Abstracts 2011


January 3, 2011 Technical Presentation
Geology & Exploration of the Ocampo Gold Silver District, Chihuahua
Peter Drobeck
V. P. Exploration, Gammon Gold

The Ocampo Mining District, in a remote canyon in western Chihuahua, is a substantial low-sulfidation epithermal mining district that is presently being exploited by open pit and underground methods. The historic production from the district is unknown as much of it went unrecorded in this remote location. In its first 5 years of production (2006 through 2010), the present operator, Gammon Gold Inc., will have produced approximately 470,000 oz Au and 17,100,000 oz Ag from the mines here.

Ocampo stratigraphy is typical of much of the Sierra Madre Occidental geomorphic region with a basal Eocene (?) Navosaigame Fm. conglomerate that is overlain sequentially by the El Salto Fm. (variably welded dacite ash flow tuff), the Ocampo Andesite (flows and flow breccias), the San Ramon Fm. (volcaniclastic sandstone), and the Sta. Eduviges Fm. (porphyritic dacite flow breccias). These units comprise the local components of the "Lower Volcanic Sequence" (LVS). The LVS units are overlain disconformably by a series of thick quartz-porphyry rhyolitic tuffs and volcaniclastics that comprise the local component of the Miocene "Upper Volcanic Sequence" (UVS).

The district has been deformed by a series of NW-striking oblique-normal faults that have left the stratigraphy tilted north-eastward. Mapping has shown that the basal contact of the UVS is deformed similarly; indicating that at least some of the district tilting is post-UVS. In addition to the predominant NW faults, there is a major WNW-striking normal fault system (PGR Fault) that intersects and cuts the NW-striking system. N20°E striking brittle structures are also common, and appear to be extension fractures.

Mineralization is characterized by low-sulfidation epithermal mineralization that in many ways is typical of the Sierra Madre Occidental province. Ore is controlled by all three faults systems: NW, WNW, and NNE. The WNW and NNE systems dip steeply south, but the NW-striking system dips both NE and SW forming complex intersection relationships with the other vein systems. Veins consist of a gangue of drusy quartz, white sugary-textured quartz, amethyst quartz, adularia, and calcite, with ore-bearing phases of argentite - electrum - argentiferous galena, pyrite, galena, sphalerite, and rare chalcopyrite. Most of the vein mineralization consists of planar arrays of hydrothermal breccias and sheeted to networked arrays of veinlets, although some classic fissure veins occur to a lesser extent. Delicately banded multi-stage fine-grained quartz and quartz-chalcedony are rarely found, and only in the uppermost elevations.

The present mines comprise a 3.5km long series of four open pits (where NW-striking veined zones intersect the WNW-striking PGR trend), and a 0.8 km diameter underground mine complex that extracts ore from six major veins and numerous smaller vein splits. Ore grade mineralization is restricted to the stratigraphic interval from Sta. Eduviges Fm to the base of the Ocampo Andesite. The highest outcrops of veins are at 2150m elevation, and the lowest known mineralization at present is at 1450m - thus there is a 700m vertical interval of known mineralization, although no one ore shoot has more than a 400m vertical interval. Ore grade veins outcrop at 2150m, so the distanced to the top of the pre-erosion ore shoots is unknown. Tilting of the UVS rocks northeastward suggests that the favorable ore horizon is probably also tilted eastward and preserved underneath post-mineral UVS volcanics.


Geophysical and Geochemical Assessment Studies at the Concealed Pebble Porphyry Cu-Au-Mo Deposit, Southwest Alaska
Eric D. Anderson, Robert G. Eppinger, Karen D. Kelley, Paul Bedrosian,
David L. Fey, Burke Minsley, Anjana Shah, and Steven M. Smith
U.S. Geological Survey, MS 973, Box 25046, Denver, CO 80225
ericanderson@usgs.gov, eppinger@usgs.gov

In collaboration with the Pebble Limited Partnership (formed in 2007 by Northern Dynasty Minerals Ltd. and Anglo American LLC), the U. S. Geological Survey (USGS) is studying the Pebble deposit and the surrounding district to refine assessment techniques for concealed mineral deposits. The mostly concealed Pebble deposit consists of two contiguous zones: the partially exposed Pebble West (PW), discovered by Cominco America in 1989, and the concealed Pebble East (PE), discovered by Northern Dynasty Minerals Ltd. in 2005. Combined indicated and inferred resources are 80.6 billion pounds of Cu, 107.4 million ounces of Au, and 5.6 billion pounds of Mo, plus significant amount of Ag, Re, and Pd.

