By William A. Rehrig & James Hardy (Consulting geologists)

Sonoran and Peru Copper

Sonoran and southern Peru porphyry copper provinces are both crossed by regional WNW-striking fault zones of strike-slip origin. In North America, the zone is known as the Texas Lineament. Major ore deposits are either directly or indirectly localized along or adjacent to these faults. Overall sinistral sense of shear along the fault zones have produced large-scale oroclines of remarkably similar tectonic character in both continents. The faults are propelled by oblique Cordilleran plate convergence during Tertiary time, and deposits are localized at key structural loci along the faults that have concentrated diavatory transpressional stresses.

Mineralization is controlled by intense hydrofracturing (copper, gold mineralization) in mineralogically distinct, "wet" plutons possibly related to large, calc-alkaline batholiths. The orientation and superposition of distinct mineralized vein and fracture sets are dependent upon: (1) Direction of regional compression, (2) secondary transtension stresses; and, (3) localized or point-source stresses.

In Peru, higher level porphyry-related copper and gold deposits are structurally more complex than they are in the Sonoran province of North America

February 2003

Ken Witherly,
President of Condor Consulting, Inc.,

Geophysical techniques have contributed significantly to exploration success since beginning of the modern era in exploration (post-WW II). An examination of the performance of airborne geophysics since its inception in the early 1950s shows that there is close tie-in between the successful use of the technique and the robustness of the geological models that were developed to guide the use of the technique. When this relationship breaks down, either due to specific environment maturity or the inappropriate selection of environments, then technique performance falls dramatically.
In the last 10 years, two new exploration models have appeared that airborne geophysics has been aggressively applied to: the search for kimberlites in the Canadian arctic, and, the detection of chalcocite blankets in the Andean porphyry copper belt. These examples will be looked at in greater detail: how the models were developed, the geophysical technique applied and outcomes. The marked difference in exploration results is sobering but to a large extent, simply reinforces what was observed in the previous 50 years of experience.
With the closing of the last decade/millennium, a new airborne technical has been introduced, gravity gradiometry. This technique's record to date will be touched on in light of past experience and some suggestions made as to how best to adapt this new technique to maximize the likelihood of discovery success.

March 2003

SEG Thayer Lindsley Lecture - Jointly Sponsored by CSM SEG Student Chapter and DREGS

Lode Gold Through Earth History - Patterns in Space and Time

Richard Goldfarb, U. S. Geological Survey, MS-973, Box 25096, Denver, CO 80225

Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800-2555 Ma and 2100-1800 Ma), a singular lack of deposits for 1200 m.y. (1800-600 Ma), and relatively continuous formation since (<600 Ma).

The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma). This worldwide process allowed preservation of gold deposits in cratons--roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate-tectonic regime led to the accretion of volcano-sedimentary complexes as progressively younger linear orogenic belts surrounding the margins of the more buoyant cratons.

The susceptibility of these linear belts to uplift and erosion can elegantly explain the overall lack of orogenic gold deposits at 1800-600 Ma, their exposure in 600-50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma.

April 2003

Magmas, Melt Inclusions, Metals, And Ores

Edwin Roedder, Professor Emeritus,
Harvard University, 47 Salt Island Road Glouchester, MA 01930

Most mineable metallic ore deposits arose from several geochemical processes that collected the small amounts of metals from igneous magmas, and then deposited them in concentrated form as ore deposits. The steps in this process are best evidenced by chemical analysis of the various fractions, most particularly, analysis of the melt (and fluid) inclusions trapped in these fractions.

May 2003

A Neoproterozoic Intrusion-related Gold System at Ad Duwayhi Saudi Arabia

Stephen G. Zahony*, Jeff L. Doebrich, James D. Leavitt, Jose S. Portacio, Jr., Alim A. Siddiqi, Joesph L. Wooden, Robert J. Fleck, Holly Stein

The Ad Duwayhi gold deposit is located in the east-central part of the Arabian shield and is the newest mining-viable gold discovery in Saudi Arabia. Exploratory core drilling of over 100 holes has identified a gold resource of greater than 1 million ounces with significant potential for expansion. Detailed mapping, drilling, geochemistry, petrochemistry, and U/Pb, 40Ar/39Ar, and Re-Os geochronology were used to characterize the mineral system and demonstrate that mineralization was related to a Neoproterozoic magmatic-hydrothermal event that included emplacement of granitic intrusive phases and generation of a gold-bearing hydrothermal system.

