THE THAMES G0LDJFIELD. ITS GEOLOGICAL FORMATION. A CHAT WITH MR PARK, F.G.S.

Te Aroha News, Volume VII, Issue 424, 30 November 1889, Page 6

THE THAMES GOLDJFIELD. ITS GEOLOGICAL FORMATION. A CHAT WITH MR PARK, F.G.S.

The Thames "Star" of IS'ov. 20, contains a report of an interview between a member of its staff and Mr James Paik, F.G.S., re garding the geological formation of the Thames goldh'eld, from which we cull the following: — " Mr Park has just competed a survey of the line ot section extending across the goldiield from Tararu to Hape Creek, which shows that the auriferous rocke of the goldfield arc arranged in what is technically known to geologists as an anticline, which means in ordinary language that the strata are fo ded in the form of a ridge. This has been ascertained by accurately noting the direction of the strike and dip of the strata at different points along the line of section. The core of the anticline is situated between the Saxon mine and the old Queen of Beauty shaft, at the mouth of the Waio-Karaka gully. From this it would appear that thedeepefct ground on the field will be found between these two point?, .so that in the event of deep level te&tin» by means of the diamond drill being undei taken, thia locality would necessarily recommend itself as a favourable point for exploration. The auriferous series ,consists of h'ne-grained tufaceous sand&tot c, alternating and passing impel coptibly into coarser - grained sandstones and breccias. The auriferous Aeins appear to be almost always found in the liner grained sandstones, which arc generally much decomposed, thus gieatly facilitating mining operations. The coarser bands of sandstone and breccias seldom, if ever, carry auriterous veins, which appear, as above stated, to be almost exclusively confined to the softer and hnergrained rocks. The aurilerous series on both sides ol the anticline is overlain, without apparently any unconformable break,by a great formation of coarse volcanic breccias and tutls, which are frequently intruded by dykes of hornblende-andesite, augite-andesite, and trachyte, fine samples of which Mr Park says may be seen on the coast north of the Tararu Point, and also in the lower valley of the Kauaeranga. This formation contains at its base or point of junction with the underlying series, traces of coal and coaly shales, indicating an approach to land conditions at the close of the auiiferous faeries. Its thickness cannot be less than one thousand feet, and it occupies by far the largest area of any other class of lock on the Peninsula. So far as he is aware, Mr Park says that no goldbearing veins have been found in thit> great volcanic formation, which has often been confounded with the auriferous series of the Thames. It is interesting to note that the Waitakerei range between the Manukau and Kaipara harbours, and the greab Pirongia range extending between the West Coast and the W aipa, are also composed of similar rocks. It is evident that during the period of their formation the Auckland Provincial district mu&t have been the scene of the most j violent, intense, and widespread volcanic activity. From stratigraphical reasons Mr Park is inclined to think that the age of this gieat barren formation composed of volcanic mateiial is probably of the lower miocene ago, and contemporary with the great volcanic outbursts which took place at Oamaru during the deposition ot the Oamaru series. So far as his observations go at present, Mr Park considers that theie are no true fissure veins on the Held — a fact which he bays may be explained by the absence of intrusive dykes cutting through the auriferous strata. He dcsciibes the reefs as bedded segregations, possessing in mo&t cases the strike and underlio of the country. A peculiarity of thepe deposits is that while the footwall may be well defined, the lode stuii generally passes into the country without any approach to a hanging-wall or defined line of demarcation. They are also, he points out, very irregular in character, being often elliptical in shape and subject to great variations in width. They will often split up into a number of parallel veins, and may receive droppers or leaderb from the walls. Looking at tho section which he has prepared, it is appaient that the anticlinal arrangement of the rocks causes the repetition of the auriferous strata, first between the Saxon mine and the Shellback Creek, and, again, between the former and Hape Creek, or, in other words, that the gold-bearing veins being worked at, say the Kuranui Hill, are the same as those at the Una Hill, and those at intervening places between the Sa>>on and Kuranui Hill find their equivalents on the opposite side of the anticline, the great dome of which has been cut down almost to tho sea level by the Karaka Creek. The enormous denudation to which the auriferous rocks of the Thames have been subjected through countless centimes suggests at once the possible, it ,nob probable, existence of alluvial leads of great wealth along the Thames foreshore on the old floor of the bay, which has been Jilled up to a depth of several hundreds of feet by muds, sands, and shelly gravels since the time of the great fault which carried down this old shoreline, thus dragging, for a time at i least, these alluvial leads out of our reach. Mr Park, who is familiar with and has reported upon many of the alluvial and reefing fields of New Zealand, bays that in places where auriferous country has been eroded to a much less degree than that of the Thames, it has resulted in the formation of alluvial deposits of great riclrness, and he expresses himself as fully satisfied that the enormous erosion to which the j '1 hames auriierous strata have been subjected, must have caused the accuirulation of wealthy deposits, but which have since been placed out of reach by the down-throw, of the Thames foreshore by the fault already alluded to. This lault, it appears, is more than of usual interest, and has not received the attention from geologists which it deserves. In the first place it is, geologically speaking, of recent date, a fact which is satisfactory pvoved, Mr Park explains, by its having drawn down the floor of the harbour, which has rapidly become filled with marine beds containing only recent shells. Again, its course in plainly marked on the surface — in fact, as plainly, Mr Park says, as the great Kaikoura fault in the Clarence Valley, a slight displacement of which, it will be remembered, last year caused the alarming earthquakes experienced at Christchurch and Hanmer Plains. Further on,|examination of the surface wheie the fault has crossed the compact rocks, the striations and markings and other evidences of slickenside3 on these are as fresh as if the faulting or sliding had only taken place yesterday. This fault, Mr Park observes, is an important factor with regard to the distribution of the gold. It cuts almost at right angles across the gold-bearing strata, which have a general north-east trend ; ifc crosses the Hape Creek below the gorge, nnd follows along the foot of the Una hill, crosses the Karaka, and passes on to the Syaiotahi hill ; from thence across the Waiotahi to the Moanata : ari hill, From there it bends slightly to the north-west, and enters the f>eu a little beyond the mouth of tho Shellback Creek. Tho dip or hade of the fault is towards tho foieshore, thus mdi

