IDEAS ON THE STRUCTURE AND DEVELOPMENT OF THE MALDON GOLD FIELD

VICTORIA, AUSTRALIA

The following paper, which I have recently (2006) found tucked away in my files, will possibly only be of interest to geologists in Victoria; and probably only of those that are interested in gold.

The Maldon Gold Field lies between Ballarat and Bendigo, twice the distance from Ballarat as from Bendigo – in the west-central part of the state – and has produced a remarkably large amount of gold for its size. Little work is now carried out there. The field is over fine-bedded sandstones, greywackes, and argillaceous material, all being thermally metamorphosed to hornfels by the adjacent Harcourt granodiorite intrusion. Mineralisation is considered to be of mesothermal origin.

In the early 1980s a new, small exploration company (of which I was a founding member) named Triad Minerals NL took up an option on the Union Hill ML and an exploration licence over the surrounding lands. It then devolved onto me to do the almost sole mapping of the various reefs and old mine workings in the field. As I had learnt a lot about the area during my Mines Department records search of the area, that led to the taking up of the area by the company, this I enjoyed doing.

A series of field visits totalling several months of field mapping led to my visiting almost all known reefs, most of which were pace and compass mapped off tape and compass base lines at a scale of 1 inch to 100 feet (1: 1200), All outcrops were recorded and in some cases reefs were tied in to one another by tape and compass lines. At the same time a deeper knowledge of old reports was gained.

I found that few, if any, people were interested in my conclusions of the geological development of the field (where is the gold was all that mattered) so, for my own record, I one day briefly noted down what I believed were the structure and development of the field. They vary radically from the reports of other writers who, in all cases have dealt only with restricted areas and extrapolated to explain the whole. However, I have no doubt that until up to 1987 I spent more time detailed mapping than any previous geologist – by a long way – and covered a larger area. And as I had proved myself both in Australia and in other countries to be a skilled field geologist with a broad and thorough knowledge of geology, I can confidently state that my interpretation of the geology of the Maldon field is much nearer the truth than any other.

So it seemed to me that I should place the 1984 notes in my web page; at least for a little while. And so they appear below. Someone just may find my conclusions worth looking into.

A, preferably geological, map of the Maldon area is needed to follow the notes.

Ideas on the Structure of the Maldon Gold Field, Victoria, Australia.

By T. Frank Lee, geologist.

Dated 19/10/1984

The strike of the Ordovician folds is very regular throughout the area, averaging between 350° and 355° with the latter figure being more common. Folding may be fairly tight, appears to be mostly asymmetrical, but appears to be over-folded only in the Union Hill and Parkin’s Reef areas. Certainly it is not over-folded at Nuggetty Gully, east of the Great Western shaft or at Gough’s Reef.

It seems to me that there are two types of mineralisation in the area. The least important is gold-bearing quartz veins paralleling the strike of the beds. These usually occur individually, are usually not of great length, and the quartz veins, which usually are a glassy white and not totally opaque, are sharply against fresh hornfels. The second type is steeply dipping, sheet-like zones called by the old-timers lode channels. Seen in the field, these lode channels are decomposed hornfels zones (weathered hornfels on my maps) which are bounded by fresh, hard hornfels. Often (usually?) the boundaries are sharp, e.g. at North of England Mine, but can grade out to fresh hornfels. These lode channels vary in width with one another and along strike, but the major zones are commonly 50 feet or more wide. All strike at 345° magnetic or there abouts.

The lode channels are each, in fact, parts of much longer parallel fault planes [zones. FL,2006] spaced at varying distances from one another. There are seven major lode channels. These are from northeast to southwest: Nuggetty, Nuggetty Extended, North of England (these three are not well developed), Eaglehawk-Alliance, Beehive-South German, Parkin’s Reef, Gough’s Reef-Charlie’s Hope (not well developed). All strike at 345° magnetic and all dip, in their major parts at 75°E. However, the structure is not simple.

The basic structure of the zones is a (in part) quartz-filled fault zone of 345°,75°E. It appears, though, that there was a west side upwards and southwards force during formation of the shears. In most parts of the zones this resulted in a sharp break. However, in each of these major zones faulting did not take place over part of the fault plane. There appears to have been some force relationship between the non-faulted sections of each zone as they progressively step off to the south and west. Instead of faulting, the zone widened and bent into the form of an elongated S. Thus a structure evolved showing wall dips and strike closely as for the surrounding, unaffected beds. The difference of 10° anticlockwise [ between the strike of these zones and the strike of the beds, F.L, 2006] suggests a west side south component of the force. This is perfectly shown on a bench at Union Hill as follows: [The bench was removed during mining in the late 1980s. Those geologists familiar with Dr. E.S.T O’Driscoll’s lineaments work will recognise the faults described to be lineaments. I was unaware of his work at the time. F.L., 2006]

With rupture of the S-shaped-contained beds beginning, a set of moderately WNW-dipping fractures formed; and in one case a complete shattering resulted (Nuggetty).This trend of cross-fractures occurred in the west-dipping zone.

In each of these major zones, then, there is basically a 345°,75°E shear within which is an S-shaped, unfaulted zone, which zone dips about 75°W. These are the “lode channels” of the old miners.