The deposit is located in the Afif composite terrain of the Arabian shield. Country rocks at the deposit consist of an older volcano-sedimentary sequence intruded by a suite consisting of diorite, granodiorite, and rhyolite to dacite porphyry of the Siham group (710 + 7 Ma to 685 + 3 Ma). Gold mineralization was closely associated, in time and space, with emplacement of a younger northwest-oriented granite body (659 + 7 Ma) and petrochemically similar square quartz porphyry (SQP), and other hypabyssal and perhaps younger phase of the granite. The granite and SQP are weakly metaluminous, within-plate, post-orogenic, oxidized intrusions correlative in time with the latest stages of regional post-orogenic Haml suite magmatism. The granitic rocks at Ad Duwayhi likely represent an apophysis on the western margin of the northwest-oriented Haml batholith.

Biotitic alteration affected Siham group intrusive rocks. Sericitic alteration was coincident in time and space with gold mineralization, and produced a quartz-sericite-calcite-pyrite-rutile mineral assemblage, found both as vein fill and wall rock alteration products. Sericite from gold-bearing veins yielded reproducible though slightly reset 40Ar/39Ar ages of 611 to 604 + 1 Ma, indicating that mineralization at Ad Duwayhi was constrained to the period 659 to 604 Ma. Re-Os dating of molybdenite from gold-bearing veins produced ages of 650 + 2 and 656 + 3 Ma, thus documenting that mineralization was related to the younger granitic suite, following the emplacement of granite and SQP that are correlative with Haml suite of magmatism.

The bulk of the bedrock-hosted gold resource at Ad Duwayhi is confined to northeast-striking, southeast-dipping mineralized zones, and primarily the main vein zone (MVZ). The difference in orientation between northeast-striking mineralized zones and northwest-oriented causative granite and SQP stocks and dikes indicate hydrothermal fluids and local SQP dikes used a preexisting northeast-striking structural fabric. Mineralization is characterized by a variety of styles that crosscut all rock units. These, in a general paragenetic sequence, include (1) steeply dipping quartz-molybdenite veinlets in and near the granite stock, (2) gold-bearing quartz-vein breccia in and along the margins of the granite stock, (3) gold-bearing sheeted quartz vein networks, and (4) massive to banded gold-rich tabular quartz veins with minor molybdenite and traces of base metal sulfides. The gold-bearing sheeted and tabular veins contain the greater part of the gold resource, are more distal to the granite stock, and are associated with SQP dikes.

Gold is intimately associated with pyrite and commonly found as inclusions and late microfracture fillings in pyrite, and to some extent base metal sulfides, and sulfosalts. Molybdenite is present in tabular veins in minor to trace quantities, and most as a fine film-like bands near vein margins. Mineralized zones at Ad Duwayhi are characterized by gold:silver ratios greater than 10:1, low base-metal contents , low sulfide content, and a relatively reduced ore mineral assemblage. Gold shows an overall correlation with molybdenum.

A reassessment of similar intrusion-hosted deposits in the Arabian Shield (e.g. the Red Hill Deposit at Sukhaybarat) is warranted in light of our findings at Ad Duwayhi. Furthermore, areas of post-orogenic plutonism in the Arabian shield, which historically have been evaluated for Sn, W, Ta, and Mo resource potential, should be considered permissive for intrusion-related gold systems.

September 2003

In Pursuit of the Extraordinary:
The Quest for Understanding the How and Where of World-Class Ore Deposits
Byron R. Berger
U.S. Geological Survey
Federal Center MS964
Denver, CO 80225-0046

Despite over a decade of debate and scientific investigation, an understanding of how and where world-class porphyry Cu and epithermal Ag, Au, and base metal ore deposits form is still an elusive prize. Whether large or small and regardless of subtype or associated igneous rock composition, deposits of a general type have indistinguishable physical and chemical characteristics (cf. Whiting et al., 1993; Clark, 1995; Sillitoe, 1997). Is the search for world-class deposits merely a roll of the dice, or can we gain the needed understanding to successfully forecast the locations of as-yet undiscovered world-class deposits?

Because world-class ore deposits-i.e., the largest 10% of deposits as ordered by metal content (Singer, 1995)-dominate the supply of many metals and are, therefore, of great economic importance, the USGS is initiating a project the goals of which are to understand how these deposits formed, why they grew to such a size, and what criteria may be used to forecast their occurrence. The underlying thesis in this research project departs from the prevailing conventional wisdom amongst economic geologists. It is commonly assumed that metal-saturated fluids are formed through processes occurring within magma reservoirs that underlie ore deposits. In contrast, our perspective is that, although magma reservoir processes are important, the dissolved metal concentrations needed to make high-grade ore bodies result from processes at the site of ore deposition. Pregnant solutions are "made" through the active coupling of deformation, chemical transport, heat transport and, importantly, heat transfer. Ore bodies can be thought of as forming within mechanically-constrained thermal-chemical "reactors."

