GEOLOGY AND MINERAL ZONING OF THE LOS PELAMBRES PORPHYRY COPPER DEPOSIT,CHILE
By W.W. Atkinson, Jr.
University of Colorado, Boulder,Colorado
Los Pelambres, located 200 km north of Santiago, Chile, is a major porphyry copper deposit within a small intrusive complex hosted by Early Cretaceous andesitic lavas. The complex consists of a preore tonalite stock that was intruded by a series of porphyries, ranging from quartz diorite to quartz monzonite. K-ar dates cannot discriminate ages of the individual units, which yielded an average age of 9.9 ± 1.0 Ma. Hypogene mineralization was introduced with quartz stockwork veining, potassic alteration, and breccia pipes. The entire complex was subjected to pervasive biotitization of hornblende. The stockworks show a sequence of types, including granular quartz veins lacking obvious alteration halos, quartz veins with K-feldspar halos, and green mica veins consisting principally of biotite and phengite. The main stage of mineralization consists of quartz veins with complex alteration halos containing quartz, K-feldspar, biotite, andalusite, and, less commonly, corundum. All these types of veins and alteration contain anhydrite, chalcopyrite, and bornite. Magnetite is present locally in the mica veins and alteration. A later stage of comb quartz veins introduced most of the molybdenum, with only minor K-feldspar and sericitic alteration. Late veins contain pyrite and quartz and have sericitic envelopes. Breccia pipes, as much as 600 m across, have matrices of igneous rocks, biotite, K-feldspar, quartz, tourmaline, magnetite, chalcopyrite, bornite, pyrite, and molybdenite. Supergene enrichment produced 560 million tonnes of enriched ore with a grade of 0.93 wt. percent Cu. Sulfide minerals occupy a bornite-chalcopyrite zone at the center of the deposit, surrounded by a pyrite-chalcopyrite zone, with increasing pyrite outward related to the sericitic alteration. Molybdenum occupies a truncated dome-shaped zone near the center of the bornite-chalcopyrite zone, which is also the location of the largest breccia pipe. Metal reserves are in excess of 3,300 million tonnes with a grade of O.63 wt. percent Cu, 0.016 wt. percent Mo, using a 0.4 wt. percent Cu cutoff.
WEATHERING-RELATED METALS MOBILITY IN COPPER
EXAMPLES OF COPPER-GRADE ENHANCEMENT AS OXIDE AND SULFIDE ASSEMBLAGES
By William X. Chavez, Jr.
Minerals & Environmental Engineering Department, New Mexico School of Mines
Copper deposits hosted by porphyry systems and andesite type mantos represent economically important sources of metals; those metals are recovered from both hypogene- and supergene-generated resources, an important point. This discussion examines the nature of weathering-related ore development, emphasizing the mineralogy and zoning present in copper deposits of northern Chile.
Porphyry copper systems represent large-volume resources of metals contained in geochemically reactive host rocks. Host-rock reactivity is a function of reduced sulfur content, quantity of mafic minerals present, abundance and type (-s) of alteration minerals, and the amount and rate of supply of weathering-related water. As such, porphyry systems display widely variable metals enhancement attributable to oxidation, leaching, and reprecipitation; because the economic viability of most porphyry copper deposits depends on the development of volumetrically significant metals (Cu, Au) enrichment, an understanding of how ore-mineral distribution is affected by host-rock reactivity is important in property evaluation and mineral processing.
Andesite [+/- dacite +/- rhyolite]-hosted copper +/- Ag deposits of central to northern Chile are characterized by bornite +/- digenite/chalcocite +/- chalcopyrite, with variable but generally low total pyrite. Alteration comprises albite, chlorite, epidote, carbonate, and specular hematite; silicification is volumetrically minor, developed only on a local basis in most systems. Because host rocks for these ore deposits are intermediate to felsic, and because alteration comprises silicates susceptible to hydrolysis by low-pH solutions, supergene oxidation and enrichment of copper (-silver) is limited: enrichment may form in-place, generating copper oxide+red hematite+/-chalcocite. Copper oxides consist of chlorides, carbonates, silicates, and minor sulfates; gypsum and anhydrite (!) are paragenetically late but are ubiquitous.
