Ag/Au Ratios in Time and Space in the Bingham Mining District, Utah: Implications for Genesis and Exploration

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

"There are two frontiers of knowledge: that which we are about to discover, that which we discovered long ago and are about to forget."

The position in time and space of precious metals in porphyry copper districts is poorly documented. In 1988, Sillitoe stated that porphyry systems commonly exhibit well-defined metal zoning with centrally located copper surrounded by Zn-Pb-Ag aureoles. He could have been describing the Bingham district, which contains a central Cu-Mo zone associated with a small porphyry stock, surrounded by copper-bearing skarns, surrounded in turn by Pb-Zn-Ag vein and replacement ores in limestone and igneous rocks, the whole pattern measuring six kilometers in diameter. Sillitoe went on to say that gold may be concentrated at several different sites within this zonal sequence--it may be concentrated with copper in centrally located porphyry stockworks and proximal calcic skarns, and is by no means restricted to a distal position beyond Zn, Pb, and Ag.

This important statement by Sillitoe, which reminds us of something discovered long ago (e.g., Boutwell, 1905) but forgotten or ignored in the copper boom of the 1970s and 80s, is the theme of my talk. I will present a description of gold and silver distribution in the Bingham mining district and compare the results with other districts of the porphyry-copper clan in an effort to achieve inter- district generality.

Gold & silver in average base-metal ores of Bingham

The main base-metal zoning pattern from pluton (Cu-Mo) to skarn (Cu) to limestone (Pb-Zn-Ag) displays a slight decrease in Cu:Au ratios by one order of magnitude and a significant overlap in Cu:Au ratios between the skarn and limestone environments In contrast, Ag:Au ratios over the same trend display an order of magnitude increase, but there is little or no overlap in Ag:Au between different ore types or host rocks; that is, Ag:Au ratios appear to be a better descriminator of both zoning and ore environment. Furthermore, there is a strong negative correlation between Ag:Au and Au content. Thus, Ag:Au ratios are considered to be the single most useful indicator of both gold potential and space-time changes in ore-forming environments; also, the focus on Ag:Au allows for comparison between the change in this variable in space and time in porphyry- related systems compared to epithermal precious metal deposits and active geothermal systems.

Major base-metal ore types at Bingham can be devided into: (1) central Cu-Mo ores in porphyry, with 0.007 - 0.014 oz/t Au, 0.06 - 0.10 oz/t Ag, and Ag:Au = 7 - 10; (2) Cu ores in skarn, with 0.03 - 0.05 oz/t Au, 0.45 - 0.60 oz/t Ag, and Ag:Au = 15 - 25; and (3) peripheral Pb- Zn-Ag ores in limestone, with 0.06 - 0.02 oz/t Au, 3.5 - 5.0 oz/t Ag, and Ag:Au = 80 - 150. The highest grades of gold in these three types of large-tonnage base-metal ores are in skarn. Within this overall pattern, however, there is considerable variation in grades of precious metals and in Ag:Au ratio that are a complex function of evolution of ore-forming processes in both space and time. Ores with unusually high gold grades (referred to here as "gold ores" irrespective of economics) can occur anywhere in space and time, but the highest grades and tonnages of gold ore occur within the copper zone rather than within or distal to the peripheral Pb-Zn-Ag zone. Within any given zone, gold ores display the lowest Ag:Au ratios. At present we have a very incomplete understanding of the details of these variations, but some broad contrasts are fairly clear.

"Gold ores" in the porphyry copper stock

There are two different types of gold concentrations in porphyry copper ores at Bingham: (1a) "Disseminated bornite-gold ores" and (1b) "Vein realgar-gold ores". Type 1a predates Type 1b and may represent the earliest introduction of gold into the Bingham system. Type 1b may represent local remobilization of gold from Type 1a by late, lower temperature, more acidic hydrothermal fluids flowing along faults.