In asking the question "How does deformation couple with magmatic and hydrothermal fluid flow to result in ore deposits?" evidence will be pieced together at various spatial scales from Sumatra to Irian Jaya to the Andes to the U.S. to New Zealand. The hypothesis is that if we can understand the forces and processes at the level at which ore bodies form, then we have a chance of establishing criteria of what to look for and measure when exploring for world-class copper and gold factories.

October 2003

MINI-SYMPOSIUM Sponsored by SEG/SGA Student Chapters - CSM, and co-sponsored by DREGS

Sediment-hosted Copper Deposits, and The Role of Salt Tectonics in Mineralization

The Congolese (Katangan) Copperbelt: Impressions from the 2003 IGCP 450 Field Trip

David Broughton and Murray Hitzman Colorado School of Mines

The Congolese Copperbelt (CCB) forms the northern portion of the Central African Copperbelt, and is hosted within Neoproterozoic Katangan Supergroup metasedimentary rocks, deformed and weakly metamorphosed to chlorite facies during the circa 500 to 580 Ma Pan-African Lufilian Orogeny. It contains known reserves plus past production of approximately 100 Mt of Cu and 7 Mt of Co (Kirkham, 1989). The high Co:Cu ratio of some Congolese deposits means that, at current metal prices, they are predominantly cobalt mines (e.g. Luiswishi: 8 Mt @ 2.5% Cu, 1.1% Co). Discovery potential for new deposits is high and, with the recently revised D.R.C. mining policy, exploration companies are actively acquiring land. The July 2003 IGCP 450 conference and field trip featured visits to deposits at Tenke-Fungurume, Kambove, Shituru, Luiswishi and Kipushi, with emphasis on their regional stratigraphic setting.

In the CCB, the Katangan Supergroup is subdivided into three major stratigraphic sequences: the lowermost Roan Group, the Nguba Group (formerly Lower Kundelungu), and the Kundelungu Group (formerly Upper Kundelungu). Throughout most of the CCB the full thickness of the Roan Group is unknown, as drill holes have not penetrated to basement, and many workers interpret the Katangan sequence as allochthonous. The Roan is divided into four subgroups: R.A.T., Mines, Dipeta and Mwashia, with the Mines Subgroup host to Cu-Co mineralization.

The R.A.T. (Roches Argilo-Talqueuses) consists of poorly bedded to massive fine-grained rocks, composed of <30% quartz, 5 to 55% dolomite, up to 55% chlorite, and up to 10% hematite (Kampunzu and Cailteux, 2002). Dolomite abundance increases and hematite decreases, upsection towards the more dolomitic Mines Subgroup. Congolese geologists interpret the R.A.T. as a sequence of shallow marine dolomitic siltstones, the distal equivalent of Lower Roan first-cycle, continental arkoses and conglomerates in Zambia. In this correlation, the mineralized Mines Group is equivalent to the Ore Shale in Zambia. Exposures seen during the excursion corroborate the petrology of the R.A.T., but no diagnostic sedimentological features were seen with which to characterize its depositional environment.

The top of the R.A.T. becomes grey-white immediately below the contact with the Mines Subgroup, and may host sulfide mineralization. This change is thought to reflect a change from oxidizing to reducing depositional environment, associated with marine transgression. It may also reflect post-depositional alteration, perhaps associated with sulfide mineralization in the overlying Mines Subgroup.

Contacts between the R.A.T., Mines and Dipeta Subgroups are typically marked by unusual conglomeratic and ,Äúfriction,Äù breccias, which are not always differentiated in the existing literature. Large areas of the R.A.T. are brecciated in situ by talc-dolomite stockworks (friction breccias), possibly related to Lufilian deformation. The conglomeratic breccias contain a variety of rounded to subangular, oxidized (red) to reduced (green) intrabasinal clasts, many derived from adjacent strata, in a dolomitic -(chloritic) matrix. Where they occur within strongly foliated zones, they are themselves relatively undeformed. The conglomeratic breccias also form piercement or diapiric structures injected as high as the Nguba Group, and can be explained by salt tectonics (DeMagnee and Francois, 1989; Jackson et al, 2003). Despite this disruption of the lowermost stratigraphy, the lithologies and the contained mineralization of the R.A.T. and Mines Subgroup apparently display a remarkable consistency along 300 kilometres of strike. In contrast, the overlying sequence of Mwashia, Nguba and Kundelungu metasedimentary rocks is relatively intact and conformable.

On the regional scale, there is an empirical relationship between the breccias and orebodies: Cu-Co deposits occur within or adjacent to breccia zones, both those hosted in Mines Subgroup, and those higher in the stratigraphy (Shituru deposit in the Mwashia Group, and Zn-Pb-Cu deposits such as Kipushi in the Nguba Group). Mineralization in the Mines Subgroup consists of early-stage disseminated sulfides and bedding-parallel Cu-(Co)-bearing veinlets, and late-stage, probably late Lufilian, coarse-grained Co-Cu ,Äì dolomite veins. Mineralized ,Äúmegablocks,Äù of Mines Group are enveloped within the breccias, and disseminated mineralization is locally present within the breccia matrix. The late veins cut the breccias at Kambove and Luiswishi. The breccias therefore appear to have been emplaced during a multi-stage mineralization process, whose absolute timing remains poorly constrained.

