Dames & Moore, 1999 - USDA Forest Service

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Quartz-rich layeis of 40 percent quartz, 30 percent plagioclase, 20 percent diopside, 4
percent hornblende, 2 percent sphene (CaTiO(Si04)) and 2 percent biotite
In the vicinity of the mineralized zone, the amphibole is altered to a black biotite, with local chlorite
((Mg,Fe)3(Si,Al)4010(OH)2.(Mg,Fe)3(OH)s) and sericite (KA12(AISi3010)(OH)2) alteration, and additional
amounts of quartz and sulfides. The increase in the biotite is usually accompanied by a decrease in the
lime-silicate (e.g., diopside) content.
6.1.1.2 Ore Deposit
Geologists at the Holden Mine originally classified the ore deposit as a vein-type deposit formed in a
shear zone (Youngberg and Wilson, 1952); However, it has recently. been re-classified as a
metamorphosed Kuroko-type VMS deposit (Dragovich and Derkey, 1994). The ore zone is interpreted to
be located within the limb of a large, northwest-trending, overturned, isoclinal fold.
Three distinct zones of mineralization, based on sulfide minerals can be distinguished (Dragovich and
Derkey, 1994):
1. The "original foatwall sulfide zone," contains pyrrhotite (Fel,S), pyrite (FeS2), biotite
and sericite. Pyrite and pyrrhotite in this zone is largely disseminated, and the contact
with the original footwall is diffise or irregular.
2. The "original footwall ore zone" which contains pyrrhotite, chalcopyrite (CuFeS2) and
gold (Au) mineralization. This is partially interbedded with the footwall sulfide zone.
Economically, it was the most important unit at the mine.
3. The "hanging wall ore zone," which contains pyrite and sphalerite (ZnS), with lesser
amounts of pyrrhotite, chalcopyrite and galena (PbS). Over 50 percent of the
mineralization in this zone is present as sulfides. The contact with the original hanging
wail rocks is sharp.
Because this sequence is overturned, the "original footwall" is now the structural hanging wall.
Other minerals identified in the deposit include: magnetite (FqOd), quartz, molybdenite (MoS3, calcite
(CaC03), bournonite (PbCuSbS3), silver (Ag), and possibly pitchblende (U4) (Youngberg and Wilson,
1952).
Post-ore intrusive "diabase" dykes cross the ore zone in several locations, varying from approximately
twenty-four percent abundance at the 2125 level to almost fifty percent at the 2500 level (Ebbutt, 1956)
(Figure 6.1-2). The dykes have a well-developed igneous texture, and are comprised of quartz-diorite.
Typical specimens are light colored, with a porphyritic medium grained texture. They contain 10 percent
-quartz, 70 percent plagioclase, 10 percent hornblende, and 5 percent chlorite, with minor biotite, apatite
(ca~(P0~)3(0H,F,Cl)), sphene, sericite and magnetite. They form sharp contacts with their host rocks
(Dubois, 1954). The intrusives appear to be related to the late-Triassic Marblemount belt (Dragovich and
Derkey, 1994). The dykes represent an important waste component of the ore and therefore the minerals
are expected to be present in the tailings.
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17693-005019Uuly 27.1999;4:42 PMPRAlT FINAL RI REPORT