ACTIVE OIL SEEP AT NEVADA GOLD MINE HOLDS INTRIGUE FOR MORE EXPLORATION

Michael L. Pinnell, Jason G. Blake
Pioneer Oil & Gas
Midvale, Utah

Jeffrey B. Hulen
University of Utah
Research Institute
Salt Lake City, Utah

An active oil seep has been discovered in one of Nevada's famous "Carlin-type" low grade disseminated gold deposits.

This unique seep, at the Yankee gold mine in White Pine County (Fig. 1), may have important implications for both oil and gas and gold exploration in the Basin and Range province of the western U.S.

The open pit Yankee mine, near the western margin of Long Valley (Fig. 1), exploits one of numerous Carlin-type gold ore bodies in the Alligator Ridge mining district; all are currently owned and operated by USMX Corp., Reno.

Characteristically for the Carlin-type deposit,1 11 12 the Yankee ores and others at Alligator Ridge are hosted by hydrothermally altered Paleozoic rocks enriched not only in micron-sized disseminated gold but also in arsenic and, locally, hydrocarbons.6 7

Yankee is unique among these deposits, however, in hosting abundant "live" oil, not only actively seeping (Plates 1 and 2) but also occurring as fluid inclusions in associated hydrothermal vein minerals.

The inclusions point to direct involvement of a convecting hydrothermal system in oil migration and entrapment.

SEEP DISCOVERY

In preparation for drilling a 7,000 ft wildcat in Long Valley in 1990, Jason Blake of Pioneer Oil & Gas contacted the Alligator Ridge mine, then owned by Kennecott Corp., to arrange for a telephone link to the well site.

Kennecott approved the link, and an employee casually mentioned a substantial area of oil impregnated, fractured, and hydrothermally-veined calcareous shale at the bottom of the newly excavated Yankee pit.

The employee said oil had oozed from these rocks and accumulated in shallow depressions in the pit floor.

Blake collected a small amount of free-flowing oil from the mine wall for geochemical analysis, while Pinnell initiated a detailed photographic and geologic study of the mine. Several hundred pounds of oil bearing rocks were also collected for detailed geochemical analysis of the oil and its host rock.

Subsequently, Hulen initiated detailed vein-mineral and fluid inclusion studies of the Yankee mine seep as part of U.S. Department of Energy funded research on the role of hydrothermal systems in evolution of Basin and Range oil fields.

Preliminary results of these geochemical and fluid inclusion studies help constrain the oil seep's origin and evolution.

GEOLOGIC SETTING

The Alligator Ridge mining district is located in the eastern Basin and Range physiographic province, characterized by narrow, northerly-trending, alternating fault block mountains and valleys, the latter thickly filled with alluvial and lacustrine sediments mostly of Miocene to Recent age.

The district, in the easternmost foothills of the Ruby Mountains-Buck Mountain chain, encompasses Yankee and numerous other Carlin-type deposits hosted by Paleozoic carbonate and siliciclastic rocks ranging in age from Cambrian through Mississippian.

These rocks form a broad, generally north-south trending anticlinal trend6 at least 10 miles in length.

The crest and 10-30 dipping west flank of the anticline expose progressively younger rocks ranging westward from the Devonian Nevada (?) formation (dolostone) through Devonian Devil's Gate formation (dolostone and limestone) and Mississippian-Devonian Pilot shale (calcareous, carbonaceous siltstone, occasional argillaceous limestone-the host rock for the Yankee mine oil seep); Mississippian-age Joana limestone, Chainman shale, and Diamond Peak formation crop out farther west (Fig. 2).

These Paleozoic rocks are locally concealed by tuffaceous sedimentary rocks of Tertiary age and by andesites that Ilchik6 speculates may be 39-24 million years old.

The low grade, Carlin-type gold orebodies of the Alligator Ridge district occur principally in silicified, decalcified Pilot shale.6

Hydrothermal veinlets within the ore bodies are dominated by quartz, kaolin, and stibnite; those distal to the orebodies are dominated by kaolin or by calcite and the arsenic sulfides orpiment and realgar.

The largest of the Alligator Ridge deposits, the Vantage group, yielded 5.8 million tons of ore averaging 0.13 oz/ton gold6-an inventory worth about $273 million at today's gold prices.

