SOCIETY OF ECONOMIC GEOLOGISTS
INTERNATIONAL EXCHANGE LECTURE - JUNE 1994

6-km Vertical Cross Section Through Porphyry Copper Deposits, Yerington District, Nevada: Multiple Intrusions, Fluids, and Metal Sources

Marco T. Einaudi, Stanford University
Stanford, California, U.S.A.

Gaps in understanding of individual porphyry copper deposits result from limited exposures of the tops, sides, and bottoms of ore zones. In order to build models of porphyry systems, investigators have relied on observations from numerous localities that are thought to represent different paleolevels. This approach does not account for variability between deposits and for important features located several kilometers below and to the side of ore zones. Missing are direct observation and analysis of: potential source regions for fluids and their dissolved components; deep pathways for inward-convecting and thermally prograding non-magmatic fluids whose presence is predicted by numerical models of mass and heat transport; the effects of prograding fluids on cycling of metals and other components; and time-space relations of intrusions and hydrothermal evolution on a system-scale.

These and other questions can be addressed in the Yerington mining district, Nevada, where post-ore tilting during extreme custal extension (150%) in Late Tertiary time rotated the Jurassic porphyry copper deposits 90_ on their sides (Proffett, 1977). Present exposures are Jurassic- age cross-sections, with up to the west. The geological data base for the district was acquired through some 40 man-years of effort, initially (1967-1975) by geologists of the Anaconda Company, and later (1975-1990) at Stanford University. Studies have been published on district geology and structure (Proffett and Proffett, 1976; Proffett, 1977; Proffett and Dilles, 1984), district copper skarns (Einaudi, 1977, 1982; Harris and Einaudi, 1982), geology and alteration-mineralization of the porphyry copper deposit at the Yerington mine (Carten, 1986), petrology and U-Pb ages of Jurassic plutonic and cogenetic volcanic rocks (Dilles, 1987; Dilles and Wright, 1988; Proffett and Dilles, 1991), and geology, alteration, fluid inclusions, and stable isotope characteristics of the porphyry copper deposit at Ann Mason (Dilles et al., 1992; Dilles and Einaudi, 1992). The focus of this contribution is on the time-space evolution of hydrothermal fluids of diverse origin that generated the porphyry copper deposits in the district. The presentation is a composite from detailed studies at the Yerington mine and Ann Mason prospect and reconnaissance study of the paleosurface in the Buckskin Range west of Yerington. The composite view extends from the paleosurface (2.5 km above the top of copper ore, where copper ore is defined as rock containing > 0.3 % Cu) downward to depths of 6 km (2 km below the bottom or copper ore), and laterally 2.5 km from the edge of ore.

Geological setting

The Yerington district is located in the western edge of the present-day Basin and Range Province in the former site of a subduction-related magmatic arc that developed along the western margin of North America in Jurassic time (Dilles and Wright, 1988). Porphyry copper deposits in the district, exposed in the Singatse Range, are associated with granite prophyry dike swarms emanating from a deep granite cupola emplaced into an earlier quartz monzodiorite batholith. Based on U-Pb dates, the batholith and its related granite dike swarms and hydrothermal systems had a total life span of less than 1 m.y. at 169-168 Ma (Dilles, 1987; Dilles and Wright, 1988). The batholith is overlain by a 2 to 3 km-thick cogenetic andesite/quartz- latite flow-dome complex with interbedded volcaniclastics (Proffett and Dilles, 1991); there is no evidence to suggest the presence of stratavolcanoes. The batholith intruded a 3 km thick sequence of Middle to Upper Triassic volcanic arc andesite-rhyolite and Upper Triassic to Middle Jurassic shelf sequence of volcaniclastic, argillaceous, and carbonate sedimentary rocks, including evaporites and eolian sandstone at the top (Einaudi, 1977). Carbonate units of Late Triassic age contain small copper skarns at distances of 3 to 5 km from porphyry ore (Einaudi, 1977; Harris and Einaudi, 1982). Basin and Range extension on normal faults dipping east has rotated the district 90_ to the west (Proffett, 1977).

The description of attitudes, directions, and the interpretation presented below is given in the context of Jurassic time, rather than present day.