The long-term aridity of the weathering environment in northern Chile and adjacent Argentina has engendered and preserved copper oxide and sulfide assemblages having substantial economic importance. Although well-developed phyllic alteration volumes have been noted to host some of the most important copper occurrences in the world (Chuquimata District, Collahuasi-Quebrada Blanca District, La Escondida-Zaldivar, El Salvador, Fortuna), newly recognized environments hosting copper sulfides and oxides in K-silicate stable assemblages now comprise significant discoveries and exploration targets. Copper deposits at El Abra, Lomas Bayas, and Buey Muerto are hosted by K-silicate stable alteration assemblages, and show significant copper oxide development despite low total sulfide contents and the presence of reactive mafic minerals showing only nominal to modest phyllic alteration overprint. Thus, although copper sulfide mineral development may be limited because of the lack of reduced sulfur required to form chalcocite-digenite covellite assemblages, these systems may, nonetheless, develop very significant copper oxide ores.
Examples of these copper-bearing systems show that copper oxidation, transport, and reprecipitation may engender economically significant ore deposits in protoliths characterized by variable pyrite: copper sulfide and K-silicate stable alteration assemblages.
DON MARIO PROJECT, BOLIVIA-ORVANA MINERALS CORP.
By Michael W. Roper
Orvana Resources Corp., Golden, CO
The Don Mario gold-copper deposit is located in eastern Bolivia, 380 km east of Santa Cruz. The property is accessible by road and is located 95 km north of a railhead along the Bolivia-Brazil railway. A 1,300-m airstrip suitable for twin engine aircraft has been constructed near the project camp. A natural-gas pipeline, now under construction, will pass within 5 km of the project. Orvana controls mineral concessions in the area totaling more than 1,500 sq km (nearly 500 sq mi).
A prominent copper and iron-stained hill was mined for copper on a small scale by the Jesuits in the 18th century. Don Mario was rediscovered in 1991 by Minere La Rosa and explored by La Barca (Battle Mountain) in 1991-92. La Barca and La Rosa drilled six RC (reverse circulation) and 14 core holes. Billiton and La Rosa formed a joint venture in 1993 and carried out regional-scale exploration. They also drilled 52 core holes from 1993 to 1995. Orvana acquired 100 percent interest in the property in April 1996.
Mineralization is hosted by the Cristal Schist Formation, one of several northwest-trending Middle Proterozoic schist belts that are part of the Brazilian Precambrian Shield near Don Mario. Gold-copper mineralization is spatially related to rocks with affinities to chemical sediments (lean iron formation and carbonates) within a prominent zone of northwest trending regional shearing.
Two principal bodies of mineralization are first, an Upper Mineralized Zone (UMZ), characterized by Au, Cu, Ag, Pb, and Zn (as oxide and sulfide mineralization) within a unique calc-silicate lithology. Second, a Lower Mineralized Zone (LMZ) is characterized by high-grade gold with associated sulfide Cu, Bi, Ag, and W, hosted in an amphibole-magnetite-garnet-cordierite schist, which may represent a failed iron formation.
Folded host strata strike northwest and have a near-vertical dip. LMZ strata have great horizontal continuity (to +1 0 km) and occur elsewhere in the region. UMZ strata are localized within a mega-boudin in a zone of shearing. Evidence suggests a Middle Proterozoic age for gold and copper mineralization.
Orvana compiled and interpreted data, then completed geochemical sampling, geophysical surveys, geologic mapping, rock sampling, and trenching, followed by RC and core drilling totaling more than 12,000 m in 1996. Orvana expanded the resource, confirmed the strike extent of the LMZ (to +5 km), and discovered similar mineralization in a separate belt of Cristal Schist-like rocks at Las Tojas, 9 km north of Don Mario. Orvana believes that Don Mario may represent an initial discovery in a new gold-copper province within the Bolivian portion of the Brazilian Precambrian Shield having a resource potential of 3 million to 10 million ounces gold.
