Metallogeny of Zn-Pb-Ag deposits in
Characteristics and genesis of the Tertiary sandstone-hosted Jinding
Zn-Pb deposit,Yunnan, China.
J. RICHARD KYLEMost Zn-Pb(±Ag) deposits are hosted by marine sedimentary
sequences and traditionally have been classified into three genetic
types based on their geologic characteristics: Mississippi Valley-type
(MVT), sedimentary exhalative (SEDEX), and sandstone-type (SST)
deposits. The origin of these deposits is considered to be an
integral process within the evolution of marine sedimentary basins,
including the development of metal-bearing saline formation waters.
In contrast, sedimentary sequences deposited in nonmarine environments,
although common on all continents and of many geologic ages, have
not been considered favorable for hosting large sediment-hosted
Zn-Pb deposits. However, the major Jinding Zn-Pb deposit occurs
in a Paleocene alluvial fan sequence within the northern Lanping-Simao
Basin that is developed in the tectonic collision zone between
the Indian plate and the Yangtze platform in southwestern China.
The Jinding mineralized zone is reported to contain over 200 million
t at 7% Zn+Pb and 5.8 g/t Ag, with 60% of the mineralization in
sandstones and the remainder in carbonate breccia and carbonate
C. E. Yager Professor of Geology
Department of Geological Sciences
The University of Texas at Austin
Austin, TX 78712-1101
Jinding is unusual because it occurs within coarse siliciclastic strata deposited along the faulted margin of a lacustrine basin. Jinding is hosted by sandstones, but those sandstones are distinctly different in depositional environment and petrologic character from those that host SST Pb deposits. Jinding is unlike those sandstone-hosted Pb deposits that originated from basinal brines, such as Laisvall and Goose Creek, as well as those sandstone-hosted Pb deposits, e.g. Yava, that have been proposed to originate from low temperature groundwater. These SST Pb deposits are hosted in basal quartz arenites that formed following episodes of prolonged tectonic stability. Also, Jinding is Zn-rich in contrast to the typical SST Pb deposits. Jinding mineralization is associated with active faulting as commonly is the case for SEDEX deposits and may be related to orogenic uplift as commonly proposed to initiate MVT mineralization. Thus, Jinding may represent a new or hybrid type of sediment-hosted Zn-Pb deposit with significant economic potential.
Existing geologic, fluid inclusion, organic geochemistry, and stable and radiogenic isotope evidence support a genetic model for the Jinding deposit involving focused discharge of metal- and petroleum-bearing brines sourced from a Mesozoic marine sequence at depth. Mixing of these heated basinal brines with shallow groundwater containing reduced sulfur from bacterial reduction of sulfate resulted in sulfide precipitation in the fan delta and streamflow-dominated fan facies.
Hot Vents, Death-beds, Earthquakes, and Impacts
Richard I. Grauch
United States Geological Survey
Denver, Colorado 80225-0046
Metal-rich black shales occur fairly frequently in the Earth's stratigraphic record. Some rare black shales are so extremely enriched that they host ore-grade concentrations of metals. The lower Cambrian of southern China and the upper Devonian of the Yukon, Canada contain metal-rich black shales that host Ni-Mo-PGE-Au occurrences and deposits. In China the ores have been mined for Mo (up to 7% Mo, ~ 4.5% cut-off, and ~ 4% Ni). The Canadian "ores" contain more Ni (up to ~ 7.4%), but much less Mo (up to ~ 0.5%). "Ore" thickness generally varies between a few millimeters and 15 centimeters (max. ~ 30 cm). The "ore" sequences are, however, laterally extensive. In China the deposits occur in a belt 1600 km long. These deposits and occurrences are temporally and spatially associated with deposits of vanadium, phosphorous, barite, zinc/lead, uranium, and stone coal.
The black shales were deposited in small, anoxic, basins strung along the continental slope of a passive margin. Thick carbonate sequences both underlie and overlie the black shales. The "ore" sequences are generally conglomeratic, but often thin, very fine-grained, layers of bedded, metal-rich sulfides occur at their base. Clasts of the thin, bedded, layers are included in the conglomerate along with abundant fossils. Apparently there is only a single "ore" sequence within any one basin. Cyclic variations in both metal content of sulfide minerals and sulfur isotope ratios suggest significant changes in the chemistry of the ore-forming fluids and that the basins were periodically open to sea water. U/Pb ages and Re/Os isotopic data indicate that the "ores" formed syndepositionally or during early diagenesis. Widespread bitumen veins indicate that burial temperatures exceeded the oil generation window.