Geology of the Pebble district consists of Jura-Cretaceous argillite, siltstone, and graywacke cut by granitic intrusions occupying a northeast-trending structural corridor. Late Cretaceous subalkalic granodiorite intrusions (91-89 Ma) are genetically related to mineralization. Quaternary glacial deposits up to 50 m thick cover PW; similar glacial material overlies approximately 300-600 m of post-mineralization Tertiary volcano-sedimentary rocks that unconformably overlie PE.

Regional- and deposit-scale aeromagnetic data provide a means of looking through the mostly nonmagnetic glacial and variably magnetic volcano-sedimentary cover to aid in the interpretation of the underlying deposit and associated geology. On a regional-scale, the reduced-to-the-pole (RTP) aeromagnetic data, comprised of five airborne magnetic surveys, show several clusters of northeast-trending magnetic anomaly highs. The Pebble deposit is located on the southeastern edge of one such cluster that occurs over the Late Cretaceous Kaskanak batholith, which is genetically related to mineralization. The dimensions of the clustered anomalies are 25-40 km long and 20-30 km wide. The clusters are aligned subparallel to the crustal-scale Lake Clark strike-slip fault and show spatial correlation with Pebble-aged intrusive rocks. The amplitude of the anomalies is on the order of 800 nT (at a measurement height of 350 m), which is similar to anomalies associated with modern-day, subduction-related volcanoes in the Aleutian Range. The 10 km upward continuation transformation shows a linear northeast trend of magnetic highs spaced about 40-50 km apart. Such spacing is similar to patterns shown by the numerous porphyry deposits in northern Chile and suggests favorability for additional discoveries in southwestern Alaska. These long wavelength magnetic highs are thus also interpreted to reflect the mainly buried, subduction-related Late Cretaceous paleo-magmatic arc.

On a deposit-scale, the RTP transformation of the high resolution airborne magnetic data, images both the Tertiary and Late Cretaceous magmatic rocks of the region. The Tertiary magmatic rocks are reflected as short-wavelength, low- to high-amplitude magnetic anomalies, suggesting their presence at or near the surface. The Kaskanak batholith appears as a long-wavelength, moderate-amplitude anomaly, with the Pebble deposit on its eastern flank. The gradient of the anomaly suggests the batholith may dip to the east. This interpretation is supported by drill hole data from the deposit area. Although potassium feldspar alteration is a common product of the hydrothermal system, a potassium feldspar-magnetite assemblage is not reported. The aeromagnetic and magnetic susceptibility data support this observation and do not suggest an increase in magnetite within the hydrothermal system.

Porphyry copper indicator minerals identified in till samples collected up and down ice from Pebble include gold, jarosite, apatite, and andradite garnet. The best vectors to the deposit include gold and jarosite. Gold grain abundance and morphology vary with distance from the deposit. Samples directly over the deposit contain 12 times the number of gold grains (largely pristine) compared to those approximately 5 km away (re-shaped and modified).

Geochemical techniques were applied to determine optimal sampling media for detecting buried mineralization. Lake, pond, stream, and seep water samples collected over the deposit contain anomalous concentrations of dissolved Cu (up to 661 Êg/l), Mo (up to 18.3 Êg/l), F (up to 2.6 mg/l) and SO4 (up to 84.8 mg/l) compared to distal samples. The highest metal concentrations are generally limited to surface waters over the locally-exposed PW. Also noted are low-level (parts per trillion) metal concentrations for Ag, As, Mo, Sb, U, V, and W in surface waters collected proximal to faults that cut the deeper PE ore body. Total metal concentrations in upper soil horizons identify the shallow PW (Au up to 281 ppb, Cu up to 1830 ppm, and Mo up to 27.1 ppm). Partial leach techniques delineate the deeper PE with anomalous Au, Cu, and Mo, particularly at sites coincident with surface projections of faults. The nonconventional pathfinder elements Cl and V are additional detectors of deep porphyry-type mineralization.

A deposit-scale self-potential ground electrical survey was conducted to investigate the possible role of electrochemical transport associated with the mineralized zone. Several large self-potential anomalies (-600mV and nearly 1 km in diameter) are present and may be related to sulfide minerals at shallow depth. Weak self-potential variability over PE is likely attributed to the greater depth of mineralization.