A 1996 scoping study to estimate ore reserves and to review development options was updated in 1997, and funding recently has been secured to complete a bankable feasibility study during 1998. Currently the deposit is estimated to contain +600,000 ounces Au, +8 million ounces Ag, and +200 million lbs Cu (+1.3 million ounces Au equiv).
Development alternatives include +/- 750 tonne/day underground mining of high-grade sulfide ore (1.4 million tonnes, 12 gm/tonne Au), with subsequent (or simultaneous) +/- 2,500 tonne/day open-pit development of near-surface oxide and transition gold-silver-copper ore (5.5 million tonnes, 3-4 gm Au equiv/tonne). Open-pit mining of the entire deposit also will be reviewed. Metal recovery with SXEW/cyanide heap leach processing of oxide and transition ores, with gravity/cyanide/flotation mill processing options for sulfide ores will be reviewed. Final project development decisions will balance the factors of: NPV (net present value), cash flow, project life, capital cost, operating cost, reserve base, production rate, and risk.
GEOLOGY AND DISCOVERY OF THE EL PEÑÓN GOLD-SILVER DEPOSITS, NORTHERN CHILE
By Charles H. Robbins and Richard C. Lorson
Meridian Gold, Reno, Nevada
El Peñón property is located in the Atacama Desert in Region II of northern Chile, about 165 km southeast of Antofagasta. Gold and silver mineralization occurs in several deposits within epithermal quartz-adularia veins that are generally strongly crushed or brecciated and hosted within rhyolite domes. The deposits currently contain a geological resource of 58 metric tons (1.6 Moz) of gold and 1,119 metric tons (34.7 Moz) of silver at average grade of 7.2 g/t Au and 140g/t Ag. A final feasibility study is currently being completed on the project.
El Peñón was a grassroots discovery in an area that had no recorded precious metal occurrences. The discovery resulted from a regional exploration program that concentrated initially on volcanic-hosted targets in the Paleocene-Eocene and Miocene-Pliocene belts in northern Chile. FMC Gold Company, which became Meridian Gold in August 1996, first initiated grassroots exploration activities in Chile in late l991. The initial reconnaissance visit to El Peñón core area was in November 1992. The rhyolitic rocks of El Peñón vein area were visually targeted by a weak but pervasive hematite staining when reconnaissance was expanded outwards from an extensive area of alteration (lithocap) 10 km north-northeast of El Peñón deposits. A poorly exposed outcrop of brecciated, delicately flow-banded rhyolite with quartz vein matrix was discovered in a small drainage. This breccia contained as much as 1 g/tAu and 28.7 g/t Ag and the indicator elements (As, Sb, and Hg) were relatively low. The area was acquired in June 1993 after a follow-up visit that extended mineralized float 1.5 km along strike with values as high as 3.5 g/t Au. A trenching program in September 1993 was followed by a 13-hole drill program in November. Six drill holes encountered >3.0 g/t Au; however, the true discovery hole was the first of the second drilling campaign that started in February 1994. Hole PP014 hit the major ore shoot of the Quebrada 0rito deposit and encountered 100 m of 10.9 g/t Au and l23.4 g/t Ag from 110 to 210 m. The true width of this shoot is as much as 22 m; it extends 270 m along strike and 120 m down dip.
Mineralization at El Peñón is associated with a low-sulfidation (adularia-sericite type) epithermal vein system that is spatially and temporally related to a rhyolitic dome complex. The mineralization, K/Ar dated as 59.4 +/ 1 .4 Ma using adularia, is strongly controlled by faults. To date, mineralized fault trends have been drill tested along a total strike length of >6 km. The mineralization remains open along most of these trends. The main north-northwest mineralized fault trends dip approximately 80°W, and north-northeast trending zones dip 65° to 75° east or west. A northwest structural trend has been identified, but to date has been found to be unmineralized. The mineralized fault offsets are dominated by a dip-slip component. Normal fault patterns have developed horst and graben features that suggest mineralization developed in an extensional environment; however, low-angle post-mineral faults exposed in the exploration decline show minor reverse-slip offsets of the main north-northwest vein trend. The entire north-trending vein zone was strongly crushed by a post-mineral tectonic event.