Yerington batholith

The batholith, genetically linked to all porphyry deposits in the district, is a composite calc- alkaline pluton, 15 X 15 km in plan view, with a floor at ~7-8 km depth and a roof at ~ 1-2 km depth where it intrudes its cogenetic volcanics. The petrologic summary that follows largely is taken from Dilles (1997). The batholith ranges in composition from early hornblende quartz monzodiorite (60 % SiO2), through hornblende- biotite quartz monzonite (66 % SiO2), to late hornblende- biotite granite (68 % SiO2). Sr isotope (87Sr/86Srinitial = 0.7040) and whole-rock O isotope (*18O = 6.8) values and major and trace element compositions indicate parent magmas were high-K andesites, probably derived by fractionation of basalt with little crustal contamination. Similar data also are consistent with granite being derived from quartz monzodiorite by crystal fractionation. All intrusive phases have an essential mineralogy of plagioclase, microperthitic K-feldspar, quartz, hornblende, biotite, magnetite, sphene, apatite, ilmenite, and zircon. In contrast with the coarse- to medium-grained equigranular textures and equidimensional shapes of the earlier intrusions, the youngest intrusions, closely associated in space and time with porphyry copper mineralization, are strongly porphyritic granite dikes with 50% aplitic (0.02- 0.05 mm) groundmass of mostly quartz and alkali feldspar. The porphyry dikes occur in three separate swarms within the batholith, each localized over individual cupolas on a deeper granite pluton whose top is at 5-6 km paleodepths. The dike swarms strike NW and have steep dips, controlled by a regional fracture pattern.

Porphyry copper deposits - Grades, sulfides, alteration

Six million tons of copper are contained in three porphyry copper deposits: 162 million tons (mt) of 0.6 % Cu in the Yerington mine; 495 mt of 0.4 wt % Cu and ~0.01 wt % Mo in the Ann Mason deposit; and > 500 mt of 0.4 wt % Cu in the Bear-Lagomarsino deposit. Gold and silver assays of drill core from the copper ore zones are typically < 0.03 and 0.17 ppm, respectively. Lead and zinc are both < 50 ppm. Sulfide mineral assemblages and percentages vary as a function of veinlet/vein types and wall-rock alteration. Below, styles of alteration-mineralization are identified by the alteration type.

Early potassic (phlogopite-Kspar) alteration at 2-4 km depth.

Most of the copper ore was deposited during potassic alteration, consisting of phlogopite-(magnetite-rutile) after hornblende and biotite, Kspar after plagioclase, and very abundant quartz-sulfide veinlets. Sulfide assemblages are bornite- magnetite-(digenite) (at Yerington mine only, where it is earliest and central with hypogene grades of 2-3 wt % Cu), bornite-chalcopyrite-magnetite, chalcopyrite, and fringing chalcopyrite > pyrite. These low-sulfur sulfide assemblages are disseminated and in quartz veinlets and there is a direct correlation between volume percent quartz veinlets and grade of copper: highest grades of copper at the Yerington mine occur in zones with 10-25 vol% quartz veinlets. The majority of veinlets occur as semi-sheeted sets parallel to the regional fracture pattern, rather than as stockworks linked to localized hydrofracting. Shreddy phlogopite after hornblende characterizes a fringing chalcopyrite > pyrite zone, where quartz veinlets are sparse, hydrothermal Kspar is absent, rock magnetite reappears, and grades drop below 0.2% Cu. At the Yerington mine, three successive, partly superimposed, potassic ore-forming events were associated with each of three early dike emplacement events. Based on direct observation, at least one of these early dikes did not vent to the surface, but terminated in an upward-flaring, igneous- matrix breccia that fingered and died-out upwards.

Early sodic-calcic (actinolite-oligoclase) alteration at 4-6 km depth.

Sodic-calcic alteration occurred at depth and laterally along the margins of granite cupolas, but overlapped upward and centrally with potassic alteration. Sodic calcic assemblages are characterized by actinolite after hornblende and biotite, oligoclase after Kspar, abundant sphene, local epidote, local tourmaline, and absence of biotite, magnetite, and sulfides. Veinlets contain oligoclase-quartz-(actinolite) locally with patchy chalcopyrite-pyrite where superimposed on potassic ore. The most intense sodic- calcic alteration yielded an oligoclase-quartz-sphene rock with halos of oligoclase-quartz-sphene-actinolite-(epidote). Weak sodic-calcic assemblages are transitional to propylitic assemblages. Detailed mapping in the Yerington mine indicates that sodic-calcic alteration at depth developed contemporaneously with potassic alteration higher in the system during each of several, separate intrusive events. Superposition of sodic-calcic onto potassic resulted in strong depletion of copper. Some late dikes post-date the termination of potassic and sodic-calcic alteration; these late dikes pre-date the late stages of hydrothermal alteration described below.