Many hypotheses have been offered to explain the genesis of these deposits; running the gamut from boloid impact, through hydrothermal exhalative, to diagenetic. The preferred hypothesis is one involving syndepositional accumulation of metals interrupted by tectonically driven currents that not only disrupted the depositional process but also modified the water chemistry in the basins. These currents ripped up some of the early metal sulfide precipitates and redeposited them as sulfide clasts along with bioclasts. Repetition of these events resulted in the formation of a widespread "ore" sequence of relatively uniform thickness with intervening thicker channels. Burial and diagenesis may have led to some redistribution of metals but not to large scale movement of the metals out of the "ore" layers. If metals continued to be added to the system during diagenesis, their accumulation was confined to the "ore" sequence
ROSARIO DE ARACA
La Paz Department, Bolivia
A WORK IN PROGRESS
Silica/Arsenic/Gold System, Bolivian Eastern Cordillera
J. W. Lindemann, CPG
The Rosario de Araca Property is located within the Andean Eastern Cordillera some 60 kilometers southeast of La Paz, Bolivia. Historical data links both the church at Sica Sica (1549) and the San Francisco Cathedral of central La Paz (1744-1753) with the family owning/exploiting Rosario in Spanish Colonial times. Rosario is situated on the northwestern flank of the Quimsa Cruz Batholith which is an Oligocene-Miocene aged pluton that is part of the northwest/southeast trending line of intrusives that characterizes the southernmost Cordillera Real segment of the Eastern Cordillera.
The Rosario property is characterized by a northwest-southeast trending, asymmetric, west -vergent anticlinal fold developed in intercalated siltites and fine-grained quartzites of lower Paleozoic age. Within the property, the fold's east limb overrides the west limb in a high-angle, reverse fault relationship. Multiphase sulphide/gold mineralization is associated with silica flooding controlled by multi-generational, west-vergent thrust structures manifested as breccias and zones of intense silicification. Although traditionally thought of as a skarn style of mineralization, current thinking characterizes the pyrrhotite-arsenopyrite-gold mineralization as high-temperature hydrothermal possibly related to a little understood volcanic event characterized by dacitic rocks cropping out on the west flanks of the batholithic rocks.
Exploration activity has focused on the property's northern sector and consists of detailed reconnaissance, geologic mapping, shallow trench and channel sampling, and diamond drilling. The 1997 diamond drilling entailed 1260.25 meters distributed over 10 holes which served to confirm the complex breccia characteristic of the Rosario structures. Analytical data afforded by these activities addressed the coarse-gold sampling problem, gold distribution, and multielement correlation. Metallic screen, total gold analyses indicates that on average, the coarse gold fraction accounts for approximately 19% of the total gold value in any given sample. Over-all gold distribution suggests that gold mineralization is polymodal but lack of data subsets of sufficient size to assure statistical validity has prevented further characterization of the different gold modes. Neither gold nor arsenic correlate with any other element nor with each other. Gold grades range from the 100's ppb to the 10's ppm but grade continuity within the structures has yet to be established. The possibility exists that native gold present at Rosario is the product of a hypogene "event" that has overprinted an earlier sulphide/gold phase(s) mobilizing and redepositing gold in elemental form.
The Rosario de Araca property remains a viable exploration target and recommended activity entails re-logging of existing core, geologic mapping in the southern sector, and petrographic and metallographic work to more clearly define the location and mineral association of the gold. Consideration should be given to a program of limited underground development to test both grade continuity within the structures and the amenability of the contained gold to concentration and recovery.
THE ZAMBIAN COPPERBELT
A NEW LOOK AT A CLASSIC WORLD-CLASS DISTRICT
HITZMAN, Murray W.,
Department of Geology and Geological Engineering,
Colorado School of Mines,
Golden, CO 80401; firstname.lastname@example.org
Department of Geology and Geological Engineering,
Colorado School of Mines,
Golden, CO 80401; email@example.com
Cu-Co mineralization in the Zambian Copperbelt is concentrated in "ore shale" units of the Proterozoic Lower Roan Group lying above a variable thickness of metaconglomerate and arkosic metasandstone. Sulfides occur in coarser-grained layers (commonly relict beds) in weakly deformed samples and along cleavage surfaces or metamorphic quartz-(carbonate) veins developed during deformation in more highly deformed samples. Cu-Co sulfides are observed to replace euhedral to anhedral (non-framboidal), diagenetic (?)pyrite. An inverse relationship between anhydrite (or scapolite in more recrystallized rocks) and sulfide abundance in many layers suggests that sulfide sulfur was supplied in part by reduction of anhydrite.