Alteration consists dominantly of quartz±adularia replacement or flooding within and near the vein zones, The silicification grades outward into quartz-sericite illite adularia. Argillic alteration is locally developed in the rhyolites; however, argillic and sericitic alteration are best developed in the ash-flow tuff and dacite-flow wall rocks of the dome complex.
Gold-silver mineralizaton is associated with a variety of quartz vein textures and grain sizes. Chalcedonic to coarse-grained quartz occurs in banded, saccharoidal, comb, and bladed carbonate-replacement vein textures. Hydrothermal breccias are common and were an important preparation feature for vein formation in the tight but brittle rhyolitic host rock. Ore shoots within the vein zones are locally as much as at least 22 m in true width. The quartz veins formed from periodically boiling low salinity fluids (±2 wt.% NaCl) between <200 and 255°C, which is typical of low sulfidation vein systems.
The mineralogy in the oxidized zone, which typically extends 250 to 280 m below the present surface, consists largely of silver halides (cerargyrite±bromine and iodine), native silver, and electrum with variable fineness. Base metals do not occur in significant quantities in the oxidized or unoxidized zones (to date the deepest intercept is 380 m below surface); however, sphalerite, galena, and chalcopyrite are present in the unoxidized zone in trace amounts, usually occurring in the early banded vein stages.
GEOLOGY AND MINERAL DEPOSITS OF ECUADOR
Ecuador exhibits a continuation of the highly prospective Andean geology from Peru northward to Columbia. The Ecuadorian Andes constitute the southern portion of the Northern Andes in an area that is transitional from the Central Andes, where there is continental lithosphere at the coast, to the Northern Andes, with oceanic crust under the coastal region. The Andes chain divides Ecuador into three geologic and geomorphologic regions-the Costa, the Sierra, and the Oriente. The Costa to the west is composed of oceanic crust and represents a fore-arc basin environment. The Sierra consists of a) the western Cordillera, a Cretaceous volcanic island arc, b) the Inter-Andean graben, and c) the Cordillera Real to the east, which is a metamorphic belt. The Oriente is composed of the sub-Andean zone, a back-arc fold/thrust belt, and the Oriente basin, a back-arc basin developed on continental crust.
Although the mining history of Ecuador dates back to the time of the Incas, it is discontinuous and poorly documented. Unlike Chile and Peru, few major discoveries were made over the centuries and a true mining industry and complementary educational system were never well developed. This was principally a result of the heavy volcanic soil and vegetative cover over much of the country. These factors have hindered exploration until the last few years when increased foreign interest in Latin America and revised mining laws in Ecuador stimulated modern activity.
Moderate petroleum resources are exploited in the northern Oriente of Ecuador and contribute substantially to the country's economic base. Nonmetallic mineral resources are found predominantly in the Sierra and the Costa, with cement production being most important. Precious- and base-metal mineralization is found from border to border in the Oriente, Sierra, and, to a lesser extent, the Costa. A wide variety of mineral deposits is present in Ecuador, including breccia pipes, porphyry systems, skarn-related mineralization, massive sulfides, shear-zone deposits, stratabound occurrences, polymetallic veins, and gold placers. Although silver, copper, zinc, and lead are important components of certain deposits, Ecuador is predominantly a gold province. A remarkably high percentage of the country's streams and rivers contain physically and geochemically anomalous gold values; however, to date, only three substantial gold-producing districts have been identified. These are the multi-million-ounce camps of Nambija, Portovelo-Zaruma, and Ponce Enriquez. Current exploration appears promising and indicates that additional deposits may yet be discovered in Ecuador.
The El Abra Porphyry Copper Deposit
Larry F. Barrett
Cyprus Amax, Englewood, Colorado
The tonnage and copper grade at El Abra are not exceptional for Chilean porphyry copper deposits, but the configuration of the deposit, its amenability to mining and SX-EW extraction, and the paucity of waste material are exceptional qualities that make it a "World Class" ore deposit that yields 225,000 tonnes of copper metal per year. One of its better assets is a nearly one percent copper core that was exposed at the surface and will supply the first five years of production at more than 100,000 tonnes per day. This core extends beneath oxidation as a hypogene bornite and chalcocite zone that is the focus of a current sulfide evaluation program. A lack of pyrite in this zone has precluded significant leaching and enrichment so that the sulfide material has essentially the same copper grade as the oxide ore that overlies it.