Cailteux, J.L.H., and Kampuzu, A.B.H, 2002. Lithostratigraphic Position and Petrological Characteristics of R.A.T. (Roches Argilo Talqueues) Subgroup Sedimentary Rocks, Neoproterozoic Katangan Belt (Congo). In: Robb, L. and Montjoie, (eds), 11 ^th Quadrennial IAGOD Symposium and Geocongress 2002, Extended Abstract Volume (CD ROM).

De Magnee, I. and Francois,A. (1988); The origin of the Kipushi (Cu,Zn,Pb) Deposit in direct relation with a Proterozoic salt diapir; Copperbelt of Central Africa, Shaba, Republic of Zaire; In: Friedrich,G.H. and Herzig, P.M. (editors); Base metal sulfide deposits in sedimentary and volcanic environments. Special Publication of the Society for Geology Applied to Mineral Deposits. No. 5; pp 74-93.

Jackson, M.P.A., Warin, O.N., Woad, G.M., and Hudec, M.R., 2003. Neoproterozoic salt tectonics during the Lufilian orogeny in the Katangan copperbelt, central Africa. Geol. Soc. Amer. Bull., v. 115, n0. 3, pp 314-330.

Kirkham, R.V., 1989. Distribution, Settings and Genesis of Sediment-hosted Stratiform Copper Deposits.

In Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., (eds.), Sediment-hosted Stratiform Copper Deposits: Geological Association of Canada, Special Paper 36, p. 3-38.

MINI-SYMPOSIUM

Copper in Katanga: Salt - the Great Simplifier

Oliver Warin Geology and Management Consultant (former Sen. V.P., Expl, B.H.P.) oliverwarin@earthlink.net

An understanding of the three dimensional geometry of the huge breccia masses and overthrust Klippen within which the copper bearing rocks of the Roan subgroup occur as floating sheet-like slabs (the ,'Fish Scales' of the early Belgian authors) in Katanga Province was essentially established years ago during the initial exploration of the province by dint of detailed geological mapping and geochemical surveys. The relatively recently published maps stand as a tribute to what can be achieved by foot-slogging hard work.

While we can be sure many hours were spent in discussion and argument in the early days of exploration, no particularly convincing explanations of the extraordinary geometry were advanced.

Meantime, as the worldwide search for petroleum was carried into the offshore, particularly of the North Sea and the shelves of the North and South Atlantic, the significance and widespread nature of the salt sequences began to be appreciated; and 'salt tectonics' (the understanding of how soft, slippery, easily squeezed, buoyant salt in a sequence affects sedimentation and focuses subsequent tectonics) was born and came to its present robust teenage - perhaps almost unnoticed by most metals geologists.

The Katangan combination of huge, dominantly vertical breccia masses, regional detachments, nappes, and floating fragments strongly urges this was a salt province on the same scale as the now well known, present-day Gulf of Mexico.

The evidence of present-day provinces with 'active' salt suggests the eventual outcome of their history is to lose their salt. We can guess that Katanga was somehow frozen with this process incomplete. In other Proterozoic metal-bearing sequences, the evidence for ,Äòsalt engineering,Äô could be overprinted, much more subtle, and take a risky leap of faith - but might be illuminating in terms of predicting further metal finds.

December 2003

Overview of Characteristics and Genesis for SHSCuDs (Sediment-Hosted Stratiform Copper Deposits)

Alex C.Brown École Polytechnique de Montréal, acbrown@ polymtl.ca

Characteristics

The SHSCuD deposit-type is characterized by: (1) widespread copper zones distributed along preferred, relatively thin stratigraphic units of major sedimentary basins; (2) disseminated fine-grained sulfide ore minerals (commonly chalcocite, bornite, chalcopyrite) concentrated along locally favorable strata; (3) variable but continuous mineralization across well-defined cupriferous zones; (4) cupriferous zones positioned immediately adjacent to and on the reducing side of a distinctly visible redoxcline (a field-mappable line separating a major thickness of footwall continental redbeds from extensive marine or lacustrine greybeds); (5) unmineralized (typically pyritic) greybeds stratigraphically above and laterally beyond the mineralized zones; (6) stratigraphically conformable ore zones, closely correlating with the abundance of pre-ore sulfide (e.g., syndiagenetic pyrite); (7) a peneconformable configuration for the outer limit of the total cupriferous zone (including both economic and uneconomic mineralization); (8) other ore-stage metals, either accompanying copper (e.g., Ag, Co) or zoned immediately beyond the peneconformable outer limit of copper mineralization (e.g., Pb, Zn, Cd, Hg, Mo); (9) a zoning of metals (and corresponding sulfides) relative to the redoxcline, distributed outward from the redoxcline according to decreasing sulfophile behaviors (Cu > Pb,Zn > Fe); and (10) a low-temperature mineral paragenesis trending from early syndiagenetic (pre-ore) iron sulfide (generally pyrite) (± sulfates) to subsequent post-sedimentary (ore-stage) cupriferous sulfides (most commonly: chalcopyrite; bornite; chalcocite).

