Geology, Mineralization, and Genesis of the La Quinua Gold Deposit, Yanacocha District, Northern Peru
Published:January 01, 2005
- PDF LinkChapter PDF
Patrick M. Mallette, Raul E. Rojas, Alberto R. Gutierrez, 2005. "Geology, Mineralization, and Genesis of the La Quinua Gold Deposit, Yanacocha District, Northern Peru", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
Download citation file:
With premining reserves plus resources of 10.2 million oz (Moz) of gold, 424 million metric tons (Mt) at 0.75 g/t, La Quinua is one of several world-class gold deposits that comprise the Yanacocha mining district in the Cajamarca province, northern Peru. Unlike volcanic-hosted high-sulfidation gold deposits that characterize much of the Yanacocha district, La Quinua is hosted by unconsolidated gravel. In 1996, Newmont and Minera Yanacocha geologists discovered gold-bearing gravel while drilling an alluvial basin to test “blind” basement targets. Exploration and development drilling proceeded rapidly; 1 yr after initial discovery the reserve plus resource stood at 7 Moz. Production began in mid-2001.
At La Quinua, there is infilling of coarse clastic sediments in two structural basins situated along the western flank of the Yanacocha Sur and Yanacocha Oeste gold deposits. Gravel fans reach a maximum thickness of 350 m on the downthrown side of the basin-bounding La Quinua fault. Sediments fine down gradient from chaotic boulder gravel in proximal facies to gravelly silt and sand in distal facies. Bedding becomes more pronounced down gradient with a decrease in bed thickness. Five deposit-scale stratigraphic units are recognized. These include, from bottom to top, (1) regolith directly overlying basement rocks, (2) high-to low-energy deposits of clay-and alunite-bearing sand and gravel, (3) low-energy deposits of organic-rich mud, peat, and bog iron, (4) ferruginous gravel, and (5) high-energy deposits of pebble-cobble-boulder gravel grading distally to fine-grained sandey silt.
La Quinua gold was derived from erosion, transportation, and deposition of gold particles and mineralized clasts from the Yanacocha Sur and Yanacocha Oeste deposits. However, a portion of the gold may have resulted from chemical mobilization and reprecipitation. Gold particles are mostly micron sized, liberated within mud matrix, and disseminated within mineralized clasts, although liberated particles to 0.2 mm have been observed. Gold is disseminated throughout the deposit with only gradual lateral and vertical grade transitions. Placer “paystreaks” have not been encountered. The La Quinua gold-trapping system was efficient; grade dilution from source deposits is <25 percent. The Ag/Au ratio is 6/1, compared to >10/1 for the Yanacocha Sur and Yanacocha Oeste deposits. Copper and iron are locally enriched in specific stratigraphic horizons. Copper values up to a few percent are associated with detrital and authigenic minerals. Copper mineralization is erratic and not considered economic and may present future ore-processing challenges. Authigenic iron is present as bog-iron lenses, ferricrete cement, and gravel matrix impregnation.
La Quinua formed in response to dynamic interaction of climate and tectonics. A subsiding tectonic basin preserved gold-bearing sediments during periods of intense mechanical weathering in the adjacent highlands. Cold alpine climatic conditions resulted in two pulses of rapid sedimentation and basin infill. Temperate conditions resulted in diastems marked by organic accumulation and surficial iron deposits. La Quinua is characterized by a paucity of channel deposits, lack of coarse-grained placer gold, and preservation of fine-grained gold. Structural offset and warping of sedimentary units indicates that basinal tectonism continued after deposition of the gravel sequence.
Gold production from La Quinua will exceed 1 Moz/yr during 5 yrs of an 8-yr mine life, with a peak of nearly 2.5 Moz predicted in 2006. Light blasting improves mining efficiency. Oxide ore is belt agglomerated prior to placement on the leach pad. The majority of gold is recovered in carbon columns with a smaller portion recovered in a Merrill-Crowe facility. Mine equipment includes Hitachi 5500EX shovels, Cat 992 loaders, and Cat 785 and 793 haul trucks.
La Quinua is located in the western portion of the Yanacocha gold district in the Cajamarca province of northern Peru (Fig. 1). The deposit lies along the Andean continental divide at an elevation of 3,700 m above sea level, 18 km north of the city of Cajamarca, and 600 km north of Lima.