Late sodic (chlorite-albite) alteration at 2- 4 km depth.

Sodic alteration was superimposed on all porphyry dikes and on potassic and propylitic zones. At deeper levels, it consists of albite after feldspars, chlorite-(sericite) after ferromagnesian minerals, rutile, sphene, and sparse pyrite > chalcopyrite. Sodic assemblages occur along fractures and as halos on sparse quartz-albite veinlets and are flanked by a zone of chloritized biotite. At higher structural levels (near 2 km depth), sodic assembalges consists of albite-sericite- (chlorite) accompanied by tourmaline, 1-2 % pyrite, minor rutile, and rare quartz-pyrite-(tourmaline) veinlets. The sericite:chlorite ratio increases upward in a probable transition to sericite-quartz-pyrite assemblages (see next).

Very late sericitic alteration and tourmaline breccias at 0-2 km depth.

Youngest sericitic alteration, consisting of sericite-quartz with 2 to 10 vol % pyrite, formed as well-defined halos on through-going pyritic fractures and pebble breccias with steep Jurassic dips. At the Yerington mine, all sericitc alteration postdated all porphyry intrusions within the 2.5 km of vertical exposure available. The volume of sericitic alteration expands upward from 1-2% of the rock in potassic ore zones at 2-3 km depths to ~50% of exposures at 1 km paleodepth. Near the paleosurface in the Buckskin Range, sericite-quartz-pyrite related to the three underlying porphyry copper centers coalesces into a regional zone of pervasive sericite. Veins are filled with pyrite and lesser chalcopyrite, hematite, quartz, and sericite. The pyrite:chalcopyrite ratios in veins are dependent on local background sulfide assemblage and content. In potassic ore zones with original bornite-magnetite at the Yerington mine, sericitic veins contain chalcopyrite-(magnetite) or chalcopyrite>pyrite and constitute ore. In contrast, in previously low-grade upper portions of the pattern, sericitic veins typically have pyrite-chalcopyrite ratios that exceed 50:1. Pyrite veins are bordered by sericite-pyrite selvages with minor quartz and rutile; some veins display an outermost chloritic (intermediate argillic) halo. In most cases, intermediate argillic alteration is not present as an outermost selvage on sericictic veins. At higher structural levels, innermost silica-pyrite halos are present bordered by outer sericite-pyrite halos.

Tourmaline breccias root at about 2.5 km paleodepth and extend upward at least 2 km. They are up to several meters wide, and contain matrix-supported clasts averaging 1 to 5 cm in largest dimension in a matrix of quartz, tourmaline, 1 % pyrite, and trace rutile. Clasts are angular to subrounded and consist of sericitic and sodic assemblages similar to near-by wall rocks, indicating lack of large-scale upward transport (e.g., these are not pebble- dikes). Tourmaline breccias cut and/or re-open pyrite- sericite veins, identifying them as the youngest event.

Advanced argillic alteration at the paleosurface.

Andesite-dacite flows, breccias, tuff, and sediment, comprising a section up to 1.5 km thick in the Buskskin Range, are the extrusive equivalents of quartz monzodiorite. These are cut by flow- banded porphyry dikes that may correlate with late granite porphyry dikes of Singatse Range exposures. Sericitic alteration is pervasive in the upper part of the section in many areas of the Buckskin. Surface outcrops are intensely leached by weathering, but limonite mapping indicates that original sulfide content increased upward, averaging 4 % py:cp > 50:1. Fine-grained specular hematite, likely hydrothermal, is abundant locally. Within zones of pervasive sericitic alteration are stratiform silica ledges, up to 75 m thick and with strike lengths up to 400 m, that replaced mostly tuff or sedimentary units. These ledges are composed largely of fine-grained quartz; textures and character of limonite suggests they contained abundant fine-grained pyrite. Advanced argillic minerals, including alunite and pyrophyllite, occur within the silica ledges and on their borders. Also present are andalusite, diaspore, corundum, and zunyite. The mineral assemblages and paragenetic sequence among these minerals has not been worked out, and the origin of alunite (supergene, hotspring, or magma-hydrothermal) is unknown.