(1a) Disseminated Bornite-gold ores. It has long been known that, discounting skarn ore, gold grade tends to be highest in any given porphyry copper deposit in the bornite zone of potassically altered rocks and commonly in the highest copper grades of such zones (e.g., Sillitoe, 1988; for Panguna, PNG, see Fig. 3 of Clark, 1990). However, details of grade, mineralogy, and tonnage of such gold ores in porphyry copper deposits remains unknown. Gold grade distribution patterns at Bingham mimic copper distribution patterns. Within the zones of highest Cu and Au grades, 25 to 60 % of the rock consists of coarse- grained quartz as irregular patches up to several cm across and as veins up to a cm wide with irregular borders; the quartz contains disseminated brown biotite flakes and bornite + chalcopyrite. Relict patches (0.25 - 1.0 cm across) of porphyry between quartz veins are totally recrystallized to fine-grained, porous sugary quartz + Kfeldspar with very abundant disseminated bornite + chalcopyrite and minor pale brown flaky biotite. These volumes of highest Cu and Au grade occur as "halos" on veins or zones of almost pure quartz with lower grades of metals, reminiscent of the "barren quartz core" at Ok Tedi, Papua New Guinea (Rush & Seegers, 1990). The restriction to porphyry, the similar grade patterns as copper, the close association with high grade copper, and the mineralogical characteristics all point to a magmatic- hydrothermal origin for the gold.

(1b) Vein Realgar-gold ores: These gold occurrences are found along NNE-striking fissures that cut porphyry. Gold appears to be localized within the fissures and does not occur at significant grades in the wall rocks of the fissures. Where fissures are closely spaced, some signifcant mining widths may occur, but continuity along strike and dip appears to be limited. In contrast with the disseminated bornite-gold ores (Type 1a), which are associated with high-temperature alteration and vein styles, the realgar-gold veins contain features that indicate deposition from late, low-temperature, relatively acidic hydrothermal fluids. These features include sericite (?) and clay alteration, high pyrite content, and cinnabar- realgar(+/enargite). These arsenic minerals are characteristic of the high sulfidation states that accompany relatively high acidity at low temperatures.

Gold ores in copper skarn

There is one style of high-grade gold mineralization in the skarn environment at Bingham: "Silica-pyrite Au-Ag ores". In contrast with the first categories, this ore type is relatively well-known (Cameron and Garmoe, 1987). Silica-pyrite gold ores, generally with elevated arsenic content, are small-tonnage ore bodies, containing a few 100,000 to 1,000,000 tons each of 1-2.4 % Cu, 0.10-0.15 oz/t Au, and 0.35 - 1.0 oz/t Ag, generally in steep NNE- striking fissures that cut copper skarn ores. The 800,000 ton Parnell gold shoot with 0.12 oz/t Au, 0.26 oz/t Ag (the lowest Ag:Au ratios in the district), is 15-30 m wide, with over 500 m of strike length and 450 m of vertical extent. This ore type is characterized by dark gray, fine-grained quartz and 30-50 % pyrite, locally abundant arsenic minerals such as arsenopyrite and tennantite, and minor carbonate and clay. Accessory minerals include relic garnet, magnetite, and chalcopyrite from the precursor skarn assemblage. Gold occurrence and mineralogy are virtually unknown. Silica-pyrite gold fissures have been found only in copper-bearing skarn within 300 m of the stock, not further out in the Pb-Zn-Ag fringe or in limestone.

Other porphyry districts - Porphyry stockwork ores

Ag:Au data from Tanama, Puerto Rico, and Dos Pobres, Arizona, two pluton-hosted porphyry deposits, are avaliable from Cox (1985) and Langton and Williams (1982), respectively. Both of these data sets reinforce the Bingham trend and display a strong negative correlation between Ag:Au and Au. Both of these deposits can be contoured in map view for Au grades and Ag:Au ratios, and both show the same pattern as Bingham: Ag:Au ratios increase and gold grades decrease outward within the central copper zone in pluton-hosted ores.