The Yanacocha gold district is well known for volcanic-hosted, high-sulfidation epithermal gold deposits (Turner, 1997; Harvey et al., 1999; Myers and Williams, 2000; Teal et al., 2002) and is currently the largest gold producer in South America. Like other large Yanacocha district gold deposits, La Quinua is world class with premining reserves plus resources of 10.2 million oz (Moz) of gold. Unlike the classic volcanic-hosted Yanacocha-style deposits, however, La Quinua is erosionally derived and hosted by relatively young unconsolidated gravel.
During 1996, Newmont and MYSRL (Minera Yanacocha SRL) geologists initiated an exploration program to evaluate bedrock-hosted gold targets beneath gravel cover in the La Quinua basin. Alteration and mineralization southwest of the Yanacocha Sur and Yanacocha Oeste gold deposits were mapped and traced to their termination by the basin-bounding La Quinua fault. A two-hole drill program was designed as an initial test for a blind deposit on the downthrown side of the La Quinua fault. These holes penetrated 130 to 160 m of gravel and encountered weakly to moderately altered bedrock that was barren of gold. Overlying gravel, however, returned gold values of 0.5 to 1.0 g/t. Two additional holes were drilled to complete the target test, and again insignificant gold was encountered in bedrock while significant gold was present in the gravel. At this time, exploration emphasis switched from bedrock targets to evaluating gravel-hosted gold mineralization. Exploration proceeded rapidly during 1997; by the end of that year 149 drill holes had been completed, a second gravel-hosted gold deposit—La Quinua Norte—had been discovered, and Newmont/MYSRL announced a reserve and resource of 3.0 and 3.9 Moz of gold, respectively. Continued development drilling during 1998 to 2000 brought the reserves plus resources to 10.2 Moz of gold. Mining began at La Quinua Central in mid-2001.
La Quinua is located in the western portion of the Yanacocha district immediately west of, and downslope from, the volcanic-hosted gold deposits of Yanacocha Sur and Yanacocha Oeste (Fig. 2). Gravel shed from highlands in the Yanacocha Sur and Yanacocha Oeste areas was transported downs-lope and filled two depositional basins separated by the locally east-west trending Andean continental divide. South of the continental divide, in the La Quinua basin, gravel fill is up to 320 m thick. This sequence of gold-bearing coarse clastic sediments hosts the La Quinua Central and La Quinua Sur gold deposits. The La Pajuela basin occurs immediately north of the continental divide where gravel fill is up to 350 m thick. Gold-bearing gravel of marginally economic grade is localized in uppermost stratigraphic units; these units host the La Quinua Norte deposit.
Both the La Quinua and La Pajuela basins are bounded on their northeast sides by the northwest-trending La Quinua fault (Fig. 3). Tectonism appears to have been episodic and has continued until recent times. The La Quinua fault is marked by a 1-to 3-m scarp in the present-day topography. The scarp is developed in gravel and bedrock, indicating recent displacement. The northwest-trending El Tapado high is a positive bedrock feature that bisects the La Quinua basin. This feature, and northwest-and northeast-trending basement faults, interpreted from drill data, dissect and separate the larger basin into smaller internal basins.
Coarse siliciclastic sediments that comprise La Quinua Central and Sur were derived from bedrock in the Yanacocha Sur and/or Yanacocha Oeste areas and issued primarily from a point source located at or near present-day Quebrada Quinua Corral. Coarse sediments in the La Pajuela basin, which hosts La Quinua Norte, were sourced from the Yanacocha Oeste area and were primarily transported along a pathway coinciding with present-day Quebrada La Pajuela. Minor net deposition occurred east of the La Quinua fault. Gravel fill thickens abruptly immediately west of the La Quinua fault, owing to tectonic accommodation. Gravel of La Quinua Central and Sur displays a modified fan-shaped morphology; the deposit is rhombohedral as a result of basin-bounding faults and is stretched down gradient. La Quinua Norte displays a classic fan-shaped morphology.
