Round Top prospect (incl. Tim's Greisen prospect), Kaiyuh Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Round Top prospect (incl. Tim's Greisen prospect)	Prospect
Kaiyuh Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Location: The Round Top prospect is on the south-central flank of the Kaiyuh Hills, in sec. 21, T. 15 S., R. 7 W., approximately 3300 feet southwest of the peak of VABM Round Top. The lobate prospect area, defined by drill holes and sediment sampling, is about 0.6 mile wide and extends northwest for approximately 1.24 mile. The coordinates are for the approximate center of the western lobe of mineralization. The location is accurate within about 300 feet. Tim's Greisen, a small, nearby occurrence interpreted by Harris (1980) to be part of the Round Top system, is included in this record. Tim's Greisen is located approximately 0.3 mile southwest of the Round Top prospect.
Geology: The Round Top porphyry Cu-Mo deposit was discovered in 1980 during a reconnaissance exploration program by Anaconda Minerals Company (Harris, 1985). Harris completed a Masters thesis at the University of Colorado on the Round Top prospect in 1985 and the following information is from that thesis, unless stated otherwise. The country rocks in the area of the Round Top prospect consist of greenschist and amphibolite, locally accompanied by blueschist-grade pelitic schist and quartzite. Lesser amounts of carbonate rocks and metavolcanic rocks are also present (Gemuts and others, 1983). All have undergone multiple periods of deformation and generally trend northeast. Outcrop in the prospect area is scarce and the geology is largely defined by rubble. It consists of a large intrusive complex that trends northwest, parallel to a major regional fault located 0.6 mile southwest of the prospect. The intrusive complex can be divided into two masses: 1) a western lobe exposed for 0.3 mile; and 2) an eastern lobe exposed for 0.25 mile. The two lobes cover an area of approximately 0.8 square mile, although much of the area is obscured by willows. Harris (1985) identified six types of porphyritic intrusive rock and an intrusive microbreccia in the complex. The two oldest porphyrys, which are quartz monzonite, appear to be the parent lithology for the microbreccia and are associated with the mineralization. They are cross-cut 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 and intrusive 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. The microbreccia comprises coarse-grained breccia that is cut by finer-grained breccia. 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 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 schists. Locally calcareous rocks have been altered to calc-silicate hornfels near the intrusive. Two small outcrops of light-green metavolcanic rocks (possibly meta-andesite) are also present. 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. 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, along with traces of pyrrhotite. Chalcopyrite content decreases outward from the zone of intense stockwork veins, and lead, zinc, and silver increase, as determined by rock, soil, and sediment sampling at the western boundary of the intrusive complex. Within the wall rock schists, iron enrichment is is marked by iron sulfides in calc-silicate hornfels. The average copper grade in calcareous schist in the 1400 East Gossan horizon is 0.78%. 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 schists near the complex have undergone calc-silicate hornfelsing and iron metasomatism. The early potassic and propylitic alteration centers around an approximately 330-foot-diameter zone of stockwork quartz veins. Potassic alteration extends west for about 3600 feet, and gradually transitions to propylitic alteration. Quartz veins in the stockwork zone make up to 90% of the rock. Older, smaller 'A'- type veins containing potassium-feldspar margins locally contain molybdenum, pyrite, and chalcopyrite. Younger, larger, 'B'- type veins usually contain molybdenum in their cores or along selvages. Sericite, anhydrite, and andalusite occur along some vein margins. The propylitic alteration consists of chlorite, epidote, calcite, actinolite, montmorillonite, and chalcedonic quartz. The later sericite-quartz-pyrite alteration is fracture controlled and pervasive through the intrusive complex. A zone of supergene enrichment underlies the oxide (gossan) zone, which typically is about 300-400 feet deep. The supergene zone contains chalcocite and sparse covellite and native Cu, as well as pyrite and chalcopyrite. Within the oxide zone, goethite, hematite, jarosite, and limonite are common. The oxidation of these rocks results in the formation of a kaolinite-sericite-quartz assemblage that is leached of all sulfides. Tim's Greisen, a nearby polymetallic occurrence, is interpreted by Harris (1980) to be part of the Round Top system (Flanigan, 1998). At this occurrence, the greisen contains locally brecciated quartz-chlorite-muscovite schist and gossan. The brecciated schist is mildly bleached, iron-stained, and altered to clay. The gossan is black or gray, massive, and extends over an approximately 250- by 820-foot area. Sooty, amorphous, yellow, black or gray, and red-orange supergene oxides fill 0.6- to 1.2-cubic-inch voids in the gossan. Lithified gossan is composed of a thick, dark, iron and manganese coating on small, foliated siliceous structures. Assays of grab samples at Tim's Greisen show values of up to 4.52% lead, 1.3% zinc, 6.15 ounces of silver per ton, 1700 ppm tin, 415 ppb gold, 1255 ppm copper, >1000 ppm arsenic, >2% manganese, 65 ppm tungsten, and 8 ppm molybdenum (Flanigan, 1998). Also see NL002, 008, 009, and 010.
Workings: This prospect was discovered by Anaconda Minerals Company in 1980. From 1980 to 1984, they drilled seven diamond-drill holes, dug numerous trenches, completed extensive geologic mapping and geophysical surveys, and conducted an extensive sediment and soil sampling program. To assist exploration, a large base camp and an airstrip were constructed (Harris, 1985).
Age: A K/Ar date on potassium feldspar from the oldest porphyry at Round Top is 74 +/- 2.8 Ma, corresponding to the age of cooling and mineralization (Harris, 1985). Although K/Ar dating is suspect, a Late Cretaceous or Early Tertiary age is probable.
Alteration: 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 schists near the complex have undergone calc-silicate hornfelsing and iron metasomatism. The early potassic and propylitic alteration centers around a 330-foot- diameter zone of stockwork quartz veins. Potassic alteration extends west for about 3600 feet, and gradually transitions to propylitic alteration. Quartz veins in the stockwork zone make up to 90% of the rock. Older, smaller 'A'- type veins containing potassium-feldspar margins locally contain molybdenum, pyrite, and chalcopyrite. Younger, larger, 'B'- type veins usually contain molybdenum in their cores or along selvages. Sericite, anhydrite, and andalusite occur along some vein margins. The propylitic alteration consists of chlorite, epidote, calcite, actinolite, montmorillonite, and chalcedonic quartz. The later sericite-quartz-pyrite alteration is fracture controlled and pervasive through the intrusive complex. A zone of supergene enrichment underlies the oxide (gossan) zone, which typically is 300-400 feet deep. The supergene zone contains chalcocite and sparse covellite and native Cu, as well as pyrite and chalcopyrite. Within the oxide zone, goethite, hematite, jarosite, and limonite are common. The oxidation of these rocks results in the formation of a kaolinite-sericite-quartz assemblage that is leached of all sulfides.

Commodities (Major) - Cu, Mo; (Minor) - Ag, Pb, W, Zn
Development Status: None
Deposit Model: Porphyry Cu-Mo, and skarn with stockworks and gossans, disseminated (Cox and Si

Other Databases

Link to USGS - Alaska:	NL011

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References