DEPOSIT TYPE
VMS [ copper, gold, and silver]
LAND STATUS
State Select
SIZE
The Licken Property is made up of twenty Prospecting Sites for a total of Thirty-two hundred [3200] acres.
PREVIOUS EXPLORATION
Government geologist have discovered an occurrence of copper and gold- bearing volcanic rocks in the low glaciated hills of the northern Talkeetna Mountains. Analyses of several composite and grab samples taken show copper values ranging to 3.71 percent and 66 ppm silver and 475 ppb gold. A government geologist [Smith] concluded that the occurrence may represent the distal edge of a larger deposit. There is a considerable amount of superficial cover in the area that may conceal the full extent of the mineralization. A government report on the Lichen Property will be put up in the near future.
FIELD PROGRAM OBJECTIVES
Planned exploration work will include soil sampling, trenching geochemical surveys and drilling shallow holes[50-100feet]. The objective is to develop the property only to the point where a Major Mining Company will want to joint venture the property.
STRATABOUND COPPER-GOLD OCCURRENCE, NORTHERN TALKEETNA MOUNTAINS, ALASKA by T.E. Smith,T.K.Bundtzen, and T.C. Trible
ABSTRACT
During the 1973 field season, an occurrence of copper and gold-bearing volcanic rocks was discovered in low glaciated hills of the northern Talkeetna Mountains.
Metallization is generally confined to a single, near vertical flow that is discontinuously exposed for over 3000 feet. Analyses of 10 composite samples over this interval show significant amounts of copper, gold, and silver. Mineralization, although somewhat remobilized, appears stratabound and may well extend beyond the limits of bedrock exposure. Additional exploration work is recommended.
INTRODUCTION
This brief paper provides prelliminary data on an interesting base-metal and precious-metal occurrence identified during the 1973 field season, which was a phase of a reconnaissance geologic mapping and mineral appraisal program carried out by T.E.Smith from 1968 to 1974, under support of the USGS and the DGGS. A summary of regional geology beyond the limits of 1 and 2 in this report is available in the DGGS Annual Report for [ Smith, 1973,p.3-6].
Copper occurrences and geochemical drainage anomalies are common in Triassic volcanic rocks of the Amphitheater Group [TRa of fig.1], which are exposed in an elongate belt north of the prospect described here. To the author's knowledge, however, this mineral occurrence is the first of it's type to be reported in the foliated lavas and metasediments of probable upper Paleozoic age that are exposed in a broad belt extending over 70 miles from the northern Talkeetna Mountains to the Richardson Highway. Most of this lowland terrane is discontinuously mantled with surficial deposits, heavily vegetated, and does not lend itself to mineral evaluation by stream-sediment geochemistry. The prospect described here was discovered while examining sparsely distributed bedrock exposures using continuous helicopter support. Surface exposures of sulfide-bearing lavas contrasted strikingly with barren rocks of the Pzv unit in that they were invariably covered with a brillant orange lichen-hence our informal name Lichen Prospect. Its presence of mineralized rocks of the Pzv unit may provide a geo-botanical guide for exploration of the remainder of the unit.
Regional Geology
Bedrock terrance near the Lichen Prospect ranges in age from upper Paleozoic to Cretaceous-including lavas of mafic to intermediate composition with interbedded sediments, pelitic sedimentary rocks, and a variety of intrusive igneous bodies. These are shown on fig.1.
The oldest layered rocks are a deformed sequence of amphibolttes with interbedded marbles [Pza] that are in apparent gradational contact with an extensive terrain of mildly schistose lavas of intermediate composition, with minor fine-grained sediments and carbonates [Pzv]. Rocks of the Pzv unit form an east-west arcuate belt across the northern part of the Copper River bassin. Fossils collected by Csejtey [1973,p 45] and by Smith and Bundtzen during the 1973 field season in the vicinity of Watana Lake support an age assignment of Upper Paleozoic for this rock sequence.In this area, the Pzv unit is in thrust contact with the Triassic Amph-theater Group, a thick sequence of 1] metabasalts and andesites [TRa] which displays both columnar jointing and pillow structures locally, and 2] cherts, argillites, volcaniclastics sediments, and limestones [TRas] which are in part fossiliferous. Fossils collected northeast of the map area from the upper part of the sequence [Moffit, 1912, p.33] established an upper Triassic age for the TRa unit. Newer fossil collections made by Smith and Bundtzen in 1973, Stout [1974, in process] confirm the earlier determination. An upper Jurassic assemblage of sandstones, siltstones, and pebble congloerates [Js] is in fault contact with the Amphitheater Group in the northwest corner of the map. Most of the lowland areas and stream valleys are covered by Quatemary surficial deposits which were not differentiated during the mapping program.