Deposit morphology and lithofacies indicate deposition in an alpine periglacial setting. Glacial striae are etched on bedrock exposed near Yanacocha Sur and a small cirque is present at the head of Quebrada Quinua Corral. Alpine glacial processes influenced sediment production and may have played a role in sediment transport as well; at certain times, deposition may have occurred in a periglacial fan environment. Sediment transport and deposition in proximal regions of the La Quinua fans were the result of episodic and possibly catastrophic debris flows, resulting in chaotic non-sorted, nongraded, and poorly stratified deposits (Fig. 4A). High rates of sediment input to the proximal fan would have led to oversteepened slopes while sediment saturation would have allowed slope failure by slumping and sliding as debris flows. Boulders within proximal facies gravels are commonly well rounded with polished and striated surfaces (Fig. 4B). The short transport distance from source area to depositional basin would argue against rounding and polishing by alluvial processes; these features are more likely the result of glacial processes and/or tumbling action during transport within viscous debris flows and subsequent creep. Little winnowing and reworking took place in proximal facies during or after deposition as indicated by a general lack of channel structures, size sorting, or clast imbrication. Sediment sorting and stratification increase progressively downslope into distal portions of the fans accompanied by a decrease in particle size (Fig. 4C). These patterns reflect the progressively greater influence of alluvial processes in distal fan environments.
The stratigraphic succession of La Quinua gravel, as typified by the La Quinua Central deposit, is presented in Figure 5. La Quinua clastic sediments as a whole represent a firstorder depositional sequence. Two internal second-order depositional sequences are recognized and are referred to herein as upper-and lower-sequence gravel. Both resulted from rapid sediment production in adjacent highlands and rapid influx to depositional basins in relatively high energy environments. The second-order depositional sequences are bounded by horizons reflecting lower energy and/or lower sediment-flux environments consisting of (1) basal regolith overlying bedrock, (2) organic and authigenic iron deposits separating upper-and lower-sequence gravels, and (3) the modern erosional surface.
A discontinuous sequence of mostly monolithic regolith consisting of weathered bedrock immediately overlies bedrock at the base of the gravel section. Regolith represents initial weathering and erosion of the La Quinua bedrock surface, possibly before the onset of tectonic activity along the La Quinua fault. Distribution of regolith is poorly known due to the paucity of drill holes that penetrate bedrock. In some cases, regolith thickens adjacent to bedrock highs, suggesting that these were areas of positive topographic relief prior to basin infill.
A thick, transported gravel sequence overlies regolith and is deposited directly on basement rocks where regolith is absent (Fig. 6). Coarse fragments consist primarily of silica-, silica-alunite-and silica-clay–altered volcanic rocks. XRD-XRF analyses suggest a similar composition within the mud matrix (Williams and Vicuña, 2000). Lower-sequence gravel reaches a maximum thickness of 220 m adjacent to the La Quinua fault and thins to a few meters in the distal fan. In proximal facies near the La Quinua fault, lower-sequence gravel is characterized by nonsorted and poorly stratified, muddy, sandy, pebble-to-cobble gravel (terminology of Folk, 1974). Proximal facies sediments were deposited subaerially in a subsiding tectonic basin with little reworking. Sediment sorting and stratification increase in medial and distal facies where interbedded sequences of pebble gravel, muddy sand, and silt are exposed in canyons and intersected in drill holes. These features, in combination with finely laminated clayey silt beds, presence of laminated and disseminated organic material, and load cast structures, suggest low-energy deposition in shallow aqueous environments interrupted by periodic high-energy depositional events.
Lower-sequence sediments are mostly white to light gray in color; iron oxides are absent to minor. In some areas, particularly in deeper portions of the basin, both primary and authigenic sulfide minerals are observed. Primary sulfides include pyrite, chalcopyrite, and covellite as mineral grains within gravel clasts and as loose grains in the sand and silt matrix. Authigenic sulfides include pyrite, marcasite, and chalcocite. Authigenic sulfides form irregular pods and disseminations within the sediments and crusts on rock fragments. Chalcanthite forms crusts on sulfidic surface exposures and is commonly observed on dry days following rain storms.
