H Block Prospect, Bristol Bay Mining District, Lake and Peninsula Borough, Alaska, USAi

Regional Level Types
H Block Prospect	Prospect
Bristol Bay Mining District	Mining District
Lake and Peninsula Borough	Borough
Alaska	State
USA	Country

Location: The H Block prospect consists of 149 State of Alaska mining claims approximately centered at the northeast corner of section 22, T. 9 S., R. 44 W., i.e., at about the center of the township. The center of these claims is about 5.3 miles west of triangulation station Yellow Hill 2, which is about 22 miles north-northeast of the village of Lovelock.
Geology: The H Block prospect is in an area of extensive glacial outwash about 8 km west of a terminal moraine complex associated with the west end of Iliamna Lake. Because bedrock is not exposed, the geology of the prospect is only known from two diamond drill holes by TNR Gold Corporation (Chapman, 2004). Il-H-01 is a vertical drill hole drilled in the center of a positive magnetic anomaly. It went through 67.1 meters (220 feet) of sand, silt, and fine gravel deposits to bedrock. Bedrock is a metasedimentary and metavolcanic(?) sequence that extended to the bottom of the hole at a depth of 208.8 meters (685 feet). The entire bedrock sequence was variably altered and mineralized. Petrographic descriptions (Chapman, 2004, Appendix B) indicate that at least some of these rocks are hornfels and calc-silicate rocks containing biotite-amphibole-plagioclase, plagioclase-clinopyroxene-quartz-sphene, and plagioclase-biotite-orthopyroxene assemblages. These rocks may be altered volcanic rocks and/or skarn. Granitic dikes were also encountered. The alteration varies from weak sericite, carbonate, and chlorite alteration to strong biotite veining and replacement. Secondary potassium feldspar was identified in and adjacent to some veins and fractures. Chalcopyrite, pyrite, and pyrrhotite occur both as disseminations and in veins and fractures but the veining is generally weakly developed. The sulfide-bearing veins commonly include quartz +/- carbonate. Some fractures contain sulfide linings and lack gangue minerals. Drill hole Il-H-02 was located about 1 km west of Il-H-01 and inclined - 70 degrees east. It targeted an IP anomaly on the flank of a positive magnetic anomaly. It intersected 85.4 meters (280 feet) of sand, silt, and fine gravel overlying medium- to coarse-grained granodiorite. This hole terminated in a fine- to medium-grained granitic intrusive (dike?) at a depth of 254.6 m (835 feet). The granodiorite varies from unaltered to extensively altered with chlorite, carbonate, and sericite replacement, local silicification, and secondary potassium feldspar and biotite adjacent to fractures. Sulfides in IL-H-02 are disseminated and fracture-controlled chalcopyrite, pyrite, and pyrrhotite similar in occurrence to that in IL-H-01. Assay results are fairly consistent over long intervals in these holes (Chapman, 2004). In Il-H-01, 141.8 meters (465 feet) contained 0.0195 gram of gold per ton and 231 parts per million (ppm) copper. The bottom 73 m (240 feet) contained 0.02 gram of gold per ton and 244 ppm copper. The best assays were 0.068 gram of gold per ton and 601 ppm copper over 1.5 meter (5 feet) and 0.044 gram of gold per ton and 709 ppm copper over 3 meters (10 feet). In Il-H-02, the entire 168.6 meters (553 feet) of bedrock averaged 0.0484 gram of gold per ton and 212 ppm copper. The high values included 3 meters (10 feet) of 0.0535 gram of gold per ton and 235 ppm copper and 1.5 meter (5 feet) of 0.037 gram of gold per ton and 1,125 ppm copper. The initial exploration shows that a porphyry copper-gold system is present at the H Block prospect. This system is buried by 200 feet or more of glacial outwash deposits where it has been drilled.
Workings: The H Block prospect was discovered as a result of regional geophysical surveys meant to locate Pebble-like (ARDF IL007) porphyry copper-gold deposits. In 2000, Rio Algom contracted a 7,468-kilometer-long (4,640 miles) aeromagnetic survey; the flight line spacing was 1,600 meters and it was flown at an altitude of 300 meters. This survey was followed up by ground IP surveys over 7 anomalies identified by the aeromagnetic survey. Two of these anomalies, H Block and D Block (DI011) were subsequently staked. BHP-Billiton Minerals International controlled these prospects when they were optioned to TNR Gold Corp. in 2002. TNR Gold Corp. in turn optioned the H Block to Geocom Resources Inc. in 2003. Geocom completed two diamond drill holes (Il-H-01 and IL-H-02) on the H Block prospect in August and September 2003. In late summer 2004, Geocom completed a 47 kilometer (28 miles) three-dimensional induced polarization-resistivity survey in the area of IL-H-01 and IL-H-02 in the H Block prospect. This survey identified several anomalies, some of which were tested in the fall of 2004 by five more drill holes (http://www.tnrgoldcorp.com/news/tnr_050605.asp; May 6, 2005). One of these holes, IL-H-06 encountered drilling problems and did not reach bedrock.
Age: The age of this mineralization is assumed to be Late Cretaceous or early Tertiary based on the general age of intermediate intrusive rocks north of the prospect (Wilson and others, 2003). The similar(?) Pebble porphyry copper-gold deposit (ARDF IL007) nearby has been dated at 89.7 Ma.
Alteration: The granodiorite varies from unaltered to extensively altered, with chlorite, carbonate, and sericite replacement, local silicification, and secondary K-feldspar and biotite adjacent to fractures.

Commodities (Major) - Cu; (Minor) - Au
Development Status: None
Deposit Model: Porphyry Cu-Au (Cox and Singer, 1986, model 20c)

Other Databases

Link to USGS - Alaska:	DI012

Other Regions, Features and Areas containing this locality

References