Barrett, T.J. and MacLean, W.H., 2000.

Chemostratigraphy, Petrography and Alteration of the Marshall Zone, Myra Falls, Vancouver Island, B.C.

Unpublished Report for Boliden-Westmin (Canada) Limited, Campbell River, B.C. 126 pp + 5 appendices.

The Marshall Zone at Myra Falls is situated about one kilometre northwest of the Battle massive sulfide deposit, within mainly felsic volcanic rocks of the Myra Formation. A total of thirty exploration holes have been drilled since the zone was discovered in 1995. Mineralization follows a semi-continuous WNW-ESE trend and ranges from polymetallic veins to semi-massive to massive sulfides (sphalerite, galena, chalcopyrite, tetrahedrite-tennantite plus barite). The most massive intersections, which are in the order of 3-10 metres in thickness, have very high Zn and significant precious metal contents (e.g. 8.3 m grading 29.3 % Zn, 1.80 % Cu, 2.3 % Pb, 7.6 g/t Au, 195.2 g/t Ag and 6.7 % Ba in hole 2020; and 6.6 m grading 22.6 % Zn, 2.09 % Cu, 2.71 % Pb, 5.2 g/t Au, 367.6 g/t Ag and 16.5 % Ba in hole 2012). There are also intervals which are dominated by barite but which are nonetheless enriched in precious metals (e.g. 5.7 m grading 1.37 % Zn, 0.11 % Cu, 0.90 % Pb, 11.5 g/t Au, 336.8 g/t Ag and 43.0 % Ba in hole 2007; and 3.5 m grading 7.39 % Zn, 0.19 % Cu, 2.29 % Pb, 6.3 g/t Au, 522.8 g/t Ag and 33.5 % Ba in hole 2023). Polymetallic veins are present both below and above the massive ores.

The present study was carried out in order to determine the sequence of volcanic stratigraphy within the Marshall Zone, to assess the position of mineralization and the nature and extent of hydrothermal alteration, and to establish correlations across the Marshall Zone. The results of the study are based on two main sets of lithogeochemical data: 1) 147 samples from 10 drill holes which were collected by T. Barrett during 10 days of field work in the spring of 2000; and 2) 183 samples from 10 main holes which were collected by Boliden Westmin geologists during drilling programs in 1998-1999. Holes sampled by T. Barrett were also relogged, and 20 samples were selected for petrographic examination by W. MacLean. In addition to the holes in the Marshall Zone, one deep basement hole at the H-W deposit was sampled by T. Barrett, while the Boliden Westmin data set includes one hole from the Myra Ridge.

The deepest drill holes in the Marshall Zone intersected ‰200 m of the Price Formation, which consists mainly of mafic volcanic rocks. Above this are 250-350 m of mainly felsic volcanic rocks, then about 100 m of mafic to intermediate, pebbly to sandy, volcaniclastic turbidites and lesser mudstones, followed by >200 m of similar but fine-grained volcaniclastic beds containing common mafic sills. The rocks have been classified lithogeochemically on the basis of immobile-element ratios into the following groups: basalt, basaltic andesite, andesite, dacite, rhyolite B and rhyolite A. Based on the new data, a series of revised geological sections has been produced. Overall, the stratigraphy in the Marshall Zone appears to be subhorizontal or to have a gentle dip to the south. The general lateral continuity of units from hole to hole suggests that no major high-angle faults are present in the holes examined.

The Price Formation shows a general progression from basalts and primitive basalts at the base, to basaltic andesites in the main central interval, and thinner andesites at the top. A thin unit of dacitic composition is locally present above this. Some of the andesitic and dacitic units could conceivably represent mixtures of mafic and felsic clastic material. The Myra Formation begins with a few tens of metres of rhyolite A, followed by a lens of rhyolite B which ranges in thickness from 30 m in the east to >120 m in the west. In the original drill logs, rhyolite B was generally logged as andesitic and belonging to the Price Formation, partly because of its commonly dark colour; a number of holes ended in this lithology and did not reach the actual Price Formation. Rhyolite B is overlain by a few tens of metres of rhyolitic fragmental rocks which range from breccias to lapilli tuffs (locally pumiceous) to volcaniclastic sandstones. These fragmental rocks are mainly of rhyolite A composition, although a few samples plot between rhyolites A and B, and probably contain material derived from both sources. The Marshall Zone mineralization lies within the fragmental interval, with the exception of hole 2025, where it lies on rhyolite B.