At the broader sedimentary basin-wide scale, SHSCuDs located at the redbed-greybed redoxcline transition are commonly found: (1) in close association with extensive evaporitic units (signaling basins most commonly formed 10 to 30 ^° north or south of the contemporaneous paleoequator; Kirkham, 1989), and (2) especially in or associated with continental rift basins filled with immature redbeds (signaling ages postdating oxidation of the Earth's atmosphere at about ~2.4 Ga) bimodal volcanic strata.

Genetic Considerations

The above characteristics are especially well-explained with an overprint depositional model at the deposit scale: copper and other associated post-sedimentary metals soluble in warm oxidized chloride brines hosted by (and probably generated within) the porous redbeds were overprinted on reduced greybeds immediately adjacent to the redoxcline. The reduced host greybeds were prepared for mineralization during sedimentation and early diagenesis with an abundance of pre- or syn-ore stage sulfide (either already reduced sulfur in iron sulfide, or sulfur available from biogenic reduction of sulfates in the presence of a reductant such as in situ carbonaceous matter, or more ephemeral sour gas). Some sulfur may have been introduced as sulfate with the infiltrating brine. Disseminated fine-grained sulfides precipitated in zoned arrays, with more-cupriferous sulfides proximal to the redoxcline, less-cupriferous sulfides in more distal portions of the cupriferous zone, and other still less-sulfophile sulfides (e.g., galena, sphalerite) in still more distal positions. Large volumes of metalliferous brines could have been driven across the redoxcline by sediment compaction, by basin-water recharge from rift margin highlands, or by thermal recycling resulting from the anomalous heat available in rift basins. Direct magmatic involvement appears to have been unnecessary. Continental-marine (or lacustrine) redoxcline transitions form a basin-scale stratigraphic metallotect. In addition, three global-scale metallotects guide explorationists to SHSCuD occurrences: continental rifts, formed at low paleolatitudes, in post-Archean strata.

Paradox Basin sandstone-hosted copper deposits generated by two episodes of basinal fluid expulsion

Jon P. Thorson, PhD Consulting Geologist, 5515 Nuthatch Road Parker, Colorado 80134jonthorson@rmi.net

The Paradox Basin of Colorado, Utah, Arizona, and New Mexico is a Pennsylvanian half-graben rift that initially received several thousand feet of evaporite, dolomite and black shale. Later Permian through Jurassic basin fill is predominantly continental redbeds and eolian deposits. In the Early Cretaceous the basin was buried beneath the marine transgression of the Interior Seaway, and sealed beneath several thousand feet of shale. Several periods of salt tectonics have deformed the evaporites into NW-trending diapirs that folded or pierced the overlying section.

Disseminated copper occurs as mineralization in porous and permeable sandstone units from Pennsylvanian to Early Cretaceous age. The most significant copper deposits are those at the Lisbon Valley project, San Juan County, Utah, where announced reserves are 36.7 million tons at 0.514% Cu in the Jurassic-Cretaceous Burro Canyon Formation and Early Cretaceous Dakota Sandstone. Additional measured and indicated resources add 26.5 million tons of similar grade; drill-inferred resources and additional exploration potential around the deposits bring the total mineral inventory to +100 million tons at 0.46% Cu.

Copper mineralization can be related to two sequential episodes of fluid expulsion from the basin. The earlier fluid was large volumes of sulfur-bearing, reducing, aqueous fluid that bleached red sandstones to white, cream, or light gray colors as hematite was reduced to pyrite or green reduced-iron clay minerals. This fluid rose up through the section along marginal breccia zones of the salt diapirs, or along faults, until lateral permeability was greater than vertical permeability, at which time fluids flowed laterally in permeable strata, behaving buoyantly and displacing connate fluid from the top down. At the margins of upwelling early basinal fluid plumes, fluid was neutralized by oxidized rock leaving roll-front redox boundaries. Some Paradox Basin copper occurrences containing associated bituminous hydrocarbons, uranium, and vanadium occur at these redox boundaries. The unoxidized Colorado Plateau uranium deposits may also be related to the early basinal fluid episode. Regional geology and burial history analyses suggest a late Jurassic age for the early fluid event as the black shales in the Paradox evaporites entered the oil window.

