GOLD FIELD.

Otago Daily Times, Issue 220, 3 September 1862, Page 6

GOLD FIELD.

NOTES by Dr. Hector relative to the Geology of the Jfanuherikia Valley. Dunedin, August 27, 1862. * To His Honor the Deputy-Superintendent. Sir,-—I have the honor to submit for your information a few notes relative "to'the Geology of the Manuherikia Valley, as I am aware that any information concerning that district, however imperfect, has a special interest and value at the present time. I visited it for a few days in May last, in the course of a rapid tour which I have now made through most of the eastern portion of the Province, and of which a short report, with illustrative sections, is in preparation. The routs by-.which" I reached the district in question, was the ordinary d.ay track by Shag Valley and the Maniototo , or Upper Taieri Plains. The furthest ■'< point I reached "was a little above the junction of the Manuherikia River with; the Molyneux, so that, excepting- from a distance, I have not seen even that portion of the newly reported auriferous country which lies on this side of the •Dunstan Range. The g-er -ral position of the very appropriately ter;i 1 v Dunstnn 'Diggings," so .'far as yet known, will be along' the course of the 31 olyneux River, where'it passes the south end of the Dunstan Range by a rugged gorge, to escape from the Valley of the Lindis into that of the Maiiuherikia, for even

with-a "very partial ■■knowledge of the country, a glance at the map shews that, in a certain sense, the Molyneux River cannot be said to have a proper valley of its own, 'hid, either to follow a rent or fissure across a succession of great valleys which lie nearly at rig-ht angles to its main course, and occupied by comparatively insig-nfiicant streams. The principal object I have in this communication is to point out this'anomaly and to shew reasons why the occurrence of gold detritus should not be too rigidly associated with the action of the river, which at first' appears the most natural view of its distribution. To understand the geology of the lower portion of the Manuherikia Valley, it must be considered in connection with" a much larger area of which it: forms a natural division. By referring to any of the excellent, maps of the country which have been issued by the Survey Department, it will be observed that westward from the head of Shag Valley the country is traversed by a succession of nearly parallel ridges which coalesce at their southern extremities to form the broad, me ssive, and elevated region of the Lamraernmir Mountains, but to the north they gradually die out as they approach the Kakanui Mountains which rise abruptly in a line nearly at right angles to them. These ridges, which are composed of highly altered sedimentary rocks, impregnated with quartz, along with the Kakanui Mountains, which are composed in part of the same altered schists, but overlaid by crystaline limestones and newer schists, thus enclose an extensive depression, having a very irregular boundary, embracing the Upper Taieri Plains, Idaburn Valley, and the Manuherikia Valley. How far this depression may extend to the north-west I cannot, at present say, but if not cut oil' by a mountain ridge extending between Mount St. Bathan's and Hawkdun Mountain, it may

be continuous with the Upper Wuitaki country. Leaving out of consideration this possible extension of its limits, we find ■that this great .basin has only two outlets for.its drainage—one by the gorg-e of the Taeiri River close to the situation of

the Highlay diggings, and the other by the gorge of the Molyneux, which commences just below* tlie point where that river receives the Manuherikia. In both these cases it is ■ important to observe that the drainage escapes by a channel which lias been, so to speak, cut through the rocky lip of the basin. Bearing* in mind the now well established fact that the original we;u 'iff down of the rocky matrix, and the mechanical liberation of its contained gold, lias been effected for the most part by long continued wave and current action, either in the sea or in large inland lakes, and that the comparatively feeble and limited action of streams has merely exerted an ever-re-curring sifting and sorting influence on the materials thus prepared, we can at once perceive that in this rock-bound basin the conditions have been most favorable for retaining and assorting the debris formed by the degradation of the surrounding schists during the submergence of the land in Tertiary times. The floor of this basin is now occupied by extensive grassy plains, sometimes presenting a wide expanse of many thousand acres in extent, but always terraced and beautifully moulded towards the margin by the action of the retiring waters, as the drainage channels were slowly cut by the two rivers before mentioned.

The divide or watershed between these two drainage systems lies between the point of Rough. Ridge and Mount Ida, and it is deserving of notice how much more efficiently the larger river, constantly supplied from great natural resources, as lately described by M'Kerrow, has done its work in cutting a drainage channel, than lias the small and intermittent Taieri River, for the level to which the plain has in this manner been reduced at where the Molyneux breaks out from it is only.(500 feet, while the outlet of the Taieri River is 900 feet above the sea.

Round the margin of the basin, resting on the flanks of the schist ranges, remnants of tertiary strata are to be found to the height of 1000 feet above its lowest present level, or 1000 feet above the sea. Did not this agree well with the thickness of the tertiary strata in other-.parts-of the Province, I might have hesitated..to'ascribe to.-river and lake action so extensive degradation even of such incoherent materials, as there must have been removed on an average, several hundred feet of thickness from an area of at least 800 square

miles

Within the basin as thus described, from the different class of agencies which have

been at work, we have two distinct groups of-deposits, which for the present I shall i simply designate as the Older Tert'iaries and the Newer Tertiaries. The older tertiaries are- the deposits which gradually filled xip the depression as it passed through the successive stages of submergence, from an estuai-y-like arm of the sea to a deeply excavated submarine valley. The earliest formed of these strata can only be seen in a few localities where they have been preserved from the succeeding denudition round the margin, and in the deep recesses of the ancient valley. They invariably consist at the base, where they rest on the schist,"of strata which indicate the neighbourhood of dry land at the period of their formation, supporting- a vigorous vegetation, which has been preserved to us as 13rown Coal, associated with finely assorted beds of clay and gravel, | indicative of current action in shallow water. Elsewhere in the Province, though not as yeb in the district I am treating of, marine shells have been discovered along with these beds.

Over this -group, havebeen deposited strata of sand and conglomerate, formed of -materials derived from the schistose rocks. The upper portion of this deposit, which is many hundred feet in thickness, consists pa rely of fragments of quartz,-doubtless from the more enduring- quality of that substance enabling- it better to survive the long-continued ■ attrition. In some cases these upper quartz pebble beds have been firmly, cemented by some process not }'et clearly understood into a hard reisting rock, which breaks up into hug-e cubical fragments, that are to be .found scattered over .slopes from which the underlying1 and more yielding- strata have been removed, or are found capping- hills of tlie.se strata, which they have preserved Such hills are the ."' White Made Hills" of the g-old iniiij-r. They are undisturbed outliers of the older tortiarie*, and when they rest on a depression in the schistose rocks, and at a considerable altitude, they often prov? very rich in gold, having1 been formed during- the period of greater depression,, when the tlia current action would manifest! v be most intense.

Indications of '' Made Hills" were seen throughout the district, and will doubtless be speedily recogni.-sed and tested by the practical miner. The mere prevalence of blocks of v cement," as it is technically termed, upon the Hanks of a wide valley, forms no sure indication of the presence, of a lead, but if the ''cement," be found strewn over a limited tract, round which the bedrook crops out on all sides, or in such a manner as to indicate the existence of an ancient valley at a higher level than the present drainage system "of the country, it may then be taken as a guide in prospecting.

The period of the greatest depression of the land, which corresponds with the close of what I term the older tertiary epoch, was marked in some districts of the Province by volcanic eruptions, during which basaltic lavas was poured forth from rents in the earth's crust, and flowed over what were then the lowest levels of the sea bottom, but which have now, owing to the preserving influence of the hard, tough, basalt, become elevated to lofty positions. Like the " White made hills," these basaltcapped hills often cover deposits of rich auriferous drift. There is evidence of volcanic ejections- during this period in the 'lower part of the Manuherikia Valley, but none of the characteristic flat-topped hills were observed. These are, however, very prevalent, in the district from the gorge of .the Taieri, at the head of Shag" Valley, southward towards Dunedin. Highlay and Flat-top Hills form very perfect examples.

It was with the first display of volcanic activity' that. the elevation of the lund commenced, and although, as is always the case, this elevatory movement was accompanied by degradation of pre-existing strata, rather than by the formation of new deposits; yet under favorable circumstances this very degradation gives rise to local deposit?, which are those I shall provisionally term, the- newer tertiaric*.

During this period, the rock-bound -basin afterwards to be drained by the Taieri and Molyneux rivers, became converted into a system of lakes, connected by streams, which, slowly excavated terraces, and deposited in a more perfectly assorted form the materials which composed the plateau. In the lakes, the sorting process would bo still more perfectly accomplished," the coarser materials being deposited at the inlet, while the finer sediment would only settle in deeper water, or towards the outlet. The Taieri Lake is the last remnant of this once great expanse of fresh water, but the material which were 'deposited in

this manner still. exist," forming ■ terraces and cliffs of shingle, gravel, clay, and fine loam. As the main exit channels of the basin were deepened, the lakes were in time drained, and their materials again assorted by the corrosion of the streams. This sorting' process, which may justly he compared to the action of a sluice in ordinary gold washing, has been continuously in operation during* the lapse of vast ages, sifting and separating, according to the specific gravity of its component parts, the debris or the scliistoze rocks, and preparing the way for the gold miner by concentrating1 the precious metal in a smaller and smaller quantity of wmhgracel. Gold being the heaviest material contained in these deposits, must be looked for as a residium, remaining behind after the lighter parts of the scliistoze debris has been removed by the action of water. When, therefore, it is found on the bars or in old channels of a stream, we must look to the detritus, or drifts, in the neighbourhood, as its immediate source, and the more they bear signs of having been well sorted, the probability is increased of finding the heavier materials, and among these, the o-old, gathered into leads towards their base.

From these considerations it will be perceived that by simple natural operations similar to those now in progress, we may have a great variety in the mode of occurrence of the gold in these so-called "alluvial drifts." I.do not underrate the agency of streams in wearing and cutting the solid rock, and also carrying forward to a lower

[ level immense quantities of detrital matter, j i But every practical miner knows how in- J adequate any.theory-of river action would j be to account for tne "gold drifts" in all i situations where they are found. Wo must rather, as I have endeavoured to show, look for their origin in a sequence of grand ' changes in the natural features of the country, dependent upon slow alterations in the relative distribution of sea and land in the first instance, followed by the formation of great inland lakes which were gradually dried up as the existing drainage systems became established. Exactly similar deposits occur in basins upon the I western "slope of the Rocky Mouu tains of North America, from an elevation of 5000 feet almost to the sea level, mid extending from California north wards into British Columbia. The absence of basalt-topped hills, and the large scale on which the terraces have been developed, give the Miinuherikia and Taiert Plains a striking resemblance to the upper valleys of British

Columbia. Before arguing- from this su-

pei'ficial rcsainblance, liowever, any. similavity in the richness of their auriferous deposits, it is important -to . comT>are the nature of the schisto/e rocks -from which the ",'old has been derived. The prevuiliii"- rock which forms the framework of the Province embraces many varieties of ■ jrneiss and-mica ■ schist, but generalh" imperfectly formed, in so far that

the evidence of mechanical origin has rarely been obliterated from the intimate particles, while in all cases the original lines of bedding remsin distinct, and mark changes in mineral character. As compared with the Lower Silurian auriferous rocks of Victoria, although they are more altered in mineral' struct ion, yet they have been subjected to much loss- meehanicul disturbance, the' ilexures and folding* of the strata forming, us a rule, only gentle undulations,., while, in Victoria, and also in California, they are generally- so abrupt that the strata appear to be vertical. The general trend'or direction, of these rocks is about N.W. by \V\, or nearly across the island. At first sight this sip-

pears as if at right angles to the "mountain

rid>.re.s; but it' wo disrogurd the minor valleys, it will be found, us I before indicated with reference, to the t-irtiurus, that :i broad massive axis of elevated land follows the above direction, with a slope northwards towards the .valley along* the ha.se of the Kakamii Mountains, and to the south towards that remarkable valley which appears to extend from the Five Itiver Plain to the mouth of the Molyneux, and which is bounded on the south" by a range of mountains which, though more broken, have a similar conformation to the Kakamii Range, It is in the -valleys included between the finger-like spaw which brunch oif to the N. E. and o. W. from this central axis that the tertiary drifts have proved most

auriferous, all the present diggings being1

in such valley*. The schists towards the central portion of the axis contain much more quartz, and have the lauiirue more contorted than those either to the south or north, but excepting* this, which is only a matter of degree, there is no observable difference. The quartz always occurs in thin layers, or circumscribed patches, or as concretionary masses, and in all cases has a clear relation to the original -composition of the sedimentary rock before it underwent any mineral or structural alteration. Even the ku'gest masses of quartz I have seen, including- those at Hitrhlsiy and Waipori, have to all appearance this limited form, and are what miners term "Hut reefs," as distinguished from the vertical veins winch occur in Victoria, the sides of which remain nearly - parallel at least for preat distances, like true mineral lodes. The absence of •••ranito, and the rarity

of ancient volcanic dykes of greenstone or porphyry (of which I have seen as yet none in .situ, ult.hoiitrli fragments liave

been obtained) forms another peculiarity of- our schistose rocks. However, il" as has been reported, there are lodes of copper in the vicinity of the Dunstan Kan^e, that factmavbo taken as a sure indication of the vicinity both of granite and of .such dykes. The only well marked variety in "the schists has Icon observed at Slug Valley and in llighlny ll'urn, where they approach inoro to the character of arfjilaeioiH slates, of a deep blue colour, and traversed by. small veins both of quartz and carbonate of lime, the former of which have more of the- true character of auriferous quartz veins than elsewhere. As yet I have seen no evidence of wh.»t is the exact i matrix of the gold in this country. The presumption that it is contained in O'.u* schist rocks is only because of the absence of other rocks of a more -likely nature so far as is yet known. The schists which bound the lower part of the Manuherikia Valley have exactly the same character as in many other places, as &r instance at Moa Flat below the gorge of the Molyneux at Mount Benger, or to take a nearer instance, as are displayed in the sea cliffs below Saddle-hill, about seven miles from Dunedin. It is this sameness in the character of the schist which impresses me with the idea that the intensity and long continuance of the operations which produced the tertiary " drifts " has had more to do with the accumulation of rich deposits of gold in certain localities than any peculiarity in the bed rock in these spots. If these views be correct, we may expect an extension of auriferous country to the north-west along both sides of the central axis which I have indicated, towards the district occupied by the large lakes, and in which the miners will find the natural agencies working large " claims " for the benefit of future generations. A sample of black -sand with gold has been submitted to me by the CommisI sioner for the Gold Fields Department, as procured from the Manuherikia country. | It consists of- — ! 88*0 magnetic iron ore. 4*5 specular iron ore. 7*5 gold, silica, and insoluable ! matters.

100*0

Tliere is no trace either of tin in the sample. The large per centage of highly magnetic iron is a very destinctive feature

from the black sand found in other parts of the Province, of which there is rarely more than 20 per cent, easily attracted by the magnet. The scales of gold when examined under the microscope are seen to be only moderately, water-worn as compared with ordinary samples of gold from river bars, but much more so than with the Waipori or Tuapeka gold. If this black sand deposit occurs in great quantity, it would seem to indicate that the natural sifting process has been carried out very perfectly, but as in that case we would then expect to have [ all the heavy mineral substances which exist in the original rock, represented in it, unless it can be shown that even the bulk of the gold has been separated from the iron sand, it seems to argue against the rock matrix being auriferous in a high degree. While I confidently expect that "gold will be found in fair quantities in the locality of the new diggings, yet I fear that this rush will prove somewhat premature, unless a wellinarked lead be soon struck. For a large body of miners to work successfully in one locality requires that the auriferous deposits be very clearly defined, as compuritively few 'of those who iloek there have any other rule to work by than doing as the} r see others do. What we know as yet of j the district, however, would lead us to ' expect, that the shallow deposits at least, will prove oxtreiaely paichy and eccentric in their distribution, and perhaps only to be detected by men who have a well practised eye for detecting likely spots. T hare to apologise for the somewhat indefinite nature of the remarks I have ventured xipoa in this communication, but my object has been merely to make you aware of what I consider to be the general plan upon which the gold drifts, especially of the M:umherikia district, have been formed, and to show what grounds there are for expecting their distribution over a larger area than the immediate vicinity of the Molyneux river. I have the honor to remain, Snt, Yours most obediently, Jasjes Hector, M.D., F.G.S., Sec.