Deposits related to reduced siliciclastic influx
A thin but laterally extensive sequence of organic and authigenic ferruginous deposits overlies lower-sequence gravels (Fig. 6). Deposits of organic-rich mud, peat, and bog iron reflect equilibrium within the depositional environment with increased vegetation cover and decreased siliciclastic input to the basin. Individual peat beds are up to a few meters thick, consist of matted plant material, and were deposited in bog environments. Some laterally continuous, organic-rich sediments may represent paleosols, similar to the modern 1-m-thick organic horizon that marks the present-day La Quinua surface. Paleosols mark third-and fourth-order depositional sequence boundaries. Horizons of nearly pure goethite overlie organic material and reach a maximum thickness of 30 m. Goethite is often porous with abundant fossil plant casts. Thinner deposits likely represent bog iron formed at or very close to the paleosurface in flooded environments. Thicker deposits may have formed as surficial terraces over iron-saturated springs as suggested by Williams and Vicuña (2000).
Coarse-grained gravel immediately overlies peat and bogiron deposits (Fig. 6). The sequence attains a maximum thickness of 250 m and represents a return to high-energy conditions with greatly increased siliciclastic sediment influx to the basins. Upper-sequence gravel consists primarily of oxidized, limonitic, silica-altered volcanic fragments. Current mine faces expose a thick section of nonsorted pebble-to-cobble gravel; occasional boulders of 3-to 4-m diam float in a muddy-sandy-gravel matrix (Fig. 4B). Proximal facies deposits are nonstratified to weakly stratified. Stratification is manifest by weak ferruginization and small-scale channel, lag, and/or talus deposits at unit tops. Strata are warped into open parallel folds likely due to postdepositional tectonic adjustments in underlying bedrock and/or differential movement along the La Quinua fault. Lenses of fine-grained, slightly gravelly silt and sand are interbedded within upper-sequence gravels. Fine-grained lenses increase in abundance and eventually merge downfan to become the predominant lithology in distal facies.
The basal portion of upper-sequence gravel contains abundant authigenic goethite and earthy hematite disseminated in the mud matrix and coating rock fragments (Fig. 6). Locally, the gravel is cemented by iron oxide-forming ferricrete (Fig. 4D). Ferruginous gravel attains a maximum thickness of 90 m adjacent to the La Quinua fault. In cross section, the body is wedge shaped and thins from proximal to distal fan. The wedge shape may indicate that the source of iron was upslope in the Yanacocha Sur-Yanacocha Oeste areas. Alternatively, or in combination with an upslope source, leaching from the overlying gravel pile could have provided iron for goethite and hematite precipitation. A thicker gravel section, as developed near the La Quinua fault, would provide more primary iron oxide and iron-bearing minerals for remobilization and precipitation at the redox boundary below. The modern redox boundary within La Quinua gravel corresponds closely with the boundary between the upper and lower gravel sequences and more precisely with organic deposits below and ferruginous deposits above (Fig. 6). Iron-bearing ground waters moving through or along the top of lower-sequence gravels would deposit iron oxides when encountering oxygenated conditions at the base of upper-sequence gravels.
There is an overall transition from predominantly goethite at the base of the ferruginous gravel horizon to predominantly hematite at the top. Within this larger goethite-to-hematite transition are smaller scale (1–10 m thick) transitions. These cycles may represent seasonal or longer duration positions of the paleowater table where goethite formed in a saturated or near-saturated oxidizing environment at the water table and hematite formed above in the vadose zone. Alternatively, hematite may have formed by dehydration of precursor goethite with seasonal or longer term lowering of the water table.
Timing of ferruginization is not precisely known but appears to have been relatively late. Ferruginization may postdate deposition of upper-sequence gravel. Notably, both goethite and earthy hematite are precipitating on natural and man-made exposures at La Quinua today.
La Quinua is currently in a state of net erosion. Grasses covering the fan surfaces form a dense root mat that limits erosional severity. However, when the vegetative root mat is breached, downcutting progresses rapidly through unconsolidated sediments carving vertical-to near-vertical–walled canyons up to 35 m deep. During the annual rainy season, and particularly during high-precipitation El Niño events, large volumes of rainwater funnel through these canyons. High-flow events cause undercutting and calving of the canyon walls. Large volumes of sediment are then transported down the canyons, but because sediment production is almost entirely internal to the fans, the overall process is destructive.
La Quinua is a mostly unconsolidated gravel-hosted disseminated gold deposit derived from erosion of nearby volcanic-hosted epithermal gold deposits. Gold is extremely fine grained and is present both as primary disseminated particles in rock fragments and as liberated silt-to clay-sized particles in gravel matrix. Because much of the gold at La Quinua appears to be physically transported from the adjacent source deposits, comparisons could be made to placer deposits. La Quinua differs from alluvial placers in that gold does not occur in localized paystreaks but instead is broadly disseminated with gradual lateral-and vertical-grade variations.
