Summary:
The Imperial gold deposit is believed to represent epithermal gold mineralization related to Tertiary-age low angle detachment faults and associated extensional faults. The epithermal gold mineralization is structurally controlled and transitional between low and high-sulphidation systems.
Mineralization and Alteration
Gold mineralization occurs primarily within haematitic and limonitic altered breccias, microfractures and gouge zones developed in the host biotite gneiss and sericite gneiss units. Minor quartz veining, very-fine grained pyrite pseudomorphs and silicified zones are also common.
The density of fractures, extent of the red-brown to yellow haematitic/limonitic coatings and pyrite pseudomorphs within the host units are notable mineralized features. Logging of coreand cuttings samples from the project site indicated no fresh pyrite or sulphide mineralization is present due to the oxidized state exhibited throughout the deposit.
The deposits were oxidized to a depth in excess of 750 ft indicating that the deposits were oxidized near surface and down dropped by faulting to their current locations.
The majority of gold mineralization occurs stratigraphically above a siliceous breccia horizon. This distinct relationship between the siliceous breccia and the overlying host rock units is traceable across the deposit. Sporadic mineralization is also noted along the cemented gravel and volcanic contacts and in fault structures within the brecciated volcanic and conglomeritic units. Low grade mineralization also occurs withinthe overlying cemented gravel units as narrow layers eroded from exposed mineralized gneissic units.
The mineralization and alteration character of the deposit varies across the deposit as described below.
East Area
Gold mineralization in the East area occurs within a west-northwest trending fault zone with a strike length of 3,200 ft, a variable width of up to 800 ft, and an average thickness of approximately 85 ft. The mineralized zone is a tabular body, predominantly flat lying to gently dipping 5° to 15° south. The mineralized body is cut by a series of east-west striking normal faults. The fault bound mineralized lenses of the tabular body are offset progressively deeper southward across the series of faults.
The east-west normal faulting may represent extension or possibly a change from a positive flower structure to a negative flower structure. It was noted that the dip of the mineralized lenses to the north steepen to 45° to 70° to the south. It was explained that the change in dip may be coincidental with the inflection of the flower structure thrust sheet where it steepens to a 60° to 70° dip to the south (Scott, 1992).
Another explanation may be that the shallow mineralized lenses were thrust over the adjacent, relatively stable stratigraphy, andthen during the extensional period, a section of the shallow mineralized lenses located along the edge of the relatively stable stratigraphy was dragged down and southward along the south dipping normal fault. The mineralized lenses are cut by north-northeast trending normal faults that drop stratigraphy to the east and west. Paleocene to recent gravels covers the East portion of the deposit, averaging approximately 200 ft in thickness.
Gold values in the East area are elevated where the pervasiveness of limonitic alteration increases and is accompanied by silicification, quartz veining, pyritization and gouge zones. The distribution of the hematitic and limonitic alteration zones within the East area exhibit a definite spatial association to the siliceous breccias. A vertical zonation is noted in several mineralized intersections associated with the breccias from limonitic to hematitic alteration moving up in the stratigraphy. The thickness of the limonitic zone is variable, ranging from 10 ft to 75 ft. Thehematitic zones are typically thicker, up to 150 ft. Hematitic and limonitic alteration show crude correlation with an increase in gold grade/thickness along linear trends oriented to the east-northeast. The linear trends are believed to reflect the presence of high angle mineralized structures. Similar structures also occur in the nearby Picacho and Mesquite mine sites.
West Area
The West area is similar to the East area and was modelled by the QP as an extension of the same mineralized body. Mineralization occurs as a tabular body made up of several zones with planar dimensions of 1,200 ft in length, 1,000 ft in width and an average thickness between 90 ft and 120 ft. Mineralization intercepts occur as shallow as 20 ft from surface and average 80 ft to 120 ft below surface.
The gold mineralization is down faulted to the south by a series of east-west trending vertical to steeply south dipping normal faults. Vertical displacement on these structures is variable from 80 ft to 260 ft. Drill data suggests that the mineralized zone is cut off to the west by a north-northeast trending structure that displaces stratigraphy down to the west. The amount of strike slip displacement is unknown on this structure. The West area gold mineralization is limited to the east by a northeast trending fault and to the east of this fault is situated the Central area. Mineralization to the north tapers into a series of discontinuous lenses or is cut off by a north dipping antithetic fault to the flower structure.
Central Area
The Central area is a down faulted block of the same stratigraphy encountered in the West and East pits. Structurally the area differs slightly from the West and East pits. Bedrock intersections occur predominantly in the shallow portion of the "flower structure". Mineralization is not as prevalent in the shallowest portion of the thrust structure in the West and East pits. This may be the result of the structural preparation of the host and explain the narrow (10 ft to 40 ft) sporadic intersections in the Singer Pit area.
Mineralization is hosted by biotite to biotite-chlorite quartz-feldspar gneisses and to a lesser degree sericite schists. Mineralization is also spatially related to a fault gouge zone that represents the fault contact between the gneissic package and underlying gravels. Gold values are associated with hematite fractured gneisses with localized zones of quartz veining, gouge zones, and to a lesser degree limonite alteration, silicification and brecciation of the host rock. Mineralization commonly occurs stratigraphically below a fine-grained, quartz-rich unit that has a variable thickness (5 ft to 180 ft). This unit, descriptively-logged as "quartzite", may represent a facies change within the gneissic package or more likely a silicified quartz feldspathic unit that acted as a cap to mineralizing fluids. The "quartzite" is fractured and altered by hematite along fractures but seldom hosts any mineralization.
A siliceous breccia unit in the Central area has mineralization occurring stratigraphically above although not directly adjacent to the breccia unit. However, in areas where the breccia appears to have a steep dip to the south mineralization may occur both above and below the breccia horizon. An example is drill hole I-11, which intersected 0.045 oz/t gold over 20 ft below the breccia.