Minor zones occur in various parts of the field between the major zones. They are of relatively short length and width. There also appear to be a few “splits”, in which a small, short zone turns off from a major zone by some 10°-20° anticlockwise.

It also appears to me that the “cigar-shaped” lode channels pitch gently southwards, as I will attempt to explain later.

Gold-bearing quartz was intruded up the faults. Where the quartz reached the lode channels development varied from zone to zone, depending on the degree of distortion and rupture. The best developed zone is probably the Eaglehawk Reef. Here the quartz continued approximately up the centre of the channel, i.e, it turned from west to east, but at the same time filled the complementary shears and irregular cross-fractures to give a halo about the main reef of much smaller quartz veins.

In other zones, quartz veins formed along the walls of the lode channel, e.g. North of England, Nuggetty; usually with the western reef best developed. Occasionally there was also a central reef. These wall reefs were joined by flat-lying to moderately dipping veins; and, depending at what elevation in the lode channel, dips of these gash reefs are west or east.

The development of quartz reef was not uniform along the zones, in fact it was quite irregular, so that the pinching and striking of the steeply dipping veins are pronounced. Also, it appears that best development of the cross-veins occurred at zones of argillaceous/arenaceous hornfels beds mixtures or in hornfels derived from greywacke. Cross-veins are not usually developed in purely argillaceous beds. The whole gave a quartz network in the lode channels and caused the carrying beds to be metamorphically degraded. The result is that when weathering of the area takes place the metamorphically degraded beds decompose much more readily than the unaffected, unstressed enclosing hornfels. It was therefore possible for the old-timers to select a potential site of mineralisation, needing only to find a lode channel by the appearance of weathering of the hornfels and then follow it to find a quartz vein. Again, the more complete the “weathering” or alteration, the more likely good mineralisation is present. [It was possible in 1984 to see how the old-timers carried this system out: the costean in degraded hornfels to locate a quartz vein, the sinking of a short shaft on the vein, and the crosscut to one or both walls of the lode channel. F.L, 2006]

As I noted earlier, it seems to me that these lode channels form in the large fault zones in an en echelon relationship (In fact the fault zones appear to be en echelon.) and that they pitch south. In a broad, and admittedly sketchy, way I see the lode channels as follows.

Nuggetty Reef, at its northern end where it is truncated by the Harcourt Granite, represents the southern and lowest part of the lode channel on this zone. It is for this reason – secondary enrichment from the previously overlying part of the channel – as much as by remobilisation of the gold by the granite which made the reef so rich. At the southern end of this lode channel, where the quartz formed a single sheet, the top of the quartz reef pitches south under Nuggetty Ridge and has been cut in Carpentaria drill holes.

At the North of England the lode channel is well developed although small, being only 47 feet [14.33 m] wide, but it dips 75°W and carries west and east wall reefs with poorly developed cross-veins between. At depth the zone curves to vertical and to the east. The Nuggetty Extended reef may have been similar but on a smaller scale. Both develop north and south into single reefs and both of these reefs pass at depth to the south. For example, the North of England was followed by old workings almost half-way to Union Hill and may have been cut by an eastern cross-cut, there.

Lahn’s Reef, west of the North of England and about on line with the Eaglehawk Reef, appears to be a short, parallel, similar structure.

These preceding zones are characterised by fairly abundant sulphides as well as free gold and so differ somewhat from the lode channels farther south. This may support the view of them being nearer the base of the S-structure.

The Eaglehawk lode channel is the best developed, both for width and for length. It stretches from the northern side of Union Hill to almost the railway station on ground surface, narrowing south, and was worked at the railway station by the Alliance shaft. The zone continues south of the Alliance as a reef zone (Carpentaria drill hole) and quartz reef (eastern cross-cut from the Great Western shaft); again seeming to be pitching south as the reef does not show on ground surface although well defined at depth.

I know virtually nothing of the Beehive-South German lode channel so cannot comment on this.

The Parkin’s Reef lode has been interpreted by others as a series of thin saddle reefs in which the western limbs have coalesced to give the Parkin’s Reef. Shatter zones at the anticlinal axis gave quartz networks. This is not greatly removed from my interpretation if it is assumed that the easterly dipping fault zone coincided with an anticlinal axis – but movement would have to be east up, wouldn’t it?

Finally there is the Gough’s Reef-Welcome Reef – (Charlie’s Hope) lode zone. This has no well-developed central or wall reef but rather the quartz has been intruded as a series of short, parallel reefs accompanied by cross-cutting, moderately dipping gash veins. It is possible that Guogh’s Reef hill marks the upper and northern limit of the lode channel and Charlie’s Hope the southern for in both cases the zones seem to pinch out (north and south, respectively).

Looking at the gold recoveries of the mines on the various lode channels, it seems to me that, with the exception of the deepest South German lode, gold was best where lodes dipped west and vertical. This would accord with what one would expect of the structure as there would be a decrease in vapour pressure within the expanded lode channels and thus an increased tendency for mineral contained in the quartz solution to precipitate. It may also account for the tendency of the quartz to be dense white in colour and of a rather granular texture (apparent on weathering) [as opposed to glassy, bluish white and non-granular quartz seen in quartz veins in the fresh, hard hornfels in various places throughout the area. FL, 2006]

[A sketch interpretation of a lode channel with reef followed. It is reproduced below. FL, 2006]