Spatial distribution of gold
Vertical and lateral distribution of gold throughout La Quinua Central is shown in Figures 3, 5, and 6. In plan view (Fig. 3) the footprint of gold mineralization is distinctly fan shaped. Upslope apices of gold shapes at La Quinua Norte and Central coincide closely with the modern Quebradas La Pajuela and Quinua Corral. At La Quinua Central, upper-sequence gravel and ferruginous gravel carry 88 percent of the gold resource reserve (Figs. 5–6). Stratigraphic distribution of gold in La Quinua sediments is attributable to gold distribution with respect to alteration facies in the source deposits. Primary gold is generally more abundant, and of higher grade, in the silica-altered volcanic rocks at Yanacocha Sur and Oeste, as opposed to silica-alunite– and silica-clay–altered rocks. Lower-sequence gravels at La Quinua are characterized by silica-clay– and silica-alunite–altered clasts, whereas upper-sequence gravels are predominantly altered only to silica. La Quinua sediments reflect original hydrothermal alteration facies in the source deposits but in an inverted vertical succession.
Gold particle size distribution
Primary volcanic-hosted gold deposits at Yanacocha Sur and Yanacocha Oeste are typified by micron-sized, disseminated, and fracture-controlled gold (Turner, 1997; Harvey et al., 1999; Teal et al., 2002). At La Quinua, gold occurs as both liberated particles in the silt and clay matrix and as disseminations within gravel fragments. Gold panning and Gemini table tests have recovered occasional sand-sized gold particles (62–220 μm) but these are rare. Metallurgical leach tests (6-in columns) indicate gold recoveries of approximately 75 percent for upper-sequence gravels and 60 to 70 percent for Lower-sequence gravels, with standard cyanide concentrations. Leach-recovery tests suggest that gold particle sizes are predominantly in the ~10-to 20-μm range (Larry Todd and César Vidal, pers. commun., 2002).
Figure 7 shows the distribution of gold within sediment-size classes in upper-sequence gravel from current mine faces. Gold grade typically increases from coarser to finer fractions. Total gold content is roughly equal in the gravel and silt plus clay fractions with a lesser percentage in the sand fraction. Results shown in Figure 7 are somewhat biased to the fine fraction because large cobbles and boulders are usually excluded from granulometric-assay tests due to collection and processing limitations. Chip samples from larger cobbles and boulders commonly contain 1.0 g/t gold.
The distribution of gold shown in Figure 7 is explained by progressive mechanical disaggregation of gold-mineralized fragments during transport. In primary hard-rock deposits at Yanacocha Sur and Yanacocha Oeste, gold is disseminated and associated with silica-altered volcanic rocks and occurs as limonite-after-sulfide plus gold fracture fillings (Harvey et al., 1999; Teal et al., 2002). As primary deposits eroded and mineralized material was transported downslope to the La Quinua basin, large fragments tended to break down along preexisting fractures. Thus, fine-grained gold and limonite that occupied the fractures were exposed to abrasion and released to the mud matrix. Original fracture-hosted gold was progressively added to the mud fraction as transportation and fragmentation continued. As rock particles were reduced in size, they ultimately reached a nominal fracture spacing. Gold grade of the sand-sized particles reflects grade of the disseminated gold component minus fracture-related gold.
Source and transport of gold
La Quinua gold was sourced in the Yanacocha Sur-Yanacocha Oeste area and transported to the La Quinua and La Pajuela basins as disseminated gold in clasts of primary ore and as silt-to clay-sized liberated gold grains. However, the presence of anomalous gold grades (>100 ppb) in both peat beds and chemically precipitated bog-iron horizons suggests that a portion of La Quinua gold may have been transported in solution and reprecipitated in the gravel. Gold in solution could have been locally derived from gold-bearing gravel or, less likely, from Yanacocha Sur-Yanacocha Oeste deposits up gradient, or both. Evidence to support solution-reprecipitation of gold include the following: (1) gradual vertical and lateral grade changes throughout the deposit; (2) higher gold grades associated with authigenic iron in the ferruginous blanket at the base of the upper-sequence gravels; (3) significant concentrations (>100 ppb) of gold in nearly pure organic and chemically precipitated sediments; and (4) presence of a gold-depleted (leached?) horizon in the uppermost 1 to 6 m of gravel over most of La Quinua. Gold mobilization and precipitation in the near-surface environment, particularly in laterites and chemical placers related to extreme chemical weathering in tropical environments, is well documented in the literature (cf. Boyle, 1987). While there is no evidence for extreme chemical weathering or tropical climatic conditions in the Yanacocha district, studies are in progress to investigate the mode of gold deposition with a particular emphasis on gold-enriched ferruginous and organic-rich sediments.
