Round Top

Prospect, Active

Alternative names

Roundtop

Commodities and mineralogy

Main commodities Ag; Cu; Mo
Other commodities Pb; W; Zn
Ore minerals argentojarosite; beudantite; chalcocite; chalcopyrite; covellite; galena; jarosite; molybdenite; native cu; pyrite; pyrrhotite; scheelite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale NL
Quadrangle map, 1:63,360-scale A-4
Latitude 64.1703
Longitude -157.5334
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Round Top prospect is about 1.4 miles southwest of VABM 1954 'Round Top' in the Kaiyuh Mountains. It is on an east-west trending ridge at the headwaters of Minnesota Creek near the center of section 20, T. 15 S., R. 7 E., of the Kateel River Meridian. The location is between the drill collars RT-3 and RT-7 drilled by Anaconda Minerals Company in 1981. The location is accurate to 100 meters.

Geologic setting

Geologic description

Outcrop in the prospect area is scarce and the geology is largely defined by rubble. The country rocks in the area of the Round Top prospect are Paleozoic to Proterozoic(?) pelitic schist and quartzite of greenschist- and amphibolite-grade, locally accompanied by blueschist-grade (Patton and others, 2009). Lesser amounts of carbonate rocks and metavolcanic rocks are also present (Gemuts and others, 1983). All rocks have undergone multiple periods of deformation and generally trend northeast. The country rocks are intruded by a large intrusive complex that trends northwest, parallel to a major regional fault about 0.6 mile southwest of the prospect. The intrusive complex can be divided into a western lode and an eastern lobe that together cover an area of about 0.8 square mile.
Harris (1985) identified six types of porphyritic intrusive rocks and an intrusive microbreccia in the complex. The two oldest porphyries, which are quartz monzonite, appear to be the parent lithology for the microbreccia and are associated with the mineralization. They are crosscut by later porphyritic intrusive rocks. A K/Ar date on a potassium-feldspar phenocryst from the oldest intrusive gave an age of 74 +/- 2.8 Ma. The microbreccia is composed of lithic fragments that are cemented and replaced by biotite, potassium feldspar, and quartz, or by chlorite and quartz, with lesser amounts of chalcedonic quartz and montmorillonite, calcite and actinolite. The biotite-potassium feldspar-quartz replacement indicates potassic alteration and suggests that the microbreccia formed during copper mineralization. Gradational contacts between the microbreccia and the older quartz monzonite porphyry indicate that the microbreccia formed from it. The second-oldest porphyry, which is also quartz monzonite, is in sharp contact with the microbreccia; textural evidence suggests that the microbreccia was emplaced in pulses along shear zones, possibly coincident with explosive degassing during cooling of the intrusive rocks.
The wall rocks of the intrusive complex are mainly fine- to medium-grained pelitic schist. Near the intrusive, the calcareous rocks have locally been altered to calc-silicate hornfels. Two small outcrops of light-green metavolcanic rocks (possibly meta-andesite) are known (Harris, 1985).
The early potassic and propylitic alteration encloses an approximately 330-foot-diameter zone of stockwork quartz veins. Potassic alteration extends west for about 3,600 feet, and gradually grades into propylitic alteration. The propylitic alteration consists of chlorite, epidote, calcite, actinolite, montmorillonite, and chalcedonic quartz. The later sericite-quartz-pyrite alteration is fracture controlled and pervasive throughout the intrusive complex (Harris, 1985).
Mineralization at Round Top occurs as: 1) veins containing molybdenite, chalcopyrite, pyrite, and pyrrhotite; 2) gossan containing argentojarosite, jarosite, and beudantite; 3) a supergene zone beneath the gossan containing chalcocite, covellite, native copper, and chalcopyrite; and 4) calc-silicate wall rocks near the intrusive that contain pyrite, chalcopyrite, and sphalerite (Harris, 1985).
Quartz veins in the stockwork zone make up to 90 percent of the rock. Older, smaller 'A'- type veins with potassium-feldspar margins locally contain molybdenite, pyrite, and chalcopyrite. Younger, larger, 'B'- type veins usually contain molybdenite in their cores or along selvages. Sericite, anhydrite, and andalusite occur along some vein margins (Harris, 1985).
Molybdenite occurs primarily in an elliptical area of quartz veins that encloses an intensely veined stockwork zone at the eastern lobe of the intrusive. Chalcopyrite mineralization also occurs in this zone, and in potassically-altered microbreccia. The dominant sulfide in this zone is pyrite with traces of pyrrhotite. Chalcopyrite content decreases outward from the zone of intense stockwork veins and there is a concentric zone of tungsten mineralization as scheelite in quartz veins (Ellis and Roberts, 2014); lead, zinc, and silver increase at the western boundary of the intrusive complex. Within the schist, iron enrichment is marked by iron sulfides in calc-silicate hornfels.
A zone of supergene enrichment underlies the oxide (gossan) zone, which typically extends to a depth of 300 to 400 feet. The supergene zone contains chalcocite, sparse covellite and native copper, as well as pyrite and chalcopyrite. Within the oxide zone, goethite, hematite, jarosite, and limonite are common in a kaolinite-sericite-quartz assemblage that is leached of all sulfides (Harris, 1985).
Geologic map unit (-157.55924456148, 64.1729520813677)
Mineral deposit model Porphyry Cu-Mo (Cox and Singer, 1986; model 21a).
Mineral deposit model number 21a
Age of mineralization A K/Ar date on potassium feldspar from the oldest porphyry at Round Top is 74 +/- 2.8 Ma. Mineralization is likely younger than this (Harris, 1985).
Alteration of deposit Alteration consists of early potassic and propylitic alteration in the microbreccia and earlier porphyritic intrusions, and later sericite-quartz-pyrite alteration throughout the intrusive complex. The deposit also has undergone supergene enrichment, and calcareous schist near the intrusive complex have undergone calc-silicate hornfelsing and iron metasomatism (Harris, 1985).

