Raintree (West, North, and South)

Prospects, Active

Alternative names

Raintree

Commodities and mineralogy

Main commodities Ag; Au; Cu
Other commodities Pb; Zn
Ore minerals chalcopyrite; galena; gold; magnetite; pyrite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale TY
Quadrangle map, 1:63,360-scale D-7
Latitude 61.9769
Longitude -152.6072
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Raintree prospect is about 1.6 miles northwest of the mouth of Portage Creek on the Skwentna River and about 0.6 mile southeast of the center of section 20, T. 22 N., R. 18 W., of the Seward Meridian. The location is accurate to within 500 feet.

Geologic setting

Geologic description

The Raintree (West, North, and South) prospect is one of several gold-copper porphyry deposits that form a belt that trends north-northwest for at least 9 miles (Wahl and others, 2008; Kiska Metals Corporation, 2010). The best known is the Whistler deposit (TY022) about two and half miles to the west-southwest. Relatively little detail is available for the Raintree prospect. It is largely covered by surficial material but it is probably similar to the adjacent Whistler prospect (TY022). The rocks in the vicinity are mainly extrusive porphyritic andesite cut by porphyritic diorite dikes.
Cominco Alaska began mineral exploration in the area in 1986 but they ended their work in 1989. From 2004 to 2006, Kennecott Exploration Company was active in the area and staked several large blocks of claims, which included the Raintree prospect. Geoinformatics Exploration Inc. acquired the property from Kennecott, and in 2009, Kiska Metals Corporation was formed by a merger of Geoinformatics and Rimfire Minerals Corporation.
There are two significant zones of gold-copper porphyry mineralization at Raintree West, a near surface zone on the east side of the Alger Peak fault and a deeper zone on the west side of the fault. The near surface porphyry mineralization is coincident with a northwest-elongate aeromagnetic (AMAG) high anomaly that measures 250 meters long and 150 meters wide, which pinches out to the northwest and southeast. The host rock to mineralization in both zones (diorite porphyry) and the general style of gold-copper mineralization and alteration (disseminated chalcopyrite and quartz-chalcopyrite veins associated with magnetite and potassic alteration) is almost identical to the Whistler Deposit. In contrast, there is a significant component of gold-silver-lead-zinc mineralization overprinting both the gold-copper porphyry mineralization and the adjacent volcanic wall-rocks associated with quartz-carbonate veins that contain sphalerite + galena + pyrite ± chalcopyrite. These veins are interpreted as intermediate sulphidation epithermal veins that typically overlie and commonly telescope onto the core of porphyry systems. The near surface zone of porphyry mineralization has a defined strike length of 200 meters, remains open to the south, and occurs from surface down to a maximum depth of approximately 170 meters, where it is truncated by post-mineral diorite porphyry intrusions and/or faulting (Roberts, 2014).
There were several (shallow?) holes drilled at the Raintree prospect before 2008. In 2008, Geoinformatics drilled two holes, including the first at Raintree West (Kiska Metals Corporation, 2008). The area is covered by up to 5 meters of gravel and the holes were sited using proprietary targeting techniques. The first hole cut 160 meters that contained 0.59 gram of gold per tonne, 6.02 grams of silver per tonne, and 0.1 percent copper, and was entirely in porphyritic diorite. In 2009, Kiska drilled a 471.6-meter hole that cut porphyritic andesite, porphyritic diorite, and trachyandesite; it bottomed in mineralization (Kiska Metals Corporation, 2009 and 2010). The best intervals were 128.7 meters that contained 0.56 gram of gold per tonne, and 6.7 grams of silver per tonne; and 97.2 meters that contained 0.61 gram of gold per tonne, 6.94 grams of silver per tonne, 0.16 percent copper, 0.25 percent lead, and 0.59 percent zinc. The entire hole averaged 0.38 gram of gold per tonne, 4.67 grams of silver per tonne, 0.09 percent copper, 0.15 percent lead, and 0.35 percent zinc. The presence of late-stage lead- and zinc-bearing quartz-carbonate veins suggests that the drilling ended at the top of a gold-porphyry system distinct from the Whistler deposit (TY022).
Geologic map unit (-152.609434242527, 61.9763204652018)
Mineral deposit model Gold-copper porphyry (Cox and Singer, 1986; model 20c)
Mineral deposit model number 20c
Age of mineralization Early or middle Tertiary; mineralization is interpreted to be coeval with or younger than the Tertiary volcanic, host rocks (Wahl and others, 2008).
Alteration of deposit Phyllic alteration is noted widely around the prospect at the surface. Alteration is typical of porphyry systems associated with relatively oxidized magma series (quartz vein stockwork, chalcopyrite-pyrite ore assemblage, presence of sulfates, core of potassic alteration with well-developed peripheral phyllic alteration zones) (Morris, 2011).