Petrographic examination of samples from a number of mines throughout the Zambian Copperbelt indicates that the "ore shales" comprise siltstones and minor shales which now consist of quartz-muscovite-biotite-(calcite, dolomite)-(feldspar)-(scapolite) phyllites to schists. The Zambian Copperbelt shows a distinct zonation of metamorphic grade varying from lowest greenschist in the north and northeast (Mufulira and Konkola areas), to middle greenschist (central Copperbelt mines, Nchanga, Chambishi, Nkana), to amphibolite (Luanshya). The metamorphic gradients between these zones appear to be rapid suggesting that these zones may represent individual thrust sheets.
At the Chambishisi deposit, the "ore shale" horizon represents a structural boundary (decollement) between contrasting structural regimes: the underlying undeformed or weakly folded footwall rocks and overlying moderately to tightly folded metasediments. Similar relationships have been observed in several of the other Zambian Copperbelt deposits. The "ore shale" in these deformation zones is commonly much more highly recrystallized than underlying and overlying rocks and displays well-developed shear textures both megascopically and microscopically. These zones generally contain coarse-grained Cu-Co sulfides and enhanced Cu-Co grades relative to surrounding "ore shale." The texture and grade of much of the "ore shale", together with sulfur isotopic values for the sulfides, indicate that sulfide dissolution and reprecipitation during metamorphism helped to produce the high-grade zones within the Copperbelt deposits. Such sulfide dissolution during metamorphism, which has rarely been documented in other areas, may have occurred in the Copperbelt because of the high salinity of metamorphic fluids. Such high salinities could have been produced by the dissolution of evaporites which are much more common within the Proterozoic sequence than is commonly recognized. The evaporite horizons may also have formed planes of weakness along which low-angle faulting took place.
This work suggests that new deposits may be found in stacked thrust repeats of "ore shale" units along decollement zones and within other stratigraphic units of the Roan Group which have undergone intense shearing. The work also hints that the Copperbelt may be allochthonous.
The Geomorphology of the Central African Plateau and its Effect on Supergene Processes in the Copperbelt area, Zambia
Engineering Dynamics, Inc.
Englewood, CO. 80110
The central African plateau is a recently uplifted peneplain which has undergone relatively little dissection. The Copperbelt, and the Proterozoic basin in which it resides, suffered extreme supergene alteration effects prior to uplift and most of these have been preserved.
These effects make exploration procedures both difficult and different. We will go through all stages of exploration and discovery of a modest sized deposit, and get into the question of supergene versus primary chalcocite in this and other deposits..
And, time permitting, we will give a brief description of how exploration in this area is actually beneficial to the environmental cause. Are you familiar with Chaetura Boehmi?
THE PALLACOS OF CERRO RICO DE POTOSI,
A NEW DEPOSIT TYPE
Paul J. Bartos
Manger - Latin American Exploration Division
274 Union Blvd., Suite 450
Lakewood, CO 80228
Manger - Latin American Exploration Division
274 Union Blvd., Suite 450
Lakewood, CO 80228
Cerro Rico de Potosi is the world's largest silver deposit, having produced between 1 and 2 billion ounces. The Cerro Rico deposit is associated with a subvolcanic dacite porphyry dome which intruded a maar complex. The maar is situated on the ring fault of an earlier, unrelated caldera. The Cerro Rico dacite porphyry is a single stage, near-surface intrusion, which does not appear to have vented. It is intensely advance argillically altered, with a thick layer of vuggy silica at the present day summit. Overlying the vuggy silica at the uppermost 10-20 meters of the mountain is a zone of pervasive fine grained silicification (jasperoid). Both vuggy silica and jasperoid contain disseminated silver mineralization, which is the source of the silver in the pallaco deposits.
Pallacos are silver-rich gravels which were deposited on the flanks of Cerro Rico. There are six major pallaco deposits scattered radially about Cerro Rico, as well as several other lesser deposits. Total tonnage of pallaco material is estimated to be in excess of 80 million tonnes. On the order of 23-30 million tonnes of pallaco are believed sufficiently mineralized so as to be potentially mineable. Present reserves are estimated at 21 million tonnes grading 3 1/2 oz Ag/ton; screening this to - 8 mesh and discarding the fines would yield 12 million tonnes averaging 5 oz Ag/ton, or over 67 million ounces. Exploration and evaluation are continuing. The majority of the present reserves are contained in the Huacajchi deposit, with additional resources in Santa Rita and Diablo.
