Host rocks, veins and alteration in reduced intrusion-related gold (RIRG) deposits display similar features and distribution across the Tombstone gold belt (TGB). The primary intrusive rocks hosting gold at these deposits are felsic to intermediate, silica-saturated, alkalic to calcic, metaluminous to weakly peraluminous magnesian (calc-alkaline) granodiorite, quartz-monzonite, monzonite and syenite. Preliminary field and petrographic observations suggest that distribution of veins, vein textures, and their correlative vein selvedge alteration assemblages show similar distribution patterns to porphyry deposit models on a more spatially restricted but observable (drill core) scale. At each of the deposits examined on four properties, sulphide-poor (3–5% sulphide minerals) quartz veins were commonly observed proximal to the causative intrusion, whereas sulphide-rich (>10% sulphide minerals) veins tend to be more distal, though are commonly observed using the same fluid pathways as earlier veins. Early sulphide-poor quartz veins commonly exhibit potassic and locally sodic alteration in vein selvedges, whereas sulphide-rich veins typically coincide with phyllic (sericitic) alteration. Sinuous veins with diffuse boundaries suggest emplacement within a higher temperature ductile regime closer to the centre of the mineralizing intrusion, whereas sharp, straight-sided quartz sulphide-rich veins are more typical and emplaced within a moderate temperature brittle regime in the intrusion carapace and hornfelsed country rocks.

The Len porphyry gold prospect is located 47 km north of Mayo, Yukon, in the Tombstone Suite intrusive belt. The area was explored as a Keno Hill-style silver prospect in the 1960s and 1970s. An arsenic-in-soil anomaly first identified in 1980 was followed up by soil geochemistry and excavator trenching in 1996. Multiple sheeted quartz-sulphide veins hosted in a previously unmapped granodiorite stock were discovered during the trenching program. A six-hole program of diamond drilling in 1997 encountered grades ranging up to 2.22 g/t gold across 18.6 m, and showed that gold mineralization is dominantly within, but not restricted to, the intrusive stock.

The Bear Paw breccia zone is located at the Clear Creek property in the Tintina gold belt, central Yukon Territory. Gold mineralization occurs in hydrothermal breccias with stockwork quartz + potassium-feldspar + sulphide veins that overprint earlier intrusive and tectonic breccias. Grades of up to 2.3 g/t gold over 31.8 m have been intersected in recent drilling. The Bear Paw breccia zone is 1.5 km from any significant-sized granitoid stock and thus is indicative of the high potential for gold mineralization outboard of Tombstone Plutonic Suite stocks.

Auriferous sheeted quartz veins and silicified shear zones occur along the margins and within adjacent hornfels zones of mid-Cretaceous Tombstone intrusions near the head of Clear Creek in the central Yukon. The lodes are the source for more than 120,000 ounces of downstream placer gold production. These lodes contain variable amounts pyrrhotite, pyrite, and arsenopyrite, with less abundant scheelite - alkali-feldspar, muscovite, biotite and tourmaline are common gangue phases. Grab samples of mineralization often contain gold grades in excess of 1 ounce per ton. Gold-to-silver ratios vary most commonly from 1:1 to 5:1. Gold-rich quartz veins cut all stocks, adjacent hornfels and associated lamprophyre dykes commonly contain greater than 1% arsenic. Bismuth, and less consistently tungsten and stibnite, characterize many of the most highly mineralized veins within and surrounding the stocks. Quartz veins along the intrusive-metasedimentary rock contact around the Pukelman stock are also enriched in lead and silver. R-mode factor analysis of multi-element geochemical data for 111 gold- and sulphide-bearing rock samples indicates that there are two geochemically distinct metal suites in the Clear Creek occurrences. The first is characterized by As-Au-Bi ± Sb, Te ore-related mineral association, which is typical of many intrusion-related deposits in the Tombstone gold belt. Less consistently, anomalous concentrations of Ag, Co, Cu, Fe, and Mo occur within these auriferous rocks. The second metal factor is defined by Ag-Bi-Pb ± As, Au and Te. It characterizes metalliferous vein samples that have uncommonly low Au: Ag ratios and may represent a second hydrothermal episode. Tungsten shows little consistent correlation with the metalliferous veins in either element suite.

Magmatism in the Nahanni region, which defines the eastern extent of the Tintina Gold Province, is generally associated with tungsten mineralization and/or gold-copper-antimony-bismuth-lead-zinc metal occurrences. Intrusions are subalkaline, granitic to granodioritic, and contain several types of textural variations and highly evolved phases. The intrusions range from large composite batholiths to small stocks with associated felsic dykes and veins. Initial U-Pb and Ar-Ar geochronology reveals ages of 97.5-95 Ma with short (0.5-1.5 m.y.) cooling periods, although the intrusion associated with the Cantung tungsten-skarn orebody cooled over a relatively long period (3 m.y.). Magmatism in the area has been interpreted as crustally derived, however, the rare earth element primitive-mantlenormalized profile revealed negative niobium, tantalum and titanium anomalies suggesting an arctype setting. Furthermore, the granites lack volumetrically significant, primary peraluminous mineralogies characteristic of S-type granites.