March 14, 2011 Technical Presentation
John L. Lufkin, Ph.D.
Department of Earth and Atmospheric Sciences
Metro State College Denver
lufk3@comcast.net

In this presentation I will discuss highlights of two books that I have been working on for the past several years, Ore Microscopy and Colorado Geology Illustrated.

Ore microscopy has important applications, not only in the routine identification and paragenesis of ore minerals in reflected light, but also in the beneficiation of ores. Yet relatively few geologists are experienced in the subject. I have been trying to revive this "lost art" for the past four years at the School of Mines, and hope to introduce the subject at Metro State College later this summer. As Paul Barton stated over 35 years ago, "it is certain that ore textures present much information, but it is equally certain that there are few areas of scientific endeavor that are more subject to misinterpretation than the study of ore textures. The interpretation of ore textures is the most maligned, most difficult, and most important aspect of the study of these (sulfide) rocks". In this talk, I will focus on the criteria of interpretation of several common textures, particularly those produced by replacement, exsolution, and simultaneous deposition, and my plans for future microprobe research.

Colorado Geology Illustrated. There are many books on the geology of Colorado, but none like this one. Although it is designed for the non-geologist and my students at Metro, I feel there is much here of interest for anyone interested in Colorado geology. I will present many pictures of Colorado geology, including some the parks, mountains, canyons, geologic hazards, and mineral deposits.

And finally, I will talk about my plans for teaching a seminar in Economic Geology at Metro State next fall - an evening course once a week, ~2 hours in length, and open to all who would like to participate. I welcome members of DREGS to contribute to this effort in one way or another.


April 4, 2011 Technical Presentation
Thoughts on Discovery
Dan Wood
Retired EGM Exploration, Newcrest Mining, Director

If pressed, many discoverers of an ore body will describe the process by which they made the discovery as an art informed by science, as will no doubt many of those who make a discovery in some other field of science; although the words they use may be different. The thesis of this talk is that mineral resource discovery is more likely if its pursuit is, indeed, practised as an art informed by science, albeit constrained by business considerations. I suggest that there are ways to enhance the art of discovery, particularly when applied by geologists with "differently wired" brains. Many of these enhancements won't necessarily be found in any text on exploration management and they include, in no particular order of importance: people, good science, experience, intuition, creativity, perseverance, visualisation, serendipity, a systems approach and drilling. Of these, only drilling is a measurable quantity. Collectively, they provide the basis for a discovery technique that relies on the ability of the geologist rather than on technology to achieve the discovery goal.


May 2, 2011 Technical Presentation
The Sulphide Queen Carbonatite Deposit: Rare Earth Resurgence at Mountain Pass, CA
John O. Landreth, Chief Geologist, Molycorp Inc.

The rare earth-bearing carbonatite at Mountain Pass, California was discovered in 1949. It became world famous for its high-grade content of rare earth elements (REE) and remains as the premier rare earth property in the Western Hemisphere.

The alkaline complex is unique in its character. It consists of ultrapotassic, silicate, alkaline igneous rocks that include seven shonkinite and syenite stocks, and hundreds of dikes that compositionally vary from shonkinite to syenite to alkali-rich granite. The final intrusive sequence includes several varieties of RE-bearing carbonatites (sovite to beforsite) that have formed a roughly tabular, sill-like body. Fenitization is the major alteration, which usually accompanies alkaline carbonatite intrusions. The alkaline complex was structurally controlled by pre-existing zones of weakness and foliation in the host Precambrian metamorphic basement.

This world-class bastnasite deposit produced REE for nearly five decades, until permits to operate the tailings storage facility expired in 2002. During the period 2002 to present, Molycorp has produced rare earth products from stockpiled material. After a hiatus of about 10 years, mining and milling operations will be restarted by Molycorp Minerals in 2012, along with new RE separation facilities.


September 2011 Technical Presentation
Canadian Mining Code 101: Highlights of Recent
Changes in the 43-101 Regulatory Framework
Craig Horlacher, Principal Geologist, Pincock Allen and Holt

The presentation will review the highlights of changes in the Canadian 43-101 regulations that went in effect in July 2011. This will be a general talk on the subject with some emphasis on the key elements of the Code that practicing economic geologist may encounter in the preparation of Technical Reports for Canadian Exchanges.


October 2011 Technical Presentation
Applying 3D GIS to Abandoned Mine Problems
James Russell RPG, GISC
GIS Coordinator
Gilpin County, CO
Abandoned mines and their secondary subsidence effects are a significant problem in small communities with limited funds and resources. Personal safety and property safety are concerns to local planning departments, but little can be done when old, unknown workings become exposed in residential areas where underground mapping is either incomplete or not available to local agencies.