The El Abra deposit occurs entirely within an intrusive complex ranging from diorite to granite and ages from 38 to 34 million years. The El Abra porphyry, a quartz monzonite porphyry, is the youngest intrusive rock and is closely associated with breccias that both pre- and post-date it. The porphyry and associated breccias occur as a cluster of north -northeast-trending dikes and irregular bodies whose extent broadly defines the extent of potassic alteration that hosts essentially all of the copper deposit.
The deposit developed in a wrench fault system defined by east-trending right-lateral faults, northeast-trending left-lateral faults, and by abundant northwest-tension fractures that were the loci for late mineral fissure veins. Chrysocolla, pseudomalachite, copper clay, and tenorite are the principal oxide ore minerals. Cuprite and native copper occur erratically in the basal part of the oxide zone, particularly in the fissure veins. Bornite, chalcocite and chalcopyrite are essentially the only copper suffide minerals and occur predominantly in stockwork quartz veins.
Geology, Alteration, and Mineralization
Cerro San Pedro Gold-Silver Deposit, San Luis Potosi, Mexico
Cambior USA, Inc.,
Winterbourne, MSc. Student, C.S.M.
The Cerro San Pedro gold-silver district in central Mexico has had intermittent production for the last 400 years, with past production estimated at 2.5 milion ounces Au and 40 million ounces Ag. Recent exploration has defined a bulk mineable deposit amenable to open-pit mining and heap leach processing. Announced reserves by the Cambior-Metallica joint venture are 55.8 million tonnes at 0.57 g/t Au and 21.8 g/t Ag (0.95 g/t Au eq), mineable at a strip ratio of 0.8:1.0. Construction is anticipated to start early in 1999.
District geology is characterized by a complexly folded Cretaceous limestone sequence which has been intruded by a granodioritic porphyry. Several episodes of premineral deformation have resulted in generally north-south fold axes cut by both north-south and younger east-west faults. lntrusion of the porphyry was localized by the north-south Mendez structures, and there is significant post-intrusion faulting. Mineralization post-dates the deformation events. In the porphyry, mineralization is in the form of stockwork veinlets with better grades near the hanging wall contact with the limestone. Mineralization in limestone is present as carbonate replacement and manto/chimneys preferentially developed along structures. There is essentially no skarn and/or marble associated with the intrusion. All ore to be mined is thoroughly oxidized with the bulk being porphyry hosted. Sulfide mineralization at similar grades persists at depth.
There are five broad alteration styles observed at Cerro San Pedro, some which are distinctly related to mineralization. The most important alteration types are hypogene sericitic and hypogene acid-sulfate. Sericite alteration is composed of fine-grained muscovite, while the acid-sulfate alteration assemblage includes alunite, jarosite, and kaolinite. The most important structural controls of hypogene alteration are the north-south Mendez-type faults and the east-west Princesa fault. Hydrothermal fluids appear to have migrated upward from depth along the Princesa fault, then north into the Mendez-type faults.
Preliminary results from geochemical studies suggest that gold and silver were introduced during different alteration events, and zones of elevated gold are often distinct from zones of elevated silver. Silver and base metals (primarily lead and zinc) appear to be part of the intense sericitic alteration event, while gold and volatile constituents (primarily arsenic and antimony) seem to be associated with the acid-sulfate alteration event. Based on crosscutting relationships, sericitic alteration (silver) appears to predate the acid-sulfate alteration (gold). Preliminary fluid inclusion work also supports two separate mineralizing events. Additional investigations, including age dating and stable isotope (S, O, H) analyses, are planned.