eating this as the downthrow side. A moment's consideration will show that the , country at the sea level is the sama, or I correspond*, with that at a height of about 300 feet above the sea on tho upper side of the fault. The effect of this wi'lbe better understood when it is stated that a shaft sunk from t^he tyloanataiari tunnel on the upper side of the fault would exp'ore lower countiy, and reach the old floor more quickly than a shaft put down on the foreshore, which, as previously observed, has been cariied down several hundiedss ot feet below its original position. Besides vertical downthrow, this fault has caused a certain amount of lateral displacement, which must always bet aken into account when cairying on explorations from the low side on to the high side of the fault. Referring to the diamond drill, Mr Park expresses his surprise that no attempts have been made to explore the low levels by this means, as he says these diills are well recognisbd aa a valuable accessory to prospecting in all metalliferous districts, their operations being rapid and cheap, and free frommany of the disadvantages encountered in sinking shafts, and for thi& leaeonjie strongly urges their adoption on the Thames guldtield. With reference to prospectingoutsicle of the present narrow limits of the field, Mr Park points out that the goldbearing rocks and their accompanying reefs ha\o a general north-east strike, which would take them past the head of Tinker's Guliy, acioss the left-hand branches of the Tararu Creek to the high langes foiming the watershed ... of the Taraiu and Kauaeianga streams. He is iully convinced that the prospects of finding good gold in the forest country just indicated are sufficiently encouraging to warrant the thorough exploration of that portion of the field. It is needless to point out that the gold already found in the Tararu watershed fully warrants this assumption. The country is broken and heavily timbered, but these obstacles could easily be overcome by constructing pack or even blazed tracks. Mr Park's suggestions with regard to going further afield are deserving of the serious consideration of those interested in the welfare and continuance of the goldfield. The next point touched upon was regarding what are commonly called ' buck ' reefs, several of which are known to exist upon the field, a'l being large bodies of stone. Mr Park mentioned that he had taken samples of quartz from several of those lodes, and they had been tested at the School of Mines. In almost every instance the results showed the existence of the piecious metal, the the assays in some , cases yielding bullion woith as much a§ £11 per ton— largely silver. Mr Park is' of opinion that some day these reefs will com- . mand attention and form a valuable as&et, and that miners and otheis interested, in \ the industry will fall back to them when . the richcfeb portions of the field are ex- ' hausted." ' 1