The second episode of fluid expulsion was smaller volumes of oxidized warm saline brine carrying copper, silver, and other base metals but no uranium and vanadium. At the Cashin Mine, located in western Colorado 15 air miles northeast of Lisbon Valley, Utah, and at the Lisbon Valley project, the mineralization deposited by this second fluid is zoned from chalcocite or hematite ± native copper, through bornite and chalcopyrite zones, to peripheral pyrite + lead and zinc. Copper sulfides replaced coaly material, earlier pyrite, calcite, or bituminous hydrocarbons. Gangue minerals in feeder fault structures are dolomite and barite. Homogenization temperatures from fluid inclusions in dolomite and barite are between 70 and 110 C; salinities are variable but as high as 21% NaCl equivalent. Fluid inclusions from the Cashin fault, feeder structure to the disseminated copper deposit, contain hydrocarbon droplets from a Pennsylvanian Paradox evaporite source. Fractures along the Lisbon Valley Fault are filled with solid bituminous hydrocarbons partially replaced by chalcocite. Paleomagnetic data and burial history analyses suggest an early Tertiary age for the second episode of fluid expulsion as Cutler Formation arkosic red beds subsided into the oil window.

Some Paradox Basin copper deposits, like those at Lisbon Valley and the Cashin Mine, are the result of both fluid expulsion episodes. The earlier fluid prepared host rocks that were unfavorable otherwise by reducing hematite to pyrite, removing calcite, and depositing hydrocarbons. The second fluid deposited copper sulfides that replaced pyrite and bituminous hydrocarbon.

The Polish Kupferschiefer, a giant Cu-Ag (and Au?) deposit: a general overview

Zbigniew Sawlowicz Institute of Geological Sciences, Jagiellonian University, Krakow, Poland zbyszek@ing.uj.edu.pl

The Polish Zechstein copper deposit is a part of the extensive European Kupferschiefer formation. Two mining districts in south-west Poland: "old"(closed in 1989) and "new" (opened in 1961) are situated in the North-Sudetic Basin and the Fore-Sudetic Monocline, separated by the Fore-Sudetic Block formed during the Cretaceous uplift. The "new" Lubin-Sieroszowice copper mining district in the Fore-Sudetic area is situated close to the SW paleomargin of the Polish Rotliegend Basin. Permo-Carboniferous (including Autunian volcanics) and Mesozoic deposits cover the crystalline basement. The uppermost part of the Lower Permian (Rotliegend) clastic sequence is represented by white and grey sandstones (Weissliegend). A thin (5-20 cm) bed of boundary dolomite overlying the Weissliegend sandstone is present only locally. The Kupferschiefer shale or mudstone of the Fore-Sudetic Monocline is mainly dolomitic, more rarely calcareous, rich in organic matter, and was deposited at the estimated water depths of a few tens of meters. It passes upward into Zechstein carbonates (the Zechstein Limestone). The Zechstein Limestone is overlain by four Zechstein cyclothems, Triassic sandstones, claystones, dolomites, limestones, and anhydrites, and Tertiary and Quaternary sediments.

The surface of the Lubin-Glogów mining district is 150 sq.km. Yearly output of three mines is about 27 mln tonns of ore, with 400,000 t of Cu and 1000 t of Ag. Ores are located mainly in the organic-rich and dolomitic shales (Kupferschiefer), in underlying uppermost part of the Weissliegend sandstones and in lowermost part of the overlying Werra dolomites. Thickness of ore horizon is between two and twenty meters. Forms and lithology of ore bodies vary from mine to mine.

Chalcocite is the dominant ore mineral and locally can constitute up to 90 vol.% of the rock. The copper ores are also characterized by significant amounts of bornite, chalcopyrite, digenite, covellite, galena, sphalerite, pyrite, tennantite and tetrahedrite. Ore minerals usually have xenomorphic shapes. In the mining district the following types of ore mineralization can be distinguished: disseminated microlites and small grains (predominating in the shales), nests, lenses, ore bands, veinlets and veins, and massive ores (predominating in the Weissliegend sandstones).

Adjacent to areas of high grade metal concentrations, the Kupferschiefer horizon is locally barren and oxidized. This cross-cutting secondary oxidation is referred to as the Rote Fäule facies. Sub-economic accumulations of Au and PGMs have been found recently at the transition zone between reducing and oxidizing areas.

A horizontal zoning, in the sequence dominated by Cu-Pb-Zn, is observed on a regional scale around Rote F§ule hematite areas. On the mine scale mainly a vertical type of zonation is visible with the idealized mineral distribution as follows (from bottom to top): pyrite chalcopyrite; bornite ; chalcocite- bornite –chalcopyrite – galena - sphalerite – pyrite.