The absolute age of the Alligator Ridge deposits is currently unknown, principally because of a lack of suitable secondary minerals for dating. Ilchik6 suggests the age of mineralization has an upper range of 39-24 million years from associated volcanics and a lower range from a potassium-argon age of 11.5 million years for alunite (potassium aluminum sulfate) from the Vantage deposits, but according to Seedorf 15 this may simply date post-mineral supergene oxidation of the deposit. Seedorf, citing Bonham2 believes the Alligator Ridge orebodies were probably formed, like most other Carlin-type deposits, during the mid-Tertiary interval 43-34 million years ago.

YANKEE MINE OIL SEEP

Live oil in the lower walls and floor of the Yankee mine open pit occurs primarily as thick coatings and vug fillings along fractures and hydrothermal veinlets disrupting argillaceous thin-bedded limestone of the Devonian-age portion of the basal Pilot shale.

Within the area of the oil seep, this limestone is bluish-gray and unoxidized; elsewhere in the pit it is thoroughly oxidized, bleached to a light tan color, and stained with trace to minor amounts of secondary geothite and hematite (Plate 3).

The limestone is locally folded, intensely fractured, and disrupted both by minor thrust faults and major high-angle normal faults; the latter probably provided ingress for circulating hydrothermal solutions.

The unoxidized limestone hosting the Yankee mine oil seep is cut by abundant, oil stained calcite veinlets that also contain primary red realgar and probable secondary yellow orpiment. The relationship of these veinlets to gold mineralization at Yankee remains to be determined.

However, at the rich Vantage deposits, about 6 miles to the north, veinlets of this composition were considered by Ilchik6 to be contemporaneous with but deposited distally to gold mineralization.

HOST ROCK, OIL GEOCHEMISTRY

Detailed chemical analyses of oil from the Yankee mine seep as well as its host rocks were completed by DGSI Laboratory, The Woodlands, Tex. (table and Fig. 3).

Maturity of the organic poor argillaceous Pilot shale limestone hosting the oil seep appears to be very close to the oil preservation limit.

By contrast, as shown by hopane and sterane isomerization as well as sterane ratio, the free oil of the seep was derived from a source rock that had reached a stage of maturity just slightly exceeding peak oil generation.

The Mississippian Chainman shale, a major petroleum source rock throughout eastern Nevada and notably at Grant Canyon and other fields in Nye County's famous Railroad Valley,13 17 is believed to have furnished the oil for the Yankee mine seep.16

Biodegradation of this oil is mild to moderate, as evidenced by the presence of n-alkanes and unaltered pristane, phytane, sterane, and terpanes.16

FLUID INCLUSIONS

In addition to occurring along fractures, in vugs, and as coatings on vein calcite, oil in the Yankee mine seep occurs inside the calcite crystals, trapped as tiny fluid inclusions.

When heated and cooled under carefully controlled conditions, these inclusions can provide a wealth of information about fluid temperatures and compositions at the time the inclusions were trapped.14

Fluid inclusions in calcite from the Yankee mine oil seep are of both primary and secondary origin. The former were trapped as the host crystals were growing; the latter were encapsulated along healed fractures after cessation of crystal growth.

Three varieties of both primary and secondary inclusions have been observed-those containing (at room temperature) pure oil and a vapor bubble, those composed of an aqueous solution and a vapor bubble, and those with both oil and aqueous solution in addition to vapor.

Upon heating to the appropriate temperature, the vapor bubbles disappear (the inclusions homogenize). The temperatures at which the bubbles vanish are the minimum temperatures of fluid entrapment.14

Since the Alligator Ridge deposits were probably formed at relatively shallow depth (1,600-3,300 ft6), fluid inclusion homogenization temperatures for the Yankee mine veins can be considered very close to actual entrapment temperatures.

Homogenization temperatures for the calcite-hosted, Yankee mine oil, oil/aqueous, and aqueous inclusions measured to date range from 75 C./167 F. to 106 C./223 F. The higher temperatures correspond to primary inclusions and reflect temperatures of the mineralizing solutions at the time of crystal growth.

Lower temperatures correspond to secondary inclusions, and document gradual cooling of the oil-bearing solutions after crystal precipitation.