Gold transport and enrichment may have resulted from both mechanical-and solution-transport mechanisms, although a mechanical mechanism is thought to have been dominant. Anomalous gold grades occur in chemically precipitated goethite (bog iron) that contains little or no dilution by detrital components, an observation indicating that gold is available and scavenged from solution. However, if gold scavenging from solution were the dominant mineralizing process, these nearly pure, chemically precipitated sediments would carry higher gold grades than purely detrital sediments of the Upper gravel sequence.
La Quinua gravel hosts a significant amount of silver. The Ag/Au ratio at La Quinua Central and Sur is ~6/1. Like gold, silver was originally sourced from the Yanacocha Sur and Yanacocha Oeste deposits where Ag/Au ratios are >10/1. Mechanical processes likely facilitated transport of silver-mineralized clasts to the basins, although Williams and Vicuña (2000) suggested supergene remobilization from the Yanacocha deposits.
Copper is present in trace amounts throughout the La Quinua deposit, to >1 percent locally in lower-sequence gravel. Copper can occur as primary copper sulfides and oxides in clasts but more abundantly as an authigenic copper sulfide overprint. Average copper grades for various stratigraphic units are given in Table 1. The highest copper concentrations consistently occur in sulfidic portions of the lower-sequence gravel where copper sulfides form an authigenic overprint on previously deposited, commonly organicrich sediments.
|Upper gravel||Bedded fines||Ferruginous gravel||Organic sediments||Lower gravel (oxidized)||Lower gravel (sulfidic)|
|Upper gravel||Bedded fines||Ferruginous gravel||Organic sediments||Lower gravel (oxidized)||Lower gravel (sulfidic)|
The accumulation of large economic concentrations of detrital gold at La Quinua depended on at least four critical local and environmental factors. These included: (1) an unlimited source of primary gold during the erosional cycle; (2) a paleoclimatic regime that favored mechanical weathering and rapid sediment production in the gold source area; (3) a short sediment transport distance that allowed little alluvial winnowing, reworking, or dilution; and (4) tectonic accommodation in the depositional basins.
Primary gold deposits at Yanacocha Sur and Yanacocha Oeste contained premining reserves plus resources of nearly 10 Moz (Harvey et al., 1999). The deposits formed in the near-surface epithermal environment (Harvey et al., 1999; Teal et al., 2002). Subsequent erosion led to exposure of economic-grade gold mineralization at the surface. The Yanacocha Sur deposit supplied gold-bearing clastic sediments primarily to the La Quinua basin while Yanacocha Oeste contributed mineralized material to the La Pajuela basin.
La Quinua gold-grade contours vector to source areas and indicate point sources coincident with the proximal fan at or very close to the mouth of Quebrada Quinua Corral and at Quebrada La Pajuela (Fig. 3).
Effect of paleoclimate on weathering and sediment production
La Quinua gravel fans are currently in a state of net erosion. Very little sediment production is occurring upslope and outside the basins to feed an aggradational system. Therefore, it is logical to conclude that La Quinua formed under a different set of climatic conditions to those operative at present. We suggest that La Quinua represents a response to Quaternary climatic cyclicity. Glacial striae on exposed bedrock at higher elevations of the Yanacocha district indicate colder climatic conditions in the recent past.