The Central City District, located about 30 miles west of Denver was the most important mining camp in the Front Range mineral belt. Between 1858 and 1920 it yielded more than $100 million worth of gold, silver, and base-metal ore (at $20 per troy ounce). Since 1859, the district has been of vital importance to the development of the Rocky Mountain region. Until the middle 1880's, the metal output of the district exceeded the combined total of all other mining districts in Colorado.

The district lies within a terrain of Precambrian crystalline rocks that are interlayered and include a generally conformable succession of gneissic, granitic, and pegmatitic units. These units are folded along northeast-trending axes that plunge to the northeast or southwest. The veins and minor stockworks that constitute the ore deposits were formed largely as fillings in the faults. The deposits contain a simple suite of metallic and gangue minerals and are valued for their gold and silver content. Veins can be mineralogically classified into two distinctive classes: pyritic or galena-sphalerite. Supergene enrichment is important in the Central City District. Gold was enriched in the upper oxidized parts of the veins, commonly to depths ranging from 50 to 175 feet, and silver and copper were locally enriched below the oxidized zone.

Surface and subsurface data has been collected by US Geological Survey during and after the boom in mine development of the late 19th and early 20th centuries, but much of this data has not been converted to digital form. Converting the subsurface historic mine data into 3D Geographic Information Systems (GIS) layers has proved quite useful in understanding the nature and extent of past mineral development beneath residential and commercial areas.

The application of geometric networks to track water flows through water utility systems can be adapted to track water flows through underground mine workings and allows for modeling flows through the underground systems. This presentation describes the process involved in data conversion, the finished product, and how this information can be used by local governments.


November 7, 2011 Technical Presentation
Guelb Moghrein Iron oxide-Copper-Gold Deposit, Mauritania
Mike Kirschbaum
Ivanplats, DRC

The Guelb Moghrein ore deposit is a metacarbonate-hosted Cu-Au occurrence located within the Mauritanide fold and thrust belt in east-central Mauritania that has previously been designated as an iron oxide-copper-gold (IOCG) deposit.

The rocks enclosing the metacarbonate orebodies are comprised of metabasalt of the Akjoujt Metabasalts unit which have experienced peak amphibolite and retrograde greenschist facies metamorphism. The St. Barbe Fault emplaces the stratigraphically lower St. Barbe Volcanic unit which has been metamorphosed to greenschist grade over the Akjoujt Metabasalt unit along a gentle to moderate west- to southwest-dipping fault zone (figure 1). This fault displays both reverse and normal kinematic indicators and dominantly ductile deformation fabrics. On the west side of the Guelb Moghrein open pit, the St. Barbe Fault propagates downward into the Akjoujt Metabasalt unit and comes in contact with the metacarbonate horizon. This contact is associated with significant thickening and elongation of the metacarbonate. In the vicinity of the Guelb Moghrein open pit, movement along the St. Barbe Fault appears to have caused the formation of brittle-ductile shear zones along the upper and lower metacarbonate contacts. Sulfide mineralization was concentrated in and immediately adjacent to metacarbonate bodies along shear zones bounding the bodies and where one of the bodies intersects a splay from the St. Barbe Fault. To the south of the open pit, the St. Barbe Fault is situated higher above the metacarbonate. In this area there is much less deformation along the metacarbonate contacts and the metacarbonate horizons have a more tabular geometry and contain less sulfides.

The Guelb Moghrein metacarbonate bodies formed from a marine limestone protolith with carbon and oxygen isotopic values similar to those of late Archean carbonates. The sedimentary carbonate body at Guelb Moghrein was subjected to deformation and hydrothermal alteration to siderite. Deformation of the metacarbonate bodies in proximity to the St. Barbe Fault resulted in both brittle and ductile deformation coupled with alteration of the metacarbonate bodies to an assemblage of magnesian siderite + magnetite Ò graphite. Wallrocks around the metacarbonate bodies in proximity to the St. Barbe fault underwent early albitization prior to or during greenschist to amphibolite grade metamorphism and later potassic alteration characterized by growth of biotite during amphibolite grade metamorphism.

The Guelb Moghrein orebodies consist of a series of coalescing siderite + magnetite lenses with chalcopyrite, pyrrhotite, cubanite, cobaltite, aresnopyrite and a variety of other Cu-, Ni-, and Co-bearing minerals. Gold commonly occurs with sulfide minerals in its native form and as electrum. The presence of graphite, formed in the metacarbonate bodies by the thermal decomposition of siderite to magnetite and graphite, was likely an important control on mineralization.