Other porphyry districts - Limestone-hosted ores

Returning to skarn- and limestone-hosted ores in the Bingham district, skarn-hosted ores show a continuum from early skarn partially retrograded to chlorite-clay- pyrite assemblages to late crosscutting silica-pyrite fissures. The overall trend with time is one of declining Ag:Au and increasing Au grade. Cu:Au is fairly constant in skarn, but drops to low values in silica-pyrite. Limestone-hosted ores display a different trend: Ag:Au ratios of Pb-Zn-Ag ores that cut skarn are broadly comparable to the skarn itself, but the ratio abruptly increases as gold grades decline into the Pb-Zn-Ag fringe in limestone. Although the relative age of the silica-pyrite-gold and Pb-Zn-Ag ores is not known, there is indirect evidence that suggests that the silica-pyrite-gold fissures are the youngest event in the district.

Comparable data for skarn and limestone-hosted ores in other porphyry districts is difficult to come by. One example is Yauricocha, Peru, described by Thompson (1960) and Petersen (1965); gold and silver data recently have been published by Alvarez and Noble (1988). At Yauricocha, ores are hosted by silica-pyrite bodies in limestone and are zoned individually and on a district scale from copper-gold-rich in the central portions to lead-silver on their margins. The trend for Au-content versus Ag:Au is the same as the trend for limestone-hosted ores at Bingham: a trend outward toward increasing Ag:Au ratios, decreasing Au and Cu, and increasing Pb+Ag.

Comparison with the Battle Mountain district, Nevada

The Battle Mountain district (Roberts, 19xx), including the Copper Canyon and Copper Basin subdistricts, are both centered on porphyry stocks and dikes that are weakly mineralized. Copper, and more recently gold, ores have been mined from sedimentary rocks on the margins of the stocks, and Pb-Zn-Ag ores are found on the fringes of the district. Production data, contoured on a district scale, displays the familiar pattern of increasing Ag:Au outwards on a scale in excess of six kilometers. All of the recent discoveries of gold deposits in the Copper Basin and Copper Canyon area plot within the lowest Ag:Au zone near the centers of mineralized patterns (e.g., Tomboy-Minnie and Fortitude gold deposits). In the Copper Canyon area, the Copper Canyon stock is not the center of metal zoning as previously thought; rather, an elongate zone of Ag:Au less than 5 defines an axis of hydrothermal activity related to a series of buried stocks. This point was made by Theodore et al. on the basis of fluid inclusion studies at the Tomboy-Minnie gold deposit which suggested that temperatures were too high to relate that deposit to the Copper Canyon stock located about 1 km to the NNW.

Space-time evolutionary scheme for precious metal ores in porphyry systems.

"Early stage" characterized by potassic alteration at 600 to 500 C in porphyry and zoned skarn formation at 500 to 400 C in limestones. Gold is first introduced with copper and without pyrite at this stage, and is generally broadly disseminated and in low amounts, preferentially concentrated in skarn. Gold and silver both display positive correlations with copper under most conditions. If skarn development occurs under unusually reducing conditions then gold is deposited at ore grades, accompanied by pyrrhotite, from chloride solutions, as may have been the case in the Fortitude hedenbergite skarn at Copper Canyon. Hydrothermal fluids carrying the same concentration of gold, but at the higher oxidation states of andradite-salite skarn, such as Bingham, would deposit less gold before entering the field of gold-bisulfide dominance below 300 C, where a reversal to retrograde solubility could occur. Under such conditions, lower grades of initial gold concentration would be expected.

"Middle stage", characterized by sericitic alteration in porphyries and retrograde alteration of skarn to actinolite, chlorite, and possibly some clays. Structures, especially faults and stock contacts, play an increasingly important role and focus the flow of retrograding fluids. This is a mildly acidic and oxidizing event, capable of remobilizing both base- and precious metals. Base metals may be scavenged under acid conditions in chloride-rich fluids from stocks and reprecipitated with pyrite in the more alkaline skarn environment. The copper-gold association in skarn is locally reworked and upgraded, with gold solubility possibly controlled by bisulfide complexing. Large portions of skarn and stock may be isolated from such events, however, as fluid conduits shift position. The absence of retrograde alteration in the Fortitude skarn suggests that this may have been the case there, as fluids were channeled into the Liberty fault system, producing gold ores associated with actinolite, chlorite, and pyrite in the Upper Fortitude and Tomboy-Minnie deposits. Pb-Zn- Ag ore deposition takes place from chloride-rich solutions at the skarn/limestone contact where structures are preferentially concentrated--these ores cut skarn, locally contain high grades of gold, and form chlorite-carbonate- clay envelopes.