Production and reserves

Workings or exploration
The area has been explored by several companies since the early 1980s and has been one of the principal interests of Kiska Metals Corporation's Whistler project. Their work has included detailed mapping, geochemical and geophysical surveys, sampling, and at least 6 drill holes, including two in 2008 and another in 2009.
There were several (shallow?) holes drilled at the Raintree prospect before 2008. In 2008, Geoinformatics drilled two holes, including the first at Raintree West (Kiska Metals Corporation, 2008). The area is covered by up to 5 meters of gravel and the holes were sited using proprietary targeting techniques. The first hole cut 160 meters that contained 0.59 gram of gold per tonne, 6.02 grams of silver per tonne, and 0.1 percent copper, and was entirely in porphyritic diorite. In 2009, Kiska drilled a 471.6-meter hole that cut porphyritic andesite, porphyritic diorite, and trachyandesite; it bottomed in mineralization (Kiska Metals Corporation, 2009 and 2010). The best intervals were 128.7 meters that contained 0.56 gram of gold per tonne, and 6.7 grams of silver per tonne; and 97.2 meters that contained 0.61 gram of gold per tonne, 6.94 grams of silver per tonne, 0.16 percent copper, 0.25 percent lead, and 0.59 percent zinc. The entire hole averaged 0.38 gram of gold per tonne, 4.67 grams of silver per tonne, 0.09 percent copper, 0.15 percent lead, and 0.35 percent zinc. The late-stage lead- and zinc-bearing quartz-carbonate veins suggests that the drilling ended at the top of a gold-porphyry system distinct from the Whistler deposit (TY022).
Kennecott Exploration Company carried out systematic exploration of the Whistler project area between 2003 and 2006, which included reconnaissance mapping and prospecting; airborne geophysical and radiometric surveying; rock, soil and stream sediments sampling; detailed mapping over selected areas and ground induced polarization (IP) surveying (Wahl and others, 2008).
Since the first hole that discovered mineralization at the Raintree West prospect (Hole RN08-06, drilled by Geoinformatics in 2008) there has been a total of 8,538 meters drilled (Roberts, 2014). Five holes were drilled by Kiska in 2009 and 2010 (Morris, 2011).
The near surface porphyry mineralization is coincident with a northwest-elongate aeromagnetic (AMAG) high anomaly that measures 250 meters long and 150 meters wide, which pinches out to the northwest and southeast. Drilling has intersected porphyry mineralization on two 100 meter-spaced east-west AMAG cross-sections (6,871,350 millinewtons and 6,871,450 millinewtons). Hole WH09-002 (drilled at an opposing azimuth to hole RN08-06) confirmed the near surface zone of mineralization discovered in hole RN08-06, returning 128.7 meters of 0.56 gram of gold per tonne and 0.16 percent copper from 59.0 meters downhole. In addition, this hole crossed to the west of the Algers Peak fault and intersected the deeper zone of porphyry mineralization returning 40 meters of 0.98 gram of gold per tonne and 0.21 percent copper from 429.0 meters downhole (Roberts, 2014).
The deep zone of mineralization on the west side of the fault has been drilled along a northwest-trending strike length of 325 meters and is open to the north, west and to depth. The AMAG and 3D IP data could not resolve the mineralization due to the limited depth penetration of these surveys and a masking effect caused by the presence of narrow, but strongly magnetic, dykes in the near subsurface. A cross-section of the AMAG anomaly at 6,871,650 millinewtons shows significant grades and widths of porphyry mineralization, and demonstrates that this strong system remains open to depth. Significant intercepts include 172.0 meters averaging 1.13 grams of gold per tonne and 0.18 percent copper from 619.0 meters in hole WH10-030 and 218.0 meters averaging 0.53 gram of gold per tonne and 0.11 percent copper from 480.1 meters in hole WH11-033 (Roberts, 2014).
Drilling at Raintree North and Raintree South has confirmed that porphyry stocks in these areas host gold-copper porphyry mineralization, with alteration and veining remarkably similar to that characteristic of the Whistler Deposit and Raintree West. Gold-copper mineralization in all three prospects occurs immediately below till cover at the top of bedrock. Significant drilling intercepts include: Raintree North: WH11-034 returned 77.3 meters averaging 0.58 gram of gold per tonne, 1.6 grams of silver per tonne and 0.19 percent copper (0.99 gram of gold per tonne equivalent) within a larger interval averaging 0.40 gram of gold per tonne, 1.53 grams of silver per tonne and 0.14 percent copper over 148.5 meters (Roberts, 2014).
In 2016, Brazil Resources Inc. (now Gold Mining Inc.) is evaluating prospects it acquired from Kiska Metals Corporation in 2015. Although no on-site work was performed in 2016 at the Whistler (TY022), Raintree West, and Island Mountain (TY018) porphyry copper–gold–silver prospects, in May of 2016, Brazil Resources Inc. released a new technical report (Giroux, 2016) announcing the first resource estimates for the Raintree West and Island Mountain (TY018) deposits, largely based on drilling by Kiska Metals Corporation between 2009 and 2011. Metal recoveries reported for the Whistler deposit (TY022) resource estimate, and assumed for the Raintree West deposit, include 85 percent for copper, 75 percent for gold, and 75 percent for silver (Athey and Werdon, 2017).
Indication of production None
Reserve estimates At a 0.6 gram of gold-equivalent per tonne cut-off grade below the 100-m elevation level, the new resource estimate for Raintree West includes an inferred resource of 51.76 million tonnes grading 0.68 gram of gold per tonne (1.13 million ounces of gold), 3.74 grams of silver per tonne (6.22 million ounces of silver), and 0.10 percent copper (114.13 million pounds of copper). At a 0.3 gram gold-equivalent per tonne cut-off above the 250-m elevation level, Raintree West also contains an inferred resource of 31.68 million tonnes grading 0.40 gram of gold per tonne (410,000 ounces of gold), 5.39 grams of silver per tonne (5.49 million ounces of silver), and 0.06 percent copper (41.91 million pounds of copper). The Raintree West resource is based on 7,078 meters of drilling in 14 holes (Giroux, 2016).
Production notes None.