Available geochronological data suggests early metamorphism, hydrothermal alteration, and sulfide mineralization occurred around 2.5Ga. A later tectonothermal event at ~1750 Ma resulted in the formation of greenschist facies assemblages in the Guelb Moghrein area and was associated with a second mineralizing event that may have resulted primarily in remobilization of pre-existing sulfides.

Guelb Moghrein was probably formed from oxidized metamorphic-hydrothermal fluids similar in terms of oxidation state, temperature, and possibly salinity to those responsible for many IOCG deposits. At Guelb Moghrein these fluids were channeled along major structures to a graphitic iron carbonate trap rock. No other IOCG deposits have similar, iron carbonate-rich host rocks suggesting that Guelb Moghrein may represent a true outlier of the IOCG class of deposits.

Figure 1 Schematic diagram showing the structural interpretation of the Guelb Moghrein area. The diagram also shows how structures influence the shape of the metacarbonate body and the distribution of sulfide minerals (red asterisks). The St. Barbe Fault system consists of the St. Barbe Fault and the West Pit Fault that forms a splay off the St. Barbe Fault.


Early Days of Oyu Tolgoi Cu-Au-Mo Porphyry
Discovery, What Lead to the Success?
Sergei Diakov, VP Exploration Americas, AngloGold Ashanti

One of the largest porphyry Cu-Au-Mo discoveries that is currently under construction by Ivanhoe and Rio Tinto with the Mongolian government has an interesting history that started 15 years ago. On September 17, 1996 a team of BHP geologists discovered a prospect called Central Oyu (or Turquoise Hill in Mongolian) in the middle of the Gobi desert approximately 600km south of the Mongolian capital Ulaanbaatar and 80 km north of the border with China.

The presentation highlights early days of the discovery, what worked and what did not work in the generative program that Magma Copper courageously took in Mongolia and BHP then continued after Magma acquisition. Little was known about the area prior to discovery of Oyu Tolgoi in 1996. It was shown as arsenic/molybdenum occurrence on regional metallogenic maps and apparently received little exploration in modern times. A series of old diggings and minor slag material found in the area testify, however, to small-scale mining during the Bronze Age.

Discovery of Oyu Tolgoi was the result of a reconnaissance program initiated in 1995 by Magma Copper Company. More than 60 copper localities in many parts of Mongolia were evaluated. In 1996, BHP acquired Magma Copper and continued the reconnaissance in western and southern Mongolia. Central Oyu Tolgoi was recognized as a porphyry copper leached capping at the end of the field season in September 1996. South Oyu was identified in April 1997. BHP completed a program of detailed mapping, sampling, and geophysics during the 1997 field season. Encouraged by favourable geological, geochemical, and geophysical (induced polarization and ground magnetics) results in the prospect area, an initial drilling campaign tested for supergene chalcocite mineralization beneath the leached cap at Central Oyu and for hypogene copper-gold mineralization at South Oyu. By the end of October 1997, BHP had completed six diamond drill holes (OT1 - OT6) totalling 1,002.4 m. OT3 was drilled in Central Oyu (2.39 percent Cu from 20.7 to 30.8m; 1.89 percent Cu from 33.1 to 50.0m), and OT4 was drilled in South Oyu (1.65 percent Cu and 0.154 ppm Au from 56 to 126m). These two holes are considered to be the discovery holes at Oyu Tolgoi. Drilling continued in 1998.

A preliminary evaluation in early 1999 showed that Oyu Tolgoi contained an in-situ mineralization of 438 Mt averaging 0.52% Cu and 0.25 ppm Au. Of these, 331 Mt 0.48% Cu and 0.30 ppm Au were contained in South Oyu. Based on these figures, Oyu Tolgoi was Mongolia's second largest porphyry copper system.

The impressive history of Oyu Tolgoi continued with Ivanhoe after BHP farmed out the project. Significant discoveries of gold mineralization at South West Oyu and then Hugo Dummett mineralization made by Ivanhoe Mines predetermined an exciting future of the mineral potential for the Mongolian Gobi making Oyu Tolgoi not only the largest porphyry in Mongolia but one of the largest porphyry discoveries in the world during the last two decades. Currently Oyu Tolgoi is under construction and expected to go into production in 2013.