Production and reserves

Workings or exploration
The Round Top porphyry prospect was discovered in 1980 during a reconnaissance exploration program by Anaconda Minerals Company (Anaconda) (Harris, 1985). Between 1980 to 1983 Anaconda completed geologic mapping, geochemical sampling, drilling, and geophysical surveys including MaxMin (small loop frequencey domain electromagnetics) survey, ground magnetics, electromagnetics (EM), gravity, and induced polarization surveys, and airborne magnetic regional survey (Ellis and Roberts, 2014).
The geochemical sampling in 1981 resulted in 874 soil samples and 730 rock samples being collected over the Round Top prospect. Results showed concentric zoning with molybdenum in the center grading outward through tungsten, to copper-lead and finally lead-silver-tin-copper. The molybdenum center anomaly coincides with the occurrence of quartz vein stockwork within quartz monzonite porphyry. The outer lead-silver halo is associated with a contact between the intrusive quartz monzonite porphyry and the schist (Ellis and Roberts, 2014).
Anaconda drilled at the Round Top prospect in 1981. They drilled 1,668.4 meters in seven drill holes. Highlights from the drilling include: hole RT-7 with 0.48 percent copper over 18.6 meters in a zone of supergene chalcocite enrichment, and 5 other mineralized zones that contained 0.36 to 1.10 percent copper; hole RT-3 had 5 intercepts with thicknesses between 26.3 to 62.7 meters that contained from 0.22 to 0.72 percent copper and up to 0.07 percent molybdenum; hole RT-2 intersected three zones of copper-silver mineralization that contained 0.98 to 1.8 percent copper and 4.5 to 23.0 grams per tonne silver (Ellis and Roberts, 2014; Western Alaska Copper and Gold, 2015).
Western Alaska Copper and Gold completed a complex resistivity induced polarization (CR-IP) survey on the Round Top prospect in 2010. Data was collected from 29 stations along two survey lines for a total of 5.4 line kilometers (Ellis and Roberts, 2014).
In 2012 Western Alaska Copper and Gold completed a soil geochemistry program collecting samples on a 500 by 100 meter grid covering the Round Top prospect, TG (NL028), and TG North (NL027) in order to determine the extent of the Round Top geochemical signature. They collected over 1,050 soil samples. The molybdenum results confirmed the concentric anomaly discovered by Anaconda in 1981. The copper soil anomaly is slightly west of the molybdenum anomaly and covers a 2.5 by 2.5 kilometer area. Western Alaska Copper and Gold also completed an airborne magnetic and radiometric survey in 2012 (Ellis and Roberts, 2014).
In 2016, Western Alaska Copper & Gold conducted exploration drilling at their Roundtop property in western Alaska, a combined porphyry copper-molybdenum, skarn, and lead-zinc-silver carbonate-replacement deposit. The 6-hole, 1,461-meter-total-length drill program tested historical drilling by Anaconda, as well as soil-geochemical anomalies and geophysical targets. Western Alaska discovered secondary chalcocite beneath the East Lobe Porphyry, including 39 meters of 0.50 percent copper within a 75-m interval of 0.31 percent copper. Chalcocite was found at depths of greater than 330 meters (Athey and Werdon, 2017).
Indication of production None

Additional comments

In the 2015 record update, Tim's Gossan or TG was separated into a separate record (ARDF number NL028).

References

MRDS Number A013473

References

Reporters C.E. Cameron (Northern Associates Inc.); D.J. Grybeck (Contractor, USGS); N.V. King (Alaska Earth Sciences); M.B. Werdon (DGGS)
Last report date 8/26/2017