THE YANACOCHA EPITHERMAL GOLD DEPOSITS, NORTHERN PERU: HIGH-SULFIDATION MINERALIZATION IN A FLOW DOME SETTING
Stephen J. Turner
Colorado School of Mines and Newmont Exploration
Yanacocha is a major, recently developed gold district at an elevation of 3,500 to 4,200 m (11,500 to 13,800 feet) in the Andean mountains of northern Peru. The district includes 3 operating heap leach Au mines with 1.75 Mozs of production in the past 3 years, and at least 5 undeveloped deposits with a total oxide Au reserve and resource of 11.25 Mozs. The Carachugo, Maqui Maqui and San Jose Sur deposits are being mined by a joint venture of Newmont Peru Limited (51 %) Buenaventura (44 %) and the IFC (5%). Despite low average grades of 1 to 2 g/t Au, production costs are very low, with cash costs of $107/oz in the third quarter of 1996. Run of mine ore is trucked directly to leach pads, with no crushing required.
The Yanacocha district has a long history of prospecting and mining activity, beginning with extensive shallow Inca and pre-Inca mining of cinnabar, and continued during Spanish colonization. Minor Au was recovered from the Carachugo area. Exploration between 1968 and 1973 by a Japanese company and St. Joe Minerals was directed at porphyry and base metal potential. BRGM followed up conventional stream sediment Pb and Zn anomalies at Yanacocha Norte, and subsequently defined a small Ag resource. In 1983 Newmont joint ventured the property and focussed exploration on the Au potential.
The Au deposits are hosted by the Yanacocha Volcanic Complex (YVC), a recently recognized, mid- to late Miocene flow dome field with associated pyroclastic rocks and minor lacustrine sedimentary rocks. This sequence unconformably overlies deformed, peneplained Cretaceous sedimentary rocks, mainly limestone, shales and quartzite; and to the southwest overlies and intrudes Lower Tertiary volcanic rocks of the Llama Formation. Hornblende andesitic fows and tuffs of the Regalado Volcanics were erupted during the waning stages of hydrothermal activity. Post-mineral ashflow tuffs of the Huambos Formation occur on the perimeter of the YVC, with minor coeval rhyodacitic intrusions in the Yanacocha Lake area.
The YVC is extensively and pervasively altered which hindered previous efforts to map the rocks. Exogenous and endogenous flow domes were emplaced into precursor tuffs, forming coalescing domes. The dome field encompasses an area approximately 19 X 7 km, which was intensely altered within less than 1 Ma of dome emplacement. Acid-sulfate alteration occurs as multiple coalescing centers which broadly correspond to the flow domes. This alteration is zoned with central massive to vuggy silica, succeeded outwardly by silica - alunite, silica - kaolinite (alunite) and mixed- layer clays (kaolinite - pynte). Least altered rocks comprise an assemblage of quartz - sericite (illite) albite ± chlorite and secondary magnetite, rather than propylitic alteration. The hypogene alteration is otherwise typical of many acid-sulfate deposits, but differs in the immense volume of altered rock, and in the amount of massive silic
Main-stage alteration was a precursor to the disseminated Au mineralization, which occurs in more siliceous alteration, and as a lower grade aureole in silica - alunite alteration. In most deposits the Au is not visible. In the sulfide zone early quartz - pyrite alteration is overprinted by a high-sulfidation assemblage of enargite, pyrite, covellite, digenite with minor sphalerite and galena. These occur in leached vugs, irregular fractures and breccia matrices. The Au is associated with Ag-bearing enargite and minor arsenian pyrite. In the oxide zone the Au is dissociated from the Cu - As - Ag assemblage, and the Cu is leached. Gold occurs in Fe oxides whereas Ag is incorporated into argentian jarosite, and As occurs in scorodite and combines with Pb in beudantite, a Pb - As sulfate.