At least two ages of igneous intrusive rocks are exposed in the map area. Hornblende quartz diorite [Jqd] of Jurassicage underlies most of the rounded hills within the big bend of the Susitna River. A per-aluminous granite series [Jgr], of late Jurassic age, crops out as a linear body in the southeast corner of the map. Hornblende gabbro bodies [Pzg], some of which are too small to be shown on the map, are considered to be of the same age as the amphibolite terrane [Pza] and are probably genetically related to it, Cretaceous magmatism is recorded by a foliated granodiorite [Kqm and Ki ] in the center of the map. Kqm probably represents a more homogeneous, felsic core of the larger complex plutonic body.
MINERAL OCCURRENCE
Host rocks for mineralization at the Lichen Prospect consist of metavolcanics, minor detrital sediments and banded quartz of probable late Paleozoic age. These rocks [Pzv] have undergone lower greenschist facies metamorphism, and many of the less competent sedimentary beds are recrystallized to green schists. The slfide-rich zone, apparently confined to a single volcanic flow, extends along a regional strike or N. 60 W. for at least 3000 feet. Surificial deposits mantle most of the area, concealing both ends of the metalliferous horizon zone.
Petrographic studies of rocks from the prospect suggest a predominant intermediate composition for the metavolcanics. Although the rocks are locally chloritized and epidotized, varying amounts of biotite, hornblende and quartz are present in the sections. Compositionally they are slightly more felsic than the Amphitheater Group [TRa] to the north; this is true of the entire Pzv unit.
The sulfide minerals are generally disseminated or are concentrated in small quartz-feldspar-epidote veinlets averaging less than 1 centimeter thick. Both the veinlets and disseminations are restricted to a zone about 5 feet wide, they seem to be confined to a single volcanic horizon that strikes N 60 W and dips nearly vertical. Chalcopyrite [CuFeS2] and bornite [CuFeS4] are the most obvious economic sulfides, but under the ore microscope, chalcopyrite, bornite, minor covellite [CuS], digenite [Cu-S] and an unusual mineral, probably idaite [Cu6FeS5] have been identified. The idaite is perhaps an alteration productof the bornite. Covellite and digenite veinlet systems cut the bornite and chalcopyrite, and supergene malachite is abundant. No native precious metals or precious metal sulfides were recognized. The mineral source of the silver and gold values present in the analyzed samples was not recognized, although higher silver values seem to correlate with the higher copper values [table 1].
A conspicuous bright-orange lichen is invariably associated with the sulfide-bearing material and actually aided the field party in deliniating the extent of the mineralized zone. The lichen is actually the free-living plant Trentepohia aurea, an alga which is a common component of the lichen genus Ionaspis. The identification was made by Dr. William Weber of the University of Colorado museum in Boulder [written comm., 1974], who stated that the lichen commonly grow on spruce twigs in a locally humid environment, but has also been found on rocks in europe. A search in several USGS publications on the subject revealed no mention of the plant associated with other mineral deposits in the state.
The association with ore minerals with a single stratum of vocanic flow rocks suggests a primary volcanogenic source may best, explain this occurrence. Hutchinson [1973,p.1229], in his classic paper on volcanogenic ore deposits, descibes a "cupreous-pyrite" type of massive sulfide deposit with high ratios of gold to silver, but containing virtually no lead or zinc. These are usually associated with mafic-to-intermediate tholelitic volcanic rocks. Felsic volcanics are usually absent, and sedimentary assemblages are volumetrically insignificant, but usually include siliceous ironstones and radiolavian cherts. Mineralization is usually irregularly distributed in the flows in pillows, pillow matrices, and breccias. The volcanic hosts are usually interpreted as being the results of deep-water quiescent fissure eruptions on the sea floor.
At the Lichen Prospect, several lines of evidence support the assignment of this prospect to this category of volcanogenic deposit:
1[ Mineralization seems to be confined to a single volcanic unit of intermediate composition that extends for several thousand feet.
2] Sediments do not make up a significant percentage of the total rock exposed at the prospect; however, a quartzite band possibly representing a metachert or siliceous metasediment is present near the mineralized zone.
3] Felsic volcanics are absent.
4] Lead and zinc are not present in significant amounts in the samples. Copper analyses average 2.48 percent.
5] The average silver to gold ratio of the analyzed samples is 4.3 to 1. This is much lower than ratios reported for deposits in Hutchinson's other categries and agrees with his " cupreous-pyrite" type deposit.
6] The nature of the mineralization seems to suggest that the sulfides before mrtamorphism were irregularly distributed through the volcanic horizon and did not constitude a uniform sheet of massive sulfides.
RECMMENDATIONS
Analyses of several composite and grab samples taken along the length of the exposure show copper values ranging to 4.5 percent and gold values to 16parts per million [tables 1 and 2 ]. Outcrops are limited at the prospect; thus a high probability exists for extensions of the mineralized zone under surficial deposits of the area, as well as for the presents of other metalliferous strata in the flow sequence. THE LICHEN PROSPECT COULD REPRESENT THE DISTAL EDGE OF A LARGER DEPOSIT. Further examination should include induced polarization surveys to delimit the extent of disseminated sulfide mineralization.
© 1997 bmurray@sunwave.net
Get your own Free Home Page