The photo is of my grandmother Mary Louisa Gaunt (not married at the time) who was born 7 Dec 1869 in New Brampton and died 27 Dec 1965 at Roosevelt Hospital, Edison, NJ, USA. She emigrated to Canada Oct 1895 and married my grandfather Edward Addy, Oct 23, 1895 in Magog, Quebec, Canada. He had come a few years earlier and was working in Magog at the time. He was born at Moorhay Farm near Old Brampton where the family had farmed for many years. In 1900 with three young children they emigrated from Canada to New Brunswick, NJ, USA where my grandfather worked for Johnson & Johnson, the pharmaceutical company (world headquarters) as a supervisor.
The photo was recently found in our attic and had been in possession of my grandparents and then our parents. I can only guess the age of the photo as taken sometime in the mid to late 1880's. Perhaps you can narrow it down further.
The card mount, the reverse of which is shown above, has a similar design to one that I have estimated (in my profile of the studio of Alfred Seaman & Sons) was used between c.1881 and 1886, but with gold ink on a glossy dark brown card instead of dark brown ink on a pale yellow or beige card (see below). It is worth noting that in very busy studios such as this one, new batches of card mounts were probably ordered on a fairly frequent basis, perhaps as often as every six months to a year. One of Alfred Seaman's sons probably started to work for him as early as 1881, but I don't believe that the firm was actually advertised as "A. Seaman & Sons" (later it was more often just "Seaman & Sons") until around 1887. This card mount merely uses the name, "A. Seaman." The reverse of the card mount lists the branches in Brewery and Burlington Streets. By May 1886, the Burlington Street branch appears to have been closed and replaced by another in Corporation Street, so we can surmise that the card mounts were at least ordered prior to that date. It also seems have been produced before to their opening of another branch studio in Ilkeston, which happened around 1886-1887.
The clothes worn by the girl/young woman, the style of the photograph and the design of the carte de visite card mount all point to a date of the early to mid 1880s, perhaps slightly earlier than the mid- to late 1880s which Bill suggested. The tight, frilled collar, with a brooch at her neck, the shape of the hat, the rounded shoulders, vertical row of buttons, and tight, narrow sleeves all suggest to me a date of between 1882 and 1886-ish. The intricate and elaborate embroidery on the tight-fitting bodice of her dress, especially, is characteristic of this period. There is a similar example on Roger Vaughan's web site, tentatively dated at 1883.
I suggest, therefore, that this portrait was probably taken between c.1884 and early 1886. It's difficult to accurately estimate the age of the young woman in your photograph, particularly as she may have been wearing a corset, but I think she is probably in her mid-teens, say between 14 and 17. This appears to fit well with your identification of the subject as your grandmother Mary Louisa Gaunt, who was born on 7 December 1869 - she would have been fifteen years old for most of 1885.
It's also worth noting that the item of studio furniture - actually a fake pillar and balustrade, probably made of wood and plaster - is identical to that pictured in a portrait of an unidentified couple sent to me by John Bradley, and included on my Seaman web page. That one is on what I have assumed to be the slightly earlier "dark brown on pale yellow" card mount, and must, I think, have been taken at around the same time as Bill's photograph. The painted backdrop may be different, although it is more likely to be just another part of the same one.