Several processes control and constrain the Au mineralization to discrete deposits within a much larger aureole of alteration. Structure is an important, but subtle control which varies in scale from the ENE trend of the YVC, to strike-slip movement on conjugate NE and NW structures which developed local zones of dilation. At the San Jose Sur deposit, a lithological control is recognized with a tabular body of Au mineralization formed at the contact between several tuff horizons and underlying rocks of a flow dome. In several deposits the Au mineralization is spatially linked with late, intra-mineral, phreatomagmatic diatreme breccias, and associated dacitic stocks and dikes. These breccias formed within and adjacent to silicified rocks with marginal fracturing and crackle-type brecciation which host Au mineralization. Carbonate-bearing argillic alteration of the matrix and clasts in the diatreme breccia, and associated dacitic intrusive rocks was caused by neutral pH fluids, which may also be linked to the Au mineralization.
NAMBIA: A KEY PLAYER IN SOUTHERN AFRICA'S MINERAL BOOM
By Brian G.Hoal
Consultant. Corrotoman Geoscience
Africa's annual mineral production is valued at US$45 billion,with more than two-thirds being contributed by southern Africa. Of the more than 400 publicly declared exploration and development projects in Africa, the majority are situated in southern rather than West Africa. Namibia, with its mineral potential and stable political and fiscal regime, provides mining companies with the basis for long-term investments carrying good returns. As recently as 1990, uranium and gem diamond production made up 9.5 percent and 7 percent, respectively, of global production. In addition, gold, copper, silver, lead, zinc, pyrite, fluorspar, and salt make up the most valuable of about 30 different commodities produced by 43 active mines Export of minerals is facilitated by one of Africa's best transport and communication infrastructures.
The importance of mining (15 percent of GNP and 43 percent of exports in 1994) in Namibia is recognized by the government in the Minerals (Prospecting and Mining) Act of 1992, which stresses the principles of security of tenure, effective exploration and exploitation of the country's mineral resources, and due concern for the environ ment. No provision for the compulsory acquisition by government of any interest in mining property or any other mineral operation is made. Direct taxes are profit based and calculated on a sliding scale (generally, 25 percent to 40 percent), royalty payments of 5 percent being levied only on commodities that would increase in value through local beneficiation. Deductible allowances include exploration and development expentiture. while exploration expenditure in not "ringfenced." The effective tax rate for foreign investors is 10 percent of declared dividends.
Exploration expenditure has risen from N$71 (US$47) million during the period 1981-85 to N$229 (US$76) million for the period 1991-95. Despite this dramatic rise in expenditure, Namibia is still emerging from its legacy of under-exploration prior to independence from South Africa in 1990 . Exploration cost/production Value ratios have been historically less than 1 percent, far below the optimum 5 percent to 10 percent cited by the World Bank. Such under-exploration must be seen in the context of a country that covers an area twice the size of Califomia,with 46 percent bedrock exposure and a host of economic mineral deposits. It should further be realized that exploration to date has conducted predominantly in areas of surface showings. This trend has changed significantfy with the government's release of high-resolution airborne geophysical data over 130.000 km2 of prospective ground. These geophysicaI surveys are ongoing and should receive continued financial support from the European Union. With increased exploration in covered areas there is significant potential for finding additional economic deposits, such as the gold skarns at Navachab (11 mt at 2.4 g/t Au), polymetallic paleokarst breccias at Tsumeb (932 mt at 4 percent Cu, 9 percent Pb, 3 percent Zn, 98 g/t Ag), and stratabound base metals at Rosh Pinah (23 mt at 7 percent Zn, 2 percent Ph). This potential for new mineraI discoveries is enhanced by the influx of innovative junior companies (mostly Australian), accessible file grant reports, and streamlined mineral-licensing procedures.
SEDIMENT-HOSTED Zn.Pb AND Au DEPOSITS IN THE PROTEROZOIC PARACATU VAZANTE FOLD BELT, MINAS GERAIS, BRAZIL
Murray W. Hitzman
Department of Geology and Geological Engineering,
Colorado School of Mines
The late Precambrian (Brasiliano) fold-and-thrust belt on the western margin of the Sao Francisco craton contains the Morro Agudo zinc-lead sulfide, Vazante zinc silicate, and Morro do Ouro gold deposits. The Zn-Pb deposits are hosted in carbonate rocks and the gold deposit is hosted in siliclastic rocks in a transitional setting between the Bambui carbonate platform to the east and a siliclastic basin to the west. Morro Agudo is in detrital carbonate rocks in the hanging wall of a normal fault system and is interpreted as an Irish-style (syndiagenetic replacement) deposit. Vazante is an unusual zinc silicate (willemite-Zn2Si04 deposit in platformal carbonate rocks stratigraphically below Morro Agudo. Morro do Ouro is in siliceous phyllites along a low-angle fault and is interpreted as a deformed sediment-hosted (Carlin type) gold deposit.