"Late Stage" involves extreme acid leaching in stocks and leads to advanced argillic alteration characterized by pyrophyllite/kaolinite-quartz-pyrite and high sulfidation state sulfide assemblages such as pyrite- bornite-chalcocite, commonly accompanied by enargite. Where these late stage fluids encounter skarn, they form massive silica-pyrite bodies encased in clay-chlorite sheaths, such as the Parnell gold shoot at Bingham. Silica- pyrite more commonly forms in limestone beyond skarn, as at Ely, Nevada; Yauricocha, Peru, Copper Basin area of the Battle Mountain district, Nevada; and Tintic, Utah. The ores in silica-pyrite may contain pyrite-enargite in extreme cases of acid sulfidation as at Yauricocha, or pyrite- tennantite-arsenopyrite in less extreme cases, as at Bingham. Both assemblages may be accompanied by high grades of gold exhibiting low Ag/Au ratios and Cu/Au ratios; the positive correlation between Cu and Au, exhibited by earlier events, breaks down. The alteration assemblages suggest that hydrothermal fluid evolution is toward extreme oxidation and sulfidation, conducive to gold precipitation from bisulfide complexes and silver and base metal leaching or non-transport.

I have emphasized Ag:Au ratios as the best indicator of geochemical environment and spatial position of gold ores in porphyry systems. And this brings me back to the beginning of the talk. Long ago we discovered that Ag:Au ratios in many types of hydrothermal systems increase laterally and toward the surface, but the emphasis of research in the past decade on epithermal deposits and the geothermal analogy (where Ag:Au decreases toward the surface) have caused us to forget. In 1923, Pryor indicated that Ag:Au increased toward the surface in the Kolar gold field. In 1935, Nolan indicated that Ag:Au increased toward the margins of the Tonopah district. More recently, in 1988, Clarke and Titley showed that Ag:Au ratios increase laterally and upwards in the epithermal ores at the Tayoltita silver mine in Mexico. It would appear that porphyry systems also display these same trends. At Bingham, Copper Canyon, Dos Pobres, Tanama, and Yauricocha, the highest grades of gold are located in the immediate fringe of the copper zone where the lowest Ag:Au values of the district are encountered.

There are two main types of low Ag:Au gold deposits hosted by porphyry-related skarn. The Fortitude is one endmember; as shown by Larry Meinert, it fits the category of gold skarns formed at high temperature in reducing environments, with little or no retrograde alteration. Presumably, gold was deposited from chloride solutions at high temperature. The other endmember gold deposit that occurs in or near skarn, the silica-pyrite gold ores, represents perhaps the lowest temperatures and the most acidic conditions in carbonate rocks developed in the evolutionary history of a given district, a fluid type that most commonly is though of as distal. Are these the same types of fluids that form the Carlin-type disseminated gold deposits in sedimentary rocks? Are the differences between these silica-pyrite ores and the Carlin-type gold ores in part a function of the fact that the former, being superimposed on copper mineralized skarn and plutons, inherit some of the metals and sulfur of earlier events, hence contain more pyrite and base-metals? Are we seeing in the epithermal deposits, the distal reversal in Ag:Au ratios predicted by Don White, a looping continuum with the active geothermal systems and their low Ag:Au sinters? The answers to such questions must await further work. Paraphrasing the words of Sillitoe and Bonham, such studies have to be focussed on districts where exposures are superb, primary ores are exposed by mining, and structural complications can be worked out.

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