The higher temperatures recorded for the primary Yankee mine inclusions are just slightly lower than those measured for similar primary oil and water inclusions in hydrothermal quartz from Grant Canyon oil field in Railroad Valley.5 17

As determined by freezing point depressions, aqueous fluids in inclusions from Grant Canyon are relatively dilute (< 2 wt % NaCl equivalent); apparent salinities for the Yankee mine inclusions have not been determined.

DISCUSSION, CONCLUSIONS

The authors' detailed research on the remarkable Yankee mine oil seep has just gotten under way, but even the preliminary findings have intriguing implications.

First, discovery of primary, "live" oil bearing fluid inclusions at Yankee demonstrates that initial oil migration and entrapment there accompanied circulation of the hydrothermal fluids that deposited the host calcite-orpiment-realgar veinlets.

If we assume that these are contemporaneous with identical veinlets at the Vantage deposits a few miles to the north, then they probably formed during mid-Tertiary time.6 15

Thus, oil had either been generated in the district prior to mid-Tertiary time, then was remobilized in the Yankee mine hydrothermal system, or the system itself (perhaps in its deeper, higher-temperature regions) was responsible for both generation and migration of oil during the mid-Tertiary.

In either case, the Yankee mine fluid inclusions provide further support for a model of Grant Canyon field' that invokes a still-circulating, moderate-to high-temperature geothermal system as a major influence in oil reservoir evolution.

The age of actively seeping oil at the Yankee mine relative to the age of associated fluid inclusion oil is unknown at present, but it should be noted that free oils and inclusion oils are intimately related at both the Yankee mine and Grant Canyon oil field.

Moreover, at Grant Canyon, entrapment temperatures for fluid inclusion oils are the same as the current reservoir temperature.5

The authors strongly suspect that at both locations the inclusion oils and free oils are genetically related and were transported to their entrapment sites in convecting, moderate-temperature hydrothermal systems.

WHERE DID THE OIL COME FROM?

The authors have shown that free oil in the Yankee mine seep (and possibly associated fluid inclusion oil) was likely sourced from the Mississippian Chainman shale,16 a unit younger than the seep's host rocks.

No other source rocks in suitable stratigraphic position are known directly beneath the seep. This fact and the relationships outlined above permit several plausible scenarios for the seep's evolution.

Based in part on proprietary oil well, surface geologic mapping, and geophysical data, Pioneer Oil & Gas geologists believe the most likely of these scenarios involves structural entrapment of Chainman-derived oil during early to mid-Jurassic time.

During post-jurassic/preLate Tertiary compression, this oil reservoir may have been overthrust by more mature Devonian to Permian rocks. Mid-Tertiary hydrothermal systems at Alligator Ridge then may have remobilized the oil.

As proposed by Moulton10 for Grant Canyon field, this remobilization may have been effected by deep hydrothermal heating, creating or enhancing a gas cap that forced a portion of the oil out of the subjacent reservoir.

The newly liberated oil would have been entrained and transported in a hydrothermal convection cell, ascending into what would later become the Yankee mine seep. Multiple hydrothermal events, quite likely in the thermally active Basin and Range,9 would repeat the process and periodically replenish the high level secondary oil accumulation.

The current presence of relatively undegraded oil in the seep implies either that deep oil was supplied to the seep site in the geologically recent past, or (perhaps less likely) that oil transported to the seep site during the Yankee mine mineralization was effectively sealed until breached by modern mining,

One important implication of this model is that deep, relatively ancient oil reservoirs remain to be tapped in the vicinity of the Alligator Ridge district.

Another, based on the authors' preliminary fluid inclusion data, is that oilbearing hydrothermal fluids locally may have played a direct role in gold mineralization.

If the authors' detailed studies in progress at the Yankee mine support this idea, it may lead to new gold discoveries at Alligator Ridge and elsewhere.

ACKNOWLEDGMENTS

The authors are grateful to USMX Corp. for providing access to Yankee mine oil seep as well as insight into its evolution. They also appreciate geologic discussions of this part of Nevada with Floyd C. Moulton. Funding for Hulen's vein mineralization and fluid inclusion research at Yankee is being provided by the U.S. Department of Energy, Office of Basic Energy Sciences (Grant No. DE-FGo2-90ER14133); this support does not necessarily constitute a DOE endorsement of the views expressed in this article.

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