There are two distinctly different types of pallacos, each with a different origin: the Huacajchi type and the Potosina type. Huacajchi type pallacos occur adjacent to Cerro Rico and contain 30- 80% coarse (> 5 cm), poorly sorted, subangular to subround clasts in a silt to clay matrix. The clasts are composed dominantly of vuggy silica altered dacite porphyry. Clast diameters locally exceed 1 meter, but 10-40 centimeters is far more typical. Laminar bedding, channeling, crude stratification and internal graded bedding indicate multiple depositional events. Huacajchi type pallacos are interpreted to be proximal debris flows caused by the transport of talus by intense rainfall.
Potosina type pallacos occur distal to Cerro Rico, and contain huge blocks of dacite porphyry and sands or tuffs of the Carocoles Formation which have been pervasively silicified to jasperoid. The boulders and blocks are subround to subangular, reach up to 20 meters in diameter and are chaotically set in a matrix of red clay or tuffaceous material. The alteration and texture of the dacite porphyry boulders contained in Potosina type pallacos are identical to that seen in the upper 10-20 meters of the Cerro Rico; it is believed these boulders came from a higher level of formation in the Cerro Rico hydrothermal system than the clasts in Huacajchi type pallacos. Sedimentary bedforms are absent in Potosina type pallacos; a single, violent depositional event is indicated. Potosina type pallacos are inferred to be of laharic origin.
There are few known analogs to the pallaco deposits, but others ought to exist.
Epithermal gold deposits: characteristics, classes and causes
Dr. Noel White
SEG International Exchange Lecturer
Reviews the characteristics of different styles of epithermal gold deposits and highlights the significance of their differences.
While the term epithermal is widely used to describe gold deposits, there remains widespread confusion about what it really means. I review the characteristics of epithermal gold deposits, highlighting and illustrating the most important differences between the two major classes and outlining their causes. But while there is good understanding of many aspects of these deposits, there remains much that is not known or understood, including some of the most fundamental questions. Also, it is certain that the umbrella term hides far more subtleties and differences than is recognized in subdividing them into two major classes. These deposits will remain a fruitful area for research for a long time to come.
Volcanic domes and gold mineralization at the Pueblo Viejo Deposit, Dominican Republic
Carl E. Nelson
Consulting Geologist & President of Recursos del Caribe, S.A
Geologic mapping at the Pueblo Viejo gold deposit in the Dominican Republic reveals a series of early Cretaceous volcanic domes that are spatially related to sulfidic silicification and gold mineralization. These domes drove separate but overlapping hydrothermal cells that together deposited over 40 million ounces of gold, 240 million ounces of silver, 3 million tonnes of zinc, and 0.4 million tonnes of copper. A total of 5.3 million ounces of gold and 24.4 million ounces of silver have been recovered from five open pits. (1975 - 1996).
In the Moore deposit, a dacite porphyry dome intrudes hydrovolcanic dacite vent breccia and carbonaceous epiclastic sediments. In the Monte Negro deposit, crumble breccias of mixed pyroclastic and epiclastic origin mantle domes of basaltic andesite porphyry. Hydrothermal alteration and gold mineralization were coeval with dome emplacement.
All rock units at Pueblo Viejo are cut by regional low angle thrust faults that formed during mid-Cretaceous obduction of oceanic lithosphere. Thrust faulting caused folding and imposed a low-grade metamorphic fabric on the Los Ranchos Formation. Metamorphic grade increases across the deposit towards the northwest-striking Hatillo thrust. Thrust faults were reactivated in post Early Oligocene time.
GENESIS OF HIGH-GRADE CARLIN TYPE GOLD ORE THROUGH THE SUPERPOSITION OF SUCCESSIVE HYDROTHERMAL, IGNEOUS, AND TECTONIC EVENTS AT THE MEIKLE DEPOSIT, CARLIN TREND NEVADA
U. S. Geological Survey, MS-973, Box 25046, Denver, CO 80225
The Meikle Mine, northern Carlin trend, Nevada, is the world's highest grade Carlin-type Au deposit. The ~5.3 million oz deposit averages 0.78 oz/ton and locally has grades greater than 10 oz/ton Au. In 1998 the mine produced 847,313 oz of gold at an average cost of $77/oz, also making it one of the lowest cost Au producers on the Carlin trend. Although Meikle averages an order of magnitude higher gold grade, its ore has all of the isotopic, geochemical, and petrographic characteristics of lower-grade ore in more typical Carlin-type deposits. In order to explain the geologic and geochemical processes responsible for its extraordinary Au content, it is necessary to understand a protracted sequence of geologic and hydrothermal events that span Devonian through mid-Tertiary time.