PETROGRAPHIC AND ISOTOPIC STUDIES OF LATE DOLOMITE CEMENTS: NEW CONSTRAINTS
THE AGE AND ORIGIN OF THE SHERMAN-TYPE MINERAL DEPOSITS OF CENTRAL
Cliff D. Taylor*,
John D. Humphrey
Richard F. Wendlandt
Colorado School of Mines
*Presently with the U.S.G.S.
The Leadville Formation, a regionally extensive platform carbonate of Mississippian age, is the principal host to two very different types of mineralization. The first, Leadville-type, are carbonate-replacement massive sulfide deposits adjacent to mid-Tertiary intrusive stocks. The second, Sherman-type, are smaller, low sulfide, Zn-Pb-Ag-Ba deposits hosted in karst features that are common along the margins of the Paleozoic Colorado Trough and are distal to mid-Tertiary intrusions. At Leadville and Gilman, the two deposit types are spatially related and occur at the intersection of: 1) a major Paleozoic basin margin, 2) the Colorado Mineral Belt, a zone of major crustal shearing and Tertiary igneous activity, and 3) an area coincident with the Colorado Mineral Belt within which the Leadville Formation is dolomitized.
The age and origin(s) of the Sherman-type Zn-Pb-Ag deposits have been controversial for over one hundred years. Proposed genetic hypotheses for the Sherman-type deposits have invoked processes that range from those involved in formation of Mississippi Valley-type (MVT) deposits, occurring in Mississippian, Pennsylvanian, or early-Tertiary time, to mid-Tertiary magmatic processes contemporaneous with formation of the Leadville-type deposits.
New petrographic and isotopic studies on dolomite cements that are associated with both deposit types reveal five stages of host dolostone, and four generations of late dolomite cements (LDC). These cements occur regionally throughout the Leadville Formation, or locally as gangue to ore in both the Leadville- and Sherman-type deposits. Two generations of paragenetically early regional LDC's and recrystallized dolostone display carbonate-staining, cathodoluminescence, and carbon and oxygen isotope characteristics (19 to 25.2 d18O and -7.3 to 1.3 d13C) consistent with formation by burial diagenetic processes during Pennsylvanian time. Two later generations of ore-related cements exhibit distinctly different petrographic and isotopic characteristics (6.4 to 12.2 d18O and -6.0 to -1.6 d13C) and are clearly unrelated temporally, spatially, and genetically to the regional LDCs. These newly recognized differences in cement paragenesis indicate formation of the Sherman-type deposits after the Pennsylvanian burial event that produced the regional LDC's. This constraint precludes the Mississippian-aged, syn-karst genetic hypothesis for Sherman-type mineralization.
FOR PORPHYRY COPPERS INTO THE 21st CENTURY
John M. Guilbert
Emeritus Professor, Department of Geosciences, University of Arizona
As the nature of the exploration business shifts more and more toward required discovery of unexposed mineral deposits, and as exploration technology is increasingly able to adapt to that reality, a whole new generation of techniques is coming into play that we all - field practitioners, managers, and educators - need to deal with and understand. Many of the newly developed or recently refined techniques also serve to optimize "bootleather" exploration as many of us have practiced it, and discovery of partially or "cryptically" exposed orebodies will also be aided by these novel methodologies. Some of the "new tricks" and extensions of old ones represent dramatic developments and improvements that have already changed (or will soon have changed) the way we carry out field work and discover new deposits and resources.
Novel approaches, the applications of which will be described and integrated include Global Positioning Systems (GPS, Omnistar, etc.), computer uses (GPS-GIS, AutoCad, image processing, field and derivative data management), the Total Station concept (VectorMap, PenMap, ArcView) and its uses, the several new portable instruments for field mineralogical and chemical analysis (PIMA-II, MapTool, and LIBS), some new geophysical techniques (Hyperspectral TM, TEM, CSAMT, etc.), novel geochemical systems that are effective for buried deposits (Mobile Metal Ion, Enzyme Leach, electrogeochem, and multielement analysis and data management) and isotope systems (Cl, Re-Os, Nd-Sm), and Quantitative Alteration Mapping. Many of these approaches will take us to places we have never been before in exploration - we enter the New Century and the New Millennium with formidable challenges, exciting developments, and expanding capabilities for mineral deposit discovery.