Character and sources of hydrothermal fluids

Pre-porphyry fluids.

Fresh rocks yield whole-rock *18O values of 6.7 to 7.0 and biotite *D values of -85 to -88, typical magmatic values, which are calculated to be in equilibrium with 700C water with *18O values of 7.4 and *D values of -65. Igneous quartz in the early equigranular phases of the batholith contains liquid- rich L-V fluid inclusions with halite, sylvite and hematite daughters, interpreted to have been trapped during crystallization of quartz monzodiorite and quartz monzonite. These fluids did not generate ore.

Fluids associated with potassic alteration.

Primary L-V fluid inclusions (+daughters) in quartz, without coexisting V-rich fluid inclusions, homogenize by halite dissolution at 150 to 550 C and yield calculated salinities of 32 to 62 wt % NaCl equiv. Whole- rock and/or Kspar have *18O = 6.5 to 8.4 and biotite has * D = -68 to -69. Values from deeper samples are calculated to be in equilibrium with magmatic water at 500 to 600 C. These data, in conjunction with the geologic and petrologic data presented above, are consistent with copper deposition and exchange of K for Na from upward flowing and cooling fluids released episodically during dike emplacement.

Fluids associated with sodic-calcic alteration.

Application of phase equilibria suggests temperatures of 360 to 480 C for oligoclase- actinolite assemblages, and peristerite solvus relations suggest temperatures greater than 375 to 400 C. Primary L-V fluid inclusions (+daughters) in quartz and epidote homogenize by halite dissolution at 200 to 375 C and yield salinities of 31 to 41 wt % NaCl equiv. Whole-rock and/or oligoclase have *18O = 5.7 to 8.4 and actinolite has *D = - 68 to -69. Therefore, sodic-calcic fluids were dominantly nonmagmatic (*18O = < 3-6 and *D = -50 at ~ 400C) and likely originated as isotopically evolved, 18O-enriched formation waters, which circulated convectively into the deep root zone of the granite stocks. These fluids, exchanging Na for K on heating, were capable of leaching and transporting precisely those components that were added to the ore zone (e.g., K, Fe, Cu, S, etc.). High salinities of these fluids may have been achieved by their interaction with evaporite, shale, and carbonate in the Triassic-Jurassic sedimentary wall rocks.

Fluids associated with sodic and sericitic alteration.

Whole-rock and/or albite have *18O = 6.7 to 9.6 and chlorite has *D = -79 to -85. Primary fluid inclusions have not been identified in vein minerals associated with sodic alteration. The youngest, sericitic, alteration displays a distinct enrichment in 18O and D relative to other alteration types: whole-rock and/or quartz have *18O = 9.8 to 10.4 and sericite has *D = -61. Possibly primary L-V fluid inclusions in quartz homogenize to liquid at 100 to 250 C and yield salinities of 2 to 13 (mostly 2 to 5) wt % NaCl equiv. Pressure corrections result in a mode for late-stage fluids of 220 ± 20 C. Tourmaline breccias contain the only vapor-rich inclusions noted in hydrothermal minerals at Yerington; their potential coexistence with L-rich fluid inclusions present in the samples, which would suggest boiling, is dicounted because hydrostatic pressure estimates are considerably less than the geologic depth estimate. Sericitic alteration was caused at high water:rock ratios by fluids with calculated (225 C) *18O = 0 and *D = -20 to - 55, whose origin most likely was 18O-shifted (heavy) coastal meteoric water or seawater.

Synthesis.

At least three hydrothermal fluids of different origin were involved in generating the alteration-mineralization patterns at Yerington. Earliest stages, cyclically repeated with each of several porphyry emplacement events, involved high- temperature flow and interaction of two fluids: (1) a cooling (thermally retrograding), saline, hydrothermal fluid of magmatic origin linked to potassic alteration and metal deposition, and (2) a heating (thermally prograding), saline, hydrothermal fluid of nonmagmatic origin linked to sodic- calcic alteration and metal leaching. These early, multiple patterns of sodic-calcic and potasssic alteration were broadly overprinted in their upper portions by (3) lower tempertaure, dilute, and convecting, nonmagmatic hydrothermal fluids of surface water origin that caused sodic and sericitic alteration.