This study documents four successive hydrothermal events that are temporally, mineralogically, geochemically, and isotopically distinct. Devonian auriferous H2S-rich brines, discharged to the seafloor through synsedimentary faults along flanks of subsiding continent-margin basins, deposited barite, minor base metals, Ag, and Au in the Devonian strata of the Roberts Mountains and Popovich Formations. At the Meikle mine, these brines pervasively dolomitized the underlying Roberts Mountains Formation in and along the vertical faults which were loci of major venting, also depositing the above metals. The products of this earlier event is cut by a series of Upper Jurassic dikes. Minor hydrothermal activity, associated with these intrusions, deposited base and precious metals in quartz veins hosted in the Goldstrike Stock. This study constrains Carlin-type mineralization to mid-Tertiary time. Finally, a Au-deficient lower temperature hydrothermal event as young as 2 Ma caused extensive, post-ore dissolution of carbonate, collapse brecciation, and precipitation of calcite and barite in resulting cavities.
These geologic events converged at the Meikle mine to produce spectacular Au grades. Prior to Carlin-type mineralization, the Devonian syndepositional hydrothermal system altered the Roberts Mountains Fm. to Fe-rich dolomite, creating an ideal host rock for the later Carlin-type fluid. These rocks were subsequently brecciated by faulting, and perhaps by the intrusion of the Late Jurassic dikes. Resulting permeability focused still later, H2S- and Au-rich, weakly acidic Carlin- type ore fluids into these rocks, where it reacted with the Fe-rich dolomite. Au-rich pyrite formed by dissolution of the dolomite and sulfidation of its contained Fe to produce Carlin-type ore. Continued removal of reactive host rock by these acidic fluids further upgraded the Au content of the ore. In places, ore stage pyrite and other insoluble minerals accumulated as cave-fill to form exceptionally high grade Au ore. Locally, the ores were modified by young post-ore dissolution and collapse brecciation.
As Economic Geologists, Where Are We Now?
Geoffrey G. Snow
Consulting Economic Geologist
Allan P. Juhas
Consulting Economic Geologist
We will attempt to characterize the situation at the end of the 1900's and provide our view of how and why we got where we are. Offering a retrospective is easy. Forecasting accurately is impossible; however we do see some trends that may accelerate in the future.
The situation in which we scientists in the business of economic geology find ourselves is not wholly of our own doing, but rather reflects influences from the environment in which we work (or wish to work). The environment is comprised of the global economy; the behavior, demands, and constraints of society in the developed and developing world; the mining industry (our customer); and the exploration business itself.
In the developed world, dollar (in real terms) and manpower resources devoted to metals exploration peaked in 1982 and has with a blip in 1987, declined since then. This, notwithstanding, the huge expansion of exploration in the developing world.
There are four primary causes for the decline in the developed world: 1. Our societies have evolved from industrial to information and services; 2 The rate of growth in demand for most metals has declined; 3. The public attitude and governmental policy toward the resource industries have changed for the worse at an accelerating rate since 1900; and 4. The behavior of the mining industry has changed from maximizing dividends and ensuring company survival to lottery-like conduct. These factors, attitudes, and policies adversely affect the economic geologists in industry, government and universities.
Countering the decline in the developed world has been a dramatic shift to foreign exploration owing to the push of the above factors and the pull exerted by under-explored countries where socialism has given way to market-driven economies. It is obvious that the dominant factor affecting mining and exploration is and will continue to be globalization. As in the past, it can be expected to reduce metal prices in real terms. It will also reduce the need for scientific exploration and reduce opportunities for most geologists from the developed countries.
We are at the end of a century that has seen terrific benefits of science and technology to society, only to have 44% of the U. S. public believe God created humans in the last 10,000 years. We see a society consumed by consumerism at the same time they object to mine development. Mining companies who mined low grades in times of high price and saved high grade for bad times, and who between 1957 and 1982 invested in exploration research and in training geologists have all but abandoned exploration and are exploiting their deposits at alarming rates to please fund managers.
Can the projected, pent-up demand of about two-thirds of the
world population be the salvation of the mining and exploration
business? It might, but up to now exploration successes have more
than kept up with demand, as is proven by low metal prices. To
their detriment, economic geologists in the developed world will
also notice the increasing tendency for international companies
to hire local professional staff.
Where will be the repository of expertise gained from the heyday of exploration? From whence will come the economic geologists who must find the deposits needed to satisfy any new demand? Who will train and mentor them? We can only guess.