Ascension of ore-bearing fluids from the Rotliegend basin (with its molasse rocks and bimodal volcanics) is commonly accepted. The main process of sulfide formation was probably bacterial sulfate reduction, possibly accompanied by thermochemical sulfate reduction. Several ore stages can be distinguished, lasting probably from early diagenesis (Zechstein) till epigenesis (Upper Jurassic?).

The origin of the Kupferschiefer deposit in Poland: facts & controversies

Zbigniew Sawlowicz Institute of Geological Sciences, Jagiellonian University, Krakow, Poland zbyszek@ing.uj.edu.pl

The discussion of the origin of the Kupferschiefer deposits in Europe, and especially the time of their formation, has divided researchers into syn- and epigeneticists. I would like to present and defend the idea that the formation of the Fore-Sudetic copper deposit was a multi-stage and long-term process. There were two significant ore stages, framing this process: (1) disseminated ore mineralization, dominating in shales, and (2) high-grade ores in the Weissliegend sandstone.

The disseminated copper sulfides can constitute up to 30-70 vol.% of the total mineralization in the organic-rich shale, where copper content can be as high as 15 wt. %. Several features suggest that this mineralization took place during early diagenesis (late Permian): (1) most of the δ³⁴S values of sulfides range from about -40 to -25‰ , indicating bacterial sulfate reduction (BSR) in an open system, closing with time; (2) in Cu-Fe-S-rich zones, iron monosulfides (precursors of pyrite) were probably replaced by copper sulfides; (3) in Cu-S-rich zones, formation of primary copper sulfide spherules and rarer framboids is observed; (4) there are specific horizons in the shale with sulfides depleted in copper, resulting probably from low-temperature leaching; (5) rhythmic copper sulfide bands in the Weissliegend sandstones, formed as the result of interaction between H2S from the overlying shale and copper ions present in the sandstone (Liesegang phenomenon), have isotopically very light sulfur; (6) sedimentological observations including: synsedimentary mineralized conglomerates in shales; truncation of well-laminated and copper-sulfide mineralized shale by non-mineralized trace fossil burrows; compaction of organic laminae around base metal cements and cementation of detrital muscovite grains.

Massive copper sulfide ores (up to 20 wt.% Cu), overlapping earlier ore types, occur in the uppermost part of the Weissliegend sandstone, especially at the “sandstone highs” (former paleodunes). These highs served as traps both for metals and sulfides. Mixed sources of sulfide can be invoked: bacterial sulfate reduction, possibly with intermediate formation of organo-sulfur compounds, and thermochemical sulfate reduction. K/Ar isotopic ages of authigenic illites, which are partly replaced by the massive ores, suggest that these ores were formed in Middle Jurassic or later.

The maximum temperature reached in the Polish Kupferschiefer, obtained using different methods, varies substantially from 70 to 140^oC, and should be evaluated with care. Most reliable data from maturation of organic matter suggest the lower end of this temperature range.

The metalliferous fluids ascended from the Rotliegend basin that provided copper and other base metals from various lithologies, including Lower Permian volcanic rocks, Carboniferous and Rotliegend sedimentary rocks, and from precipitates within the Rotliegend sediments related to hydrothermal activity and gaseous emissions. The main fluid pathways are recorded by hematite alteration of the Rote Faule, which enlarged with time, such that earlier copper sulfides were replaced and then reprecipitated as upgraded mineralization on the down-flow side of the redox boundary. Early sulfide mineralization could be related to compaction-driven upward migration of fluids containing preconcentrated leachable copper-bearing components from the upper part of the Rotliegend sediments. Convective fluid flow, responsible for the later ore stages, developed gradually from the local scale to the basin-wide scale. It caused the leaching of base metals from different rocks within the Rotliegend basin and supplied metals over a long period of time.

November 2003

Context, Characteristics and Genesis of the Salta Sediment-Hosted Stratiform Copper-Silver Mineralization, Northwestern Argentina

Alex Brown
Department of Civil, Geological and Mining Engineering
École Polytechnique, Montréal, Québec, acbrown@polymtl.ca

The Juramento sediment-hosted stratiform copper-silver deposit in the Province of Salta, northwestern Argentina, is perhaps the best known of several such copper occurrences in central South America. This particular deposit has reserves of 11 million tonnes grading 0.83 % Cu and 19 g/t Ag. Mineralization of a similar type extends into adjacent areas of Chile, Bolivia and Paraguay, forming a possible "Salta copperbelt". Although the mineralization examined to date is low grade and was complexly disturbed by Andean tectonics, the full extent of the copperbelt leaves ample ground for further exploration.