The Morro Agudo deposit (17 million tonnes of S 14 percent Zn and 1.53 percent Pb) occurs within the hanging wall of a synsedimentary normal fault system that controls local carbonate facies distribution. The deposit consists of a series of stacked, stratiform sulfide (sphalerite-galena) lenses. Sulfides are concentrated in dolomitized grainstone units which had greater porosity and permeability than other units in the local stratigraphic section. Mineralization occurred largely by progressive replacement of diagenetically modified coated grains and resulted in the preservation of original sedimentary textures; minor late mineralization resulted in wall rock dissolution and infilling by colloform sphalerite and coarse-grained galena-dolomite-pyrite. The entire deposit is zoned vertically with sphalerite and galena-rich zones deep in the system, progressively changing to sphalerite-pyrite-enriched zones at shallower levels. The uppermost sulfide lens contains early, replacive silica-hematite ("iron formation"), which is cut and replaced by iron sulfide and then sphalerite galena. Individual sulfide lenses also are zoned, decreasing in grade upward and basinward away from normal "feeder" faults. Sulfide distribution and textures indicate that mineralization at Morro Agudo predated the Brasiliano deformation event. Although Morro Agudo has been classified as a Mississippi Valley-type deposit, the sulfide textures, relationship of sulfide lenses to faults, and paragenesis indicate that it is an Irish-type (syndiagenetic replacement) deposit that formed within several million years of host-rock sedimentation. Like the Irish deposits, Morro Agudo formed at temperatures between 100° and 250°C. Sulfur isotopic studies suggest that metal deposition at Morro Agudo, like the Irish deposits, is the result of the mixing of a sulfur-rich, metal-poor fluid (seawater) with a metal-rich, sufur-poor (basinal) fluid.
The Vazante deposit (8 million tonnes of 23 percent Zn) occurs within a northeast-trending, 40- to 70-meter-wide fault zone cutting platformal carbonate sediments that are approximately 700 meters stratigraphically below the sediments hosting the Morro Agudo deposit. Mineralization at Vazante forms a series of centimeter-to+5-meter-wide veins within the fault zone which have a lateral extent of more than 5 kilometers and a vertical extent in excess of 500 meters. The fault zone has a sharp lower contact and a complexly broken and brecciated hanging wall. The overall offset of units across the fault indicates normal movement and it is probable that the fault was initiated at the same time as the normal faults at Morro Agudo. Minor kinematic indicators within the Vazante fault zone display, however, clear evidence of reverse movement, suggesting reactivation of the fault during Brasiliano compression. The host rock at the time of normal faulting and mineralization was thoroughly cemented and hydrothermal minerals formed either by wholesale wall rock replacement or infilling of fractures. The veins at Vazante contain willemite, hematite, quartz, and sphalerite, with trace amounts of galena, apatite, and barite. Willemite and sphalerite appear to have co-precipitated. Ferroan dolomitization of the host platform dolostones extends as far as 300 meters from the veins in the hanging wall of the fault. Similar alteration is restricted to within 25 meters of the footwall of the veins. The reasons for the dominant precipitation of willemite rather than sphalerite is still unclear. The abundance of hematite and silica intergrown with zinc minerals suggest however that mineralization involved an ore fluid with a relatively high oxygen fugacity that underwent rapid cooling.