An idealized genetic model is presented in Figure 8 that relates depositional-erosional cycles to paleoclimatic variability. In a colder climatic regime, mechanical weathering— from freeze-thaw cycles and alpine glacial scouring—was predominant, and vegetation to stabilize rock and soil was reduced. Both factors led to increased sediment production, although sediments might remain ice bound during maximum cold periods. As climate moderated, with melting of snow pack and alpine glaciers, sediment supply to lower elevations increased, resulting in net aggradation in adjacent basins. With further climatic moderation, vegetation cover became established to stabilize soil and regolith; sediment supply from adjacent highlands was minimized, and the basins reached depositional-erosional equilibrium. Net erosion— like that currently occurring at La Quinua—occurred during extended periods of moderate to high precipitation coupled with sparse vegetation cover and low to nil sediment supply to the basin.
La Quinua is a relatively young deposit in comparison to the ~11 Ma (Turner, 1997) volcanic-hosted deposits of the Yanacocha district. Depositional patterns in the gravel deposits can be directly related to present-day topography, but the absolute timing of basin infill is unknown. Peat samples from the interval separating the upper-and lower-sequence gravels exceeded the limitations of 14C dating techniques at >44,710 BP. It is reasonable to conclude, however, that La Quinua formed as a response to high-frequency climatic fluctuations during the Quaternary. Investigations of Holocene climatic variation in the central Andes of southern Peru and Bolivia demonstrate that climatic perturbations occurring at 1,000-to 20,000-yr frequencies greatly affected the geologic record at Lake Titicaca and Salar de Uyuni (Baker et al., 2000; Cross et al., 2000). The La Quinua depositional system may represent a response to similar climatic fluctuations, at similar or lower frequencies. Climatic perturbations of varying frequency may be reflected by first-, second-, third-, and possible fourth-order depositional sequences that fill the La Quinua and La Pajuela basins. Additional work is needed to place La Quinua in a chronostratigraphic context and compare depositional patterns to regional paleoclimatic models.
Sediment transport and deposition
Average gold grade of La Quinua gravel suggests a remarkable lack of dilution with respect to source deposits at Yanacocha Sur and Yanacocha Oeste. Average grade of the global premine reserve at La Quinua Central is 0.78 g/t Au. This compares to ~1.0 g/t Au at Yanacocha Sur and suggests less than 25 percent dilution during transport. To allow for such minimal dilution, erosional processes must have been extremely focused on the gold deposits at Yanacocha Sur and Yanacocha Oeste. In addition, a short and direct sediment pipeline was necessary to deliver ore-grade material to the La Quinua and La Pajuela basins. It is equally possible that the upper, eroded portions of the Yanacocha deposits were of higher grade than the current reserve (L. Teal, pers. commun., 2002). Even so, focused erosion and short transport distance were necessary to minimize dilution by nonmineralized material.
If La Quinua had been exposed to significant alluvial reworking and winnowing before burial, economic concentrations of fine-grained, micron-sized gold would not have been preserved. Mine faces and natural exposures in canyons dissecting the fan indicate relatively few back-filled alluvial channels. The lack of channels and channelized surfaces suggests that the aggradation rate of the fan systems greatly outpaced erosion and alluvial reworking, particularly in proximal and medial facies where the bulk of the gold reserve occurs.
Preservation of sediments and gold in La Quinua depended on continuous basin subsidence to accommodate sediment influx and minimize alluvial winnowing and reworking; this was provided by recurring movement along basin-bounding and intrabasinal faults. Tectonic accommodation, coupled with a high sediment supply, allowed rapid burial and preservation of the La Quinua gravels and contained fine-grained gold.
Reserves and resources
La Quinua is scheduled for an 8-yr mine life. Over 1 Moz of gold will be mined annually during five of those years, with a peak of nearly 2.5 Moz in 2006. Reserves plus resources at end of 2000 for the combined deposits (La Quinua Norte, Central, and Sur) totaled 424 Mt at an average grade of 0.75 g/t Au (10.2 Moz).
Premine reserves plus resources for La Quinua Central were 366 Mt at an average grade of 0.78 g/t Au (9.2 Moz). The waste/ore ratio within the original pit limits was 0.67. The reserve and resource were calculated from core holes (8.31-cm-diam) and a lesser number of reverse-circulation drill holes. Drill spacing was 50 m in the upper portion of the deposit, and roughly 100 m in the lower parts. Gold grade was determined by fire assay. La Quinua Norte is only marginally economic at a US $325 gold price due to low grade and a relatively high mud content that adversely affects processing. End of 2000 resources at La Quinua Norte amounted to 30 Mt at an average grade of 0.53 g/t Au (0.5 Moz). La Quinua Sur could be mined as an extension of La Quinua Central but further materials testing is needed. End of 2000 resources at La Quinua Sur amounted to 27 Mt at an average grade of 0.50 g/t Au (0.4 Moz). Neither La Quinua Norte nor La Quinua Sur was included in end of 2002 reserve and/or resource.