Additional comments

The Whistler Deposit (TY022), which is within 3 kilometers of the Raintree prospect, has an existing 43-101 compliant resource: 2.25 million ounces of gold equivalent (Indicated category) and 3.35 million ounces of gold equivalent (Inferred category) (Morris, 2011). However, Raintree seems to be a different system from Whistler based on the presence of late-stage lead- and zinc-bearing quartz-carbonate veins (Kiska Metals Corporation, 2010).

References

References

Kiska Metals Corporation, 2008, Geoinformatics announces new discovery at its Whistler project in Alaska: http://www.kiskametals.com/s/Geo_Archive.asp?ReportID=357944 (News release, November 4, 2008).
Kiska Metals Corporation, 2010, Whistler project overview; http://www.kiskametals.com/s/Whistler.asp?ReportID=370036 (as of February 26, 2010).
Morris, R.J., 2011, Resource Estimate Update for the Whistler Gold Copper Deposit and Results of Property Wide Exploration, Yentna Mining District, Alaska: Technical Report (43-101) for Kiska Metals Corporation, 133 p. (posted on www.sedar.com, March 18, 2011) http://www.sedar.com/GetFile.do?lang=EN&docClass=24&issuerNo=00007774&fileName=/csfsprod/data115/filings/01703222/00000003/i%3A%5CSEDAR%5CKMTchRpAM.PDF (as of January 8, 2015).
Roberts, Michael, 2014, Whistler Project Executive Summary Report, Kiska Metals Corporation: http://www.kiskametals.com/i/pdf/Kiska_Whistler_ExecutiveSummary_24Jan2014.pdf (as of March 10, 2014).
Wahl, George, Coutture, Jean-Francois, and Keller, G.D., 2008, Mineral resource estimation, Whistler copper-gold project, Alaska Range, Alaska: Technical report for Geoinformatics Exploration, Inc., 124 p. (posted on www.sedar.com, February 22, 2008) http://www.sedar.com/GetFile.do?lang=EN&docClass=24&issuerNo=00007774&fileName=/csfsprod/data86/filings/01219440/00000001/s%3A%5Cgxl222.pdf (as of January 8, 2015).
Reporters D.J. Grybeck (Contractor, U.S. Geological Survey); V.C. Zinno (Alaska Earth Sciences, Inc.); M.B. Werdon (DGGS)
Last report date 8/26/2017