The Morro do Ouro deposit (42 million tonnes of 0.65 g/t Au) occurs within a deformed and weakly metamorphosed sequence of carbonaceous siltstones, shales, and impure sandstone. The sediments are currently graphitic phyllites and argillaceous quartzites. The deposit is located above a low-angle fault that separates these siliclastic host rocks of the Paracatu Formation from underlying carbonates of the Vazante Formation. Gold occurs as 1- to 800-micron particles associated with quartz boudins and disseminated pyrite and arsenopyrite within the phyllite. Detailed mapping of quartz boudins, ranging in size from several millimeters to approximately 15 centimeters, suggests that they represent quartz-sulfide veins and veinlets, which were disrupted during metamorphism and low-angle faulting. The deposit thus formed prior to Brasiliano deformation and consisted of a quartz-sulfide vein stockwork with weak wall-rock alteration. The source of hydrothermal fluids is not known but was probably related to a deep-seated magmatic center that is now separated from the sedimentary host rocks owing to faulting. The host rocks, grade, metal association(Au-As), and restriction of gold to quartz veins and silicified zones suggest that Morro do 0uro is a low-to moderate-temperature sediment-hosted gold deposit similar in many respects to the Carlin deposits of the western United States.
THE REMARKABLE MINERAL BELTS OF PERU
Richard L. Nielsen
Consulting Geologist, Golden, Colorado
More than 100 companies are actively exploring and developing mines in Peru. The Peruvian Government estimates that US$200 million will be spent on exploration and more than US$500 million on development projects in 1997. Two large world-class gold developments are Yanacocha (Newmont/Buenaventura) and Pierine (Barrick). Major copper projects are Cerro Verde (Cyprus), Titanya (BHP), Quellaveco (Anglo American), Antamina (Rio Algom/Inmet), Toquepala/Cuajone (Southern-Peru/ASARCO), La Grenja (Cambior), Canariaco (Placer Dome), and Cerro Corona (Rennison). Several lead-zinc-silver deposits are being mined by moderate-sized Peruvian companies, but significant funding for exploration and development comes from Doe Run, Cominco, Teck, and Phelps Dodge.
Exploration costs in Peru, about twice the level of those in North America, have encouraged many large companies to reduce levels of generative exploration and rely on junior companies to define prospects. Peru definitely is the current "hot" exploration area: The government is pro-development: Peru has a history and culture that includes mining; laws and regulations are reasonable; increasingly, companies that provide assays, drill services, helicopter support, and other services are multiplying; and, most important, geologic setting is highly favorable for discovery of world class deposits. The last point will be the principal focus of this talk.
Geologic setting of Peru is complex but it is clear that most precious metal and base-metal mineralization is associated with subduction-driven magmatic arc activity of Mesozoic and Cenozoic age that helped form the Andean mountain chain. Host rocks, which are intruded and covered by igneous rocks, consist of accreted crystalline Precambrian blocks, Paleozoic marine sediments, strongly folded and intruded by late Paleozoic plutons, and a metallogenetically important sequence of Jurassic and Cretaceous marine sedimentary rocks that exhibit a transition from eugeosynclinal to miogeosynclinal to shelf facies, from west to east. Magnificent folds and thrust faults deformed these rocks shortly before intrusion of late Cretaceous and Tertiary igneous rocks. At least two periods of deformation during Tertiary time elevated the mineral belts, subsequent erosion produced exposures of mineralization and served to produce supergene copper enrichment in some areas.
A remarkable and potentially useful feature is the presence of well-defined and coherent mineral belts associated with belts of magmatic activity:
- Submarine andesites and basalts of Jurassic age along the
present coastal margin of Peru have copper-pyrite massive sulfide
deposits and iron-rich copper skarns and iron-rich replacements.
- The coastal batholith of late Cretaceous to early Tertiary
age, granodiorite-quartz monzonite-granite in composition, is
host for relatively small gold-copper-quartz veins and small
disseminated and skarn gold-copper deposits.
- Largest and most important porphyry copper deposits are associated
with Eocene Caldera batholith (51-58 m.y.) in the southwest,
from Chile to north of Arequipa. Distal gold veins and breccia
pipes are present near the porphyries.
- A belt of important copper-gold-zinc-iron skarns is associated
with the Oligocene (21- 38 m.y.) Abancay batholith in the south
part of the country. Mineralization is confined to skarns developed
in mid-Cretaceous limestones; igneous rocks are altered but contain
only weak traces of sulfides.
