Summary:
The Sisson deposit can be defined as an intrusion related, structurally controlled, bulk tonnage tungsten-molybdenum deposit. Deposits of this type have general hydrothermal similarities to porphyry copper deposits. These types of deposit form in convergent margin to collisional tectonic environments and are related to highly evolved granitic melts formed from continental crust.
Mineralization in the Sisson deposit is related to several types of alteration and veins, each of which has a distinct relationship to metal introduction. The mineralization has an approximate strike length of 1,900 m, average width of 650 m and average depth of 350 m. Tungsten and molybdenum are the metals of principal economic interest, whereas several other metals, including copper, zinc, lead, arsenic, and bismuth, occur in geochemically anomalous but subeconomic concentrations.
Early Sodic-Calcic Alteration and Amphibole Veins
The earliest hydrothermal feature at Sisson comprises amphibole veins and associated albite-actinolite (sodic-calcic) alteration. These features occur almost exclusively in gabbro and, to a lesser extent, in quartz diorite. They are most abundant in the west-central part of Zone III and extend into the Ellipse Zone. The AA veins and sodic-calcic alteration contain a few percent pyrrhotite-pyrite but completely lack molybdenum and contain only trace scheelite. The veins are mostly between two and 20 mm in width, have diffuse contacts with host rocks and contain mostly actinolite, lesser albite, minor calcite, and trace to minor pyrrhotite and/or pyrite. Alteration envelopes are mineralogically similar and can be up to several times the width of the associated vein. Where vein density is high, the alteration envelopes coalesce to pervasive alteration.
Early Biotite and Biotite-Sulphide Alteration
This alteration mostly spans the contact between gabbro and volcanic rocks on the west side of Zone III. This alteration is commonly intense and texture-destructive. Similar alteration is erratically distributed through gabbro west of the contact, but is rare in quartz diorite. Sulphide concentration ranges from trace to 10% and manifests cubiform pyrite locally accompanied by pyrrhotite. Biotite- sulphide alteration typically contains neither scheelite nor molybdenite.
Calc-Silicate Alteration
Drilling has intersected mineralized calc-silicate alteration over lengths between 0.5 m and three meters. This alteration occurs both in gabbro and in volcanic rocks but mostly at depths greater than about 300 m. The timing of this alteration relative to other hydrothermal effects has not been constrained. The alteration is massive and pervasive and associated veins have not been observed. The mineralogy comprises various combinations of red-brown garnet, green pyroxene, calcite, epidote, and possibly amphibole, and up to a few percent pyrrhotite and lesser pyrite. Disseminated scheelite is ubiquitous but molybdenite has not been observed. Relicts of granite dyke within this alteration suggest that it may be endoskarn.
Quartz-Scheelite Veins (Type QW)
Quartz-scheelite (QW) veins represent the earliest stage of significant tungsten mineralization at Sisson. These veins are typically less than five millimeters in width, have sharp to locally diffuse contacts with host rocks, and range from planar to strongly curviplanar. They contain clear to milky quartz, scheelite, low concentrations of pyrite and/or pyrrhotite, and rare molybdenite. Alteration envelopes are mostly less than one centimeter in width and comprise biotite in mafic host rocks and, more rarely, sericite in felsic host rocks. The alteration envelopes commonly contain disseminated scheelite, particularly when hosted by gabbro or quartz diorite. The density of these veins diminishes markedly below about 300 m depth in the center of Zone III, and at shallower and deeper levels in the north and south parts of the deposit, respectively.
Quartz-Molybdenite (Type QM) and Quartz-Feldspar (Type QF) Veins
Quartz-molybdenite veins consistently cut QW veins. They represent the main stage of molybdenum mineralization and, except in the north part of Zone III, also contain high concentrations of scheelite. They are most abundant in the center of Zone III, of intermediate abundance in the south part of Zone III and the Ellipse Zone, and are rare to absent in Zones I and II, the north part of Zone III, and in gabbro from the west-central part of Zone III.
The QM veins are mostly one centimeter to five centimeters in width, but veins greater than 20 cm wide are common. They are planar to curviplanar and have sharp contacts with wall rocks. The QM veins are dominated by clear, locally epitaxial quartz. Molybdenite can form up to 10% of these veins but has a clotty distribution along their length. Most QM veins contain less than 3% pyrite, rarely accompanied by pyrrhotite. They contain low to locally high concentrations of scheelite, although in some cases tungsten mineralization is related to younger type SX veins which exploited fractured QM veins. Sericite alteration envelopes less than one centimeter in width are only locally present and rare biotite envelopes occur in mafic host rocks.
Type quartz-feldspar (QF) veins are very similar to QM veins but occur mostly at depths below about 250 m. They are distinguished by selvages of white K-feldspar, contain a dark green mineral that may be grossular garnet, have a significantly lower concentration of molybdenite and scheelite than normal QM veins, and are mostly less than three centimeters in width. Vein types QM and QF are generally contemporaneous.
Quartz Shear (Type QS) and Sulphide-Rich (Type SX) Veins
The QS veins are the main style of mineralization in Zones I and II and in the strongly mineralized contact between gabbro and volcanic rocks in Zone III. They are common throughout the Turnbull Mountain Formation in Zone III but are less common in gabbro in Zone III and in the Ellipse Zone. The QS veins and associated alteration commonly attain grades between 0.25% WO3 and 1.00% WO3, although they constitute only a very small portion of the overall deposit.
The QS veins range from planar to curviplanar and contacts with host rock are typically sharp. Lensoidal forms occur in outcrop in Trench 1. The QS veins contain mostly white quartz cut by myriad brittle fractures that reflect structural disruption during and after vein formation. Sulphide concentration can attain 15% and comprises mostly pyrrhotite and lesser pyrite, 1% to 3% chalcopyrite, and highly variable concentrations of arsenopyrite, galena, sphalerite, and bismuth minerals. These veins host almost all of the wolframite in the deposit, which occurs as black, prismatic crystals that are variably to completely replaced by scheelite. Molybdenite has not been observed.
The SX veins are widely distributed through the Turnbull Mountain Formation but are rare in the Ellipse Zone and gabbro in Zone III. Relationships exposed in Trench I suggests that SX veins may occur as sheeted swarms of planar fractures oriented at conjugate angles to and surrounding QS veins. Individual SX veins are fracture fills that rarely exceed three millimeters in width and which are enclosed by intense sericite-quartz-sulphide alteration envelopes identical to those related to QS veins. The SX veins are mineralogically similar to QS veins.
Carbonate Veins (Type QC)
Late carbonate veins are planar, up to one centimeter in width, and lack alteration envelopes. They contain calcite or dolomite, quartz, pyrite and minor green to purple fluorite. In a few rare cases, they contain minor scheelite, but molybdenite has not been observed. This same assemblage locally forms open-space infill of small voids in late SX and QS veins. These veins are widespread but volumetrically and economically insignificant.