The study area is centred on the 91.2 ± 0.9 Ma Scheelite Dome quartz-monzonite stock of the Tombstone Plutonic Suite (TPS). This stock and associated dykes and sills intrude highly deformed metasedimentary strata of the Yusezyu Formation of the Neoproterozoic to Lower Cambrian Hyland Group. The emplacement of TPS intrusions post-dates regional greenschist-facies metamorphism and multiple phases of ductile deformation related to the Tombstone strain zone. Although the Scheelite Dome stock hosts auriferous, sheeted quartz veins, extensive soil geochemistry indicates that the bulk of the gold resource is hosted in the variably hornfelsed metasedimentary rocks immediately south of the stock. The associated gold-in-soil anomaly forms an east-trending corridor of anomalous gold values (>80 ppb) approximately 6 km long by 1.5 km wide, with a more weakly defined eastern continuation. Where metasedimentary bedrock is exposed in the corridor, gold is hosted in fault-vein arrays, and less commonly as disseminated grains and in replacement zones. The styles and distribution of mineralization are largely controlled by brittle structures; a phase of east-west shortening was largely coeval with gold mineralization. R-mode factor analysis of multi-element geochemical data indicates two geochemically distinct metal suites within the area of the gold-in-soil anomaly at Scheelite Dome. The first suite, characterized by Au-Te-Bi ± W ± As, possesses the stronger gold association and is typical of intrusion-related gold occurrences elsewhere in the Tombstone gold belt. The second suite displays a metal association of Ag-Pb-Zn-Cd-Sb ± Cu ± Au, which is more characteristic of mid-Cretaceous Ag-Pb-Zn mineralization in the Keno Hill district, located approximately 60 km to the east-northeast. Field observations, combined with soil geochemistry, suggest that the different metal associations are paragenetically related. However, the possibility of two distinct hydrothermal events cannot yet be ruled out.

Due to the depletion of traditional economic gold placer deposits in unglaciated areas of the Yukon, the study of the relationship of placer gravels to overlying glacial sediments in ice covered regions is important to future exploration activities. The livingstone Creek area in south central Yukon has supported placer mining operations for over 90 years. Present activity is centred on gold-bearing gravels buried by thick Pleistocene glacial deposits. The placer gravels are associated with coarse interglacial stream and gulch deposits and they are overlain by fine-grained, proximal, glaciolacustrine sediments. During the last glaciation, damming of gold-bearing, high gradient interglacial tributary stream channels by main valley ice caused rapid environmental changes. Depositional processes dominated over erosion in the ice-marginal lakes, and thick sequences of fine-grained suspension deposits, debris flow sediments, and deltaic sands and gravels accumulated. In addition, when glaciers expanded and overrode the area, these thick deposits protected the underlying placer gravels from subglacial erosion and dilution. Subglacial tills were probably deposited by lodgement, meltout and flow. During deglaciation, ice-marginal sedimentation again dominated. Post-glacial streams later cut through the glacial sediments and re-exposed the gold-bearing gravels. Stratigraphic, sedimentologic and geomorphic evidence from the Livingstone Creek area suggests that small tributary valleys, oriented transverse to the former direction of ice flow in the adjacent main valleys would make good exploration targets in regions of new placer interest.

The Cynthia property overlies a large (greater than 2x2 km) area of gold mineralization related to a Cretaceous Tombstone Suite quartz monzonite intrusive body. The mineralization is controlled by two district-scale fault zones and is especially intensive in the area of their intersection, located above and adjacent to the intrusive body. These larger structures host abundant gold-bearing massive and drusy quartz and chalcedony veins, zones of intense stockwork and strong brecciation, as well as numerous mineralized felsic dykes. The gold grades within the mineralized structures are commonly in the range of 200 ppb to 2.0-3.0 g/t Au, with higher (up to 16 g/t Au) values attributed to the fault intersection area. Multi-staged gold mineralization found in the quartz veins, stockwork and altered dykes is associated with sulphide minerals (mainly pyrite and arsenopyrite) and elevated As, Bi and Ag values. A later mineralizing episode produced sulphide mineral-bearing chalcedony and drusy quartz veins, with gold concentrations accompanied by elevated Sb, Hg, Ag and Pb values, indicating the affi nity of epithermal style gold mineralization. The property is considered to represent a bulk-tonnage exploration target, with potential of the structures to host a major gold deposit. During the 2002 exploration program, the prospect has been advanced to a drill-ready stage.

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