The most extensive and continuous belt of mineralization follows the crest of the Andes from the Ecuador border to south of Lima, and continues as broken segments southeasterly into Chile. This belt, as wide as tens of kilometers and more than 1600 kilometers long, contains deposits that generally yield dates of 8 to 15 m.y., and is the site of many recent discoveries and development projects. Porphyry copper-gold deposits are satellites to the Miocene Pomahuaca batholith, exposed from Cajamarca north to Ecuador. World-class disseminated gold in high-level acid-sulfate systems and polymetallic veins and manto are present in andesite and dacite volcanics of Miocene age.
Precious metal and polymetallic ores of the Miocene belt are rich in arsenic and antimony, which suggests regional enrichment in those elements. A second unusual feature is the close aerial association of porphyry copper and high-level gold systems. These metallogenic features are thought to be an artifact of strong anisotropy of Peruvian lithologic patterns and complex intrusive history. The basal unit of upper Jurassic-Cretaceous sedimentary sequence of the central Peru basin contains dark, organic-rich, pyritic shale, argillite and quartzite of the Chimu Formation. This unit may be geochemically anomalous in arsenic, antimony, zinc, silver, and gold, as are many black shales, and this may be a source for the metals. Miocene Peruvian porphyry copper deposits are in plutons that moved up from deep crustal levels, while nearby gold and polymetallic ores with high levels of arsenic and antimony may be associated with igneous rocks that have experienced a complex intrusive history of reactions and assimilation with the Chimu Formation at moderated crustal levels before final emplacement at high levels and subsequent hydrothermal circulation.
PRECAMBRIAN GOLD DEPOSITS - THEIR ORIGIN, AGE AND DISTRIBUTION
By Charles H. Thorman
CTGS International, Inc., Denver, CO.
Change in the Brazilian Constitution in 1995 has been the impetus for the latest Brazilian gold rush; it again allowed foreign majority ownership in mining companies. This is a reversal of the 1988 constitutional nationalistic move that restricted foreign ownership to a minority position. The political climate is good, taxation is equal to or better than other Latin American countries, and Brazil's infrastructure is above average.
From a geologic standpoint, our knowledge of Brazil's Precambrian terrane is more or less akin to what was known about Canada in the 1950's or early 1960's. Little detailed modern mapping has been done and no gold deposit has been thoroughly studied, though several are the subject of considerable thesis studies and much is coming from these efforts. Many questions remain regarding the age of mineralization, type of mineralization, etc. of these deposits. Is what we have at our disposal adequate to make projections and keep our spirits up? I certainly think so, and apparently, so do many explorationists and investors.
Gold production in Brazil by mining companies increased threefold in the past decade (seven major new mines being put into production) and totaled more than 10 million ounces from 1982 to 1995. Ninety three percent (93%) of this production was derived from 17 mines. However, garimpo production during the past 10 years has declined more than threefold. Garimpo production is considered to have exceeded 15,000,000 ounces (official) and may well have exceeded 24,000,000 ounces (estimated). Production peaked in the late 1980's and now is in a strong decline, in large part because the more readily mined alluvial, colluvial, and deeply weathered bedrock the garimpeiros can work is giving out.
So, if you want to work in Brazil, where do you go? To the Amazon jungle? The Amazon has been a major focus for most newcomers as well as established Brazilian companies because of the impressive garimpeiro gold production. Pará and Mato Grosso states produced about 12,400,000 and 6,124,000 ounces, respectively (±50% and ±25%, respectively, of the total garimpeiro production) from 1982 to 1995. Does one go to the garimpos to try to deal with the garimpeiros, who are becoming more and more sophisticated and are willing to deal with juniors and majors as mining operations for the garimpeiro become more and more difficult? Does one try to buy into the land that has been tied up by the mad rush the past few years in staking every tiny gold show in and around the major and minor garimpos? Does one try to JV or buy property that has bona fide hard data to show there is actually something worth looking at? Or, does one undertake a grass roots program? Is it viable to chase the garimpeiro activity? Obviously, many companies and investors think so. Numerous major and junior companies hold millions of hectares adjacent to garimpos. The cost of retaining large blocks for exploration/speculation greatly increased at the beginning of 1997 and is going to be a positive stimulus for exploration. Exploration costs, etc. are about double those outside the jungle.
