The Index occurrence is at an elevation of 1950 metres on Index Creek, a minor tributary of Gainer Creek, which flows southwest into Lardeau Creek. It is part of a northwesterly trending linear cluster of eleven crown granted mineral claims that cover the Mollie Mac [082KNW036], White Quail [082KNW037], Hidden Treasure [082KNW106] and Index [082KNW038] prospects. The Index (L.3956) claim is in a cirque at the head of the creek, which drains to the northwest from Redcliff Peak.
The Index area saw limited exploration prior to 1917, when a 46 metres long crosscut was driven to develop a 12.2 metre zone of "concentrating ore". By the time Northern Inland Resources Limited acquired the property, in 1956, the workings consisted of a short adit and small inclined shaft driven into opposite sides of a band of "Molly Mac" limestone, extensive areas of past stripping, a caved shaft and two old prospect pits. Similar workings probably extended down the hill on to the adjoining tenures. Northern Inland Resources drilled two holes on the Index claim in 1956 and a further eight the following year, for an aggregate total of 547.2 metres. In 1960, the claim was held by G.C. Short.
The Trout Lake area is underlain by a thick succession of sedimentary and volcanic rocks of the Badshot Formation and Lardeau Group near the northern end of the Kootenay arc, an arcuate, north to northwest trending belt of Paleozoic and Mesozoic strata that is now classified as a distinct, pericratonic, terrane. The arc rocks are bordered by Precambrian quartzite in the east and they young to the west, where they are bounded by Jurassic-age intrusive complexes. They were deformed during the Antler orogeny in Devonian-Mississippian time and were refolded and faulted during the Columbian orogeny, in the Middle Jurassic. A large panel, the "Selkirk allochthon", was later offset to the northeast by dip-slip motion along the Columbia River Fault.
The Badshot Formation is composed of a thick Cambrian limestone that is a distinctive marker horizon in the Trout Lake area. It is underlain by Hamill Group quartzite and it is overlain by a younger assemblage of limestone, calcareous, graphitic and siliceous argillite and siltstone, sandstone, quartzite and conglomerate, and also mafic volcanic flows, tuffs and breccias, all of which belong to the Lardeau Group. The rocks are isoclinally folded and intensely deformed, but only weakly metamorphosed. They occur as intercalated beds of marble, quartzite and grey, green and black phyllite and schist. Fyles and Eastwood (EMPR BULL 45) subdivided the group into six formations (Index, Triune, Ajax, Sharon Creek, Jowett and Broadview) of which the lowermost (Index) and uppermost (Broadview) are the most widespread. The Triune (siliceous argillite), Ajax (quartzite) and Sharon Creek (siliceous argillite) are restricted to the Trout Lake area. The Jowett is a mafic volcanic unit.
The Index area is in a similar stratigraphic and structural setting to that of the Hidden Treasure [082KNW106] and White Quail [082KNW037] prospects lower down the hill. The tenure is underlain by green phyllite, metatuff and (Molly Mac) limestone near the top of the Index Formation, and by black siliceous argillites, cherts and phyllites of the overlying Triune Formation. Upslope to the southwest, the section passes through Ajax quartzite into siliceous argillite and phyllite of the Sharon Creek Formation. The Triune and Ajax formations thin dramatically to the southeast and the Ajax Formation pinches out on the Index claim (L.3956). The rocks are isoclinally folded and highly deformed. They have an axial plane schistocity that strikes to the northwest and dips at a steep angle to the southwest. The Index area is underlain by northwest trending and steeply southwest dipping schists intercalated with bands of limestone near the top of the Index Formation. On the Index tenure, the "Molly Mac" limestone is in two parallel bands that are thought to be fold repetitions of each other. The southwest band is barren. It forms the core of a small anticline and disappears under phyllite to the southeast. The northeast band is mineralized. It is between 15.24 and 27.43 metres wide and pinches out to the south of the claim.
The Index showings are found in siderite alteration zones in two fault off-set segments of limestone. One segment extends 914 metres from the cross-fault to the end of outcrop. The other is exposed around the prospect shaft, 122 to 152 metres south of the cross-fault. The fault, itself, contains a 0.3 metre wide vein of quartz and galena where it cuts the northeast band. The larger, northern, mineralized segment has seen the most exploration. It is composed of a single limestone band that contains three ragged, siderite replacement zones. One is 2.4 to 3.0 metres wide and follows the footwall of the limestone for over 300 metres. One is on the hanging wall of the limestone. It is ragged but approximately 1.8 metres wide and extends for 152 metres. The third is in the middle of the limestone. It is 1.52 metres wide and extends for approximately 46 metres. In each, galena is irregularly disseminated and locally forms lenses or veinlets a few centimetres long and a few millimetres wide. Chip samples from an adit dump assayed 58.3 grams per tonne silver, 15.1 per cent lead and 6.1 per cent zinc (Starr; Report on the Mollie Mac Group, White Quail Group and Hidden Treasure Group, 1933).