Application to further understanding

There are many implications of the Yerington study that require further work and application to other districts. Among the more important are the sources of metals in ores, recognition of prograde fluid paths, and sources of hydrothermal fluids.

Sources of metals and other components.

In spite of the overwhelming evidence presented in numerous studies over the past decades for direct magmatic sources of many of the components in porphyry copper ore zones, the evidence from Yerington indicates that potential contributions from external sources should continue to be sought. For example, the point of transition from heating to cooling of nonmagmatic fluids is the point of transition from sodic-calcic (metal leaching) to potassic (metal deposition). Comparison of multiple analyses of fresh and altered rocks at Yerington indicates that up to 50 ppm Cu was leached during sodic-calcic alteration. This leaching could be the source of up to 30% of the copper deposited in the Ann Mason ore zone. Ultimately, though, this copper had its source in the magma of the Yerington batholith.

Prograde fluid paths.

In spite of the fact that most porphyry copper deposits are associated with multiple intrusions and therefore are likely to undergoe cyclical heating and cooling, most models of porphyry copper deposits deal only with unidirectional cooling. The prograde cycle rarely has been recognized, in part because it is most likely to be found at depths below ore zones. At El Salvador, the andalusite-corundum assemblages on the margin of the late porphyry intrusion have been interpreted as the result of prograding solutions (Gustafson and Hunt, 1975; Hemley et al., 1980). Prograding fluids are capable of destroying ore zones and likely contribute to the variability of metal-distribution patterns observed in porphyry deposits.

Sources of hydrothermal fluids.

It has been generally accepted that hydrothermal fluids in porphyry copper deposits have two sources: magmatic and meteoric. The Yerington study and recent studies of Bingham and Tintic, Utah (Bowman et al., 1987; Norman et al., 1991) indicate that this two-fluid view is simplistic.

References

Carten, R.B. (1986) Sodium-calcium metasomatism: chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit: Econ. Geol., v. 81, p. 1495-1519.

Dilles, J.H. (1987) The petrology of the Yerington batholith, Nevada: Evidence for the evolution of porphyry copper ore fluids: Econ. Geol., v. 82, p. 1750-1789.

Dilles, J.H., and Wright, J.E. (1988) The chronology of early Mesozoic arc magmatism in the Yerington district, Nevada, and its regional implications: Geol. Soc. America Bull., v. 100, p. 644-652.

Dilles, J.H., and Einaudi, M.T. (1992) Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada--A 6- km vertical reconstruction: Econ. Geol., v. 87, p. 1963-2001.

Dilles, J.H., Solomon, G.C., Taylor, H.P., Jr., and Einaudi, M.T. (1992) Oxygen and hydrogen isotopes characteristics of hydrothermal alteration at the Ann-Mason porphyry copper deposit, Yerington, Nevada: Econ. Geol., v. 87, p. 44-63.

Einaudi, M.T. (1977) Petrogenesis of the copper- bearing skarn at the Mason Valley mine, Yerington district, Nevada: Econ. Geol., v. 72, p. 769-795.

Einaudi, M.T. (1982) Description of skarns associated with porphyry copper plutons, southwestern North America, in Titley, S.R., ed., Advances in the geology of porphyry copper deposits, southwestern North America: Tucson, Univ. Arizona Press, p. 139-184.

Harris, N.B., and Einaudi, M.T. (1982) Skarn deposits in the Yerington district, Nevada: metasomatic skarn evolution near Ludwig: Econ. Geol., v. 70, p. 877-898.

Proffett, J.M. (1977) Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of basin and range faulting: Geol. Soc. America Bull., v. 88, p. 247-266.

Proffett, J.M., and Dilles, J.H. (1984) Geologic map of the Yerington district, Nevada: Nevada Bur. Mines Geology, Map 77.

Proffett, J.M., and Dilles, J.H. (1991) Middle Jurassic volcanic rocks of the Artesia Lake and Fulstone Spring sequences, Buckskin Range: Geol. Soc. Nevada, Field trip 16 guidebook compendium, v. 2, p. 1031-1036.

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