Archean and Proterozoic greenstone belts occupy only 1.3% of Brazil, yet they have been the backbone of Brazil gold production, accounting for 49% in the past 14 years. Though the area is small, it still holds the highest potential for major deposits. However, these belts are largely staked and the likelihood of getting ground here is through an acquisition or JV. Proterozoic cover rocks are preserved in fold belts that surround Archean nuclei and comprise about 7% of Brazil. Morro do Ouro, the only Proterozoic cover rock-hosted deposit, has produced 13% of Brazil's gold in the past 14 years.
Where are the new, big deposits going to be found? Greenstone and sedimentary-rock hosted deposits have large reserves and production, whereas only one shear zone deposit (Fazenda Brasileiro) and one quartz vein deposit (Novo Astro) have important production records. Serra Leste (CVRD), the eastern underground extension of Serra Pelada, will probably be a ±5,000,000 ounce deposit in a greenstone setting and Igarapé Bahia may prove to be one of the largest in Brazil. To date, the best terrane to explore for major gold deposits appears to be: 1) Archean-Early Proterozoic greenstone belts (with some iron formation/chemical rock affinity); 2) shear zone-quartz vein settings in the Amazon craton; and 3) Late Proterozoic siliciclastic Proterozoic terranes that are weakly to moderately deformed.
METALLOGENY OF THE GUIANA SHIELD
Consulting Economic Geologist, Minerals Evaluation
The Guiana Shield is one of the great crystalline Precambrian cratons of the Earth, covering about l million square kilometers between the Amazon and Orinoco rivers. The shield covers parts of six countries and is mostly covered by tropical rain forest. The crystalline area is very sparsely populated and is incompletely explored both geologically and geographically.
The Imataca Complex of undoubted Archean age(+300 Ma) and various gneiss and granulite terranes of possible Archean parentage are the oldest rocks in the shield. A widespread granite-greenstone terrane, dated at 2700-2000 Ma, underlies much of the north and east, and contains most of the lode gold deposits. This terrane is succeeded by a variety of weakly metamorphosed volcanic and sedimentary units in the 2000-1700 Ma range. The youngest Precambrian unit is the Roraima Group of fluvial clastics, 1700-1500 Ma, which caps much of the central shield, Phanerozoic rocks occur around the edges, but within the shield proper they are known only from the Takutu Rift, where Triassic-Jurassic sedimentary rocks are preserved, in Triassic dikes, and in a few small alkalic intrusives of probable Phanerozoic ages. Although there are many similarities with the West African Craton, a comprehensive pre-Atlantic reconstruction is difficult to achieve without resorting to fragmentation of the Guiana Shield during Atlantic rifting.
The Guiana Shield has produced important quantities of gold, diamonds, iron, and manganese, in addition to bauxite, which mainly occur in surrounding Cenozoic sedimentary rocks. The bauxites and manganese ores were formed by residual enrichment of aluminous and manganiferous protores, respectively. Iron ores, dominantly hematitic, are mined from the surface-enriched zones of Archean Algoma(?)-type BIF's (Banded Iron Formaton) of the lmataca Complex. Until 20 years ago, all the diamonds and the great bulk of gold were won from alluvial deposits in river gravels, derived in turn from either paleo-placer Roraima sources (diamonds and some gold) or lode sources (most of the gold). Base-metal deposits are notably sparse, as in West Africa.
Important lode gold mineralization in the shield is almost entirely confined to the lower Proterozoic greenstones, variously known as the Pastora, Barama-Mazaruni, or Marowijne terranes. The cumulative production of 800-900 tonnes Au has been derived mainly from alluvial and small eluvial deposits, with perhaps 300 tonnes produced from lodes and adjacent eluvium at El Callao (Venezuela), Omai (Guyana), Lawa, GrosRosebel (Surinam), and St. Elie (FrenchGuiana). A wide variety of specific deposit styles occurs, including shear-zone deposits, quartz lodes, and porphyry-hosted stockworks.