Our research, carried out as a masters level study (Durieux, 2000), considered the environments of synsedimentary host-rock deposition and its burial diagenesis. The syndiagenetic environment is that of shallowing-upward sequences on a low-gradient ramp platform, probably lacustrine rather than marine. Evaporites are minimal but are evident in nodules and single crystals of gypsum-anhydrite and their pseudomorphs in the lower Yacoraite carbonates. The host-rock is rich in organic matter and contains abundant disseminated syndiagenetic pyrite resulting from biogenic sulfate reduction.

Early diagenesis increased porosity by dissolution of primary carbonate which completed the ground preparation for subsequent base-metal influx and deposition. The ore-forming fluid is interpreted to have been an oxidized low-temperature chloride brine carrying copper, lead, zinc and silver and originating in the underlying Pirgüa continental redbeds. The metals were deposited before silicification and subsequent closure of the Yacoraite porosity by deeper burial. The ore minerals (chalcocite, bornite, chalcopyrite, tennantite-tetrahedrite, galena and sphalerite) were precipitated by replacement of syndiagenetic pyrite and gypsum. The ore metals and their sulfides are zoned, with copper-(silver)-rich sulfides more proximal to the basal Yacoraite unit and lead- and zinc-sulfides located progressively higher in the Yacoraite strata. Ore-metal emplacement clearly followed most syn-and early-diagenetic events and occurred before silicification and advanced burial diagenesis.

Sulfur isotopes are typical of syndiagenetic biogenic fractionation for pyrite; however, ore sulfides are isotopically heavier and could have been derived from in-situ sulfates (gypsum) or from an influx of sulfate with the ore fluids. Reduction of sulfate is unlikely to have occurred at this stage: there is no evidence for high-temperature inorganic reduction and biogenic reduction would have been inefficient after syndiagenetic burial. A sour-gas source of sulfide is unlikely because of the variable isotopic values.

Reference
Durieux, C.G. (2000) Diagenetic mineralization of the Juramento sediment-hosted stratiform Cu-Ag deposit,Salta district, northwestern Argentina: M.Sc.A. thesis, École Polytechnique de Montréal, 152 p.

December 2003 Technical Presentation

Paradigm Shifts in Exploration Geochemistry

Jeffrey A. Jaacks PhD.,
Managing Member, Lodestone Resources,
8493 East Foxhill Place, Centennial, CO 80112 USA
Email: j.jaacks@att.net

In the last decade, many changes have occurred within the mining industry. Companies have downsized and corporate mergers have transformed the industry. Geochemistry staffs at the larger companies have been reduced in numbers and exploration geochemical research has plummeted. The role of the geochemist within industry has evolved to accommodate these changes.

At the same time, the field of exploration geochemistry has undergone a dramatic evolution with several breakthroughs in the areas of analytical chemistry, data analysis and interpretation, and information management utilizing GIS. While the typical explorationist thinks in terms of geochemistry playing an important role in reconnaissance (through the use of stream sediment sampling) and property evaluation (using rock/soil samples), the knowledge and use of biogeochemistry, selective extractions, and soil-gas have revolutionized local-scale exploration and evaluation. In the last decade biogeochemistry has become an accepted tool within the exploration community. Introduction of ICP-MS into production laboratories, revolutionized analysis and allowed us to reexamine the area of selective extractions. Several companies have started to utilize soil-gases to explore for deposits beneath exotic cover.

As these new tools have been utilized in different terrains over mineral deposits, we are starting to observe common patterns and consistent signatures, which cause us to question the accepted beliefs for migration of metals in the secondary environment. There has been a paradigm shift within the geochemical community from thinking about geochemistry in terms of anomalies to thinking in terms of geochemical process accompanied by the utilization of pattern recognition to identify process.

Coincident patterns regardless of media type, collected at the same scale, suggests that geochemists need to examine the role of geomicrobiology and vapor geochemistry and how element migration is affected by these factors and expressed in geochemical signatures above buried ore deposits. New concepts of migration involving the role of organic compounds, biologically mediated metal migration, and nano-particle movement via carrier gases needs to be investigated. New analytical technologies need to be developed that can utilize this knowledge to develop new reconnaissance-scale tools in a cost effective manner to screen large areas quickly.

As we move into the next decade, companies seek to rejuvenate their exploration programs by exploring frontier areas or developing new ore bodies under cover but within sight of mining operations. In frontier areas, we should be developing new portable analysis instruments where the key issue is not detection of subtle anomalies, but rapid screening for anomalies within detection limit capabilities of this instrumentation. In brownfields exploration there is another frontier area looking for additional deposits often buried by exotic cover. The company that effectively utilizes selective extractions, soil-gas, and understands and incorporates pattern recognition to identify process to delineate targets will enjoy a greater degree of discovery