Mining and processing methods
La Quinua is a large-scale, open-pit, heap-leach mining operation. The ultimate pit at La Quinua Central will measure 2,100 × 2,100 m. Mine equipment includes Hitachi 5500EX shovels, Cat 992 loaders, and Cat 785 and 793 haul trucks. Approximately 200,000 t of ore plus waste are mined daily. Unconsolidated gravel-hosted ore is drilled and lightly blasted to improve digging efficiency. Blasthole spacing varies according to material type from 5.8 × 6.7 to 7.7 × 8.9 m. AuFA and CuCN assays from blastholes, and granulometric properties of ore-grade material, form the basis for ore polygon generation.
Clean ore (>65% gravel) is routed directly to the heap-leach pad as run-of-mine material. Ore with a lower gravel content (<65% gravel) is routed through a belt agglomeration circuit before being placed on the leach pad. The agglomerator consists of two production lines with a design capacity of 123,000 t/d. Ore throughput is at optimum moisture content of 8 to 9 percent with crushing to nominal 15-cm-diameter and addition of 1.0 to 2.0 kg/t of cement. An automated sampling circuit installed on agglomeration belts allows grade reconciliation between the mine model and the leach pad.
Agglomerated ore is truck dumped on the leach pad. Hitachi excavators are used to fluff the upper 1.5 m of each final lift surface to decompact the ore. The La Quinua leach pad will ultimately be stacked to a height of 120 m and will contain over 400 Mt of material. Pregnant solution from the pad is collected in ponds and pumped through carbon columns and a Merrill-Crowe facility for gold recovery.
La Quinua geologists provide routine ore control on a 24hr basis. Primary duties include mapping dig faces, ore blocking, and sampling ore and waste for potential acid generation, moisture content, granulometric analyses, and gold grade. Ore-to-waste splits are based on granulometric properties of the materials and on presence of cyanide-soluble copper sulfides. Ore and/or waste gold grade cutoffs are sensitive to the price of gold and have varied between 0.25 to 0.35 g/t for gravel with a mud (silt + clay) content of less than 30 percent and cyanide soluble copper levels of <250 ppm. Material with a lower gold grade, and/or higher mud content, and/or greater cyanide soluble copper is routed as waste.
The discovery of the La Quinua gravel-hosted gold deposits resulted from exploration geologists who were allowed to pursue target concepts and drill holes. In the final analysis, the fact that the resulting discovery was unlike the original exploration concept is irrelevant. What is relevant is that the geologists were not blinded by a single deposit model, and that corporate management was bold enough to pursue gold production from a deposit type that was outside the realm of past experience. During this decade, La Quinua will rank as a world-class gold producer. It will be the largest producing mine in the Yanacocha district, which in turn is the largest producing gold district in South America. From an exploration perspective, there is no reason not to expect more “La Quinuas” in terranes with similar geologic histories.
The authors thank Newmont Mining Corporation and Minera Yanacocha SRL for permission to publish this paper. Valuable suggestions were provided through reviews by Richard Garnett, René Morocco, and Richard Sillitoe. We are indebted to many individuals who have furthered our understanding of the La Quinua and the Yanacocha gold district. Specifically, we thank present and past members of the “La Quinua team:” Brian Arkell, Bernardo Aznarán, Rafael Bartra, Edelmira Calderón Vicuña, Nilton Champy, Bruce Harvey, Joel Melgar, Luis Morales, Richard Pilco, Rita Pinto, Miguel Rutti, Jimmy Trejo, José Trujillo, César Velazco, and Cindy Williams. Special thanks go to Doris Davalos and Cecilia Ventocilla who drafted the figures. Martin Hayes provided background music. Lastly, we are all indebted to Steve Moore and José Trujillo for their original hard work and persistence—and especially for having the presence of mind to assay the gravel.
Figures & Tables
Andean Metallogeny: New Discoveries, Concepts, and Updates
A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.
Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo