THE GEOLOGY OF THE DEEP LEADS OF OTAGO.

Otago Witness, Issue 2200, 30 April 1896, Page 16

THE GEOLOGY OF THE DEEP LEADS OF OTAGO.

By W. GtBEEN.

For reasons whioh may afterwards appear^ it Is high time that something definite be published about onr deep leads. If there are any prospects of this character existing on the goldfields of Otago, .the geologioal fasts ooncerning the deposits should be particularised and made known for the timely encouragement of mmmg -enterprise.

la this article the writer does not aim at supplying thiß need, bat only seeks to call Attention to the subjeot by pointing oat in b general way where to look for and what to expect of the nature of deep leads on these goldfieldß. FORMATION OV LEADS. The term " deep leacl " is commonly understood to mean a stratum ox bed or ran of payably aoriferotu gravel lying deep beneath other strata, and continuous in length for •ome distance. £m account of deep leads being composed of water-worn gravels, thej are usually supposed to represent jold river beds or ooveridop .watercourses, and in California are termed »' dead rivers." But there is.much reason to ■ believe that this popular theory of the formation of the leads has in many eases been misapplied. This view is soggested by Redactions from the published reports of rariooß leads, as well bb by personal obser* rations on goldfklds where the leads are wrought. With regard to the oonrßes some of these KKsaHed" 1 dead fivers ° or leads are soppoaed ' (o have had, an American .authority says, »tiangely enongh, .that '"In California they us more conrmoßly at right angles to the Bourses of the modern "rivers." Another authority states that " an Instance ' Vas been known of a deep lead trending in Ihe -opposite direction to the flow o! fihe nrodem stream ttbove it."

Accounts of such strange freaks of nature us the above instances Incline us to doubt the reliability of those .geoUigioal records. The vagaries of onr Uew .Zealand rivers st» notorious, and their achievements in obtaining courses through all sorts of Impassable oonntry is little short of the wonderful, while the ancient beds of these itreams bnt trilling deviations from the modem conrses- as compared with tha wayward character ;of those foreign defunct .rivers.

IWe have frequently -observed in Victoria — the land of deep leads— that the notion is very prevalent among mineri, like aa oldtime prejudice, that all the leads were onoe water-oourses, and this theory is often advocated In defence of natural causes. In a ponntry bo little elevated and so flat as Amjfcralia it is difficult to conjecture whence the fljnamio £oxo9 d those ancient eo-callaa

rivers oould be derived which would enable them to move and grind gravels of so ooarse a character as we us.ye s<'<m taken out of the leads. When we compare the erosive effects of the New Zealand rivers with the pony efforts of the Australian streams, we are struck with the vast superiority of the former. As we .travel through the plains of the Hopkins, tha Wimmera, the Lodden, and the Murray we oanndt help contrasting the scarcity and gmallness of the gravels moved by these modern rivers with the character- of the w»ah from the leads — the supposed ancient streams of these districts — while the superior quality of the gold-bearing gravels of the latter is without comparison. 'The gravel raised, from those leads is very uniformly oomposed of quartz pebbles, indicating a relation to the rooks of the' neighbourhood, and is generally devoid of that travelled variety of stones whioh always characterises the river gravels. This faot seems to have escaped the notics of those who uphold the dead-river theory of the formation of the leads. In addition to the foregoing negative evidence, it should not be over-looked that very commonly there overlies the leads a regularly stratified series of thick beds of pipe-olay and quartz drift sand, in diatinot and separate beds, interstratified with and overlying the wash of the lead. Such order of deposition as this rivers are unable to perform.

VICTORIAN LEADS. It may not be out of place here to go a little farther and describe the origin of the Victorian leads. It must be admitted as a geological faot that the most .elevated and most ancient of the auriferous gravel deposits of that colony are of marine origin, and undoubtedly have been sea beaohes, not river beds. These gravels in broken lines carry their levels as beachesthrough the country for miles, following the windings and turnings of the main ranges, round whioh they trend with the utmost regularity; and they may betaken to correspond with those so-called "dead river* " iwhioh iron at " right angles to the flow, of the modern ones." In most cases' the channels of the leads were naturally hollowed oat by the combined wave and tidal action of the reoeding miooene ocean while the land was slowly rising. The local small gully streams, also brought into play, added their mite to complete the operation by cutting oat portions of the upper lines of gravels and redepositiDg their debris to enrich the gntters of the leads now forming. The lower leads, representing the accumulated gravels from higher sources, would obviously be the richest. 1 The voloanio outbursts which occurred while the land was rising filled the hollows and gullies in their vicinity with streams of lava, and in the subsequent denudation of the surface of the land, these lines of ooveredup leads 'being protected by the coating of , hard basaltio lava, resisted the atmospheric wear, and eventually in many situations i became the higher ground. Thos we seethe need for the use of the diamond drill to test the lines of aariferous grajela covered by hard basaltic rooks on the Victorian goldfields — conditions scarcely likely to be found in Otago, if indeed in New Zealand. OTJB OLD EIVEB BEDS. Oar New .Zealand rivers have been gradually catting their bsds deeper and deeper -in the rocks through long periods of time, and portions of their old courses at higher 'Jevels still remain in the vicinity. These old channels are recognised by .wornout passes through xooky points and suchlike situations, and as low fiats and terrabes where the riv«r takes a bend. These lineß would no doubt be riph in gold in proportion to the length of time that «aoh was used as the river bed, the lower Uvels and the more confined passes being probably the riohost. Examples of these deserted river beds are to be seen along the course of the Molynenx' and other rivers. Goal Creek Flat, the roadway entering the Teviot Gorge below Roxburgh township, and the Horseshoe Bend are some instanoes of old courses of the Molynenx. In the Blacks district, portions of the Manuherikia's old bed are to be seen capping the tops of some of the low spurs of the Raggedy Range hi a line between the two gorges, Upwards of 200 ft above the river's present level in the Blacks Flat. Some of these old oouraea of the river, at lower levels, are said to have furnished very rich claims in the early days. Although these old river beds ate Jeadß of a class, yet it iB not to this source that we look for typical examples of deep leads. The Otago leads distinctly represent TWO CLASSES OT DEEP LEADS. .These were formed under somewhat different conditions — viz., as marine and as lacustrine deposits. The auriferous gravels of the latter class, which possess the character of deep leads in Central Otago, are represented by rich portions of the old lake strata buried beneath recent alluvium from the hillß. In valleys which tare not river valleys, these beds of auriferous gravels lie at the foot of the hills all round, practically undisturbed — at least on all sides hemmed in by ranges. In the river valleys the river has, most likely, cut awaj the greater part of the lake shore gravels on one aide or other of the valley, and consequently the opposite side is the one to look to for any representation of the deep lead. An example of the former oircamstance is eeen in the situation of Ida Valley, and of the latter in the Manukerikia Valley. There are ciroumstances which tend]

to enrich tha gravels of these leads looally, I which will b8 referred to farther on, but, ' generally considered, those alluvial diggings which have been wrought on a "false "bottom " may be expected to have good leads immediately underneath those sites, or in their near vicinity. Among the localities •where these leads are supposed to occur, or are .known to exist, might be .mentioned German Hill, Biaotos No. 3, Drybread and Tiufcerß.Oambriaas and Bt.Bafcb.anß, and a host of other places too numerous to mention LEADS OF THE OLD LAKE STRATA are very distinctive, poeseiuing many peculiarities, of which the following might be noticed. The stratum of gravel comprising the wash iof the lead lies -next the bedrock, and is overlaid by a stratified series of rather thick beds of pipeclay and fine quartz drift sand, with a seam of lignite or -osrbonaoaous matter of variable thickness as a constant member of the series. The wash is generally composed of quartz pebbles, without admixture of .foreign rock, while a characteristic colour marks the whole series in eaoh locality. In one distriot the beds of plpeolay and drift are white, in another yellow, and in another blalsh, This peculiarity, of course, suggests a common origin for each of these beds in these several loaalities. It has been almost invariably recorded that where those distinctive beds of the leads occur the bedrock is much deoompoeed, and of the same colour and charactsr as would be neoeasarj to form those beds resting upon it. The 'only conclusion then is that those peculiar strata were formed by disintegration-of the bedrock on the spot or in the immediate locality. Evidence of this geological faot is easily obtained in districts where the formation exists. As this olass of leads lies at the fjoto E the ranges, and has its coarse parallel with the line of range conforming to the character of a beach or' shore, that terra " head of the lead," as used in Australians not appropriate to any part of them. DISTUBBED POSITION OF THE STRATA. - One remarkable feature of the lake deposits is the disturbed position of the strata. Instead ofthe beds retaining their normal position, dipping gently toward the .middle of tjhe valley, they are -usually contorted and folded by compression from the hills, until in some localities the seam of auriferous gravel is quite vertical, in others it seems to dip into the hill, and in another it is so bent up as to form a V, having a gutter-like bottom more or less narrow.

At the foot of the Dans tan Rscge in the Manuherikia Valley these strata, as seen in the claims at Tinkers and Drjbread, are nearly vertical, and appear to hold this position right through the valley .to St. Bathans, The synclinal position of the bsd of auriferous wash at Blacks No. 3 —the Ida Valley deep lead— caused it to be regarded as a " gutter " of the true Australian type. Under any circumstances, the bed of wash may be expected to thin oat towards the middle of the valley, and alio to snflfcr considerable variation in thickness and change of character in its longitudinal direction.

In many places round these inland valleys and plains it will no doubt be found that sections of the lake shore deposits have been oat oat by the gaily streams from the ranges, and oarried in lines towards the middle of the valley, forming A KIND OP GULLY LEADS. Bat these last must be considered as branch leads, whioh carry from rather than to the main leads, like the Australian leads of this olass. The linos of -these gaily leads will most likely coincide very much with the courses of the present creeks, making across tha valley or plain towards the natural outlet or river. In aomo instances they will be xapresented by a thin stratum of gravelly wash , spread over a considerable area on one or both sides of the present creek's coarse. In other cases, old creak beds at higher levels than the present ones may be oonsidered to contain the auriferous gravels oolleoted as above stated.

From the fact that these valleys and plains do not represent basins that are filling up, bat rather reservoirs which have drained off, and -whose natural outlets are gradually becoming lower, we may conclude that in the old lake basins we have no leaS gutters appreciably deeper than the course* uf the present creeks. . iEADS OP MAKINE OBIGUN. Near the seaboard a class of auriferous gravels and cements of marine -origin occurs in positions along the ioot of the higher ranges facing the sea, also capping many of the iower hills and covering considerable areas of tableland and plateau, while in basins, hollows, and gullies having natural drainage from such positions will lie .the richest gold deposits of all.

These last will bear greatest resemblance to the famous gutter leads of Victoria. Probably Gabriel's Gully may be taken to represent this class, but not having intimate knowledge of the geological Barroundingsof this historical spot, this statement is given in doubt of its correct application. The Tuapeka district may bs considered to include extensive deposits cf auriferous gravels of marine origin, and irregnlar " outliers " of this same class, with rich" gullies leading down from these spots, may be expected to exist here and there through the coast country eastward towards Shag Valley. And as the auriferous character and lines of deposition of these marine gravels come to be better understood, we may ezpeot to find Other @zteusta9 deposits, approaching in

value those of the Blue Spur, Wctheistones, and other celebrated localities.

Prospecting for rich patches of these gravels should be undertaken in likely situations along tracks of country pear -the janotion of the mica schists with the eruptive rocks of the ooast v gravels of a deoided qnartzy character lying -upon decomposed schists being the moat promising. GOLD PBOSPEOTB OP THE XBADS. With regard to the gold proapeota of these Otago lead gravels, as above described, it may be confidently asserted .that they ate in all situations anriieronf, while nearly all the known outcrops — in some districts termed "the granite" — have been .proved payable, and a jEevr of these even richly so, as, for instance, the Ida Valley deep lead, which j was discovered by following -down the rich outcrop of gravels from ledge to ledge till the very rioh wash of the bottom was' reached. It is Baid that upwards of an ounce ; of gold to the bucket waa obtained in some , parts of the original mine. The known richnoes of these gravels in the Tuapeka district is further evidence of the payable nature of the beds. - , GOED-BBABING BELTS, ranging from a -quarter-mile to a milß wide, strike across the mica schist country in about a N.W. and B.E. direction, and the areas of lead gravels lying within the limits or these auriferous zones mnet be accounted .the riokefit deposits. > These interesting tracts of country rook are not difficult to trace out by the belts of chlorite schists, which deem to ooinoidc with | them ; and there is little doubb chat the gold of tha best parts of the leads was derived by the disintegration of the decomposed mica ' and chlorite schists within these belts. * Considering that vast deposits of, these lead gravels must exist round the old lake basin* of the interior, as well as at the foot of the mountains of the seaboard, the wonder is that 30 little has been done towards developing any of them. PKOBPEOTINO with tho ordinary boring-rods, to prove the oh&raoter of tho deposits and locate sites. for shafts, is the most reasonable method, and one which we' have seen carried out generally in Victoria Tvhen no hard rock was anticipated. Ab there generally exiats a lot of water in the .fine drift — more than in the wash gravels, and a clay -Beam commonly intervenes between these bedp, it may be i more economical to place the shaft in the soft-rock country at the nearest point on the hillside and drive aoroaa in it, tapping the waah beneath, and so avoid the risk of losing shafts by sinking through the watery strata overiyiDg the lead. My article has come to a close, but the subject; is by no means exhausted; bat enough has been said to prove the existence of extensive gold deposits under our feet waiting to be developed. And the statements mode herein should not be readily get down as visionary, but it should be understood that they are based on geological facts sot to be gainsaid. One more remark, however, which must not be omitted, is the singular fact that owing to the position occupied by these deposits at the foot of the mountains, they are commonly covered by a depth cf good alluvial soil, which makes the location a coveted one by the agriculturist;. And it seems a great pity for the mining interest that more pains are not taken by the Government to ascertain the auriferous nataro of these lands before they are alienated from the Grown.