Above the rhyolitic fragmental interval is the main sequence of rhyolite A, which is 100-250 m thick. Most of the lower half of this sequence is massive to autobrecciated, flow banded rhyolite. The upper half contains local pyroclastic or volcaniclastic intervals as well as massive rhyolite (all of rhyolite A composition). The mafic volcaniclastic beds which overlie rhyolite A range from basaltic andesite to heterolithic Œandesite¹. A number of late mafic sills or dykes occur within the Price Formation and felsic volcanic sequence. They are mainly high-Mg-Cr basalts or high Fe-Ti basalts, and are little altered, and probably represent feeders for higher mafic formations. An exception is a 30 m-thick basalt sill or flow which overlies Marshall Zone mineralization in hole 2025; parts of it are brecciated, altered and mineralized. This mafic unit is compositionally similar to basalts which occur in the Price Formation, but apparently was emplaced slightly later, during the accumulation of the overlying felsic debris.

Alteration in the Marshall Zone is generally characterized by K-Ba-Zn additions, Na losses, and silica gains or losses. The most widespread anomalies are Na losses, which have affected most of the rocks in all of the holes examined. There are, however, a few areas in the rhyolites which show limited Na losses or have gained Na, even close to mineralization. Additions of Ba occur up to 100 metres away from the main zone of mineralization, and additions of Zn up to about 50 m away. Silica anomalies take the form of either mass gains or mass losses, the largest of which occur within about 100 m of the mineralized zone. Although only a few holes penetrated deep into the Price Formation, Ba and Zn additions in these holes are present as much as 100 m below the top of this formation. The upper part of the Price Formation and locally overlying Œdacitic¹ rocks show some of the largest K gains.

The overall composition of least altered volcanic rocks in the Marshall Zone (and elsewhere at Myra Falls) is typical of calc-alkaline mafic to felsic sequences in mature island-arcs and their associated back-arc basins. The mineralogy and metal contents of the Marshall Zone ores are directly analogous to massive sulfide deposits on the Valu Fa Ridge, in the back-arc basin of the Tofua arc in the southwest Pacific, where a series of sphalerite-barite white smokers lie upon a basement of basaltic andesite, andesite and dacite. The more massive sulfide intersections in the Marshall Zone are interpreted as the remains of small clusters of sphalerite-barite chimneys.

Massive sulfides in the Marshall Zone are situated 100-150 m above the Price Formation, and therefore can be classified as an "upper" level of massive sulfides. Vein systems are present either as stockworks in underlying rhyolites B and A, or in overlying rhyolite A. No massive sulfides are present at the ŒH-W¹ contact between the Price Formation and overlying felsic volcanics in the southern part of the Marshall Zone, but in the northern part, this contact is still untested.

Examination of lithogeochemical data for volcanic rocks at the Battle deposit (Robinson et al., 1996) suggests that the lower hangingwall contains pyroclastic rhyolite B overlain by rhyolite A volcaniclastic rocks, then massive flows of rhyolite A. An "upper" massive sulfide horizon is locally present at, or somewhat below the contact between rhyolite A volcaniclastic and massive rocks, and is compositionally similar to Marshall Zone mineralization. Thus, the Marshall Zone probably can be correlated with the "upper" massive sulfide horizon at Battle. Given the tendency for mineralization at Myra Falls to occur semi-continuously along linear trends, the existence of largely unexplored areas west and east of the Marshall Zone, the very high total metal contents of the massive sulfide intervals that have been intersected, and the likely presence of an untested ŒH-W¹ contact extending to the north, further exploration in this area seems warranted.

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