SXEW operation), Santa, Atacama Kozan and other operations in the Tierra Amarilla area, Cinabrio, Cabildo, Cerro Negro, and Pudahuel (closed). Small-scale operations are numbered in the hundreds and include the many small mines which ship hand sorted oxide ore to government-operated leaching plants.


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Tectonics and Gold Metallogeny
R.J. Goldfarb
United States Geological Survey, Denver Federal Center, Denver, Colorado 80225-0046, USA

The temporal pattern for different types of gold deposits will vary with evolving global tectonic geodynamics, such that a particular deposit type will tend to have a characteristic time-bound nature. Factors bearing on the age distribution of a particular type of gold deposit include uneven preservation, data gaps, and long-term secular changes in the Earth System.

The distribution of gold-rich porphyry and epithermal deposits is skewed towards the late Cenozoic (Fig. 1). The ores are associated with subvolcanic plutonic complexes and shallower parts of oceanic and continental arcs in the convergent margins of the circum-Pacific and Tethyan of southern Europe. Most deposits that formed in the upper few km of crust before ca. 20-30 Ma, were uplifted and eroded, and thus lost from the geologic record, although significant exceptions date back through all Phanerozoic orogens, and even to the Archean. Carlin-type deposits are only widely recognized in Nevada (Tertiary) and perhaps along the SW edge of the Yangtze craton (Jurassic), so knowledge about these remains too limited to confidently relate the ores to major global tectonic patterns.

Orogenic gold deposit formed in medium-grade metamorphic belts tens of millions of years subsequent to host rock deposition. The deposits in both eastern China and Sonora are hosted in high-grade rocks and provide global anomalies where deposits post-date host rock metamorphism by billions of years, leading to revisions in the ore genesis model. Preserved orogenic gold deposits correlate in time with addition of new oceanic lithosphere to craton margins during supercontinent growth at ca. 2.8-2.55 Ga, 2.1-1.75 Ga, and 650-35 Ma (Fig. 2). Major lithospheric instabilities controlling ore formation include thickening by terrane accretion, subduction of a spreading ridge, rollback or delamination of subducted oceanic lithosphere, or Precambrian plume events. The ca. 3.0 Ga timing of stabilization of subcontinental lithospheric mantle (SCLM) below the Kaapvaal craton indicates that the Witwatersrand gold ores cannot be Late Archean orogenic deposits.

The IOCG deposits represent the one group of gold ores in intracratonic settings, typically 100-200 km inland from the craton margins, where extension and anorogenic magmatism occur between areas of Archean and Proterozoic SCLM. The partial melting of metasomatized SCML, either by mantle underplating or plume episodes, leads to IOCG development in buoyant and refractory Precambrian cratons, such that even shallowly formed deposits have been preserved.

Fig. 1 Temporal distribution of major gold deposit types
Fig. 2 Orogenic gold vs. crustal growth.