FAQ- 4. How do the outer limits of lead-zinc orebodies fade out?
Some early work in studying the lead-zinc orebodies eg Croxford (1962) was hampered by the lack of mine development into the lateral extremities of the orebodies. This was rectified during the late 1980's with continued mining north of 7060mN. In particular, it allowed the investigation of lead-zinc orebodies as they approached the hinge of the Mount Isa Fold. (Perkins, 1987). A summary of the conclusions of that paper are presented here, together with accounts of the features of the outer limits of the orebodies.
Summary of features indicative of epigenetic origin for lead-zinc orebodies
Characteristics of the lead-zinc mineralization as observed in hand specimen and microscopically which indicate a late replacement origin are summarized in Figure 12. This example shows three textural types of sphalerite. The first occurs as very fine (50-100μm ) aggregates along the layering, with density variation occurring abruptly across sub-mm dolomite veins. The second occurs as much larger aggregates along bands with more coarsely crystalline dolomite, which still show relict layering, and the third is as aggregates along the cross-cutting dolomite veins themselves. These types would be regarded traditionally as 1) primary syngenetic/diagenetic sphalerite 2) recrystallization of primary sphalerite along bedding and 3) remobilization of sphalerite into cross-cutting veins during deformation. Examination of the distribution and textures of the sphalerite aggregates however, in particular the change in density across veins, the continuity of sphalerite aggregates from the bedded area into the walls of the veins, and the cross-cutting relationship of sphalerite across dolomite deformation lamellae, indicate only one generation of sphalerite which is later than the veins and alteration dolomite. An example of the continuity of galena and sphalerite from microveinlets to replacement of fine dolomite along bedding is shown in Figure 13.
Figure 12. Different textural types of sphalerite in a single specimen.
Sphalerite aggregates along bedding extend along a 2.5mm wide zone with density highest at (A), lowest between the next two thin veins and intermediate above them. Sphalerite along the central zone (B) is preferentially replacive after the clean white crystalline dolomite. In the zone on the left (with relict bedding) sphalerite (C) is interstitial to, and locally traverses, clean white "saddle dolomites" which overgrow inclusion-rich dolomite. In the veins eg (D), sphalerite aggregates are replacive after clean dolomites.. DDH K754 E. Decline #1, 15 Level 96.7m.
Figure 13. Sulphides along bedding and microveinlets.
Fine dolomite-quartz veinlet with sphalerite (a) and galena (b) extending from the veinlet into irregular replacive patches along bedding. Part reflected-part transmitted light. Blue colour the result of staining for ferroan dolomite. Field of view 2.2mm. 8 orebody 6910N, 2380 RL. Same sample as Perkins (1987) fig. 16a. and 18.
Fading out of the northern limits of lead-zinc orebodies
In syngenetic/early diagenetic models of lead-zinc ore formation it could be expected that ore grades would show a gradual diminution in a northerly direction towards the very low geochemistry in holes such as QZ10 (see Painter et al. fig. 1). In addition, enrichment would be expected in the Mount Isa Fold, if more ductile pre-deformation sulphides mobilized into fold hinges in the manner indicated for Hilton Mine by Valenta (1994). The observed lead and zinc grades, however, show a quite different relationship.
Ore grades in 7 orebody on 11 Level diminish abruptly from 7.0m. at 3.7% Pb and 8.2% Zn at 7270N to 500ppm Pb and 2400ppm Zn in the Mount Isa fold hinge only 45 metres north. The sharpness of this front can be as marked as that illustrated in Figure 14. Here a high-grade lead-zinc zone containing strongly folded non-laminated layers narrows down from >800mm to 460mm along a abrupt front with gently folded partially dolomitized layers containing two distinct generations of veins. The sub-horizontal veins are distinctly fibrous dolomite veins and overprint the more carbonaceous veins showing dilation in a vertical direction. At the sulphide boundary galena-rich sulphides appear to have transgressed and replaced the fibrous dolomites over a short distance.
Figure 14. Abrupt termination of high-grade lead-zinc zone.
The high-grade zone on the left of the figure ends in a front shown by boundaries and arrows. Non-laminated siltstone layers (eg. A) can be traced into the veined area on the right which contains no visible galena and sphalerite. 7 orebody (below the F/W TMB) 9 Level 7070mN looking NE.
Similar relationships are observed in the vertical plane. At approximately 7200mN, a development face in 7 orebody (9C sublevel) contained 10% Zn across the face extending 0.4m below the 7 O/B F/W TMB. 12m directly above on 9E sublevel, the same zone extending 0.8m above the TMB contained no visible sphalerite. The dip was slightly shallower at the upper heading indicating proximity to the Mount Isa Fold. These observations indicate that the northern limits of lead-zinc orebodies are controlled by the change in orientation of bedding as the anticlinal hinge of the Mount Isa Fold is approached. It is indicative of local dilation along some bedding planes during shear associated with folding, allowed dolomitization and silicification along bedding which is subsequently replaced by sulphides.